**3.2. Epidemiology**

The transmission dynamic of this zoonotic vector‐borne disease is complex: the seasonal activity of the vector species directly impacts on the transmission process of the parasite, whereas the length of the activity period and sandfly vectors abundances are affected by the

Since its emergence as an epidemic in Kairouan in 1982 [1], the disease has spread in several parts of Tunisia, particularly in the central and southern parts [3]. The epidemics are cyclic, and annually, 2000–3000 cases are reported. Although ZCL is usually self‐curing and not life‐threatening, individual cases may be psychologically and socially damaging, especially women with indelible scars that skin lesions leave on their faces. For these reasons, the epidemics are considered as a major public health priority, which remains an unresolved

Disease prevention and control are difficult because of the complexity of CL epizoology, and the few options available for effective vector control [4]. Furthermore, ZCL dynamic is influ‐ enced by environmental, demographic and human behavioral factors. In fact, in the recent decade, several new foci have been reported indicating the potential spread of the disease in Tunisia. Understanding the epidemiology, the distribution and the ecological structure of the

"*In Tunisia, CL was historically confined to the oasis of Gafsa and its surroundings (South‐West Tunisia) where the disease was typically sporadic and occasionally epidemic, particularly in French soldiers that camped in the Gafsa region in the late 19th century. This cutaneous affection was named 'clou de Gafsa'. In 1982, a CL outbreak occurred near the dam of Sidi Saad (Kairouan governorate, Central Tunisia) that had been just finished*" [2]. Over the next few years, the disease continued its contiguous spread to the western, eastern and southern parts of the country, leading to the emergence of several new foci every year [3]. Therefore, a notable increase in the inci‐ dence of CL cases was reported, ranging from 1 to 10 thousand new cases recorded annually depending on environmental changes and the pool of susceptible humans. This form of CL

To date, ZCL represents the dominant Tunisian CL form in terms of burden of disease, and it is mainly distributed over the central and southern arid and semiarid regions of the country,

In 2002, 15 governorates from 23 were considered as endemic in Tunisia. Between 1998 and 2007, the total number of CL cases reported to health authorities during this period

was 57,591. The global yearly incidence of CL is almost 20–30 per 100,000 persons [5].

Tunisian ZCL is a crucial prerequisite for applying efficacious control measures.

environmental conditions that influence their life cycle [2].

**2. Recall in the history of CL in Tunisia**

was identified to have a zoonotic transmission cycle.

**3. Epidemiology and ecology**

**3.1. Disease burden and distribution**

where is responsible for seasonal epidemics and regional outbreaks [4].

problem until now in Tunisia.

80 The Epidemiology and Ecology of Leishmaniasis

Tunisian CL epidemiology is characterized by an autumn‐summer seasonality with an inci‐ dence peak from October to December. Transmission of *L. major* by the phlebotomine sandfly vector occurs during the summer months, and active lesions in humans tend to emerge during the autumn and winter months.

A previous study conducted by Toumi and colleagues in Sidi Bouzid showed the seasonality of the incidence of ZCL during the same cycle with an inter‐epidemic period ranging from 4 to 7 years [9].

A recent study on the epidemiology of CL was conducted in five neighboring villages in cen‐ tral Tunisia, which were classified into one old and four emergent foci [6]. The prevalence and the incidence findings of this study illustrated an interesting phenomenon long known about CL; two foci as close as a few kilometers apart can have disparate prevalence and incidence statistics. The major risk factor for CL infection found in this study was the past history of transmission in a given geographic area. These findings provide additional evidence that the prevalence of the infection increases with length of residence in endemic areas, as an indicator of time exposure to the parasite [10]. This study suggests that people who resided in the old focus acquired a relative protection due to the presence of continuous boost of the immune system by exposure to infectious sandfly bites. Moreover, the higher rate of infection in the old focus may be due to a higher density and infection rate of rodent reservoirs and, consequently, a higher infection rate of vector sandflies.

