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## Meet the editor

Dr Samuel Okware is a medical doctor and public health specialist with a PhD in emerging infections. He heads health research in Uganda as the Director General of the Uganda National Health Research Organization. Previously he led the containment of several major disease outbreaks of Ebola and Marburg virus diseases in Uganda. He has led pioneering research on HIV/AIDS, which contributed to one of the earliest successful HIV/AIDS

Control Programs in Africa. He has supported many countries to set up national programs for AIDS control as a public health consultant. Dr Okware has international experience and was, until recently, a member of the WHO Expert Committee on Research and Development.

Contents

**Section 1**

**Section 2**

*and Tom Vincent*

*by David A. Schwartz*

**Section 3**

**Section 4**

**Section 5**

*by Samuel Ikwaras Okware*

**Preface III**

Introduction **1**

**Chapter 1 3**

Ecology **7**

**Chapter 2 9**

Clinical Features **29**

**Chapter 3 31**

Immunology of Filoviruses **49**

**Chapter 4 51**

Diagnostics for Filoviruses **69**

**Chapter 5 71** Molecular Diagnostics of Ebola Patient Samples by Institut Pasteur de Dakar

*Barre Soropogui, Gamou Fall, Cheikh Fall, N'Faly Magassouba, Lamine Koivogui,* 

*by Oumar Faye, Cheikh Tidiane Diagne, Amadou Diallo, Emily Meyer,* 

*Sakoba Keita, Cheikh Loucoubar, Mamadou Diop, Manfred Weidmann,* 

Maternal Filovirus Infection and Death from Marburg and Ravn Viruses: Highly Lethal to Pregnant Women and Their Fetuses Similar to Ebola Virus

Introductory Chapter: Emerging Challenges in Filovirus Control

*by Jean-Paul Gonzalez, Marc Souris, Massamba Sylla, Francisco Veas* 

Essay on the Elusive Natural History of Ebola Viruses

Interaction of Ebola Virus with the Innate Immune System *by Felix B. He, Krister Melén, Laura Kakkola and Ilkka Julkunen*

Mobile Laboratory in Guinea 2014–2016

*Ousmane Faye and Amadou Alpha Sall*

## Contents


**Chapter 6 85** The Emerging Challenges in Transmission and Detection of Filovirus Infections in Developing Countries *by Samuel Okware*

Preface

Filoviruses have been identified as new emerging, highly fatal pathogens. They mainly comprised the Marburg virus and the Ebola virus. There is no known cure and mortality remains very high. The Marburg virus infection was first identified in 1967 in Marburg Germany among laboratory workers who were accidentally infected by imported green monkeys. The first outbreak of Ebola virus disease occurred in 1976 in Kikwit, in the Democratic Republic of the Congo on the northern border with South Sudan. Since then several major outbreaks have occurred in Equatorial Africa, especially in the Democratic Republic of the Congo, Sudan, Angola, and Uganda. In 2014 to 2015, a major Ebola outbreak occurred in West Africa during which over 23,000 cases and nearly 10,000 deaths occurred. The outbreak spread to several countries and constituted an unprecedented global health emergency. Since then, the scientific knowledge on these pathogens has been increasing but a lot more

This book discusses the emerging challenges of the filovirus infections Ebola and Marburg virus disease. It reviews the complex ecology and role of bats and other suspected intermediate hosts, including pigs. Also examined are the challenges for other modes of transmission and shortcomings in detection. The immunology and pathogenesis of the infection and the impact on the host's immune system are discussed. The rare occurrence of the Marburg infection in pregnancy and its longterm consequences of sexual spread among survivors is further examined. These discussions have the potential to support future management of outbreaks and guide research for vaccines and drugs against Ebola and Marburg disease. The book also includes historical perspectives of these viruses and discussions on the sources of major outbreaks. It also examines their complex ecology and natural habitats. The role of bats as reservoirs of infection is reviewed and evidence incriminating them as a significant source of infection is discussed. Other possible reservoirs and intermediate hosts are also examined. The challenges for the modes of spread are discussed against new scientific findings. The further discovery of the infection in pigs raises the possibility of an intermediate host or additional reservoirs. The potential for the infection to enter the food chain is examined. Evidence is also presented suggesting sexual transmission of filovirus infection with details of long-term persistence of infection in the body tissues of survivors. The book also gives an extensive review of the immunology of the infection and the impact of their interactions with the host's immune system. The pathogenesis of the diseases is analyzed. The shortcomings in procedures for early detection and diagnosis are discussed. New diagnostic tools using dried reagents adapted for field diagnosis of

There is little information on the Marburg virus disease in pregnancy. The book describes a rare occurrence and in particular the impact of pregnancy on this infection. It also reveals the very high infectiousness and fatality rate during this period compared with known Ebola outbreaks. The role of pregnancy and the products of contraception, including the fetus, in enhancing transmission is discussed. A rare review is made of the long-term sustained sexual transmission of Marburg virus infection. Evidence for severe testicular damage and dysfunction is also

on the ecology and treatment remains elusive.

Ebola infection are suggested and described.

## Preface

Filoviruses have been identified as new emerging, highly fatal pathogens. They mainly comprised the Marburg virus and the Ebola virus. There is no known cure and mortality remains very high. The Marburg virus infection was first identified in 1967 in Marburg Germany among laboratory workers who were accidentally infected by imported green monkeys. The first outbreak of Ebola virus disease occurred in 1976 in Kikwit, in the Democratic Republic of the Congo on the northern border with South Sudan. Since then several major outbreaks have occurred in Equatorial Africa, especially in the Democratic Republic of the Congo, Sudan, Angola, and Uganda. In 2014 to 2015, a major Ebola outbreak occurred in West Africa during which over 23,000 cases and nearly 10,000 deaths occurred. The outbreak spread to several countries and constituted an unprecedented global health emergency. Since then, the scientific knowledge on these pathogens has been increasing but a lot more on the ecology and treatment remains elusive.

This book discusses the emerging challenges of the filovirus infections Ebola and Marburg virus disease. It reviews the complex ecology and role of bats and other suspected intermediate hosts, including pigs. Also examined are the challenges for other modes of transmission and shortcomings in detection. The immunology and pathogenesis of the infection and the impact on the host's immune system are discussed. The rare occurrence of the Marburg infection in pregnancy and its longterm consequences of sexual spread among survivors is further examined. These discussions have the potential to support future management of outbreaks and guide research for vaccines and drugs against Ebola and Marburg disease. The book also includes historical perspectives of these viruses and discussions on the sources of major outbreaks. It also examines their complex ecology and natural habitats. The role of bats as reservoirs of infection is reviewed and evidence incriminating them as a significant source of infection is discussed. Other possible reservoirs and intermediate hosts are also examined. The challenges for the modes of spread are discussed against new scientific findings. The further discovery of the infection in pigs raises the possibility of an intermediate host or additional reservoirs. The potential for the infection to enter the food chain is examined. Evidence is also presented suggesting sexual transmission of filovirus infection with details of long-term persistence of infection in the body tissues of survivors. The book also gives an extensive review of the immunology of the infection and the impact of their interactions with the host's immune system. The pathogenesis of the diseases is analyzed. The shortcomings in procedures for early detection and diagnosis are discussed. New diagnostic tools using dried reagents adapted for field diagnosis of Ebola infection are suggested and described.

There is little information on the Marburg virus disease in pregnancy. The book describes a rare occurrence and in particular the impact of pregnancy on this infection. It also reveals the very high infectiousness and fatality rate during this period compared with known Ebola outbreaks. The role of pregnancy and the products of contraception, including the fetus, in enhancing transmission is discussed. A rare review is made of the long-term sustained sexual transmission of Marburg virus infection. Evidence for severe testicular damage and dysfunction is also

**II**

**Chapter 6 85** The Emerging Challenges in Transmission and Detection of Filovirus Infections

in Developing Countries *by Samuel Okware*

reported in some detail. The discussions on the ecology, reservoirs, immunology, and the challenges in case detection have the potential to assist in the management of future outbreaks of filovirus infections as well as guide for future research agenda for related vaccines and drugs.

The editor of this book would like to thank the authors for their contributions. I also thank the Project Manager Lada Bozic for the constant valuable assistance during the preparation of this book. Many thanks also go to the IntechOpen team their support in publishing this book.

> **Dr Samuel Okware** Director General, Uganda National Health Research Organisation, Entebbe, Uganda

> > **1**

Section 1

Introduction

Section 1 Introduction

**3**

**Chapter 1**

**1. Introduction**

*Samuel Ikwaras Okware*

Introductory Chapter: Emerging

Infectious diseases in history have made a significant contribution to morbidity and mortality as well as disability worldwide. Nearly a quarter of the estimated 60 million reported deaths in the world each year are related to infectious diseases [1]. The influenza outbreak of 1918–1919 was the worst such incident in living memory during which nearly 40 million people died worldwide. The unprecedented *Black Death* plague outbreak in the mid 1300 equally killed millions of people. Filoviruses as emerging infections appear to be on the same path, with an ever-increasing significant global impact on public health, human traffic and commerce. The recent West African Ebola outbreak which affected 10 countries in West Africa, Europe and the USA has demonstrated its capacity to be a global threat with profound psychological, emotional and mental repercussions. Its highly virulent nature over the years and the recent West African Ebola outbreak generated considerable panic and unprecedented global public health emergency [2]. Filoviruses are comprised of the Marburg and Ebola viruses. Since the discovery of the Marburg virus disease in the 1967, and the Ebola virus disease in 1976, over 50 filovirus disease outbreaks due to *Marburgvirus and Ebolavirus* have occurred. Some 37 Ebola virus disease and 14 Marburg virus disease outbreaks occurred mostly in Africa. The recent 2013–2016 outbreak in West Africa was so far the largest and most devastating. At the end of the epidemic, about 28,000 cases and 11,000 deaths were recorded. The case fatality for both viruses is very high (34–90%). There is yet no known cure. Since July 2018, the second largest EBOV outbreak is devastating Eastern Democratic Republic of the Congo with over 1600 cases and 1000 deaths reported by October 2019 [3]. New data from the West African epidemic suggests an expansion of our understanding on ecology and geographical scope of these viruses. The pattern of occurrence and its

negative impact on the economy, society and development is emerging.

Understanding the ecology and virology of filoviruses helps in designing strategies for prevention and control. Filoviruses are non-segmented negative-stranded RNA viruses. They belong to the family *Filoviridae* in the order *Mononegavirales.* There are five genera in the filovirus family: the *Marburgvirus*, *Ebolavirus*,

*Cuevavirus*, *Striavirus*, and *Thamnovirus.* The Marburg virus and the Ebola virus are the most virulent to humans, while TAFV cause very limited disease and RESTV only asymptomatic infections. The *Ebolavirus* has five species known to cause disease in humans: *Zaire ebolavirus* (EBOV), *Sudan ebolavirus* (SUDV), *Tai Forest ebolavirus* (TAFV), *Reston ebolavirus* (RESTV), and the *Bundibugyo ebolavirus* (BDBV). In addition the *Bombali ebolavirus* was recently discovered in fruit bats in

**2. Perspectives on ecology and transmission**

Challenges in Filovirus Control

#### **Chapter 1**

## Introductory Chapter: Emerging Challenges in Filovirus Control

*Samuel Ikwaras Okware*

#### **1. Introduction**

Infectious diseases in history have made a significant contribution to morbidity and mortality as well as disability worldwide. Nearly a quarter of the estimated 60 million reported deaths in the world each year are related to infectious diseases [1]. The influenza outbreak of 1918–1919 was the worst such incident in living memory during which nearly 40 million people died worldwide. The unprecedented *Black Death* plague outbreak in the mid 1300 equally killed millions of people. Filoviruses as emerging infections appear to be on the same path, with an ever-increasing significant global impact on public health, human traffic and commerce. The recent West African Ebola outbreak which affected 10 countries in West Africa, Europe and the USA has demonstrated its capacity to be a global threat with profound psychological, emotional and mental repercussions. Its highly virulent nature over the years and the recent West African Ebola outbreak generated considerable panic and unprecedented global public health emergency [2]. Filoviruses are comprised of the Marburg and Ebola viruses. Since the discovery of the Marburg virus disease in the 1967, and the Ebola virus disease in 1976, over 50 filovirus disease outbreaks due to *Marburgvirus and Ebolavirus* have occurred. Some 37 Ebola virus disease and 14 Marburg virus disease outbreaks occurred mostly in Africa. The recent 2013–2016 outbreak in West Africa was so far the largest and most devastating. At the end of the epidemic, about 28,000 cases and 11,000 deaths were recorded. The case fatality for both viruses is very high (34–90%). There is yet no known cure. Since July 2018, the second largest EBOV outbreak is devastating Eastern Democratic Republic of the Congo with over 1600 cases and 1000 deaths reported by October 2019 [3]. New data from the West African epidemic suggests an expansion of our understanding on ecology and geographical scope of these viruses. The pattern of occurrence and its negative impact on the economy, society and development is emerging.

#### **2. Perspectives on ecology and transmission**

Understanding the ecology and virology of filoviruses helps in designing strategies for prevention and control. Filoviruses are non-segmented negative-stranded RNA viruses. They belong to the family *Filoviridae* in the order *Mononegavirales.* There are five genera in the filovirus family: the *Marburgvirus*, *Ebolavirus*, *Cuevavirus*, *Striavirus*, and *Thamnovirus.* The Marburg virus and the Ebola virus are the most virulent to humans, while TAFV cause very limited disease and RESTV only asymptomatic infections. The *Ebolavirus* has five species known to cause disease in humans: *Zaire ebolavirus* (EBOV), *Sudan ebolavirus* (SUDV), *Tai Forest ebolavirus* (TAFV), *Reston ebolavirus* (RESTV), and the *Bundibugyo ebolavirus* (BDBV). In addition the *Bombali ebolavirus* was recently discovered in fruit bats in

Sierra Leone and Kenya. Also a new distinct filovirus, the *Dianlovirus* genus, has been proposed following the recent discovery of the *Měnglà virus* (MLAV) in fruit bats in China [4], demonstrating further the expanding geographical scope of the Ebola virus ecology. It is yet to be fully determined whether the impact of population pressures such as deforestation and forest encroachment or subsequent climate change has also leveraged the ecosystems for transmission.

The origin of the infection and its life cycle is partly elusive. It is generally accepted that the infection is a zoonosis linked to wildlife reservoirs principally fruit bats and non-human primates. Studies indicate that such bats may be the ultimate reservoirs of this infection. Epizootics in wildlife have also been reported prior to outbreaks. It is also suggested that in endemic countries, non-human primates and other animals including pigs, dogs, duikers and even arthropods may be involved in the cycle linking wildlife infection to humans [5]. Direct contact during hunting and eating bush meat facilitates rapid spread. At community level, funeral ceremonies of the victims amplify further transmission. In reported outbreaks of Marburg virus infection in Kenya, Angola, the Congo Republic and the DRC, Uganda has demonstrated cave-dwelling fruit bats as the source of infection. Serological ecological studies also have showed sero-positivity in asymptomatic individuals in selected communities in the DRC. Among the pigmy population nearest to the forest, the sero-positivity for EBOV is high and nearly 10-fold. Sexual transmission of Ebola infection among survivors raises concerns. Questions remain on the role linking these observations particularly the role of asymptomatic individuals in the community outbreak initiation.

#### **3. Challenges on case detection**

The clinical features of Ebola virus disease and Marburg virus disease have been consistent and the basis for the case definition in the detection of cases and contact tracing. The clinical features are typically high-grade fever associated with severe bleeding tendencies and followed by a rapid descent to multiple organ failure, shock and death within days. However these symptoms are nonspecific and mimic several many tropical conditions such as malaria which is so endemic and responsible for up to a quarter of the patient load in typical low-resource settings. This may undermine the timely detection of cases in outbreak management and affect the implementation of contact tracing using the WHO syndrome based on the case definition and criteria. The WHO has outlined the case definition criteria of an *alert* case, a *suspected* case, a *probable* case and a *confirmed* case. The first three are based on clinical symptom assessment, and only the confirmed case depends on laboratory confirmation with RT-PCR or IgG antibody and virus antigen for Ebola virus and Marburg virus. However, the specificity and the positive predictive values in reported laboratory tests have not been accurately determined. The challenge in making a diagnosis is that the positive predictive value of the criteria may differ from outbreak to outbreak. Studies are therefore needed for concurrent validation of the case definition at localised field conditions and identification of cross-reactions in asymptomatic individuals. Concurrent validation studies should be carried out during outbreak management and containment. Therefore the search for new and accurate diagnostic methods needs to be addressed.

