SHORT REPORT Use of sentinel chickens to study the transmission dynamics of West Nile virus in a sahelian ecosystem V. CHEVALIER 1 *, R. LANCELOT 2 , A. DIAI ¨ TE 3 , B. MONDET 4 AND X. D E LAMBALLERIE 5 1 CIRAD, UPR ‘ Epidemiology and ecology, Campus International de Baillarguet, Montpellier, France 2 CIRAD, Campus International de Baillarguet, Montpellier, France 3 ISRA, Laboratoire National d’Elevage et de Recherche Ve ´te ´rinaire, Dakar-Hann, Se ´ne´gal 4 IRD, UR 178, Dakar-Hann, Se ´ne´gal 5 Unite ´ des Virus E ´ mergents, Faculte´ de Me ´decine, Universite´ de la Me ´diterrane ´e, Marseille, France (Accepted 25 April 2007; first published online 11 June 2007) SUMMARY During the 2003 rainy season, a follow-up survey in sentinel chickens was undertaken to assess the seasonal transmission of West Nile virus (WNV) in a sahelian ecosystem : the Ferlo (Senegal). The estimated incidence rate in chickens was 14% (95% CI 7–29), with a very low level of transmission between July and September, and a transmission peak occurring between September and October. Comparing these results with the estimate obtained from a previous transversal serological study performed on horses the same year in the same area, the relevance of sentinel chickens in estimating the WNV transmission rate is highlighted. Conventionally adult mosquito populations disappear during the dry season but WN disease was shown to be endemic in the study area. The mechanisms of virus maintenance are discussed. West Nile fever (WNF) is a mosquito-borne disease caused by a flavivirus (genus Flavivirus, family Flaviviridae). The transmission cycle involves birds (amplifying hosts) and mosquitoes which may also transmit West Nile virus (WNV) to other species, in- cluding humans and horses. Most infections in hu- mans are asymptomatic or lead to a non-specific mild febrile syndrome. Rare, albeit severe, neurological forms have been reported in humans during the last two decades [1] and heavy losses were observed in naive horse populations in different parts of the world. WNF has often been associated with wetlands. However, the virus is also observed in drier areas. For example, it was isolated from Aedes and Culex mosquitoes in the Ferlo (Senegal) [1] between 1988 and 1995. A serological survey in horses showed that the infection was endemic in this sahelian ecosystem [2]. We report here the results of a follow-up survey in sentinel chicken flocks implemented in the Ferlo region to assess the seasonal transmission dynamics of WNV. The study was undertaken in Barkedji (14x 52k W, 15x 16k N), a village located in the Ferlo. The climate is hot and dry with an annual rainfall ranging from 300 to 500 mm, occurring between July and September. The Ferlo was an effluent of the Senegal River which dried up in 2000 B.C. During the rainy season, temporary ponds appear in the Ferlo bed: they start to fill up in July and remain flooded until October–December. These ponds and their environment constitute a favourable ecosystem for WNV. Culex and Aedes are the dominant mosquito genera and are considered the * Author for correspondence: Dr V. Chevalier, CIRAD, UPR ‘Epidemiology and ecology’, Campus International de Baillarguet TA 30/G, 34398 Montpellier, France. (Email : chevalier@cirad.fr) Epidemiol. Infect. (2008), 136, 525–528. f 2007 Cambridge University Press doi:10.1017/S0950268807008801 Printed in the United Kingdom