They also demonstrate in this survey that CL prevalence typically increases with age, presum‐ ably because of the acquisition of immunity. Furthermore, in established endemic areas of CL in Tunisia, the risk of infection was found to be strongly associated with the presence of fam‐ ily history of disease and increases significantly with the number of past ZCL cases among other persons in the same household. This finding indicates a significant clustering of ZCL transmission within households.

It has been traditionally believed that Leishmania infection is associated with agricultural activities in Tunisia [6]. Environmental changes, whether natural or man‐made, in land use and cover, urbanization and unplanned settlements, have probably created suitable condi‐ tions for domestic transmission cycle that shifted the risk of infection from sylvatic environ‐ ment to rural settlements [11].

#### **3.3. Parasitology**

#### *3.3.1. Leishmania parasite*

The majority of *L. major* strains isolated in Tunisia belong to the MON‐25 zymodeme [3, 12]. It is largely recognized that the population structure of pathogens is influenced by different evolutionary factors, particularly during invasion of new ecosystems [13].

Over the past few decades, we reported the emergence of newly evolved *L. major* species that are genetically best adapted to the environment in Tunisia. The result of this selective process may explain the increasing chances of parasitic transmission to humans and the rise of ZCL cases in endemic regions over the last years.

Indeed, a recent study conducted in Sidi Bouzid to evaluate the temporal organization of *L. major* genetic diversity in Tunisia [14] showed that the historical *L. major* population was genetically less diverse than the current one with a significant genetic differentiation across time. In 20 years, the genetic drift in *L. major* population has played a major role in the increase in species diversity.

The same study suggested also that the parasite transmission process does not follow a vertical south‐north gradient as presumed from results of other research. In fact, the disease seems to have spread from Gafsa to Kairouan and then to Sidi Bouzid. Human settlement strategies and rodent population dynamics could lead to this nongradual spatial spread.

#### *3.3.2. Vector*

Leishmania parasites are transmitted from a vertebrate host to another vertebrate host by a tiny 2‐ to 3‐mm‐long insect vector, the phlebotomine sandfly. Only the female sandfly bites vertebrates can therefore transmit the parasite.

*Phlebotomus papatasi Scopoli* (Diptera: Psychodidae) is the main vector of *L. major* in Tunisia.

The geographical distribution of this vector was assessed by Chelbi I. et al. in September 2006 by setting CDC light traps placed in peridomestic areas in a transect from the north to the south of Tunisia [15]. Their data verify the remarkably spatial correlation between the sandfly vector density and ZCL cases. Both of them were abundant in the arid and Saharan biocli‐ matic zones and rare in the humid, subhumid and semiarid bioclimatic zones.

In the same period, another study was conducted in central Tunisia to assess the popula‐ tion density of *P. papatasi* using sticky traps (ST) [16]. Based on the sticky traps capture data, *P*. *papatasi* showed a peak of density in early spring and again in the autumn, while the low‐ est densities were recorded in the late summer. However, the peak incidence of ZCL cases in the governorate of Sidi Bouzid took place in December, three months after the fall of sandfly density, indicating a close temporal relationship with the abundance of *P*. *papatasi* [16].

In Tunisia, the percentage of *P. papatasi* females naturally infected with *L. major* likely increases over the summer and peaks to 7.9% in the fall [17], corresponding to seasonal prevalence peak in *P. obesus* of 70% [18].