The factors behind the emergence of new pathogens are complex but are facilitated partly by the enabling interaction between the host and the agent in the supported by conducive environmental factors. Apparently for emerging infections, pathogens evolve and create new phenotypic properties that adapt infectious agents to new or old hosts. The genetic variation may lead to increased virulence

**5**

*Introductory Chapter: Emerging Challenges in Filovirus Control*

and infectivity. Understanding these factors including the immunology and the interactions between the filovirus and the host immune system is critical. Such knowledge will support the development of better diagnostics and tools. These tools will facilitate surveillance and outbreak management. It will provide evidence for

This book reviews and discusses known filovirus outbreak experiences. In particular, it examines opportunities and the missing links in the ecology, the natural history, immunology and the interactions with the host innate immune systems and other infections. It examines the potential benefits that would shape future research priorities. Such efforts could lead to quality and timely outbreak detection. Early detection and early action appear to be best approach, but such strategies should

The epidemiology and the ecology together with the life cycle remain elusive. Studies are required to improve early detection to facilitate quick action. The unpredictability of the outbreaks suggests that basic epidemiological research for prevention and control should be carried out before, during and after outbreaks. Understanding the gaps in the ecology and the natural cycles of the filoviruses as well as its reservoirs will lead to the development of better strategies for prevention, control and management of future outbreaks. Campaigns directed at communities and tourists would be of benefit. Studies are needed to improve future forecasting of outbreaks. The impact of these viruses on the economy and society in general is an important area for future research. There is therefore a need for a strong global strategy that ensures international and interagency collaboration. International efforts are required to coordinate research to develop preventive strategies and tools. Support is required to support national efforts to build health systems for surveillance and emergency disease preparedness. A Global Health framework for coordination and financing of research into emerging infections will support and facilitate containment at national and global levels. The chapters in this book have tried to discuss some of these challenges and made suggestions for future research.

the development of effective drugs and vaccines against the infection.

*DOI: http://dx.doi.org/10.5772/intechopen.90653*

use evidence for prevention and control.

**4. Conclusion**

**Author details**

Samuel Ikwaras Okware

Uganda National Health Research Organisation, Entebbe, Uganda

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: okwares@gmail.com

provided the original work is properly cited.

#### *Introductory Chapter: Emerging Challenges in Filovirus Control DOI: http://dx.doi.org/10.5772/intechopen.90653*

and infectivity. Understanding these factors including the immunology and the interactions between the filovirus and the host immune system is critical. Such knowledge will support the development of better diagnostics and tools. These tools will facilitate surveillance and outbreak management. It will provide evidence for the development of effective drugs and vaccines against the infection.

This book reviews and discusses known filovirus outbreak experiences. In particular, it examines opportunities and the missing links in the ecology, the natural history, immunology and the interactions with the host innate immune systems and other infections. It examines the potential benefits that would shape future research priorities. Such efforts could lead to quality and timely outbreak detection. Early detection and early action appear to be best approach, but such strategies should use evidence for prevention and control.

#### **4. Conclusion**

*Emerging Challenges in Filovirus Infections*

community outbreak initiation.

**3. Challenges on case detection**

Sierra Leone and Kenya. Also a new distinct filovirus, the *Dianlovirus* genus, has been proposed following the recent discovery of the *Měnglà virus* (MLAV) in fruit bats in China [4], demonstrating further the expanding geographical scope of the Ebola virus ecology. It is yet to be fully determined whether the impact of population pressures such as deforestation and forest encroachment or subsequent climate

The origin of the infection and its life cycle is partly elusive. It is generally accepted that the infection is a zoonosis linked to wildlife reservoirs principally fruit bats and non-human primates. Studies indicate that such bats may be the ultimate reservoirs of this infection. Epizootics in wildlife have also been reported prior to outbreaks. It is also suggested that in endemic countries, non-human primates and other animals including pigs, dogs, duikers and even arthropods may be involved in the cycle linking wildlife infection to humans [5]. Direct contact during hunting and eating bush meat facilitates rapid spread. At community level, funeral ceremonies of the victims amplify further transmission. In reported outbreaks of Marburg virus infection in Kenya, Angola, the Congo Republic and the DRC, Uganda has demonstrated cave-dwelling fruit bats as the source of infection. Serological ecological studies also have showed sero-positivity in asymptomatic individuals in selected communities in the DRC. Among the pigmy population nearest to the forest, the sero-positivity for EBOV is high and nearly 10-fold. Sexual transmission of Ebola infection among survivors raises concerns. Questions remain on the role linking these observations particularly the role of asymptomatic individuals in the

The clinical features of Ebola virus disease and Marburg virus disease have been consistent and the basis for the case definition in the detection of cases and contact tracing. The clinical features are typically high-grade fever associated with severe bleeding tendencies and followed by a rapid descent to multiple organ failure, shock and death within days. However these symptoms are nonspecific and mimic several many tropical conditions such as malaria which is so endemic and responsible for up to a quarter of the patient load in typical low-resource settings. This may undermine the timely detection of cases in outbreak management and affect the implementation of contact tracing using the WHO syndrome based on the case definition and criteria. The WHO has outlined the case definition criteria of an *alert* case, a *suspected* case, a *probable* case and a *confirmed* case. The first three are based on clinical symptom assessment, and only the confirmed case depends on laboratory confirmation with RT-PCR or IgG antibody and virus antigen for Ebola virus and Marburg virus. However, the specificity and the positive predictive values in reported laboratory tests have not been accurately determined. The challenge in making a diagnosis is that the positive predictive value of the criteria may differ from outbreak to outbreak. Studies are therefore needed for concurrent validation of the case definition at localised field conditions and identification of cross-reactions in asymptomatic individuals. Concurrent validation studies should be carried out during outbreak management and containment. Therefore the search

for new and accurate diagnostic methods needs to be addressed.

The factors behind the emergence of new pathogens are complex but are facilitated partly by the enabling interaction between the host and the agent in the supported by conducive environmental factors. Apparently for emerging infections, pathogens evolve and create new phenotypic properties that adapt infectious agents to new or old hosts. The genetic variation may lead to increased virulence

change has also leveraged the ecosystems for transmission.

**4**

The epidemiology and the ecology together with the life cycle remain elusive. Studies are required to improve early detection to facilitate quick action. The unpredictability of the outbreaks suggests that basic epidemiological research for prevention and control should be carried out before, during and after outbreaks. Understanding the gaps in the ecology and the natural cycles of the filoviruses as well as its reservoirs will lead to the development of better strategies for prevention, control and management of future outbreaks. Campaigns directed at communities and tourists would be of benefit. Studies are needed to improve future forecasting of outbreaks. The impact of these viruses on the economy and society in general is an important area for future research. There is therefore a need for a strong global strategy that ensures international and interagency collaboration. International efforts are required to coordinate research to develop preventive strategies and tools. Support is required to support national efforts to build health systems for surveillance and emergency disease preparedness. A Global Health framework for coordination and financing of research into emerging infections will support and facilitate containment at national and global levels. The chapters in this book have tried to discuss some of these challenges and made suggestions for future research.

#### **Author details**

Samuel Ikwaras Okware Uganda National Health Research Organisation, Entebbe, Uganda

\*Address all correspondence to: okwares@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **References**

[1] Fauci AS, Touchette NA, Folkers GK. Emerging infectious diseases: A 10-year perspective from the national institute of allergy and infectious diseases. Emerging Infectious Diseases. 2005;**11**:519-525

[2] Centers for Disease Control and Prevention. 2014-2016 Ebola Outbreak in West Africa. 2018. Available from: https://www.cdc.gov/vhf/ebola/ history/2014-2016-outbreak/index.html [Accessed: 9 July 2019]

[3] Centers for Disease Control and Prevention. Ebola Outbreak in eastern Democratic Republic of Congo Tops 1000 cases. CDC Newsroom. 2019. Available from: https://www.cdc.gov/ media/releases/2019/s0322-ebolacongo.html [Accessed: 16 April 2019]

[4] Yang X-L, Tan CW, Anderson DE, et al. Characterization of a filovirus (*Měnglà virus*) from Rousettus bats in China. Nature Microbiology. 2019;**4**:390-395

[5] Leroy EM, Rouquet P, Formenty P, et al. Multiple Ebola virus transmission events and rapid decline of central African wildlife. Science. 2004;**303**:387-390

**7**

Section 2

Ecology

Section 2 Ecology

**6**

*Emerging Challenges in Filovirus Infections*

[1] Fauci AS, Touchette NA, Folkers GK. Emerging infectious diseases: A 10-year perspective from the national institute of allergy and infectious diseases. Emerging Infectious

**References**

Diseases. 2005;**11**:519-525

[Accessed: 9 July 2019]

2019;**4**:390-395

2004;**303**:387-390

[2] Centers for Disease Control and Prevention. 2014-2016 Ebola Outbreak in West Africa. 2018. Available from: https://www.cdc.gov/vhf/ebola/

[3] Centers for Disease Control and Prevention. Ebola Outbreak in eastern Democratic Republic of Congo Tops 1000 cases. CDC Newsroom. 2019. Available from: https://www.cdc.gov/ media/releases/2019/s0322-ebolacongo.html [Accessed: 16 April 2019]

[4] Yang X-L, Tan CW, Anderson DE, et al. Characterization of a filovirus (*Měnglà virus*) from Rousettus bats in China. Nature Microbiology.

[5] Leroy EM, Rouquet P, Formenty P, et al. Multiple Ebola virus transmission

events and rapid decline of central African wildlife. Science.

history/2014-2016-outbreak/index.html

**9**

**Chapter 2**

**Abstract**

**1. Introduction**

Essay on the Elusive Natural

*Jean-Paul Gonzalez, Marc Souris, Massamba Sylla,* 

This chapter presents a review of what is known about the natural history of the Ebolaviruses in Central and West Africa as well as in the Philippines. All the previous hypotheses on the natural cycle of Ebolavirus are revisited. Also, the main factors driving the virus natural cycle are summarized for the different ecosystems where the Ebolavirus is known to have emerged, including the virus species, the date of emergence, the seasonality, the environmental features, as well as the potential risk and associated factors of emergence. The proposed hypothesis of the Ebolavirus natural cycle prevails an inter-species spillover involving several vertebrate hosts, as well as biotic and abiotic changing environmental factors among other original features of a complex natural cycle. It is also compared with other virus having such type of cycle involving chiropteran as potential reservoir and vector and presenting such original inter-outbreak epidemiological silences. Ultimately, these observations and hypotheses on Ebolavirus natural cycles give some insight into the potential drivers of virus emergence, host co-evolution, and a spatiotemporal dimension of risk leading to identify high risk areas for preventing

History of Ebola Viruses

emerging events and be prepared for an early response.

**Keywords:** Ebolavirus, bats, chorology, natural cycle, host, one health

It has been several decades since an unknown fever dramatically emerged, close to the Ebola river, a small tributary of the great Ubangi river in the heart of the Congolese tropical forest of Africa. Since that time, even though the virus responsible for this new hemorrhagic fever has been identified and characterized, the natural history of the eponymic Ebolavirus remains largely unknown. The cradle of the virus remains enigmatic and the emergence of the Ebola fever unsolved. Indeed, the arcane of Ebolavirus natural history is still hypothesized, thanks to an elusive virus that always risen where it was not expected, violent and devastating, and surprising local populations and health systems, as well as the international scientific community. This Ebolavirus eco-epidemiology remains complex while the Ebola fever (alias Ebolavirus Disease) can be considered as an exemplary disease that can be eventually comprehended only with a transdisciplinary approach that has recently been promoted as a One Health concept. Indeed, it is only when we take into account all disease and virus drivers, including

*Francisco Veas and Tom Vincent*

#### **Chapter 2**

## Essay on the Elusive Natural History of Ebola Viruses

*Jean-Paul Gonzalez, Marc Souris, Massamba Sylla, Francisco Veas and Tom Vincent*

#### **Abstract**

This chapter presents a review of what is known about the natural history of the Ebolaviruses in Central and West Africa as well as in the Philippines. All the previous hypotheses on the natural cycle of Ebolavirus are revisited. Also, the main factors driving the virus natural cycle are summarized for the different ecosystems where the Ebolavirus is known to have emerged, including the virus species, the date of emergence, the seasonality, the environmental features, as well as the potential risk and associated factors of emergence. The proposed hypothesis of the Ebolavirus natural cycle prevails an inter-species spillover involving several vertebrate hosts, as well as biotic and abiotic changing environmental factors among other original features of a complex natural cycle. It is also compared with other virus having such type of cycle involving chiropteran as potential reservoir and vector and presenting such original inter-outbreak epidemiological silences. Ultimately, these observations and hypotheses on Ebolavirus natural cycles give some insight into the potential drivers of virus emergence, host co-evolution, and a spatiotemporal dimension of risk leading to identify high risk areas for preventing emerging events and be prepared for an early response.

**Keywords:** Ebolavirus, bats, chorology, natural cycle, host, one health

#### **1. Introduction**

It has been several decades since an unknown fever dramatically emerged, close to the Ebola river, a small tributary of the great Ubangi river in the heart of the Congolese tropical forest of Africa. Since that time, even though the virus responsible for this new hemorrhagic fever has been identified and characterized, the natural history of the eponymic Ebolavirus remains largely unknown. The cradle of the virus remains enigmatic and the emergence of the Ebola fever unsolved. Indeed, the arcane of Ebolavirus natural history is still hypothesized, thanks to an elusive virus that always risen where it was not expected, violent and devastating, and surprising local populations and health systems, as well as the international scientific community. This Ebolavirus eco-epidemiology remains complex while the Ebola fever (alias Ebolavirus Disease) can be considered as an exemplary disease that can be eventually comprehended only with a transdisciplinary approach that has recently been promoted as a One Health concept. Indeed, it is only when we take into account all disease and virus drivers, including biotic and abiotic factors of the natural and human environments, that some mechanisms of the Ebolavirus disease emergence, such as spread and circulation, can be ultimately unveiled. For that, we have collected all information available, often estimated, from the time and place of the virus emergence long before the emerging event was identified as it and the epidemic phase was brought to public attention. Moreover, when available we also collect all data on potential natural and accidental hosts, weather and environment chorology, among other multiple factors potentially involved.

Historically, Ebolavirus emerged in Central Africa in the late 1970s, and has re-emerged most recently with the active epidemic (April 2019) in the eastern Democratic Republic of Congo (DRC), by encompassing more than 24 epidemic events from Central to West Africa, to imported infected monkey from Asia to Virginia, and the emerging new Ebola species of the Philippines archipelago [1].

Among the negative sense RNA viruses of the *Filoviridae* family five genera are known, including *Cuevavirus, Ebolavirus, Marburgvirus*, *Thamnovirus*. Among the *Ebolavirus* genus, five Ebolavirus (EBOV) species have been identified [2].

Ebolavirus' (EBOV) first emergence occurred in 1976, as two different EBOV species in two different places in sub Saharan Africa. The Zaire Ebolavirus (ZEBOV) species and the Sudan Ebolavirus (SUDV) were detected concomitantly, a few weeks apart, respectively in the Northeastern Equator province of the Democratic Republic of Congo, DRC (alias Zaire), and in the Bahr el Ghazal province of South Sudan. On the 26th of August 1976 ZEBOV was isolated from missionaries and local villagers of the Yambuku, in the rain forest close to the Ebola river. However, earlier in June 1976, the SUDV had broken out among cotton factory workers in Nzara, Sudan (now in South Sudan) [3].

Then, in 1989, the Reston Ebolavirus species surprisingly (RESTV) emerged in the US (!) and was identified during an outbreak of simian hemorrhagic fever virus in crab-eating macaques from Hazleton Laboratories (now Covance) of Reston county, Virginia. Such primate specimens were found to be recently imported from the Philippines. Then, in 1994 a fourth new species of Ebolavirus was isolated from chimpanzee leaving in the Tai Forest of Côte d'Ivoire and named Côte d'Ivoire ebolavirus (CIEBOV). Finally, in November 2007, a fifth Ebolavirus species, was detected from infected patients in Uganda in the Bundibugyo District and was subsequently identified by the eponymic name of Bundibugyo Ebolavirus [4].

Briefly and extraordinarily among the world of the viruses, the filovirus virion presents a bacilliform (filamentous) shape, like a Rhabdovirus, but presents unique pleomorphic figures with branches and other tortuous shapes. Ebolaviruses have also an unusual and variable long length - up to 805 nanometers (only some plant virus can compete to this filamentous extensive length). However, the internal structure is more classical with a ribonucleoprotein nucleocapsid, a lipid envelope and seven nanometers size spikes. The genome is non-segmented, single stranded RNA of negative polarity with lengths of about 18.9 kb that code for seven proteins, each one having a specific function [5].

