Combining Hydrology and Mosquito Population Models to Identify the Drivers of Rift Valley Fever Emergence in Semi-Arid Regions of West Africa Vale ´ rie Soti 1,2,3 , Annelise Tran 1,2 *, Pascal Degenne 2 , Ve ´ ronique Chevalier 1 , Danny Lo Seen 2 , Yaya Thiongane 4 , Mawlouth Diallo 5 , Jean-Franc ¸ois Gue ´ gan 6,7 , Didier Fontenille 6 1 CIRAD, UPR AGIRs, Montpellier, France, 2 CIRAD, UMR TETIS, Montpellier, France, 3 CIRAD, UR SCA, Montpellier, France, 4 ISRA/LNERV, BP 2057, Dakar-Hann, Senegal, 5 Institut Pasteur de Dakar, BP 220, Dakar, Senegal, 6 IRD, UMR MIVEGEC (IRD 224, CNRS 5290), Universite ´ de Montpellier, Montpellier, France, 7 EHESP, Montpellier, France Abstract Background: Rift Valley fever (RVF) is a vector-borne viral zoonosis of increasing global importance. RVF virus (RVFV) is transmitted either through exposure to infected animals or through bites from different species of infected mosquitoes, mainly of Aedes and Culex genera. These mosquitoes are very sensitive to environmental conditions, which may determine their presence, biology, and abundance. In East Africa, RVF outbreaks are known to be closely associated with heavy rainfall events, unlike in the semi-arid regions of West Africa where the drivers of RVF emergence remain poorly understood. The assumed importance of temporary ponds and rainfall temporal distribution therefore needs to be investigated. Methodology/Principal Findings: A hydrological model is combined with a mosquito population model to predict the abundance of the two main mosquito species (Aedes vexans and Culex poicilipes) involved in RVFV transmission in Senegal. The study area is an agropastoral zone located in the Ferlo Valley, characterized by a dense network of temporary water ponds which constitute mosquito breeding sites. The hydrological model uses daily rainfall as input to simulate variations of pond surface areas. The mosquito population model is mechanistic, considers both aquatic and adult stages and is driven by pond dynamics. Once validated using hydrological and entomological field data, the model was used to simulate the abundance dynamics of the two mosquito species over a 43-year period (1961–2003). We analysed the predicted dynamics of mosquito populations with regards to the years of main outbreaks. The results showed that the main RVF outbreaks occurred during years with simultaneous high abundances of both species. Conclusion/Significance: Our study provides for the first time a mechanistic insight on RVFV transmission in West Africa. It highlights the complementary roles of Aedes vexans and Culex poicilipes mosquitoes in virus transmission, and recommends the identification of rainfall patterns favourable for RVFV amplification. Citation: Soti V, Tran A, Degenne P, Chevalier V, Lo Seen D, et al. (2012) Combining Hydrology and Mosquito Population Models to Identify the Drivers of Rift Valley Fever Emergence in Semi-Arid Regions of West Africa. PLoS Negl Trop Dis 6(8): e1795. doi:10.1371/journal.pntd.0001795 Editor: Assaf Anyamba, NASA Goddard Space Flight Center, United States of America Received March 16, 2012; Accepted July 7, 2012; Published August 21, 2012 Copyright: ß 2012 Soti et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study was funded by EU Grant GOCE-2003-010284 EDEN (Emerging Diseases in a changing European eNvironment) and Nevantropic SAS. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: annelise.tran@cirad.fr Introduction Rift Valley fever (RVF) is a vector-borne disease caused by a virus (RVFV) belonging to the Bunyaviridae family, genus Phlebovirus, that affects domestic livestock (e.g., sheep, cattle, camels, and goats) and humans. In humans, RVF can take different forms [1]. Most human cases are characterized by a ‘dengue-like’ illness with moderate fever, joint pain, and headache. In its most severe form, the illness can progress to hemorrhagic fever, encephalitis, or ocular disease with significant death rate. In livestock, it causes abortion and high mortality of newborns and thus induces important direct and indirect economic impacts. Since the first isolation of RVFV in Kenya in 1930 [2], major RVF outbreaks have been reported in Egypt in 1977– 1978 [3] and 1993 [4], in the Senegal River Valley in 1987 [5,6], in Madagascar in 1990 [7] and 1992 [8], and in northern Kenya and Somalia in 1997, 1998 and 2007 [9]. In 2000, RVF cases were reported for the first time outside the African continent, in Saudi Arabia and Yemen [10]. Recently, a new wave of RVF epidemics occurred in 2006 and 2007 in East Africa (Kenya, Somalia and Tanzania) [11,12], in Sudan in 2007 [13], in Madagascar in 2008 [14], and in Southern Africa in 2010 [15]. Two main modes of transmission of RVFV are suspected: i) a direct transmission from infected ruminants to healthy ruminants or humans, (ii) an indirect transmission through the bites of infected mosquito vectors [16]. The respective contribution of these different transmission routes remain unevaluated [17]. However, it is assumed that the transmission by the bite of infected mosquitoes is the main infection mechanism during inter- epizootic periods [18]. PLOS Neglected Tropical Diseases | www.plosntds.org 1 August 2012 | Volume 6 | Issue 8 | e1795