Rift Valley fever (RVF) is a viral disease that affects humans and domestic and wild ruminants [1–4]. The RVF virus (RVFV) is a member of the Phlebovirus genus (Bunyaviridae family). It is transmitted by mosquito bites, and also through contact with viremic fluids from infected ruminants to healthy ruminants or humans . Most human cases are characterized by a ‘dengue-like’ illness with moderate fever, joint pain, and headache. But in its most severe form, the illness can progress to hemorrhagic fever, encephalitis, or ocular disease with significant death rate. Animals such as sheep, goats, and cattle are primarily affected. RVF causes abortions in pregnant females (80-100%), and high mortality of newborns, thus inducing important direct and indirect economic losses [6, 7].
Since the first isolation of RVFV in Kenya in 1930 , major outbreaks have been occurred in African countries. In Eastern Africa, RVF outbreaks have been reported from 1977 to 2007 in Egypt, Kenya, Somalia, Tanzania, Somalia, and Sudan [9–14]. In 2000, the first RVF cases outside the African continent were reported in Saudi Arabia and Yemen . In Southern Africa, several large-scale epidemics were observed since 2010 in South Africa, Botswana, and Namibia [16–18]. In West Africa, the two major RVF outbreaks occurred in 1987 and 2010 in the Senegal River basin [19–21]. Since 1987, several RVF serologic surveys showed a continuous low-level circulation of RVF and an enzootic transmission in this region, especially in Northern Senegal [22–31].
In East Africa, RVF outbreaks are known to be linked with above normal autumn rainfall periods [32, 33], but in West Africa the drivers of RVF emergence remain poorly understood [34, 35]. In the semi-arid regions of Northern Senegal the main candidate vectors of RVFV are Aedes (Aedimorphus) vexans arabiensis and Culex poicilipes (Diptera: Culicidae) mosquitoes [22, 24, 34, 36–38]. The temporary ponds which are filled up during the rainy season (July-October) are favourable larval and resting habitats for these two species. However, those ponds are also the main water resources for sedentary and transhumant herds. Compounds, including human habitation and ruminants night pens are thus generally settled in the close vicinity of these ponds . RVF mosquito vectors having a nocturne activity for host-seeking , compounds either for humans or animals are considered as risk areas for RVFV transmission [28, 39, 41].
Nevertheless, a previous study demonstrated a strong spatial heterogeneity in RVFV transmission at local scale around Barkedji village, in the Ferlo Region in Senegal . That study identified water surface area and water body location (inside and outside of the Ferlo riverbed) as risk factors explaining the spatial variation of serological incidence in small ruminants. However, other factors related to vegetation in and around water bodies that could be potentially linked to mosquito density and distribution [42–46], were not investigated.
High spatial resolution (decametric) remote sensing had been successfully used to identify biotopes of vectors of different vector-borne diseases [42–45]. Here, we used sub-metric spatial resolution imagery to characterize favourable habitats to the reproduction and spread of RVF vectors, Aedes vexans and Culex poicilipes mosquitoes, and to identify pond-related landscape risk factors explaining the spatial heterogeneity of RVF incidence rate in small ruminants observed at a local scale.