Ebolaviruses are known for their high case-fatality rate (CFR) with always less than 2/3 of survivors among the identified cases. ZEBOV, the most frequently isolated Ebolavirus species during the outbreaks, has the highest CFR, up to 90% in some instances, with an average of 83% for the past 37 years. The Uganda BDBV outbreak had a mortality rate of 34%. RESTV imported to the US did not cause disease in exposed human laboratory workers. The scientist performing the necropsies on CIEBOV infected chimpanzees got infected and developed a Dengue-like fever, fully recovered 6 weeks after the infection while treated in Switzerland.

**11**

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

journey of a deadly Ebolavirus outbreaks.

**2.1 The Ebolavirus species emerging events**

were tested for Ebolavirus without success [6, 7].

epizootics in the island of Luzon (Philippines) [9].

90 km NW from the village [6].

chimpanzees preyed [10].

**2. When Ebolavirus raised his head in the heart of darkness**

Dates and time make History. Indeed, the various reports on the emergence of Ebolavirus in Africa show discrepancies and lack accuracy, for multiple reasons (remote event, reports by different person or team, at different time…) but the only way to forge the history is to label the events with date, time and the environmental factors observed. On July 27, 1976, the first (known) victim to contract Ebolavirus was a cotton factory worker from Nzara, Sudan. Then, in Zaire (DRC) on September 1, 1976, the first Ebolavirus (Zaire ebolavirus, ZEBOV) victim was a teacher who had just returned from a family visit to northern Zaire (6 Jennifer Rosenberg Internet). These two events were the very beginning of the boundless

When the virus becomes epidemic in a human population, it does so weeks or months after the emergent event of the virus switching from its silent transmission in a natural cycle to a zoonotic/epidemic manifestation, revealed to the local health system. Let us see in more detail such emerging events of Ebolavirus species (ICTV,

Sudan ebolavirus (SEBOV) occurred when the first recorded SUDV broke out among cotton factory workers in Nzara, South Sudan in June 271,976. This was indeed, the first case of Ebolavirus infection recorded and confirmed and also reported as potentially exposed to chiropteran. Indeed, at the Nzara Cotton Manufacturing Factory this first patient was a cloth room worker where bats (mostly *Tadarida -* mops *- trevori*) have a large population in the roof space of their premises. He died in the Nzara hospital on July 6, 1976. Local animals and insects

Zaire ebolavirus (ZEBOV) was reported in the Mongala district of the Democratic Republic of Congo (DRC; alias Zaire) in August 1976, when a 44-yearold schoolteacher of the Yambuku village, became the first recorded case of Ebolavirus infection in DRC. Also, the schoolteacher travel earlier in August 1976 near the Central African Republic border and along the Ebola River, estimated

Reston ebolavirus (REBOV) had its first emerging event as an imported infected cynomolgus monkey (*Macaca fascicularis*) in October 1989 imported from a facility in the Philippines (Mindanao Island) to Reston, Virginia, USA, where the primate got sick and the virus isolated [8]. In the Philippines, in several instances, the virus was found to infect pigs, in June and September 2008 ill pigs were confirmed to be infested by REBOV (Ecija and Bulacan, Manila island), as well during 2008–2009

Cote d'Ivoire ebolavirus (CIEBOV) was isolated for the first time, and as an only known appearance, in November 1994, from wild chimpanzees presenting severe internal bleeding of the Taï Forest in Côte d'Ivoire, Africa. A researcher became infected when practicing a necropsy on one of these primates, he developed a dengue syndrome and survived. At that time, many dead chimpanzees were discovered and tested positive for Ebolavirus. However, the source of the virus was believed to be of infected western red colobus monkeys (*Piliocolobus badius*) upon which the

Bundibugyo ebolavirus (BDBV) was then discovered during an outbreak of Ebolavirus in the Bundibugyo District (Bundibugyo and Kikyo townships), on August 1st, 2007, in Western Uganda (Towner et al. [11]). BDBV second emerging

2018) as there were reported or sometime interpreted, in time and place.

*Emerging Challenges in Filovirus Infections*

factors potentially involved.

Sudan (now in South Sudan) [3].

each one having a specific function [5].

biotic and abiotic factors of the natural and human environments, that some mechanisms of the Ebolavirus disease emergence, such as spread and circulation, can be ultimately unveiled. For that, we have collected all information available, often estimated, from the time and place of the virus emergence long before the emerging event was identified as it and the epidemic phase was brought to public attention. Moreover, when available we also collect all data on potential natural and accidental hosts, weather and environment chorology, among other multiple

Historically, Ebolavirus emerged in Central Africa in the late 1970s, and has re-emerged most recently with the active epidemic (April 2019) in the eastern Democratic Republic of Congo (DRC), by encompassing more than 24 epidemic events from Central to West Africa, to imported infected monkey from Asia to Virginia, and the emerging new Ebola species of the Philippines archipelago [1]. Among the negative sense RNA viruses of the *Filoviridae* family five genera are known, including *Cuevavirus, Ebolavirus, Marburgvirus*, *Thamnovirus*. Among the

Ebolavirus' (EBOV) first emergence occurred in 1976, as two different EBOV species in two different places in sub Saharan Africa. The Zaire Ebolavirus (ZEBOV) species and the Sudan Ebolavirus (SUDV) were detected concomitantly, a few weeks apart, respectively in the Northeastern Equator province of the Democratic Republic of Congo, DRC (alias Zaire), and in the Bahr el Ghazal province of South Sudan. On the 26th of August 1976 ZEBOV was isolated from missionaries and local villagers of the Yambuku, in the rain forest close to the Ebola river. However, earlier in June 1976, the SUDV had broken out among cotton factory workers in Nzara,

Then, in 1989, the Reston Ebolavirus species surprisingly (RESTV) emerged in the US (!) and was identified during an outbreak of simian hemorrhagic fever virus in crab-eating macaques from Hazleton Laboratories (now Covance) of Reston county, Virginia. Such primate specimens were found to be recently imported from the Philippines. Then, in 1994 a fourth new species of Ebolavirus was isolated from chimpanzee leaving in the Tai Forest of Côte d'Ivoire and named Côte d'Ivoire ebolavirus (CIEBOV). Finally, in November 2007, a fifth Ebolavirus species, was detected from infected patients in Uganda in the Bundibugyo District and was subsequently identified by the eponymic name of Bundibugyo Ebolavirus [4].

Briefly and extraordinarily among the world of the viruses, the filovirus virion presents a bacilliform (filamentous) shape, like a Rhabdovirus, but presents unique pleomorphic figures with branches and other tortuous shapes. Ebolaviruses have also an unusual and variable long length - up to 805 nanometers (only some plant virus can compete to this filamentous extensive length). However, the internal structure is more classical with a ribonucleoprotein nucleocapsid, a lipid envelope and seven nanometers size spikes. The genome is non-segmented, single stranded RNA of negative polarity with lengths of about 18.9 kb that code for seven proteins,

Ebolaviruses are known for their high case-fatality rate (CFR) with always less than 2/3 of survivors among the identified cases. ZEBOV, the most frequently isolated Ebolavirus species during the outbreaks, has the highest CFR, up to 90% in some instances, with an average of 83% for the past 37 years. The Uganda BDBV outbreak had a mortality rate of 34%. RESTV imported to the US did not cause disease in exposed human laboratory workers. The scientist performing the necropsies on CIEBOV infected chimpanzees got infected and developed a Dengue-like fever, fully recovered 6 weeks after the infection while treated in

*Ebolavirus* genus, five Ebolavirus (EBOV) species have been identified [2].

**10**

Switzerland.

### **2. When Ebolavirus raised his head in the heart of darkness**

Dates and time make History. Indeed, the various reports on the emergence of Ebolavirus in Africa show discrepancies and lack accuracy, for multiple reasons (remote event, reports by different person or team, at different time…) but the only way to forge the history is to label the events with date, time and the environmental factors observed. On July 27, 1976, the first (known) victim to contract Ebolavirus was a cotton factory worker from Nzara, Sudan. Then, in Zaire (DRC) on September 1, 1976, the first Ebolavirus (Zaire ebolavirus, ZEBOV) victim was a teacher who had just returned from a family visit to northern Zaire (6 Jennifer Rosenberg Internet). These two events were the very beginning of the boundless journey of a deadly Ebolavirus outbreaks.

#### **2.1 The Ebolavirus species emerging events**

When the virus becomes epidemic in a human population, it does so weeks or months after the emergent event of the virus switching from its silent transmission in a natural cycle to a zoonotic/epidemic manifestation, revealed to the local health system. Let us see in more detail such emerging events of Ebolavirus species (ICTV, 2018) as there were reported or sometime interpreted, in time and place.

Sudan ebolavirus (SEBOV) occurred when the first recorded SUDV broke out among cotton factory workers in Nzara, South Sudan in June 271,976. This was indeed, the first case of Ebolavirus infection recorded and confirmed and also reported as potentially exposed to chiropteran. Indeed, at the Nzara Cotton Manufacturing Factory this first patient was a cloth room worker where bats (mostly *Tadarida -* mops *- trevori*) have a large population in the roof space of their premises. He died in the Nzara hospital on July 6, 1976. Local animals and insects were tested for Ebolavirus without success [6, 7].

Zaire ebolavirus (ZEBOV) was reported in the Mongala district of the Democratic Republic of Congo (DRC; alias Zaire) in August 1976, when a 44-yearold schoolteacher of the Yambuku village, became the first recorded case of Ebolavirus infection in DRC. Also, the schoolteacher travel earlier in August 1976 near the Central African Republic border and along the Ebola River, estimated 90 km NW from the village [6].

Reston ebolavirus (REBOV) had its first emerging event as an imported infected cynomolgus monkey (*Macaca fascicularis*) in October 1989 imported from a facility in the Philippines (Mindanao Island) to Reston, Virginia, USA, where the primate got sick and the virus isolated [8]. In the Philippines, in several instances, the virus was found to infect pigs, in June and September 2008 ill pigs were confirmed to be infested by REBOV (Ecija and Bulacan, Manila island), as well during 2008–2009 epizootics in the island of Luzon (Philippines) [9].

Cote d'Ivoire ebolavirus (CIEBOV) was isolated for the first time, and as an only known appearance, in November 1994, from wild chimpanzees presenting severe internal bleeding of the Taï Forest in Côte d'Ivoire, Africa. A researcher became infected when practicing a necropsy on one of these primates, he developed a dengue syndrome and survived. At that time, many dead chimpanzees were discovered and tested positive for Ebolavirus. However, the source of the virus was believed to be of infected western red colobus monkeys (*Piliocolobus badius*) upon which the chimpanzees preyed [10].

Bundibugyo ebolavirus (BDBV) was then discovered during an outbreak of Ebolavirus in the Bundibugyo District (Bundibugyo and Kikyo townships), on August 1st, 2007, in Western Uganda (Towner et al. [11]). BDBV second emerging event was observed in the DRC in August 17, 2012 in Isiro, Pawa and Dungu, districts of the Province Orientale [11].

With the exception of REBOV in Philippines and CIEBOV in West Africa, all other EBOVs species emerged in the Central African region. Also, all EBOVs are known to emerged in the tropical rain forest during the inter-season between dry and rainy seasons. Also, REBOV appears to actively circulate in the tropical rain or moist deciduous forest of the Philippines [12].

#### **2.2 From Central Africa to West Africa**

#### *2.2.1 Concurrent emergences of Ebolaviruses*

On several occasions, concurrent emerging events of Ebolavirus have been observed. Indeed, such events occurred in places geographically distant, independent, and unconnected. The Ebolavirus was isolated and the strains different, even they belonged to the same species of Ebolavirus, altogether in favor of a different origin from an elusive natural reservoir, thus eliminating the notion of leaping from one site to the other. In that matter, the following observations are a paradigm: From its inceptive emergence the Ebolavirus was identified in Sudan at the cotton factory and a few days later at Yambuku, Zaire. The Ebola Sudan and Ebola Zaire viruses emerged concurrently in 1976 in the Congo basin of Central Africa; More than 20 years later the virus emerged and reemergence from 1994 to 1996 in a different places in Gabon, in a successive and timely overlapping events but in unconnected areas from where different strains of the same EBOVZ were isolated [13]; More recently, during the 2014–2016 dramatic Ebolavirus disease (EVD) emergence of in West Africa where the virus emerged in late December 2013 of a 18-month-old boy from the small village of Meliandou (Guéckédou district, South-Eastern Guinea) believed to have been infected by bats [14], concurrently, in August 2013, the Ebolavirus reemerged in the Equator province of DRC - different places and different strain of ZEBOV [15].

It is remarkable that most of these emerging events occurred during or close to the end of the rainy season which generally stretches from August to October in the domain of the Congo basin tropical rain forest.

Altogether, these observations are in favor of environmental factors of emergence favoring, when they occur synchronously in the same place, the spillover of the virus from its hidden natural cycle to an accidental and susceptible host. Therefore, these plural and concomitant emerging events play against the theory of Ebola virus diffusing in oil spot in Central Africa [16]. This original pattern of concurrent emergences could explain also the relative stability of the virus strains which remain for years in the same environment, and the interepidemic silences which require several fundamentals (i.e. concurrent risk factors) to be broken.

#### *2.2.2 An unexpected broader domain of Ebolavirus circulation*

The first evidence that showed that Ebola virus had previously circulated in areas without any known cases of disease came in 1977, near the Ebola outbreak in Tandala, DRC, just 200 miles west of the first known cases in 1976 [17]. Blood samples obtained from individuals in areas with no previous symptoms of Ebola were found to contain antibodies for Ebolavirus, indicating a previous or ongoing infection with that virus. Because subclinical illness is always a possibility with viral infections, the presence of these Ebolavirus-specific antibodies could only be explained by exposure to the virus, which is somewhat reasonable in an area that is

**13**

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

Ebola, despite never exhibiting symptoms [19].

areas of EBOVs distribution in Southeast Asia [24].

mainly on the African continent [25].

as Ebolavirus?

endemic to the disease. But how do we know the true endemic zone of a virus such

In the early 1980's, research based at the Pasteur Institute in Bangui, Central African Republic, demonstrated for the first time that the population of central Africa presented natural antibodies against the Ebolavirus strains of Zaire and Sudan [3, 4]. Research also showed for the first time that several mammal species had Ebolavirus-reacting antibodies, including rodents, dogs, and others. Initially, the scientific community was skeptical of the findings, due to the type of antibody tests used, and because the prevalence of these antibodies was unbelievably dispersed and at a high level of prevalence. However, a 1989 follow-up study confirmed methodology and preliminary observations, and expanded the results to include similar observations in Cameroon, Chad, Gabon, and Republic of Congo (the latter two of these countries would have their first Ebola outbreaks in 1994 and 2001, respectively) [5]. Moreover, such Ebolavirus antibody prevalence was found in West Africa (e.g. Senegal, Chad, Sierra Leone), preceding the catastrophic 2014–2016 Ebolavirus outbreak [18]. Subsequent studies have determined that 20–25% of persons living in or near the Congolese rain forest are seropositive for

Today, Ebola antibody prevalence is widely distributed across the African continent in the absence of severe clinical presentation and/or outbreak manifestation. A 1989 study even found Ebola Zaire antibodies among people living in Madagascar, an island country that has never had a single known case of Ebola, and which has been geographically separated from continental Africa for 100 million years [20]. Risk mapping, including ecological and geographical distribution <10-13 cm/s first hour, and extended, highly sensitive and specific environmental and biogeographical models based on EBOVs susceptible mammalian biogeography in Africa, show a robust and precise potential distribution of EBOVs in Africa that clearly overlap the African tropical rain forest biome of the Guinea-Congo forests (including the Congo basin rain forest, and the Occidental relic of the Congolese rain forest spreading from Guinea to Ghana) and the southern band of the Sudan-Guinea Savanna [21]. Also, as a result of potential Ebolavirus (or Ebolavirus antigen) exposure, serological markers have been found in vertebrates outside of Africa. With the exception of Philippines, where REBOV is known to circulate in monkeys and pigs, thus showing its ability to infect multiple animal species, in several instances serological evidence of Ebolavirus exposure has been detected in many vertebrates, particularly chiropterans [9]. Definitely, bat populations in Bangladesh and China present antibodies against ZEBOV and REBOV proteins [22, 23]. Ultimately, it appears that EBOVs are widely distributed throughout Africa, West and Central, and Asia. Moreover, risk mapping of filovirus ecologic niches suggests potential

The unexpected detection of REBOV first in Virginia, for the reason we know,

Philippines gave a rethinking of the entire family of Ebola viruses previously known

and then the astonishing discovery of its circulation and natural cycle in the

Endemic zones are primarily based on where disease can most likely be expected, and are determined by historical accounts of disease, as well as supplemental information such as where animals or insects that might transmit the disease are located. With respect to the Ebola virus, outbreaks that occur in Central Africa, in or near the Congo River Basin, are expected; outbreaks that take place elsewhere are unexpected and can be problematic, as was the case for the 2014–2016 West African outbreak. And yet, scientists have highlighted the presence of Ebola

antibodies well outside the endemic zone for disease for decades.