## *3.3.3. Reservoir*

system by exposure to infectious sandfly bites. Moreover, the higher rate of infection in the old focus may be due to a higher density and infection rate of rodent reservoirs and, consequently,

They also demonstrate in this survey that CL prevalence typically increases with age, presum‐ ably because of the acquisition of immunity. Furthermore, in established endemic areas of CL in Tunisia, the risk of infection was found to be strongly associated with the presence of fam‐ ily history of disease and increases significantly with the number of past ZCL cases among other persons in the same household. This finding indicates a significant clustering of ZCL

It has been traditionally believed that Leishmania infection is associated with agricultural activities in Tunisia [6]. Environmental changes, whether natural or man‐made, in land use and cover, urbanization and unplanned settlements, have probably created suitable condi‐ tions for domestic transmission cycle that shifted the risk of infection from sylvatic environ‐

The majority of *L. major* strains isolated in Tunisia belong to the MON‐25 zymodeme [3, 12]. It is largely recognized that the population structure of pathogens is influenced by different

Over the past few decades, we reported the emergence of newly evolved *L. major* species that are genetically best adapted to the environment in Tunisia. The result of this selective process may explain the increasing chances of parasitic transmission to humans and the rise of ZCL

Indeed, a recent study conducted in Sidi Bouzid to evaluate the temporal organization of *L. major* genetic diversity in Tunisia [14] showed that the historical *L. major* population was genetically less diverse than the current one with a significant genetic differentiation across time. In 20 years, the genetic drift in *L. major* population has played a major role in the increase

The same study suggested also that the parasite transmission process does not follow a vertical south‐north gradient as presumed from results of other research. In fact, the disease seems to have spread from Gafsa to Kairouan and then to Sidi Bouzid. Human settlement strategies and rodent population dynamics could lead to this nongradual spatial spread.

Leishmania parasites are transmitted from a vertebrate host to another vertebrate host by a tiny 2‐ to 3‐mm‐long insect vector, the phlebotomine sandfly. Only the female sandfly bites

evolutionary factors, particularly during invasion of new ecosystems [13].

a higher infection rate of vector sandflies.

82 The Epidemiology and Ecology of Leishmaniasis

transmission within households.

ment to rural settlements [11].

cases in endemic regions over the last years.

vertebrates can therefore transmit the parasite.

**3.3. Parasitology**

*3.3.1. Leishmania parasite*

in species diversity.

*3.3.2. Vector*

Many ecological studies to investigate Tunisian CL reservoir hosts were realized. Three rodent species carrying *L. major* in Tunisia were identified: *Psammomys obesus* Cretzschmar 1828 (*P. Obesus*) with a major part in amplifying the transmission, *Meriones shawi* Duvernoy 1842 (*M. shawi*) and *Meriones libycus* Lichtenstein 1823 (*M. libycus*).

*Psammomys Obesus* is the main reservoir host of *L. major* and the source of epidemics in the central Tunisia [19]. It was naturally infected in Tunisia [18, 20]. Its local distribution is governed by that of the halophytic Chenopodiaceae on which it depends for food [21–22]. *Meriones libycus* was suggested to have a role to propagate the parasite between *P. obesus* colo‐ nies because of their common migration, thus increasing the distribution of the parasite [23].

In the other hand, *M. shawi* is the reservoir host of *L. major* in some parts of Tunisia [5, 24]. This rodent has a particular ability to tolerate long period of water deprivation that allow survival in clay and sandy deserts, arid steppes and mountain valleys. It may also be found in cultivated fields [25].

It is a terrestrial rodent, mainly active at night, that nests in deep and complex underground burrows with several food storage cavities [26]. These saline ecological biotopes are discon‐ tinuous in distribution in the center and south of Tunisia, thus leading to a fragmentation of the populations of sand rat.

Furthermore, natural infection by *L. major* has been identified in a specimen of least weasel (*Mustela nivalis*) in Sidi Bouzid [27]. This finding might imply just an incidental infection of the *Mustela nivalis* by the *L. major* parasite. However, further research on larger samples of this animal is needed to verify its role as a potential reservoir host for CL caused by *L. major*.

#### *3.3.4. Transmission of CL*

ZCL is transmitted to humans by sandflies vectors when they are in close contact with infected reservoir hosts, as a result of activities including agricultural practices, housing and residence in close proximity to active colonies of rodents [28].

