**11. Epidemiology**

The virus is maintained in endemic areas of Africa and South America by enzootic transmis‐ sion between mosquitoes and monkeys, and obviously the epidemiology of the disease re‐ flects the geographical distribution of the mosquito vectors [57].

#### **11.1. Transmission**

The enzootic transmission cycle involves tree-hole-breeding mosquitoes such as *Aemagogus janthinomys*(South America) and *Ae. africanus*(Africa), and nonhuman primates. Infection of mosquitoes begins after ingestion of blood containing a threshold concentration of virus (~3.5 log 10 ml¯1), resulting in infection of the midgut epithelium. The virus is released from the midgut into the hemolymph and spreads to other tissues, notably the reproductive tract and salivary glands. A period of 7-10 days is required between ingestion of virus and virus secretion in saliva (the extrinsic incubation period), after which the female mosquito is capa‐ ble of transmitting virus to a susceptible host.

Vertical transmission of virus occurs from the female mosquito to her progeny and from congenitally infected males to females during copulation. Virus in the egg stage provides a mechanism for virus survival over the dry season when adult mosquito activity and hori‐ zontal transmission abate. The virus is maintained over the dry season by vertical transmis‐ sion in mosquitoes. Ova containing virus survive in dry tree-holes and hatch infectious progeny mosquitoes when the rains resume [58].

**11.2. Transmission cycle patterns and ecology**

geographic area.

be determined.

rest of its life [63].

In Africa, three transmission cycles can be distinguished: the sylvatic, urban, and savan‐ nah cycles. In South America, only sylvatic and urban cycles have been identified (Figure 3). In all the three cycles, yellow fever virus is transmitted between primates by diurnally active tree hole-breeding mosquitoes. Neither the virus nor the clinical disease differs in these three cycles, but identifying the type of transmission cycle is important for disease control. In all of these cycles, endemic and epidemic disease patterns can occur [59]. Syl‐ vatic yellow fever (YF) in South America is maintained in an epizootic cycle between nonhuman primates and Haemagogus and Sabethes mosquitoes, tree-hole breeding species that reside in the forest canopy. Humans are infected incidentally in the sylvatic cycle when they inhabit or work in the forest where infected mosquitoes are present [60].

Yellow Fever Encephalitis: An Emerging and Resurging Global Public Health Threat in a Changing Environment

http://dx.doi.org/10.5772/46041

219

In the ''Jungle'' or ''Sylvatic'' cycle, the virus is transmitted among monkeys by tree-hole breeding mosquitoes. Humans are infected incidentally when entering the area (e.g., to work as foresters) and have what is termed ''jungle yellow fever''. The main vector in Af‐ rica is *Aedes africanus*, while in South America it is Haemagogus species. Other mosquito species involved in transmission include *Ae. africanus, Ae. furcifer, Ae. vittatus, Ae. luteoce‐ phalus, Ae. opok, Ae. metallicus,* and *Ae. simpsoni* in Africa, and Sabetheschloropterus in South America. The primate species acting as vertebrate hosts of the virus also differ by

The ''Urban'' cycle involves transmission of YF virus between humans by *Ae. aegypti*, a domestic vector that breeds close to human habitation in water, and scrap containers in‐ cluding used tires in urban areas or dry savannah areas. In this situation, the disease is known as ''urban yellow fever'' [49]. YF is transmitted in urban cycles between humans and the container-breeding, anthropophilic mosquito *Aedes aegypti* [15]. In Africa, a third cycle is recognized, the intermediate or savannah cycle, where humans in the moist savan‐ nah regions come into contact with the jungle cycle. This has been referred to as the ''Zone of Emergence.'' Although YF is considered to be a mosquito-borne disease, Am‐ blyommavariegatum ticks have been shown to be naturally infected with the virus in cen‐ tral Africa [61]. The significance of this observation in the ecology of YF virus has yet to