*Emerging Challenges in Filovirus Infections*

districts of the Province Orientale [11].

moist deciduous forest of the Philippines [12].

**2.2 From Central Africa to West Africa**

*2.2.1 Concurrent emergences of Ebolaviruses*

ent strain of ZEBOV [15].

domain of the Congo basin tropical rain forest.

*2.2.2 An unexpected broader domain of Ebolavirus circulation*

event was observed in the DRC in August 17, 2012 in Isiro, Pawa and Dungu,

With the exception of REBOV in Philippines and CIEBOV in West Africa, all other EBOVs species emerged in the Central African region. Also, all EBOVs are known to emerged in the tropical rain forest during the inter-season between dry and rainy seasons. Also, REBOV appears to actively circulate in the tropical rain or

On several occasions, concurrent emerging events of Ebolavirus have been observed. Indeed, such events occurred in places geographically distant, independent, and unconnected. The Ebolavirus was isolated and the strains different, even they belonged to the same species of Ebolavirus, altogether in favor of a different origin from an elusive natural reservoir, thus eliminating the notion of leaping from one site to the other. In that matter, the following observations are a paradigm: From its inceptive emergence the Ebolavirus was identified in Sudan at the cotton factory and a few days later at Yambuku, Zaire. The Ebola Sudan and Ebola Zaire viruses emerged concurrently in 1976 in the Congo basin of Central Africa; More than 20 years later the virus emerged and reemergence from 1994 to 1996 in a different places in Gabon, in a successive and timely overlapping events but in unconnected areas from where different strains of the same EBOVZ were isolated [13]; More recently, during the 2014–2016 dramatic Ebolavirus disease (EVD) emergence of in West Africa where the virus emerged in late December 2013 of a 18-month-old boy from the small village of Meliandou (Guéckédou district, South-Eastern Guinea) believed to have been infected by bats [14], concurrently, in August 2013, the Ebolavirus reemerged in the Equator province of DRC - different places and differ-

It is remarkable that most of these emerging events occurred during or close to the end of the rainy season which generally stretches from August to October in the

Altogether, these observations are in favor of environmental factors of emergence favoring, when they occur synchronously in the same place, the spillover of the virus from its hidden natural cycle to an accidental and susceptible host. Therefore, these plural and concomitant emerging events play against the theory of Ebola virus diffusing in oil spot in Central Africa [16]. This original pattern of concurrent emergences could explain also the relative stability of the virus strains which remain for years in the same environment, and the interepidemic silences which require several fundamentals (i.e. concurrent risk factors) to be broken.

The first evidence that showed that Ebola virus had previously circulated in areas without any known cases of disease came in 1977, near the Ebola outbreak in Tandala, DRC, just 200 miles west of the first known cases in 1976 [17]. Blood samples obtained from individuals in areas with no previous symptoms of Ebola were found to contain antibodies for Ebolavirus, indicating a previous or ongoing infection with that virus. Because subclinical illness is always a possibility with viral infections, the presence of these Ebolavirus-specific antibodies could only be explained by exposure to the virus, which is somewhat reasonable in an area that is

**12**

endemic to the disease. But how do we know the true endemic zone of a virus such as Ebolavirus?

Endemic zones are primarily based on where disease can most likely be expected, and are determined by historical accounts of disease, as well as supplemental information such as where animals or insects that might transmit the disease are located. With respect to the Ebola virus, outbreaks that occur in Central Africa, in or near the Congo River Basin, are expected; outbreaks that take place elsewhere are unexpected and can be problematic, as was the case for the 2014–2016 West African outbreak. And yet, scientists have highlighted the presence of Ebola antibodies well outside the endemic zone for disease for decades.

In the early 1980's, research based at the Pasteur Institute in Bangui, Central African Republic, demonstrated for the first time that the population of central Africa presented natural antibodies against the Ebolavirus strains of Zaire and Sudan [3, 4]. Research also showed for the first time that several mammal species had Ebolavirus-reacting antibodies, including rodents, dogs, and others. Initially, the scientific community was skeptical of the findings, due to the type of antibody tests used, and because the prevalence of these antibodies was unbelievably dispersed and at a high level of prevalence. However, a 1989 follow-up study confirmed methodology and preliminary observations, and expanded the results to include similar observations in Cameroon, Chad, Gabon, and Republic of Congo (the latter two of these countries would have their first Ebola outbreaks in 1994 and 2001, respectively) [5]. Moreover, such Ebolavirus antibody prevalence was found in West Africa (e.g. Senegal, Chad, Sierra Leone), preceding the catastrophic 2014–2016 Ebolavirus outbreak [18]. Subsequent studies have determined that 20–25% of persons living in or near the Congolese rain forest are seropositive for Ebola, despite never exhibiting symptoms [19].

Today, Ebola antibody prevalence is widely distributed across the African continent in the absence of severe clinical presentation and/or outbreak manifestation. A 1989 study even found Ebola Zaire antibodies among people living in Madagascar, an island country that has never had a single known case of Ebola, and which has been geographically separated from continental Africa for 100 million years [20].

Risk mapping, including ecological and geographical distribution <10-13 cm/s first hour, and extended, highly sensitive and specific environmental and biogeographical models based on EBOVs susceptible mammalian biogeography in Africa, show a robust and precise potential distribution of EBOVs in Africa that clearly overlap the African tropical rain forest biome of the Guinea-Congo forests (including the Congo basin rain forest, and the Occidental relic of the Congolese rain forest spreading from Guinea to Ghana) and the southern band of the Sudan-Guinea Savanna [21].

Also, as a result of potential Ebolavirus (or Ebolavirus antigen) exposure, serological markers have been found in vertebrates outside of Africa. With the exception of Philippines, where REBOV is known to circulate in monkeys and pigs, thus showing its ability to infect multiple animal species, in several instances serological evidence of Ebolavirus exposure has been detected in many vertebrates, particularly chiropterans [9]. Definitely, bat populations in Bangladesh and China present antibodies against ZEBOV and REBOV proteins [22, 23]. Ultimately, it appears that EBOVs are widely distributed throughout Africa, West and Central, and Asia. Moreover, risk mapping of filovirus ecologic niches suggests potential areas of EBOVs distribution in Southeast Asia [24].

The unexpected detection of REBOV first in Virginia, for the reason we know, and then the astonishing discovery of its circulation and natural cycle in the Philippines gave a rethinking of the entire family of Ebola viruses previously known mainly on the African continent [25].

From these observation and facts, the potential circulation of EBOVs in its natural cycle appears much wider than expected, while the emerging events we can witness appears to be only a tip of the iceberg in the wide Congolese tropical rain forest.

#### **2.3 From the index case to the epidemic chain, outbreak, and pandemic**

The fundamentals of emergence are changing in the heart of the rainforest and elsewhere: changing times, when the means of transmission switch from foot to motorbike, when knowledge conveyance has switched from paper reporting to the internet.

Let us examine the risk of expansion for Ebolavirus. Indeed, the factors of transmission of the virus to man and man to man are essential to take into account in this context. Moreover, it is extremely important to note that these factors are subject to permanent changes in societies whose trade and means of communication are drastically changing as a result of health systems, responses and preparedness for epidemics at national and international levels, policies, and the economy.

So, with the experience gained for more than 40 years, the strategies of struggle are clearly defined, but the societal changes that are taking place make their application difficult and sometimes impossible (e.g., the 2019 outbreak in the DRC, where political institutions have prevented an adapted response). Situation and the epidemic are perpetuated.

There is also a growing means of communication, both smartphones and motorized transport, to travel more quickly as ever, between the epidemic zone of EVD and the family [26].

Thus, during the emergence of the Ebola virus in West Africa, all of this means of communication played a fundamental role in the regional spread of the epidemic, until it became a pandemic risk when the virus was exported to other countries of the African continent and, outside Africa in Europe and North America [27].

#### **3. A strange iteration of epidemic events with unexplained virus disappearance**

It is known for several other transmitted viruses that during the inter-epidemic silences several factors can be responsible. In general mass herd immunity (natural of due to acquired immunization i.e. vaccine) of the permissive hosts force the virus in its natural cycle without apparent clinical manifestation in the hosts (e.g. Most by the arbovirus classically yellow fever, Dengue, Japanese encephalitis, West Nile, Zika etc.).

The *Paramyxoviridae* and *Rhabdoviridae* are the two other viral families in the order Mononegavirales, genetically closely related to the Filoviridae and having chiropteran as reservoir and/or vector [28]. Indeed, it is interesting to note that megachiropteran fruit bats are reservoirs of Hendra and Nipah viruses of the Paramyxoviridae family [29]. When, Microchiroptera bats are the probable ancestors of all rabies virus variants of the Lyssavirus genus in the family Rhabdoviridae and infecting presently terrestrial mammals [30]. Both also present this cryptic interepidemic silences that has not been yet clearly understood. The Nipah emerged one time in Malaysia (1999), thought to have its original cycle in PNG, and ultimately reemerged more than 3500 km away in Bangladesh in 2001. From its inception, again the Marburgvirus (the closest to EBOVs in the family of Filovirus), emerging events from an expected natural foci occurred within the path of time

**15**

**Figure 1.**

*emerging event.*

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

mysterious (**Figure 1**).

then… silence.

including 4 to 11 years of inter-epidemic silences occurring mostly in distant sites of

If one were to describe the history of Ebola outbreaks, one could simply construct a timeline, with a point on the line for each outbreak. You could create this timeline with a varying number of points, depending on your methodology, but regardless of how you built your timeline, there would be spaces between these points. This is due to the nature of Ebola; it appears, it disappears, and it appears again. To the Ebola virus, these gaps are periods of convalescence. To us, they are periods of absence and mystery, and one of these gaps stands out as the most

The CDC lists five Ebola outbreaks in the late 1970's. The "first" Ebola outbreak took place in 1976, though we now recognize the event as two simultaneous and separate outbreaks. Between June and November 1976, 284 cases (151 deaths) of Ebola Sudan occurred near what is now Nzara, South Sudan; between September and October 1976, 318 cases (280 deaths) of Ebola Zaire occurred near what is now Yambuku, Democratic Republic of Congo (DRC). In November 1976, a researcher in England that was working with samples from the Nzara outbreak accidentally infected himself; CDC lists this accident as the third Ebola outbreak (the individual recovered). In June 1977, a child became sick and died from Ebola Zaire in Tandala, DRC though there was only one confirmed case, subsequent epidemiological investigations of the area uncovered several other historical, probable cases. Finally, between July and October 1979, 34 cases (22 deaths) of Ebola Sudan occurred, unbelievably, in Nzara, Sudan – the same community where the first cases of Ebola emerged just 3 years prior. In the span of just 39 months, the terror of Ebola had introduced itself to the world five times (638 cases, 454 deaths) and

Ebola would not reappear for 10 whole years, and even then, the subtype was Ebola Reston, which we now know does not affect humans. Though CDC lists four Ebola Reston outbreaks between 1989 and 1992, the world would not see another case of Ebola virus disease in humans until late-1994, in Gabon. Even then, the outbreak (52 cases, 31 deaths) was mischaracterized as yellow fever for several months. Perhaps the virus's long absence from the spotlight had removed it from the collective consciousness in 1994, certainly in the presence of those pathogens that

This fifteen-year disappearance of Ebola, particularly in light of its frequent and severe outbreaks in the late 1970's, has perplexed researchers for decades.

*Timeline of Ebolavirus emergence. Emerging events (bars) red = EBOV; blue = SEBOV; green = BDBV; horizontal axis = years 1972–2018; vertical axis = no value. Numbers above brackets = years of silent inter-*

had been circulating and consuming our attention in the meantime.

Eastern and South Africa (Uganda, Zimbabwe, Angola, Kenya).

*Emerging Challenges in Filovirus Infections*

forest.

internet.

epidemic are perpetuated.

and the family [26].

**disappearance**

Zika etc.).

From these observation and facts, the potential circulation of EBOVs in its natural cycle appears much wider than expected, while the emerging events we can witness appears to be only a tip of the iceberg in the wide Congolese tropical rain

The fundamentals of emergence are changing in the heart of the rainforest and elsewhere: changing times, when the means of transmission switch from foot to motorbike, when knowledge conveyance has switched from paper reporting to the

Let us examine the risk of expansion for Ebolavirus. Indeed, the factors of transmission of the virus to man and man to man are essential to take into account in this context. Moreover, it is extremely important to note that these factors are subject to permanent changes in societies whose trade and means of communication are drastically changing as a result of health systems, responses and preparedness for

So, with the experience gained for more than 40 years, the strategies of struggle are clearly defined, but the societal changes that are taking place make their application difficult and sometimes impossible (e.g., the 2019 outbreak in the DRC, where political institutions have prevented an adapted response). Situation and the

There is also a growing means of communication, both smartphones and motorized transport, to travel more quickly as ever, between the epidemic zone of EVD

Thus, during the emergence of the Ebola virus in West Africa, all of this means of communication played a fundamental role in the regional spread of the epidemic, until it became a pandemic risk when the virus was exported to other countries of the African continent and, outside Africa in Europe and North America [27].

It is known for several other transmitted viruses that during the inter-epidemic silences several factors can be responsible. In general mass herd immunity (natural of due to acquired immunization i.e. vaccine) of the permissive hosts force the virus in its natural cycle without apparent clinical manifestation in the hosts (e.g. Most by the arbovirus classically yellow fever, Dengue, Japanese encephalitis, West Nile,

The *Paramyxoviridae* and *Rhabdoviridae* are the two other viral families in the order Mononegavirales, genetically closely related to the Filoviridae and having chiropteran as reservoir and/or vector [28]. Indeed, it is interesting to note that megachiropteran fruit bats are reservoirs of Hendra and Nipah viruses of the Paramyxoviridae family [29]. When, Microchiroptera bats are the probable ancestors of all rabies virus variants of the Lyssavirus genus in the family Rhabdoviridae and infecting presently terrestrial mammals [30]. Both also present this cryptic interepidemic silences that has not been yet clearly understood. The Nipah emerged

one time in Malaysia (1999), thought to have its original cycle in PNG, and ultimately reemerged more than 3500 km away in Bangladesh in 2001. From its inception, again the Marburgvirus (the closest to EBOVs in the family of Filovirus), emerging events from an expected natural foci occurred within the path of time

**3. A strange iteration of epidemic events with unexplained virus** 

**2.3 From the index case to the epidemic chain, outbreak, and pandemic**

epidemics at national and international levels, policies, and the economy.

**14**

including 4 to 11 years of inter-epidemic silences occurring mostly in distant sites of Eastern and South Africa (Uganda, Zimbabwe, Angola, Kenya).

If one were to describe the history of Ebola outbreaks, one could simply construct a timeline, with a point on the line for each outbreak. You could create this timeline with a varying number of points, depending on your methodology, but regardless of how you built your timeline, there would be spaces between these points. This is due to the nature of Ebola; it appears, it disappears, and it appears again. To the Ebola virus, these gaps are periods of convalescence. To us, they are periods of absence and mystery, and one of these gaps stands out as the most mysterious (**Figure 1**).

The CDC lists five Ebola outbreaks in the late 1970's. The "first" Ebola outbreak took place in 1976, though we now recognize the event as two simultaneous and separate outbreaks. Between June and November 1976, 284 cases (151 deaths) of Ebola Sudan occurred near what is now Nzara, South Sudan; between September and October 1976, 318 cases (280 deaths) of Ebola Zaire occurred near what is now Yambuku, Democratic Republic of Congo (DRC). In November 1976, a researcher in England that was working with samples from the Nzara outbreak accidentally infected himself; CDC lists this accident as the third Ebola outbreak (the individual recovered). In June 1977, a child became sick and died from Ebola Zaire in Tandala, DRC though there was only one confirmed case, subsequent epidemiological investigations of the area uncovered several other historical, probable cases. Finally, between July and October 1979, 34 cases (22 deaths) of Ebola Sudan occurred, unbelievably, in Nzara, Sudan – the same community where the first cases of Ebola emerged just 3 years prior. In the span of just 39 months, the terror of Ebola had introduced itself to the world five times (638 cases, 454 deaths) and then… silence.

Ebola would not reappear for 10 whole years, and even then, the subtype was Ebola Reston, which we now know does not affect humans. Though CDC lists four Ebola Reston outbreaks between 1989 and 1992, the world would not see another case of Ebola virus disease in humans until late-1994, in Gabon. Even then, the outbreak (52 cases, 31 deaths) was mischaracterized as yellow fever for several months. Perhaps the virus's long absence from the spotlight had removed it from the collective consciousness in 1994, certainly in the presence of those pathogens that had been circulating and consuming our attention in the meantime.

