It’s risky to wander in September: Modelling the epidemic potential of Rift Valley fever in a Sahelian setting

Estimating the epidemic potential of vector-borne diseases, along with the relative contribution of underlying mechanisms, is crucial for animal and human health worldwide. In West African Sahel, several outbreaks of Rift Valley fever (RVF) have occurred over the last decades, but uncertainty remains about the conditions necessary to trigger these outbreaks. We use the basic reproduction number (R0) as a measure of RVF epidemic potential in Northern Senegal, and map its value in two distinct ecosystems, namely the Ferlo and the Senegal river delta and valley. We consider three consecutive rainy seasons (July-November 2014, 2015 and 2016) and account for several vector and animal species. Namely, we parametrize our model with estimates of Aedes vexans arabiensis, Culex poicilipes, Culex tritaeniorhynchus, cattle, sheep and goats abundances. The impact of RVF virus introduction is assessed every week, in 4367 pixels of 3,5km2. The results of our analysis indicate that September was the month with highest epidemic potential in each study area, while at-risk locations varied between seasons. We show that decreased vector densities do not highly reduce R0 and that cattle immunity has a greater impact on reducing transmission than small ruminants immunity. The host preferences of vectors and the temperature-dependent time interval between their blood meals are crucial parameters needing further biological investigations.HighlightsSeptember is a period of high Rift Valley fever epidemic potential in northern Senegal regardless of the year, but exact locations where epidemics might start change between rainy seasons.Decreased vector densities during the rainy season did not highly reduce the epidemic potential of at-risk locations.High levels of immunity in cattle populations reduce more Rift Valley fever virus transmission than a high immunity in small ruminants in our study area. This aspect should be investigated further for targeted vaccination campaigns.Precise estimates of vector feeding preferences and the temperature-dependent lenght of their gonotrophic cycle are key to ensure a good detection of at-risk pixels.