Urban cycle epidemics develop from anthroponotic, also known as human-to-human, transmission in which humans serve as the sole host reservoir of the peridomestic *Ae. ae‐ gypti* mosquito vector. Urban epidemics occur when anicteric but viremic persons who are not yet severely ill, travel from jungles and savannas to cities where they infect local *Ae. aegypti* mosquitoes, a species that is abundant in urban areas and in areas where hu‐ mans store water. When YF is identified in any setting, the likelihood that it resulted from human-to-human transmission or its possible introduction into an urban setting must be rapidly assessed to determine the need for emergency vaccination [62]. The in‐ trinsic incubation period in human beings is between two and six days. The extrinsic in‐ cubation period in a mosquito varies from four to 18 days (average 12 days), with the temperature and humidity. Once the mosquito becomes infective, it remains so for the

**Figure 3.** Yellow Fever Transmision cycles in Africa and South America

#### **11.2. Transmission cycle patterns and ecology**

Vertical transmission of virus occurs from the female mosquito to her progeny and from congenitally infected males to females during copulation. Virus in the egg stage provides a mechanism for virus survival over the dry season when adult mosquito activity and hori‐ zontal transmission abate. The virus is maintained over the dry season by vertical transmis‐ sion in mosquitoes. Ova containing virus survive in dry tree-holes and hatch infectious

progeny mosquitoes when the rains resume [58].

218 Encephalitis

**Figure 3.** Yellow Fever Transmision cycles in Africa and South America

In Africa, three transmission cycles can be distinguished: the sylvatic, urban, and savan‐ nah cycles. In South America, only sylvatic and urban cycles have been identified (Figure 3). In all the three cycles, yellow fever virus is transmitted between primates by diurnally active tree hole-breeding mosquitoes. Neither the virus nor the clinical disease differs in these three cycles, but identifying the type of transmission cycle is important for disease control. In all of these cycles, endemic and epidemic disease patterns can occur [59]. Syl‐ vatic yellow fever (YF) in South America is maintained in an epizootic cycle between nonhuman primates and Haemagogus and Sabethes mosquitoes, tree-hole breeding species that reside in the forest canopy. Humans are infected incidentally in the sylvatic cycle when they inhabit or work in the forest where infected mosquitoes are present [60].

In the ''Jungle'' or ''Sylvatic'' cycle, the virus is transmitted among monkeys by tree-hole breeding mosquitoes. Humans are infected incidentally when entering the area (e.g., to work as foresters) and have what is termed ''jungle yellow fever''. The main vector in Af‐ rica is *Aedes africanus*, while in South America it is Haemagogus species. Other mosquito species involved in transmission include *Ae. africanus, Ae. furcifer, Ae. vittatus, Ae. luteoce‐ phalus, Ae. opok, Ae. metallicus,* and *Ae. simpsoni* in Africa, and Sabetheschloropterus in South America. The primate species acting as vertebrate hosts of the virus also differ by geographic area.

The ''Urban'' cycle involves transmission of YF virus between humans by *Ae. aegypti*, a domestic vector that breeds close to human habitation in water, and scrap containers in‐ cluding used tires in urban areas or dry savannah areas. In this situation, the disease is known as ''urban yellow fever'' [49]. YF is transmitted in urban cycles between humans and the container-breeding, anthropophilic mosquito *Aedes aegypti* [15]. In Africa, a third cycle is recognized, the intermediate or savannah cycle, where humans in the moist savan‐ nah regions come into contact with the jungle cycle. This has been referred to as the ''Zone of Emergence.'' Although YF is considered to be a mosquito-borne disease, Am‐ blyommavariegatum ticks have been shown to be naturally infected with the virus in cen‐ tral Africa [61]. The significance of this observation in the ecology of YF virus has yet to be determined.

Urban cycle epidemics develop from anthroponotic, also known as human-to-human, transmission in which humans serve as the sole host reservoir of the peridomestic *Ae. ae‐ gypti* mosquito vector. Urban epidemics occur when anicteric but viremic persons who are not yet severely ill, travel from jungles and savannas to cities where they infect local *Ae. aegypti* mosquitoes, a species that is abundant in urban areas and in areas where hu‐ mans store water. When YF is identified in any setting, the likelihood that it resulted from human-to-human transmission or its possible introduction into an urban setting must be rapidly assessed to determine the need for emergency vaccination [62]. The in‐ trinsic incubation period in human beings is between two and six days. The extrinsic in‐ cubation period in a mosquito varies from four to 18 days (average 12 days), with the temperature and humidity. Once the mosquito becomes infective, it remains so for the rest of its life [63].