Meriones species, even though a minor reservoir of Leishmania in Tunisia, is thought to contribute to the dispersal of Leishmania because of their large range compared to Psammomys which is more restricted. In addition, Meriones species of rodents tend to live in close proximity to human settlements, and their main food source is gramineae.

Human activities that interfere with the ecologic niche of reservoirs such as deforestation and destruction of natural habitats can change the epidemiology of ZCL. Emergence of ZCL epidemics can take place when humans invade the territory of Psammomys [29] or the incidence can be reduced when burrows of rodents and chenopods are properly destroyed.

Epidemics of CL in Tunisia may be associated with migration and the introduction of nonim‐ mune people into lands with existing transmission [30]. Prediction of such outbreaks depends on the availability of ecological information and one valuation of development areas before implementation of projects or population movements. Noticeable increase in the number of CL cases has been observed when susceptible population migrate to formerly unsettled areas located near *L. major* reservoir host biotopes [5].

Poverty and CL transmission risk are tied closely together. Poor hygiene and inadequate sani‐ tation facilities (e.g., lack of wastewater treatment and disposal, open sewerage) may favor the proliferation of sandflies which increase human‐vector exposure. Crowding and proximity of people play also a role in attracting sandflies.

CL is a climate‐sensitive disease, occupying a characteristic "climate space" that is strongly affected by changes in rainfall, atmospheric temperature and humidity [31]. Climate conditions affect the leishmaniasis complex components (parasite‐reservoir‐vector) and their ability to interact, persist and establish in new ecosystems.

In Tunisia, there are two climate types. It is typically Mediterranean in the north where the terrain is mountainous, with hot, dry summers and mild winters, whereas the southern part close to the Sahara experiences a hot desert climate with high humidity. Annual average precipitation in the northern region reaches a high of 1500 mm, while rainfall in the extreme south averages less than 200 mm [7].

A previous Tunisian study [31] indicated that the occurrence of significant environmental changes driven by agricultural development projects created suitable conditions that did not previously exist for the emergence of ZCL. Toumi and colleagues confirmed in their study [9] that the risk of disease in Sidi Bouzid is mainly influenced by the humidity related to the months of July to September during the same season and mean rainfall lagged by 12–14 months.

Another Tunisian study [7] showed that the most important climate risk factors explaining the variability of CL incidence over time are precipitation and temperature. The decadal increase in the number of ZCL occurrence in the region suggests that changes in climate increased minimum temperatures sufficiently and created conditions suitable for endemicity that did not previously exist. The gradual warming trend in Tunisia resulted in the extension of the hot season, and as a consequence, there was an elongation of the transmission period and the exposure to the parasite bites [7].

Climate change may influence geographical distribution of both sandflies and rodent densities. Ambient temperature is one of the most important factors affecting developmental times and survival of sand flies [32].

Low and high temperatures are key in limiting the distribution of *P. papatasi* and its activ‐ ity [33, 34]. It cannot tolerate the extreme conditions of temperature and low humidity. Temperatures above a critical range suppress ZCL incidence by limiting the vectors' repro‐ ductive activity. The highest densities of *P. papatasi* are associated with temperatures between 25 and 28°C [16]. In fact, sand flies have adaptations to help them live in thermal preference conditions. They spend most of their lives in protected refuges, such as rodents burrows, animal shelters, wells, cracks and crevices in walls and floors [35, 36], to avoid prolonged exposure to extreme weather events.

In the other hand, higher rainfall in endemic area in Tunisia would generally increase the vegetation abundance such as chenopods, a halophytic plant that represents the strict diet of *P. obesus*. These environmental conditions are suitable for both rodents and sandflies to reproduce in large numbers and survive in abundance throughout the winter diapause to the following cycle. Therefore, following an extension of the sand rat and sandflies populations, the pool of the parasite transmissible from the reservoir to the vector could lead to a higher human exposure risk to Leishmania‐infected sandfly bites over the next season [9].
