The Ferlo area (north-central Senegal) is characterized by a system of temporary ponds favorable to arboviruses among which West Nile fever (WNF) was already identified. During the rainy season in 2003, a serological study was undertaken on horses to assess the activity of the WNF virus (WNFV) in Barkedji (Ferlo). The observed serological prevalence rate was 78.3% for neutralizing antibodies, with a 95% confidence interval (CI) of [64.0, 92.7]. This prevalence rate significantly increased with age (P = 10(-5)). This study confirmed that WNF was endemic in the Ferlo. The transmission risks depended on the introduction of the WNFV in the ecosystem--probably with migrating birds, on its amplification in hosts and on the vector-population dynamic. Further studies are needed to investigate how the cycle is initiated in Barkedji at the beginning of the rainy season and the impact of climatic variations on the risk of transmission of WNF. A surveillance system should be implemented: (a) to assess the clinical impact of the WNF on human and equine populations, (b) to provide an early detection of virulent strains, and (c) to assess the risk of WNF transmission to disease-free ecosystems via migrating birds.