Spatio-temporal dynamics of Rift Valley fever virus circulation in sentinel livestock herds (2021-2024) before the initial fatal human outbreaks in Senegal

Background: Multiple detections of Rift Valley fever virus (RVFV) infections in humans, animals and mosquito vectors suggest permanent circulation of RVFV in Senegal. This study investigated the spatial and temporal dynamics of RVFV circulation in central (Sahelian) and southeastern (Sudanian) Senegal from March 2021 to June 2024. Methods: A total of 976 initially seronegative sheep and goats from sentinel herds in 10 districts and 2 areas were monitored for 40 months. Serum samples were collected quarterly and tested by ELISA, while mosquitoes were collected monthly and screened for RVFV RNA by RT-PCR. Results: Active RVFV circulation was detected in both areas during the four years preceding the 2025 outbreak. Overall, 2989 sera were tested, and 7.2% (95% CI: 5.6-9.0%) of animals seroconverted, while no RVFV RNA was detected from 65,830 mosquitoes. Seroconversion was higher in goats than in sheep (OR = 2.6; 95% CI: 1.5-4.6; p < 0.001) and during the rainy than the dry season (dry: 9.1% vs rainy: 5.6, p = 0.023). Conclusion: Enzootic RVFV circulation preceded the 2025 outbreak. Sentinel herd surveillance, integrated with entomological and environmental monitoring, may improve RVFV risk assessment within a One Health framework.