This fifteen-year disappearance of Ebola, particularly in light of its frequent and severe outbreaks in the late 1970's, has perplexed researchers for decades.

#### **Figure 1.**

*Timeline of Ebolavirus emergence. Emerging events (bars) red = EBOV; blue = SEBOV; green = BDBV; horizontal axis = years 1972–2018; vertical axis = no value. Numbers above brackets = years of silent interemerging event.*

The mystery lay, to some extent, within the lack of complete knowledge of the virus reservoir, though scientists are now having their long-held suspicions in bats confirmed. It's hard to detect disease when you cannot pinpoint the source. Surveillance and reporting have been another confounding element. How many times in that fifteen-year period was an illness misdiagnosed as yellow fever, dengue hemorrhagic fever, or some other similar illness, because of lack of knowledge or diagnostic capabilities, or simply because there was no health care around? We will probably never be able to answer this question. Finally, our perceived zone of endemicity at the time was limited to northern DRC and southern Sudan. Was the virus appearing elsewhere, unbeknownst to us? We certainly were not expecting it to emerge in Gabon in 1994, and Uganda in 2000, and West Africa in 2014 [31].

Scientists today continue to be perplexed by the emergence of the virus. What brings Ebola out from its hiding place? Is its emergence/re-emergence tied to climate change? globalization? the changing interface between humans and wildlife? If it has to do with any of these increasingly significant factors, how do they explain the fifteen-year disappearance?

These days, the virus comes and goes with some predictability—since 2000, outbreaks have approached a near-annual incidence, sometimes skipping a year, sometimes lasting more than a year. The periods between outbreaks are growing shorter. Is this because our capability to detect Ebola outbreaks is improving, or is the virus able to infect humans more frequently? One thing is for sure: the world knows that when one outbreak ends, another will eventually follow, and we need not wait 15 years.

#### **4. Toward the discovery of the natural cycle of the Ebolaviruses**

#### **4.1 The discovery of a putative natural reservoir of Ebolavirus**

Since the ZEBOV and SEBOV emergence, extended field studies have been conducted to discover the reservoir of EBOVs [32] including the 1976 first recorded DRC outbreaks and Sudan, the 1979 outbreak in DRC in 1979 and 1995 following the Kikwit outbreak, the same year in the Tai Forest and in 1999 in the Central African Republic [33–38] . A total of more than 7000 vertebrates and 30,000 invertebrates were sampled and tested for the presence of EBOVs. Limited finding was inconclusive for an potential EBOVs reservoir status among all these animals. Moreover, while several animal species (Bats, birds, reptiles, mollusks, arthropods, and plants) were experimentally infected with ZEBOV, only two fruit bat species (*Epomophorus* spp. and *Tadarida* spp.) developed a subclinical transient viremia [39]. If these results were not confirmed in the natural settings, they indicated the potential for chiropteran to be natural for EBOVs [40].

Also, historically, the first documented case of EVD in Sudan in 1976, the index case was located (by the World Health Organization) in a cotton factory far from the forest block, where the only wild significantly abundant species was an insectivorous bat species [21].

Since the discovery of EBOV in 1976, more than half of the epidemic outbreaks caused by EBOVs have broken down between Gabon and the DRC. Following the successive EBOV outbreaks in Gabon from 1995 to 2001 affecting several animal species non-human primates, and wild ungulates and responsible of the dramatic decline of great apes (gorilla and chimpanzee) populations in the region (Leroy et al. [16]), researchers engaged several missions of captures of wild animals in the forest areas affected by the recent past epidemics. Also, 1030 animals were captured and analyzed, only three species of fruit bats were found infected with the ZEBOV by PCR including: *Hypsignathus monstrosus*; *Epomops franqueti;* and *Myonycteris* 

**17**

*4.2.1 The actors*

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

potential reservoir of EBOVs [41].

*torquata*. Moreover, antibody reacting anti-Ebola were detected in these species as well as for the genus *Myonycteris* spp. leading ultimately to design Chiropteran as a

Since then, many studies have converged in favor of the role of chiropters in maintaining EBOV in the wild (Caron et al. [42], Leendertz). In addition, a recent study of bats in Sierra Leone showed the association of an EBOV like with several species of bats (*Mops condylurus* and *Chaerephon pumilus*) from the Molossus family [43]. Moreover, a potential direct exposure to Ebola infected fruit bats was also reported as a putative index case of large epidemics [44, 45]. Moreover, further studies reported on direct infection of natural hosts (primates) by EBOV infected bats as highly plausible, given that bats, especially fruit bats, are frequently hunted and consumed as bushmeat by human when *Cercopithecus* species hunt roosting bats for consumption [46] also preying on bats has been reported in *Cercopithecus ascanius* and *C. mitis* (East Africa) as well as bonobos (DRC) [47]. It is also possible that different modes of exposure to Ebola virus could lead to different antibody profiles, that is, contaminated fruit vs. contact with infected bats during hunting [44, 47, 48]. Altogether, several fruit bats (*Epomophorus wahlbergi*) and insectivorous bats (*Chaerephon pumilus, Mops condylurus*) experimentally survive to EBOV infections [39], EBOV RNA and/or anti EBOV reacting antibodies were detected also in several other fruit bat species (*Epomops franqueti, Hypsignathus monstrosus, Myonycteris torquata*, *Eidolon helvum, Epomophorus gambianus, Micropteropus pusillus, Mops condylurus, Rousettus aegyptiacus, Rousettus leschenaultia*) giving more insight of the potential

for chiropteran to be a potential host or reservoir host of EBOVs [22, 49, 50]. Interestingly, REBOV was also found associated with the bats in its natural habitat of the Philippines [51]. Also, again in this same *Filoviridae* family, Marburg viruses in Africa are clearly associated with bats [32, 52] as well as the Cueva virus in Europe [53]. While REBOV has been find associated with fruit bats, *Roussetus* spp. (Pteropodid family), each filovirus genus is associated with a specific chiropteran group including: Marburgvirus with a specific fruit bat, *Roussetus aegyptiacus* (Pteropodid family); and Cuevavirus with insectivorous bat, *Miniopterus schreibersii*

(Miniopterid family); except for *Thamnovirus* isolated form fresh water fish.

which are among the most severe of the emerging viruses [54, 55].

been identified as a potential virus reservoir,

**African raining forest**

Moreover, several virus groups are known to hold bat-borne viruses including the coronaviruses, hantaviruses, lyssaviruses, lassa virus, Henipavirus, filovirus

Conclusively, this was the first evidence of chiropteran as a potential reservoir and/or vector of EBOV, while several wild animals, in particular great apes were find highly sensitive to EBOV infection. Also, if several species of chiropteran have

**4.2 The most complete figure of a putative Ebolavirus natural cycle in the central** 

From all above observations, records and historical events of EBOVs emerging events, several fundamentals of emergence have been identified as well putative time and space of such events where, that is when the virus jump from the cryptic natural cycle of the reservoir-vector to manifest itself clearly as an open index case of infection in a susceptible host and the potential opening epizootic or epidemic chain.

Again, from the literature numerous vertebrates appears to be permissive to infection by EBOVs, however, due to their ethology, including environmental habits, societal structure, density and their ability of intra and interspecies to mingle.

*Emerging Challenges in Filovirus Infections*

the fifteen-year disappearance?

The mystery lay, to some extent, within the lack of complete knowledge of the virus reservoir, though scientists are now having their long-held suspicions in bats confirmed. It's hard to detect disease when you cannot pinpoint the source. Surveillance and reporting have been another confounding element. How many times in that fifteen-year period was an illness misdiagnosed as yellow fever, dengue hemorrhagic fever, or some other similar illness, because of lack of knowledge or diagnostic capabilities, or simply because there was no health care around? We will probably never be able to answer this question. Finally, our perceived zone of endemicity at the time was limited to northern DRC and southern Sudan. Was the virus appearing elsewhere, unbeknownst to us? We certainly were not expecting it to emerge in Gabon in 1994, and Uganda in 2000, and West Africa in 2014 [31]. Scientists today continue to be perplexed by the emergence of the virus. What brings Ebola out from its hiding place? Is its emergence/re-emergence tied to climate change? globalization? the changing interface between humans and wildlife? If it has to do with any of these increasingly significant factors, how do they explain

These days, the virus comes and goes with some predictability—since 2000, outbreaks have approached a near-annual incidence, sometimes skipping a year, sometimes lasting more than a year. The periods between outbreaks are growing shorter. Is this because our capability to detect Ebola outbreaks is improving, or is the virus able to infect humans more frequently? One thing is for sure: the world knows that when one outbreak ends, another will eventually follow, and we need not wait 15 years.

**4. Toward the discovery of the natural cycle of the Ebolaviruses**

Since the ZEBOV and SEBOV emergence, extended field studies have been conducted to discover the reservoir of EBOVs [32] including the 1976 first recorded DRC outbreaks and Sudan, the 1979 outbreak in DRC in 1979 and 1995 following the Kikwit outbreak, the same year in the Tai Forest and in 1999 in the Central African Republic [33–38] . A total of more than 7000 vertebrates and 30,000 invertebrates were sampled and tested for the presence of EBOVs. Limited finding was inconclusive for an potential EBOVs reservoir status among all these animals. Moreover, while several animal species (Bats, birds, reptiles, mollusks, arthropods, and plants) were experimentally infected with ZEBOV, only two fruit bat species (*Epomophorus* spp. and *Tadarida* spp.) developed a subclinical transient viremia [39]. If these results were not confirmed in the natural settings, they indicated the

Also, historically, the first documented case of EVD in Sudan in 1976, the index case was located (by the World Health Organization) in a cotton factory far from the forest block, where the only wild significantly abundant species was an insec-

Since the discovery of EBOV in 1976, more than half of the epidemic outbreaks caused by EBOVs have broken down between Gabon and the DRC. Following the successive EBOV outbreaks in Gabon from 1995 to 2001 affecting several animal species non-human primates, and wild ungulates and responsible of the dramatic decline of great apes (gorilla and chimpanzee) populations in the region (Leroy et al. [16]), researchers engaged several missions of captures of wild animals in the forest areas affected by the recent past epidemics. Also, 1030 animals were captured and analyzed, only three species of fruit bats were found infected with the ZEBOV by PCR including: *Hypsignathus monstrosus*; *Epomops franqueti;* and *Myonycteris* 

**4.1 The discovery of a putative natural reservoir of Ebolavirus**

potential for chiropteran to be natural for EBOVs [40].

tivorous bat species [21].

**16**

*torquata*. Moreover, antibody reacting anti-Ebola were detected in these species as well as for the genus *Myonycteris* spp. leading ultimately to design Chiropteran as a potential reservoir of EBOVs [41].

Since then, many studies have converged in favor of the role of chiropters in maintaining EBOV in the wild (Caron et al. [42], Leendertz). In addition, a recent study of bats in Sierra Leone showed the association of an EBOV like with several species of bats (*Mops condylurus* and *Chaerephon pumilus*) from the Molossus family [43]. Moreover, a potential direct exposure to Ebola infected fruit bats was also reported as a putative index case of large epidemics [44, 45]. Moreover, further studies reported on direct infection of natural hosts (primates) by EBOV infected bats as highly plausible, given that bats, especially fruit bats, are frequently hunted and consumed as bushmeat by human when *Cercopithecus* species hunt roosting bats for consumption [46] also preying on bats has been reported in *Cercopithecus ascanius* and *C. mitis* (East Africa) as well as bonobos (DRC) [47]. It is also possible that different modes of exposure to Ebola virus could lead to different antibody profiles, that is, contaminated fruit vs. contact with infected bats during hunting [44, 47, 48].

Altogether, several fruit bats (*Epomophorus wahlbergi*) and insectivorous bats (*Chaerephon pumilus, Mops condylurus*) experimentally survive to EBOV infections [39], EBOV RNA and/or anti EBOV reacting antibodies were detected also in several other fruit bat species (*Epomops franqueti, Hypsignathus monstrosus, Myonycteris torquata*, *Eidolon helvum, Epomophorus gambianus, Micropteropus pusillus, Mops condylurus, Rousettus aegyptiacus, Rousettus leschenaultia*) giving more insight of the potential for chiropteran to be a potential host or reservoir host of EBOVs [22, 49, 50].

Interestingly, REBOV was also found associated with the bats in its natural habitat of the Philippines [51]. Also, again in this same *Filoviridae* family, Marburg viruses in Africa are clearly associated with bats [32, 52] as well as the Cueva virus in Europe [53]. While REBOV has been find associated with fruit bats, *Roussetus* spp. (Pteropodid family), each filovirus genus is associated with a specific chiropteran group including: Marburgvirus with a specific fruit bat, *Roussetus aegyptiacus* (Pteropodid family); and Cuevavirus with insectivorous bat, *Miniopterus schreibersii* (Miniopterid family); except for *Thamnovirus* isolated form fresh water fish.

Moreover, several virus groups are known to hold bat-borne viruses including the coronaviruses, hantaviruses, lyssaviruses, lassa virus, Henipavirus, filovirus which are among the most severe of the emerging viruses [54, 55].

Conclusively, this was the first evidence of chiropteran as a potential reservoir and/or vector of EBOV, while several wild animals, in particular great apes were find highly sensitive to EBOV infection. Also, if several species of chiropteran have been identified as a potential virus reservoir,

#### **4.2 The most complete figure of a putative Ebolavirus natural cycle in the central African raining forest**

From all above observations, records and historical events of EBOVs emerging events, several fundamentals of emergence have been identified as well putative time and space of such events where, that is when the virus jump from the cryptic natural cycle of the reservoir-vector to manifest itself clearly as an open index case of infection in a susceptible host and the potential opening epizootic or epidemic chain.

#### *4.2.1 The actors*

Again, from the literature numerous vertebrates appears to be permissive to infection by EBOVs, however, due to their ethology, including environmental habits, societal structure, density and their ability of intra and interspecies to mingle.

Altogether primates appear highly susceptible to EBOVs infection including nonhuman primate apes, gorilla and chimpanzee, but also cercopithecids (e.g. colobus) but also small wild ungulates (e.g. forest duikers) and eventually domestic animals (e.g. dogs) [32, 56–58].

One can summarize that EBOVs natural hosts belongs to chiropteran as a potential host reservoir represented mostly by Pteropodidae in Africa (REBOV and Roussetus; Bombali virus and Molossidae), and as secondary natural or accidental wild and domestic hosts including several other mammals: primates (Colobus, Cercopithecus), non-human primates (Gorilla, chimpanzee), wild ungulates (duikers) and, human primates. Also this needs to be taken into account with respect to other permissive species to EBOVs, indeed, as an example, if Roussetus spp. was shown to carry EBOVs reacting antibodies more recently *R. aegyptiacus* bats were demonstrated to unlikely able to maintain and perpetuate EBOV in nature while the natural transmission of filovirus in *R. aegyptiacus*, resulting viral replication and shedding are unknown [59].

#### *4.2.2 The stages*

The African Rain forest of the Congolese basin appears to be the epicenter of EBOVs emerging events. More than 80% of the emerging events of EBOVs occurred in the Tropical zone under the influence of the (Intertropical converging zone, ITCZ) from five degree North to 5 degrees south and oscillating as much as 40 to 45° of latitude north or south of the equator based on the pattern of land and ocean beneath it [28] (**Figure 2**).

Temperature and precipitation data for Africa (average data computed from 1960 to 1990, 300 m resolution [HIJ 05]) were integrated with the distribution map of the emergent events of the Ebola virus and the values calculated for each of the emergence points [60].

On all emergence points, the temperature at the time of emergence is not significantly different from the average annual temperature over 30 years. The difference in temperature between the moment of emergence and the average temperature (of 30 years monthly average) of the hottest month does not show any difference either. Emergence would not be directly related to temperature.

When we compare Ebolavirus emerging events time and the rainfall, there is strict quantitative correlation between rainfall and emergence: Most of the emergent events (93.8%) occurred during the rainy season (**Figure 2**). For precipitation values, there is a slightly statistically significant (p = 0.02) positive difference between the average precipitation of the month of emergence and the average of the monthly average precipitation (over 30 years), indicating that precipitations are higher when emergences occur. There is an even more statistically significant (p = 0.003) positive difference when considering precipitation of the month preceding the emergence. Emergence is therefore likely to be associated with rainfall intensity and the rainy season. 10/32 emergences occur at the beginning of the rainy season, 9/32 in the middle, and 11/32 at the end. Only 2/32 emergences occurred in the dry season.

