Unreported Rift Valley fever virus circulation during 2023-2024 El Niño event detected by slaughterhouse-based surveillance in southern Kenya.

Rift Valley fever (RVF) is a zoonotic arbovirus, and livestock cases are often underreported in endemic countries due to reliance on passive clinical surveillance. During the 2023-2024 El Niño event, Kenya experienced widespread flooding, but no RVF outbreaks were reported in the southern regions. We implemented slaughterhouse-based surveillance in southern Kenya from May 2023-June 2024, using five consecutive cross-sectional surveys. Cattle, sheep, and goats were tested for anti-RVFV IgG and IgM antibodies, with concurrent recording of post-mortem lesions. Using age estimates from dentition, catalytic models estimated the force of infection (FOI) over time and spatial analysis assessed the Loitokitok sub-county for hotspots. Among 955 animals, 10.2% were IgG-positive, with seroprevalence and FOI increasing after El Niño rains, reaching 22.6% by May 2024. Six animals (0.6%) were IgM-positive, indicating recent infection, with cases detected in 3/5 sampling periods, including before the rains. All recently infected IgM-positive animals were deemed fit for slaughter and none had lesions. Adult animals in endemic areas, void of clinical and pathological signs, may therefore play a role in the silent spread and maintenance of RVFV. Slaughterhouse-based surveillance offers a practical and scalable platform for improving RVF detection and monitoring livestock in endemic regions.