When referring to land use (**Figure 3**) the temperature at the 6 emergence points in "Cropland" is highly significantly less (p = 0.005) than 15% (21.6°C) at temperature (24.4°C) to the 9 points in "Tree cover, broadleaved, evergreen, closed to open", however the average temperature of the Cropland (21.6°) is to a degree less, significantly lower (p = 0.01) than that of the "Tree cover" (24.5°C).

**19**

*in Africa.*

**Figure 3.**

**Figure 2.**

*Environmental factors surrounding Ebolavirus emerging event: Land use and places of Ebola virus emergence in Africa from 1976 to 2014. Land use from ESA 2015, 300 m resolution; red circle = putative place of the Ebola virus emergence (index case). Estimated Ebola emergence places are superimposed on the land use layer. The identification of the land use types were 32 points (red circle) representing the putative places of Ebolavirus emergence are superimposed and are distributed as follows: (1) cropland: 6, (2) herbaceous cover: 5, (3) cropland mosaic: 5 (> 50% natural vegetation vs. <50% tree, shrub, herbaceous cover), (4) tree cover with: (a) 15% of broadleaved, evergreen, closed to open: 9, (b) 15–40% of broadleaved, deciduous, open: 2, (5) flooded, fresh or brackish water: 1, (6) urban areas: 3, and (7) water bodies: 1. The limitations of this interpretation are linked to the accuracy of the location of Ebolavirus emergence sites (from literature and reports) and, to the evolution of vegetation cover over the past decades since the first emergence of the Ebolavirus occurred* 

*Emerging events of Ebolavirus and climate since the Ebola fever inception in Africa. Left = annual rainfall; right = annual temperature. To illustrate the association temperature/rainfall and emergence, the month of May was chosen because it is at this time of the year that we observe the most emergent events of the Ebola virus. Temperature and rainfall are expressed as an annual average for the period under consideration. The precise location of 32 Ebola emergent events are here integrated into the global climatic map of Africa. Only 30-year average values per month of rainfall are available for the study period (ref.: WorldClim world* 

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

*databases) as well for the average monthly temperature.*

Ultimately, taking into account these environmental factors, when we look for an association between the emergent events of the Ebola virus and the

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

#### **Figure 2.**

*Emerging Challenges in Filovirus Infections*

(e.g. dogs) [32, 56–58].

shedding are unknown [59].

beneath it [28] (**Figure 2**).

emergence points [60].

*4.2.2 The stages*

Altogether primates appear highly susceptible to EBOVs infection including nonhuman primate apes, gorilla and chimpanzee, but also cercopithecids (e.g. colobus) but also small wild ungulates (e.g. forest duikers) and eventually domestic animals

One can summarize that EBOVs natural hosts belongs to chiropteran as a potential host reservoir represented mostly by Pteropodidae in Africa (REBOV and Roussetus; Bombali virus and Molossidae), and as secondary natural or accidental wild and domestic hosts including several other mammals: primates (Colobus, Cercopithecus), non-human primates (Gorilla, chimpanzee), wild ungulates (duikers) and, human primates. Also this needs to be taken into account with respect to other permissive species to EBOVs, indeed, as an example, if Roussetus spp. was shown to carry EBOVs reacting antibodies more recently *R. aegyptiacus* bats were demonstrated to unlikely able to maintain and perpetuate EBOV in nature while the natural transmission of filovirus in *R. aegyptiacus*, resulting viral replication and

The African Rain forest of the Congolese basin appears to be the epicenter of EBOVs emerging events. More than 80% of the emerging events of EBOVs occurred in the Tropical zone under the influence of the (Intertropical converging zone, ITCZ) from five degree North to 5 degrees south and oscillating as much as 40 to 45° of latitude north or south of the equator based on the pattern of land and ocean

Temperature and precipitation data for Africa (average data computed from 1960 to 1990, 300 m resolution [HIJ 05]) were integrated with the distribution map of the emergent events of the Ebola virus and the values calculated for each of the

On all emergence points, the temperature at the time of emergence is not significantly different from the average annual temperature over 30 years. The difference in temperature between the moment of emergence and the average temperature (of 30 years monthly average) of the hottest month does not show any difference either.

When we compare Ebolavirus emerging events time and the rainfall, there is strict quantitative correlation between rainfall and emergence: Most of the emergent events (93.8%) occurred during the rainy season (**Figure 2**). For precipitation values, there is a slightly statistically significant (p = 0.02) positive difference between the average precipitation of the month of emergence and the average of the monthly average precipitation (over 30 years), indicating that precipitations are higher when emergences occur. There is an even more statistically significant (p = 0.003) positive difference when considering precipitation of the month preceding the emergence. Emergence is therefore likely to be associated with rainfall intensity and the rainy season. 10/32 emergences occur at the beginning of the rainy season, 9/32 in the middle, and 11/32 at the end. Only 2/32 emergences occurred in

When referring to land use (**Figure 3**) the temperature at the 6 emergence points in "Cropland" is highly significantly less (p = 0.005) than 15% (21.6°C) at temperature (24.4°C) to the 9 points in "Tree cover, broadleaved, evergreen, closed to open", however the average temperature of the Cropland (21.6°) is to a degree less, significantly lower (p = 0.01) than that of the "Tree cover" (24.5°C).

Ultimately, taking into account these environmental factors, when we look

for an association between the emergent events of the Ebola virus and the

Emergence would not be directly related to temperature.

**18**

the dry season.

*Emerging events of Ebolavirus and climate since the Ebola fever inception in Africa. Left = annual rainfall; right = annual temperature. To illustrate the association temperature/rainfall and emergence, the month of May was chosen because it is at this time of the year that we observe the most emergent events of the Ebola virus. Temperature and rainfall are expressed as an annual average for the period under consideration. The precise location of 32 Ebola emergent events are here integrated into the global climatic map of Africa. Only 30-year average values per month of rainfall are available for the study period (ref.: WorldClim world databases) as well for the average monthly temperature.*

#### **Figure 3.**

*Environmental factors surrounding Ebolavirus emerging event: Land use and places of Ebola virus emergence in Africa from 1976 to 2014. Land use from ESA 2015, 300 m resolution; red circle = putative place of the Ebola virus emergence (index case). Estimated Ebola emergence places are superimposed on the land use layer. The identification of the land use types were 32 points (red circle) representing the putative places of Ebolavirus emergence are superimposed and are distributed as follows: (1) cropland: 6, (2) herbaceous cover: 5, (3) cropland mosaic: 5 (> 50% natural vegetation vs. <50% tree, shrub, herbaceous cover), (4) tree cover with: (a) 15% of broadleaved, evergreen, closed to open: 9, (b) 15–40% of broadleaved, deciduous, open: 2, (5) flooded, fresh or brackish water: 1, (6) urban areas: 3, and (7) water bodies: 1. The limitations of this interpretation are linked to the accuracy of the location of Ebolavirus emergence sites (from literature and reports) and, to the evolution of vegetation cover over the past decades since the first emergence of the Ebolavirus occurred in Africa.*

characteristics of the places of these emergences (i.e. land use, temperature, rainfall) it turns out that the emergences are always in the zone of heavy rainfall, but nevertheless do not follow the moving of the rainy season. Moreover, these emergences remain always and remarkably close enough to the Equator, therefore in the equatorial forest area with a high hygrometry, and a moderate annual temperature. However, the temperature at the time of emergence is not significantly different from the average annual temperature (at the points of emergence) which does not allow to distinguish seasonal effect in the emergence-temperature relationship. Conclusively, we did not identify a seasonality associated with the time of emergence, however the emerging events occur in specific geographic zone characterized by several environmental factors. Finally, the emergence zones are in areas of Land Use with specific temperatures not related to seasonality. Ultimately, it is also remarkable that all these emerging events occurred in an area with a highly potential presence of apes, virus-sensitive hosts.

#### *4.2.3 Fundamentals and domains of emergence: a theory for a natural cycle of EBOVs in Africa*

Also, the EBOVs species are closely genetically related, their seems to occur by foci in nature. The host appears to be the same, natural or accidental, and the transmission done by direct contact with infected hosts or its biological products [50, 61]. Altogether, in the early 2000s, before the identification of chiropteran as a potential host-reservoir of the EBOVs, a hypothetic natural cycle was described empirically based on seasonal environmental climatic factors [55]. Then, taking into account bats as a potential reservoir-host, the question of virus transmission was central to consider while environmental factors appears to play a major role to the host and their natural cycle (Chiropteran physiology) (climate/fructification, chorology, bats physiology). Several factors of emergence were then listed including: Chronic infection, infected organs, virus shedding, close encounters between reservoir and susceptible hosts, food and water resource, seasonality, chorology (i.e. causal effect between geographical phenomena – season) in the tropical rain forest and the spatial distribution of chiropteran (i.e. index site of Ebola emerging events).

Epidemiological field surveys indicate that mass mortalities of apes and monkey species due to Ebola virus often appear at the end of the dry season, a period when food resources are scarce. Restricted access to a limited number of fruit-bearing trees can lead to spatiotemporal clustering of diverse species of frugivorous animals, such as bats, nonhuman primates, and other terrestrial species foraging on fallen partially eaten (by bats) fruits. These aggregates of wild animal species favor the contact between infected and susceptible individuals and promote virus transmission. The dry season aggregation of reservoir host species involved in natural maintenance cycles, augmented by incidentally infected secondary hosts serving as sources for intra- and interspecific transmission chains independent of repeated spillover from the reservoir host, provides an ecological setting for amplifying enzootic transmission of Ebola virus when a vertebrate hosts are concentrated around a scarce number of water sources [62].

In addition to this dietary impoverishment, there are behavioral and physiological events occurring among bats during the tropical dry favor the contact frequency and intimacy between bats, which can promote transmission of Ebola virus to others and increase R0. As an example, megachiropteran fruit bats breeding activities and intraspecific competitions between males and grouped *kidding* of females favor the contact between individuals. Moreover, pregnancy can involve physiological changes among female bats that alter immune functions and eventually favor virus

**21**

**Figure 4.**

shedding. Parturition among the African megachiropteran bats occurs throughout the year, although seasonal peaks provide birthing fluids, blood, and placental tissues, potentially Ebolavirus infected, falling on the ground as a medium highly

*chiropter to duikers; and (8) consumption of chiropteran infected food by shrew or wild pig.*

*(A) Understanding Ebolavirus enzootic and epidemics. Red arrows = cycles of transmission; dashed square = a putative natural cycle of Ebolavirus in Central Africa (see B). Fruit bats are considered to be a putative reservoir of Ebola virus in Central Africa after 2004; In 2009, several non-human primate epizootic are reported; 1976 was the first emerging events and subsequent epidemic chains in remote area of the rain forest and close by; 2012 showed a dramatic spread of the virus associated with motorized transportation and ground network; In 2014 urban epidemics are reported as well as a pandemic risk and become an international public health emergency. (B) Putative natural cycle of Ebolavirus in Central Africa. Red arrow indicates Ebolavirus transmission. Numbered red circle of transmission: (1) sylvatic inter- and intra-species transmission; (2) chiropteran migration; (3) chiropter to primate (close contact of dejection); (4) primate inter species (Cercopithecus/ chimpanzee); (5) primate to primate (non-human primates); (6) non-human primate epizootic (gorillas); (7)* 

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

*Emerging Challenges in Filovirus Infections*

potential presence of apes, virus-sensitive hosts.

*EBOVs in Africa*

Ebola emerging events).

around a scarce number of water sources [62].

characteristics of the places of these emergences (i.e. land use, temperature, rainfall) it turns out that the emergences are always in the zone of heavy rainfall, but nevertheless do not follow the moving of the rainy season. Moreover, these emergences remain always and remarkably close enough to the Equator, therefore in the equatorial forest area with a high hygrometry, and a moderate annual temperature. However, the temperature at the time of emergence is not significantly different from the average annual temperature (at the points of emergence) which does not allow to distinguish seasonal effect in the emergence-temperature relationship. Conclusively, we did not identify a seasonality associated with the time of emergence, however the emerging events occur in specific geographic zone characterized by several environmental factors. Finally, the emergence zones are in areas of Land Use with specific temperatures not related to seasonality. Ultimately, it is also remarkable that all these emerging events occurred in an area with a highly

*4.2.3 Fundamentals and domains of emergence: a theory for a natural cycle of* 

Also, the EBOVs species are closely genetically related, their seems to occur by foci in nature. The host appears to be the same, natural or accidental, and the transmission done by direct contact with infected hosts or its biological products [50, 61]. Altogether, in the early 2000s, before the identification of chiropteran as a potential host-reservoir of the EBOVs, a hypothetic natural cycle was described empirically based on seasonal environmental climatic factors [55]. Then, taking into account bats as a potential reservoir-host, the question of virus transmission was central to consider while environmental factors appears to play a major role to the host and their natural cycle (Chiropteran physiology) (climate/fructification, chorology, bats physiology). Several factors of emergence were then listed including: Chronic infection, infected organs, virus shedding, close encounters between reservoir and susceptible hosts, food and water resource, seasonality, chorology (i.e. causal effect between geographical phenomena – season) in the tropical rain forest and the spatial distribution of chiropteran (i.e. index site of

Epidemiological field surveys indicate that mass mortalities of apes and monkey species due to Ebola virus often appear at the end of the dry season, a period when food resources are scarce. Restricted access to a limited number of fruit-bearing trees can lead to spatiotemporal clustering of diverse species of frugivorous animals, such as bats, nonhuman primates, and other terrestrial species foraging on fallen partially eaten (by bats) fruits. These aggregates of wild animal species favor the contact between infected and susceptible individuals and promote virus transmission. The dry season aggregation of reservoir host species involved in natural maintenance cycles, augmented by incidentally infected secondary hosts serving as sources for intra- and interspecific transmission chains independent of repeated spillover from the reservoir host, provides an ecological setting for amplifying enzootic transmission of Ebola virus when a vertebrate hosts are concentrated

In addition to this dietary impoverishment, there are behavioral and physiological events occurring among bats during the tropical dry favor the contact frequency and intimacy between bats, which can promote transmission of Ebola virus to others and increase R0. As an example, megachiropteran fruit bats breeding activities and intraspecific competitions between males and grouped *kidding* of females favor the contact between individuals. Moreover, pregnancy can involve physiological changes among female bats that alter immune functions and eventually favor virus

**20**

#### **Figure 4.**

*(A) Understanding Ebolavirus enzootic and epidemics. Red arrows = cycles of transmission; dashed square = a putative natural cycle of Ebolavirus in Central Africa (see B). Fruit bats are considered to be a putative reservoir of Ebola virus in Central Africa after 2004; In 2009, several non-human primate epizootic are reported; 1976 was the first emerging events and subsequent epidemic chains in remote area of the rain forest and close by; 2012 showed a dramatic spread of the virus associated with motorized transportation and ground network; In 2014 urban epidemics are reported as well as a pandemic risk and become an international public health emergency. (B) Putative natural cycle of Ebolavirus in Central Africa. Red arrow indicates Ebolavirus transmission. Numbered red circle of transmission: (1) sylvatic inter- and intra-species transmission; (2) chiropteran migration; (3) chiropter to primate (close contact of dejection); (4) primate inter species (Cercopithecus/ chimpanzee); (5) primate to primate (non-human primates); (6) non-human primate epizootic (gorillas); (7) chiropter to duikers; and (8) consumption of chiropteran infected food by shrew or wild pig.*

shedding. Parturition among the African megachiropteran bats occurs throughout the year, although seasonal peaks provide birthing fluids, blood, and placental tissues, potentially Ebolavirus infected, falling on the ground as a medium highly

attractive and readily available to scavenging terrestrial mammals [50, 56, 63] (**Figure 4A** and **B**).

#### **5. If we had to conclude**

Based on historical data and observations, the presented hypothesis of the natural cycle of Ebolavirus emergence prevail an inter-species spillover as the complex natural cycle involving several hosts (reservoir, vector, amplifier), as well as biotic and abiotic factors in a changing environment among other original features.

Although the natural cycle of EBOVs remains in the darkness of the rain forest, strong findings and comparative analysis of close parents of the filovirus throw some light to a potential natural cycle of EBOVs in Africa. EBOVs clearly appear linked to chiropteran and dependent for merging events in the environmental factors. Indeed, it appears that filoviridae are often associated with chiropteran while the emergence of the virus strains occurs as a sparse focus with a silent period of cryptic virus circulation. When virus transmission, i.e. spillover, from a hidden natural cycle, to accidental hosts occurs, it happened in a specific time-frame often linked to the season.

One can retain is that the EBOVs complex natural cycle is yet not on entirely elucidated and certainly dependent on environmental factors – associated with a specific environment of the chiropteran species incriminated (i.e. Different territories, different cycle) - leading to multiple, sometime concurrent, temporally and timely emergence in focus.

Although, other hypothesis has been suggested elsewhere including the Ebola virus Disease as an arthropod borne disease among others [42], there is important fundamental matters to consider as well before providing more.

However, beyond these hypotheses, fundamental questions subsist in order to go further learn. We can cite in particular the mystery of kin between the Reston virus of Asia and the Ebola viruses of Africa, would there not be a missing link in a geographic area yet to discover. Do the filovirus exist in the Americas hidden in the darkness of the tropical forest? Also, the Ebolavirus seems genetically stable, related to particular species of chiropter, was it to think about a co-evolution of the host and the virus in this closed environment of the forest of the tropical? Today, with the endless epidemic unfolding in the DRC, should we revisit our tools and strategy of struggle in an ever-changing world? [64].

#### **Acknowledgements**

We sincerely thank for their supports, brings to all the authors of this deep and never-ending research and scientific thought around an outstanding and fascinating subject: Georgetown University, Centaurus Biotech LLC., The DHS Emeritus Center for Emerging Zoonotic and Animal Diseases at Kansas State University.

#### **Conflict of interest**

All authors do not have any conflict of interest whatsoever with this published manuscript.

**23**

**Author details**

and Tom Vincent6

Washington, DC, USA

6 CRDF Global, USA

2 Centaurus Biotech LLC, USA

4 Ministry of Health, Senegal

Jean-Paul Gonzalez1,2\*, Marc Souris3

, Massamba Sylla4

1 Division of Biomedical Graduate Research Organization, Department of Microbiology and Immunology, School of Medicine, Georgetown University,

3 Institute of Research for Development (IRD), Bondy, France

5 Faculty of Pharmacy, Montpellier University, France

provided the original work is properly cited.

\*Address all correspondence to: jpgonzalez2808@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

, Francisco Veas2,5

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

*Emerging Challenges in Filovirus Infections*

(**Figure 4A** and **B**).

linked to the season.

timely emergence in focus.

**Acknowledgements**

**Conflict of interest**

manuscript.

**5. If we had to conclude**

attractive and readily available to scavenging terrestrial mammals [50, 56, 63]

Based on historical data and observations, the presented hypothesis of the natural cycle of Ebolavirus emergence prevail an inter-species spillover as the complex natural cycle involving several hosts (reservoir, vector, amplifier), as well as biotic and abiotic factors in a changing environment among other original features.

Although the natural cycle of EBOVs remains in the darkness of the rain forest, strong findings and comparative analysis of close parents of the filovirus throw some light to a potential natural cycle of EBOVs in Africa. EBOVs clearly appear linked to chiropteran and dependent for merging events in the environmental factors. Indeed, it appears that filoviridae are often associated with chiropteran while the emergence of the virus strains occurs as a sparse focus with a silent period of cryptic virus circulation. When virus transmission, i.e. spillover, from a hidden natural cycle, to accidental hosts occurs, it happened in a specific time-frame often

One can retain is that the EBOVs complex natural cycle is yet not on entirely elucidated and certainly dependent on environmental factors – associated with a specific environment of the chiropteran species incriminated (i.e. Different territories, different cycle) - leading to multiple, sometime concurrent, temporally and

Although, other hypothesis has been suggested elsewhere including the Ebola virus Disease as an arthropod borne disease among others [42], there is important

However, beyond these hypotheses, fundamental questions subsist in order to go further learn. We can cite in particular the mystery of kin between the Reston virus of Asia and the Ebola viruses of Africa, would there not be a missing link in a geographic area yet to discover. Do the filovirus exist in the Americas hidden in the darkness of the tropical forest? Also, the Ebolavirus seems genetically stable, related to particular species of chiropter, was it to think about a co-evolution of the host and the virus in this closed environment of the forest of the tropical? Today, with the endless epidemic unfolding in the DRC, should we revisit our tools and strategy

We sincerely thank for their supports, brings to all the authors of this deep and never-ending research and scientific thought around an outstanding and fascinating subject: Georgetown University, Centaurus Biotech LLC., The DHS Emeritus Center for Emerging Zoonotic and Animal Diseases at Kansas State University.

All authors do not have any conflict of interest whatsoever with this published

fundamental matters to consider as well before providing more.

of struggle in an ever-changing world? [64].

**22**

#### **Author details**

Jean-Paul Gonzalez1,2\*, Marc Souris3 , Massamba Sylla4 , Francisco Veas2,5 and Tom Vincent6

1 Division of Biomedical Graduate Research Organization, Department of Microbiology and Immunology, School of Medicine, Georgetown University, Washington, DC, USA

2 Centaurus Biotech LLC, USA

3 Institute of Research for Development (IRD), Bondy, France

4 Ministry of Health, Senegal

5 Faculty of Pharmacy, Montpellier University, France

6 CRDF Global, USA

\*Address all correspondence to: jpgonzalez2808@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[2] ICTV. Filoviridae. 2019. Available from: https://talk.ictvonline.org/ictvreports/ictv\_online\_report/negativesense-rna-viruses/mononegavirales/w/ filoviridae

[3] Kuhn JH, Andersen KG, Baize S, Bào Y, Bavari S, Berthet N, et al. Nomenclature- and database-compatible names for the two Ebola virus variants that emerged in Guinea and the Democratic Republic of the Congo in 2014. Viruses. 2014;**6**(11):4760-4799. DOI: 10.3390/v6114760

[4] MacNeil A, Farnon EC, Morgan OW, et al. Filovirus outbreak detection and surveillance: Lessons from Bundibugyo. The Journal of Infectious Diseases. 2011;**204**:S761-S767

[5] Kiley MP, Bowen ET, Eddy GA, Isaäcson M, Johnson KM, McCormick JB, et al. Filoviridae: A taxonomic home for Marburg and Ebola viruses? Intervirology. 1982;**18**(1-2):24-32

[6] Anonymous. WHO/ INTERNATIONAL STUDY TEAM. Ebola haemorrhagic fever in Zaire, 1976. Bulletin of the World Health Organization. 1978;**56**(2):271-293

[7] Anonymous. WHO/ INTERNATIONAL STUDY TEAM. Ebola haemorrhagic fever in Sudan, 1976. Bulletin of the World Health Organization. 1978;**56**(2):247-270

[8] Centers for Disease Control. Ebola-Reston virus infection among quarantined nonhuman primates– Texas, 1996. Morbidity and Mortality Weekly Report. 1996;**45**:314-316

[9] Miranda MEG, Lee N, Miranda J. Reston ebolavirus in humans and animals in the Philippines: A review. The Journal of Infectious Diseases. 2011;**204**(suppl\_3):S757-S760. DOI: 10.1093/infdis/jir296

[10] Le Guenno B, Formenty P, Wyers M, Gounon P, Walker F, Boesch C. Isolation and partial characterisation of a new strain of Ebola virus. The Lancet. 1995;**345**(8960):1271-1274

[11] Towner JS, Sealy TK, Khristova ML, Albariño CG, Conlan S, Reeder SA, et al. Newly discovered ebola virus associated with hemorrhagic fever outbreak in Uganda. PLoS Pathogens. 2008;**4**(11):e1000212. DOI: 10.1371/ journal.ppat.1000212

[12] Miranda ME, Ksiazek TG, Retuya TJ, Khan AS, Sanchez A, Fulhorst CF, et al. Epidemiology of Ebola (subtype Reston) virus in the Philippines, 1996. The Journal of Infectious Diseases. 1999;**179**(Suppl 1):S115-S119

[13] Georges AJ, Leroy EM, Renaud AA, et al. Ebola hemorrhagic fever outbreaks in Gabon, 1994-1997: Epidemiologic and health control issues. The Journal of Infectious Diseases. 1999;**179**:S65-S75

[14] Baize S, Pannetier D, Oestereich L, Rieger T, Koivogui L, Magassouba N, et al. Emergence of Zaire Ebola virus disease in Guinea. The New England Journal of Medicine. 2014;**371**(15):1418- 1425. DOI: 10.1056/NEJMoa1404505

[15] WHO. Disease Outbreak News. 2018a. Available from: https://www. who.int/ebola/situation-reports/drc-2018/en/ [Accessed: 13-01-19]

**25**

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

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[25] Rollin PE, Williams RJ, Bressler DS, Pearson S, Cottingham M, Pucak G, et al. Ebola (subtype Reston) virus among quarantined nonhuman primates recently imported from the Philippines to the United States. The Journal of Infectious Diseases. 1999;**179**(Suppl 1):S108-S114

[26] Wauquier N, Bangura J, Moses L, Humarr Khan S, Coomber M, Lungay V, et al. Understanding the emergence of ebola virus disease in Sierra Leone: Stalking the virus in the threatening wake of emergence. PLOS Currents.

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[28] Monath TP. Ecology of Marburg and Ebola viruses: Speculations and directions for the future research. The Journal of Infectious Diseases.

[29] Yob JM, Field H, Rashdi AM, Morrissy C, van der Heide B, Rota P, et al. Nipah virus infection in bats (order Chiroptera) in peninsular Malaysia. Emerging Infectious Diseases.

[30] Badrane H, Tordo N. Host switching in Lyssavirus history from the Chiroptera to the Carnivora orders. Journal of Virology.

[31] Tom Vincent. EBOLA: FIFTEEN YEARS OF SILENCE. 2019. Available from: http://oneill.law.georgetown.edu/

[32] Pourrut X, Souris M, Towner JS, Rollin PE, Nichol ST, Gonzalez JP,

ebola-fifteen-years-of-silence/

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[19] Becquart P, Wauquier N, Mahlakõiv T, Nkoghe D, Padilla C, Souris M, et al. High prevalence of both humoral and cellular immunity to *Zaire ebolavirus* among rural populations in Gabon. PLoS One. 2010;**5**(2):e9126

[21] Olivero J, Fa JE, Real R,

[22] Olival KJ, Islam A, Yu M, Anthony SJ, Epstein JH, Khan SA, et al. Ebola virus antibodies in fruit bats, Bangladesh. Emerging Infectious Diseases. 2013;**19**:270-273. DOI:

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[20] Vincent T. Ebola Footprint—Broader Than You Think. Available from: http:// oneill.law.georgetown.edu/the-ebolafootprint-broader-than-you-think/

Farfan MA, Marquez AN, Vargas JM, et al. Mammalian biogeography and the Ebola virus in Africa. Mammal Review.

[23] Yuan JF, Zhang YJ, Li JL, Zhang YZ, Wang LF, Shi ZL. Serological evidence of ebolavirus infection in bats, China. Virology Journal. 2012;**9**:236. DOI:

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*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

[16] Leroy EM, Rouquet P, Formenty P, Souquiere S, Kilbourne A, Froment JM, et al. Multiple Ebola virus transmission events and rapid decline of central African wildlife. Science. 2004a;**303**:387-390

[17] Heymann DL, Weisfeld JS, Webb PA, Johnson KM, Cairns T, Berquist H. Ebola hemorrhagic fever: Tandala, Zaire, 1977-1978. The Journal of Infectious Diseases. 1980;**142**:372-376

[18] O'Hearn AE, Voorhees MA, Fetterer DP, Wauquier N, Coomber MR, Bangura J, et al. Serosurveillance of viral pathogens circulating in West Africa. Virology Journal. 2016;**13**(1):163

[19] Becquart P, Wauquier N, Mahlakõiv T, Nkoghe D, Padilla C, Souris M, et al. High prevalence of both humoral and cellular immunity to *Zaire ebolavirus* among rural populations in Gabon. PLoS One. 2010;**5**(2):e9126

[20] Vincent T. Ebola Footprint—Broader Than You Think. Available from: http:// oneill.law.georgetown.edu/the-ebolafootprint-broader-than-you-think/

[21] Olivero J, Fa JE, Real R, Farfan MA, Marquez AN, Vargas JM, et al. Mammalian biogeography and the Ebola virus in Africa. Mammal Review. 2016;**47**:24-37

[22] Olival KJ, Islam A, Yu M, Anthony SJ, Epstein JH, Khan SA, et al. Ebola virus antibodies in fruit bats, Bangladesh. Emerging Infectious Diseases. 2013;**19**:270-273. DOI: 10.3201/eid1902.120524

[23] Yuan JF, Zhang YJ, Li JL, Zhang YZ, Wang LF, Shi ZL. Serological evidence of ebolavirus infection in bats, China. Virology Journal. 2012;**9**:236. DOI: 10.1186/1743-422X-9-236

[24] Peterson AT, Bauer JT, Mills JN. Ecologic and geographic distribution of filovirus disease. Emerging Infectious Diseases. 2004;**10**:40-47. DOI: 10.3201/ eid1001.030125

[25] Rollin PE, Williams RJ, Bressler DS, Pearson S, Cottingham M, Pucak G, et al. Ebola (subtype Reston) virus among quarantined nonhuman primates recently imported from the Philippines to the United States. The Journal of Infectious Diseases. 1999;**179**(Suppl 1):S108-S114

[26] Wauquier N, Bangura J, Moses L, Humarr Khan S, Coomber M, Lungay V, et al. Understanding the emergence of ebola virus disease in Sierra Leone: Stalking the virus in the threatening wake of emergence. PLOS Currents. 2015;**7**

[27] Ebola virus cases in the United States. Available from: https://en.wikipedia.org/wiki/ Ebola\_virus\_cases\_in\_the\_United\_States

[28] Monath TP. Ecology of Marburg and Ebola viruses: Speculations and directions for the future research. The Journal of Infectious Diseases. 1999;**179**:S127-S138

[29] Yob JM, Field H, Rashdi AM, Morrissy C, van der Heide B, Rota P, et al. Nipah virus infection in bats (order Chiroptera) in peninsular Malaysia. Emerging Infectious Diseases. 2001;**7**(3):439-441

[30] Badrane H, Tordo N. Host switching in Lyssavirus history from the Chiroptera to the Carnivora orders. Journal of Virology. 2001;**75**:8096-8104

[31] Tom Vincent. EBOLA: FIFTEEN YEARS OF SILENCE. 2019. Available from: http://oneill.law.georgetown.edu/ ebola-fifteen-years-of-silence/

[32] Pourrut X, Souris M, Towner JS, Rollin PE, Nichol ST, Gonzalez JP,

**24**

*Emerging Challenges in Filovirus Infections*

[1] CDC. Page last reviewed. Content source: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), Division of High-Consequence Pathogens and Pathology (DHCPP), Viral Special Pathogens Branch (VSPB). 2019. Available from: https://www.cdc.gov/vhf/ebola/

[8] Centers for Disease Control. Ebola-Reston virus infection among quarantined nonhuman primates– Texas, 1996. Morbidity and Mortality Weekly Report. 1996;**45**:314-316

[9] Miranda MEG, Lee N, Miranda J. Reston ebolavirus in humans and animals in the Philippines: A review. The Journal of Infectious Diseases. 2011;**204**(suppl\_3):S757-S760. DOI:

[10] Le Guenno B, Formenty P, Wyers M, Gounon P, Walker F, Boesch C. Isolation and partial characterisation of a new strain of Ebola virus. The Lancet. 1995;**345**(8960):1271-1274

[11] Towner JS, Sealy TK, Khristova ML, Albariño CG, Conlan S, Reeder SA, et al. Newly discovered ebola virus associated with hemorrhagic fever outbreak in Uganda. PLoS Pathogens. 2008;**4**(11):e1000212. DOI: 10.1371/

[12] Miranda ME, Ksiazek TG, Retuya TJ, Khan AS, Sanchez A, Fulhorst CF, et al. Epidemiology of Ebola (subtype Reston) virus in the Philippines, 1996. The Journal of Infectious Diseases. 1999;**179**(Suppl 1):S115-S119

Renaud AA, et al. Ebola hemorrhagic fever outbreaks in Gabon, 1994-1997: Epidemiologic and health control issues. The Journal of Infectious Diseases. 1999;**179**:S65-S75

[14] Baize S, Pannetier D, Oestereich L, Rieger T, Koivogui L, Magassouba N, et al. Emergence of Zaire Ebola virus disease in Guinea. The New England Journal of Medicine. 2014;**371**(15):1418- 1425. DOI: 10.1056/NEJMoa1404505

[15] WHO. Disease Outbreak News. 2018a. Available from: https://www. who.int/ebola/situation-reports/drc-2018/en/ [Accessed: 13-01-19]

10.1093/infdis/jir296

journal.ppat.1000212

[13] Georges AJ, Leroy EM,

history/2014-2016-outbreak/index.html

[2] ICTV. Filoviridae. 2019. Available from: https://talk.ictvonline.org/ictvreports/ictv\_online\_report/negativesense-rna-viruses/mononegavirales/w/

[3] Kuhn JH, Andersen KG, Baize S, Bào Y, Bavari S, Berthet N, et al.

DOI: 10.3390/v6114760

2011;**204**:S761-S767

1982;**18**(1-2):24-32

[6] Anonymous. WHO/ INTERNATIONAL STUDY

[7] Anonymous. WHO/ INTERNATIONAL STUDY TEAM. Ebola haemorrhagic fever in Sudan, 1976. Bulletin of the World Health Organization.

1978;**56**(2):247-270

Isaäcson M, Johnson KM, McCormick JB, et al. Filoviridae: A taxonomic home for Marburg and Ebola viruses? Intervirology.

Nomenclature- and database-compatible names for the two Ebola virus variants that emerged in Guinea and the Democratic Republic of the Congo in 2014. Viruses. 2014;**6**(11):4760-4799.

[4] MacNeil A, Farnon EC, Morgan OW, et al. Filovirus outbreak detection and surveillance: Lessons from Bundibugyo. The Journal of Infectious Diseases.

[5] Kiley MP, Bowen ET, Eddy GA,

TEAM. Ebola haemorrhagic fever in Zaire, 1976. Bulletin of the World Health Organization. 1978;**56**(2):271-293

filoviridae

**References**

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[34] Arata AA, Johnson B. Approaches towards studies on potential reservoirs of viral haemorrhagic fever in southern Sudan. In: Pattyn SR, editor. Ebola Virus Haemor- Rhagic Fever. Amsterdam: Elsevier/Netherland biomedical; 1977. pp. 191-202

[35] Leirs H, Mills JN, Krebs JW, Childs JE, Akaibe D, Woollen N, et al. Search for the Ebola virus reservoir in Kikwit Democratic Republic of the Congo: Reflections on a vertebrate collection. The Journal of Infectious Diseases. 1999;**179**:S155-S163

[36] Morvan JM, Deubel V, Gounon P, Nakoune E, Barriere P, Murri S, et al. Identification of Ebola virus sequences present as RNA or DNA in organs of terrestrial small mammals of the Central African Republic. Microbes and Infection. 1999;**1**:1193-1201

[37] Reiter P, Turell M, Coleman R, Miller B, Maupin G, Liz J, et al. Field investigations of an outbreak of Ebola hemorrhagic fever Kikwit Democratic Republic of the Congo, 1995: Arthropod studies. The Journal of Infectious Diseases. 1999;**179**:S148-S154

[38] Formenty P, Boesch C, Wyers M, Steiner C, Donati F, Dind F, Walker F, Le Guenno B. Ebola virus outbreak

among wild chimpanzees living in a rain forest of cote d'Ivoire. The Journal of Infectious Diseases. 1999;**179**(Suppl 1): S120-S126

[39] Swanepoel R, Leman PA, Burt FJ. Experimental inoculation of plants and animals with Ebola virus. Emerging Infectious Diseases. 1996;**2**:321-325

[40] Xavier P, Gonzalez JP, Leroy E. Spatial and temporal patterns of Ebola virus antibody prevalence in the putative bat species reservoir. The Journal of Infectious Diseases. 2007;**196**(Suppl 2):S176-S183

[41] Eric L, Kumulungui B, Pourrut X, Rouquet P, Yaba P, Délicat A, et al. Fruit bats as reservoirs of Ebola virus. Nature. 2005;**438**(7068):575-576

[42] Caron A, Bourgarel M, Cappelle J, Liégeois F, De Nys HM, Roger F. Ebola virus maintenance: If not (only) bats, what Else? Viruses. 2018;**10**(10):549. DOI: 10.3390/v10100549

[43] Goldstein T, Anthony SJ, Gbakima A, Bird BH, Bangura J, Tremeau-Bravard A, et al. The discovery of Bombali virus adds further support for bats as hosts of ebolaviruses. Nature Microbiology. 2018;**3**:1084-1089

[44] Leroy EM, Epelboin A, Mondonge V, Pourrut X, Gonzalez JP, Muyembe-Tamfum JJ, et al. Ebola outbreak associated with direct exposure to fruit bats in Luebo, Democratic Republic of the Congo, 2007. Vector Borne and Zoonotic Diseases. 2009;**9**(6):723-728

[45] Marí Saéz A, Weiss S, Nowak K, Lapeyre V, Zimmermann F, Düx A, et al. Investigating the zoonotic origin of the west African Ebola epidemic. EMBO Molecular Medicine. 2015;**7**(1):17-23. DOI: 10.15252/emmm.201404792

**27**

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

the virus in Spain. Viruses. 2019;**11**:360.

[54] Calisher CH, Childs JE, Field HE, Holmes KV, Schountz T. Bats: Important reservoir hosts of emerging viruses. Clinical Microbiology Reviews.

[55] Gonzalez JP, Pourrut X, Leroy E. Ebolavirus and other filoviruses. In: Childs JE, Mackenzie JS, Richt JA, editors. Wildlife and Emerging Zoonotic Diseases: The Biology, Circumstances and Consequences of Cross-Species Transmission. New York, NY, USA: Springer; Heidelberg, Germany; 2007.

[56] Allela L, Bourry O, Pouillot R, Délicat A, Yaba P, Kumulungui B, et al. Ebola virus antibody in dogs and human risk. Emerging Infectious Diseases.

[57] Ayouba A, Ahuka-Mundeke S, Butel C, Mbala Kingebeni P,

DOI: 10.1093/infdis/jiz006

[59] Paweska JT, Storm N,

2014;**3**:e04395

10.3390/v8020029

Loul S, Tagg N, et al. Extensive serological survey of multiple African non-human primate species reveals low prevalence of IgG antibodies to four Ebola virus species. The Journal of Infectious Diseases. 2019.

[58] Pigott DM, Golding N, Mylne A, et al. Mapping the zoonotic niche of Ebola virus disease in Africa. eLife.

Grobbelaar AA, Markotter W, Kemp A, Jansen van Vuren P. Experimental inoculation of Egyptian fruit bats (*Rousettus aegyptiacus*) with Ebola virus. Viruses. 2016;**8**(2). pii: E29. DOI:

[60] Hijmans RJ, Cameron SE, Parra JL, Jones PG, Jarvis A. Very high-resolution interpolated climate surfaces for global land areas. International Journal of Climatology. 2005;**25**:1965-1978

DOI: 10.3390/v11040360

2006;**19**:531-545

pp. 363-388

2005;**11**(3):385-390

[46] Tapanes E, Detwiler KM, Cords M. Bat predation by

Cercopithecus monkeys: Implications for zoonotic disease transmission. EcoHealth. 2016;**13**:405-409

[47] Bermejo M, Illera G, Sabater P. Animals and mushrooms consumed by bonobos (pan paniscus): New records from Lilungu (Ikele), Zaire. International Journal of Primatology.

[48] Leendertz SAJ, Gogarten JF, Düx A, Calvignac-Spencer S, Leendertz FH. Assessing the evidence supporting fruit bats as the primary reservoirs for Ebola viruses. EcoHealth.

[49] Olival KJ, Hayman DT. Filoviruses in bats: Current knowledge and future directions. Viruses. 2014;**6**(4):1759- 1788. DOI: 10.3390/v6041759

[50] Gonzalez JP, Pourrut X, Leroy E. Ebolavirus and other filoviruses. Current Topics in Microbiology and Immunology.

[51] Jayme SI, Field HE, de Jong C, Olival KJ, Marsh G, Tagtag AM, et al. Molecular evidence of Ebola Reston virus infection in Philippine bats. Virology Journal. 2015;**12**:107. DOI:

[52] Pawęska JT, Jansen van Vuren P, Kemp A, Storm N, Grobbelaar AA, Wiley MR, et al. Marburg virus infection in Egyptian Rousette bats, South Africa, 2013-20141. Emerging Infectious Diseases. 2018 Jun;**24**(6):1134-1137. DOI: 10.3201/eid2406.172165

[53] de Arellano ER, Sanchez-Lockhart M, Perteguer MJ, Bartlett M, Ortiz M, Campioli P, et al. First evidence of antibodies against Lloviu virus in Schreiber's bent-winged insectivorous bats demonstrate a wide circulation of

10.1186/s12985-015-0331-3

1994;**15**:879-898

2016;**13**(1):18-25

2007;**315**:363-387

*Essay on the Elusive Natural History of Ebola Viruses DOI: http://dx.doi.org/10.5772/intechopen.88879*

[46] Tapanes E, Detwiler KM, Cords M. Bat predation by Cercopithecus monkeys: Implications for zoonotic disease transmission. EcoHealth. 2016;**13**:405-409

*Emerging Challenges in Filovirus Infections*

et al. Large serological survey showing cocirculation of Ebola and Marburg viruses in Gabonese bat populations, and a high seroprevalence of both viruses in Rousettus aegyptiacus. BMC among wild chimpanzees living in a rain forest of cote d'Ivoire. The Journal of Infectious Diseases. 1999;**179**(Suppl 1):

[39] Swanepoel R, Leman PA, Burt FJ. Experimental inoculation of plants and animals with Ebola virus. Emerging Infectious Diseases.

[40] Xavier P, Gonzalez JP, Leroy E. Spatial and temporal patterns of Ebola virus antibody prevalence in the putative bat species reservoir. The Journal of Infectious Diseases. 2007;**196**(Suppl 2):S176-S183

[41] Eric L, Kumulungui B, Pourrut X, Rouquet P, Yaba P, Délicat A, et al. Fruit bats as reservoirs of Ebola virus. Nature. 2005;**438**(7068):575-576

[42] Caron A, Bourgarel M, Cappelle J, Liégeois F, De Nys HM, Roger F. Ebola virus maintenance: If not (only) bats, what Else? Viruses. 2018;**10**(10):549.

DOI: 10.3390/v10100549

2018;**3**:1084-1089

[43] Goldstein T, Anthony SJ, Gbakima A, Bird BH, Bangura J, Tremeau-Bravard A, et al. The discovery of Bombali virus adds further support for bats as hosts of ebolaviruses. Nature Microbiology.

[44] Leroy EM, Epelboin A,

Mondonge V, Pourrut X, Gonzalez JP, Muyembe-Tamfum JJ, et al. Ebola outbreak associated with direct exposure to fruit bats in Luebo, Democratic Republic of the Congo, 2007. Vector Borne and Zoonotic Diseases. 2009;**9**(6):723-728

[45] Marí Saéz A, Weiss S, Nowak K, Lapeyre V, Zimmermann F, Düx A, et al. Investigating the zoonotic origin of the west African Ebola epidemic. EMBO Molecular Medicine. 2015;**7**(1):17-23. DOI: 10.15252/emmm.201404792

S120-S126

1996;**2**:321-325

Infectious Diseases. 2009;**9**:159

[33] Breman JG, Johnson KM, van der Groen G, Robbins CB, Szczeniowski MV, Ruti K, et al. A search for Ebola virus in animals in the Democratic Republic of the Congo and Cameroon: Ecologic, virologic, and sero- logic surveys, 1979-1980. The Journal of Infectious Diseases.

[34] Arata AA, Johnson B. Approaches towards studies on potential reservoirs of viral haemorrhagic fever in southern Sudan. In: Pattyn SR, editor. Ebola Virus Haemor- Rhagic Fever. Amsterdam: Elsevier/Netherland biomedical; 1977.

[35] Leirs H, Mills JN, Krebs JW, Childs JE, Akaibe D, Woollen N, et al. Search for the Ebola virus reservoir in Kikwit Democratic Republic of the Congo: Reflections on a vertebrate collection. The Journal of Infectious Diseases. 1999;**179**:S155-S163

[36] Morvan JM, Deubel V, Gounon P, Nakoune E, Barriere P, Murri S, et al. Identification of Ebola virus sequences present as RNA or DNA in organs of terrestrial small mammals of the Central African Republic. Microbes and

Infection. 1999;**1**:1193-1201

[37] Reiter P, Turell M, Coleman R, Miller B, Maupin G, Liz J, et al. Field investigations of an outbreak of Ebola hemorrhagic fever Kikwit Democratic Republic of the Congo, 1995: Arthropod studies. The Journal of Infectious Diseases. 1999;**179**:S148-S154

[38] Formenty P, Boesch C, Wyers M, Steiner C, Donati F, Dind F, Walker F, Le Guenno B. Ebola virus outbreak

1999;**179**:S139-S147

pp. 191-202

**26**

[47] Bermejo M, Illera G, Sabater P. Animals and mushrooms consumed by bonobos (pan paniscus): New records from Lilungu (Ikele), Zaire. International Journal of Primatology. 1994;**15**:879-898

[48] Leendertz SAJ, Gogarten JF, Düx A, Calvignac-Spencer S, Leendertz FH. Assessing the evidence supporting fruit bats as the primary reservoirs for Ebola viruses. EcoHealth. 2016;**13**(1):18-25

[49] Olival KJ, Hayman DT. Filoviruses in bats: Current knowledge and future directions. Viruses. 2014;**6**(4):1759- 1788. DOI: 10.3390/v6041759

[50] Gonzalez JP, Pourrut X, Leroy E. Ebolavirus and other filoviruses. Current Topics in Microbiology and Immunology. 2007;**315**:363-387

[51] Jayme SI, Field HE, de Jong C, Olival KJ, Marsh G, Tagtag AM, et al. Molecular evidence of Ebola Reston virus infection in Philippine bats. Virology Journal. 2015;**12**:107. DOI: 10.1186/s12985-015-0331-3

[52] Pawęska JT, Jansen van Vuren P, Kemp A, Storm N, Grobbelaar AA, Wiley MR, et al. Marburg virus infection in Egyptian Rousette bats, South Africa, 2013-20141. Emerging Infectious Diseases. 2018 Jun;**24**(6):1134-1137. DOI: 10.3201/eid2406.172165

[53] de Arellano ER, Sanchez-Lockhart M, Perteguer MJ, Bartlett M, Ortiz M, Campioli P, et al. First evidence of antibodies against Lloviu virus in Schreiber's bent-winged insectivorous bats demonstrate a wide circulation of

the virus in Spain. Viruses. 2019;**11**:360. DOI: 10.3390/v11040360

[54] Calisher CH, Childs JE, Field HE, Holmes KV, Schountz T. Bats: Important reservoir hosts of emerging viruses. Clinical Microbiology Reviews. 2006;**19**:531-545

[55] Gonzalez JP, Pourrut X, Leroy E. Ebolavirus and other filoviruses. In: Childs JE, Mackenzie JS, Richt JA, editors. Wildlife and Emerging Zoonotic Diseases: The Biology, Circumstances and Consequences of Cross-Species Transmission. New York, NY, USA: Springer; Heidelberg, Germany; 2007. pp. 363-388

[56] Allela L, Bourry O, Pouillot R, Délicat A, Yaba P, Kumulungui B, et al. Ebola virus antibody in dogs and human risk. Emerging Infectious Diseases. 2005;**11**(3):385-390

[57] Ayouba A, Ahuka-Mundeke S, Butel C, Mbala Kingebeni P, Loul S, Tagg N, et al. Extensive serological survey of multiple African non-human primate species reveals low prevalence of IgG antibodies to four Ebola virus species. The Journal of Infectious Diseases. 2019. DOI: 10.1093/infdis/jiz006

[58] Pigott DM, Golding N, Mylne A, et al. Mapping the zoonotic niche of Ebola virus disease in Africa. eLife. 2014;**3**:e04395

[59] Paweska JT, Storm N, Grobbelaar AA, Markotter W, Kemp A, Jansen van Vuren P. Experimental inoculation of Egyptian fruit bats (*Rousettus aegyptiacus*) with Ebola virus. Viruses. 2016;**8**(2). pii: E29. DOI: 10.3390/v8020029

[60] Hijmans RJ, Cameron SE, Parra JL, Jones PG, Jarvis A. Very high-resolution interpolated climate surfaces for global land areas. International Journal of Climatology. 2005;**25**:1965-1978

[61] Mackensie J, Mills J, editors. Review. In: Wildlife and Emerging Zoonotic Diseases. Springer-Verelag CRC Press. Advances in Virology ch20

[62] Shaman J, Day JF, Stieglitz M. Drought-induced amplification of Saint Louis encephalitis virus Florida. Emerging Infectious Diseases. 2002;**8**:575-580

[63] Pourrut X, Kumulungui B, Wittmann T, Moussavou G, Delicat A, Yaba P, et al. The natural history of Ebola virus in Africa. Microbes and Infection. 2005;**7**(7-8):1005-1014

[64] Gonzalez JP, Souris M, Valdivia-Granda W. Global spread of hemorrhagic fever viruses: Predicting pandemics. Methods in Molecular Biology. 2018;**1604**:3-31. DOI: 10.1007/978-1-4939-6981-4\_1

**29**

Section 3

Clinical Features
