Incidental Discovery of Yellow Fever Cases during a West Nile Case Investigation in the Kidira Health District (Senegal)

Introduction: Sentinel surveillance of arboviral diseases (4S network) in the Tambacounda region (Senegal) revealed an autochthonous case of yellow fever detected during the investigation of a confirmed West Nile case in Sansanding (Kidira district). The objective of this study was to describe the circumstances of the yellow fever case discovery, its sociodemographic and clinical characteristics, its various contacts, and the environmental factors likely to increase the outbreak. Methods: A descriptive investigation study was conducted, combining active case finding, collection of clinical and vaccination data, blood sampling from suspected cases, and entomological/environmental surveys of at-risk households. The analysis was primarily narrative and descriptive. Results: One confirmed case of yellow fever (a 40-year-old woman) was identified with a favorable clinical outcome. Three suspected contacts were tested; no positive results for arboviral diseases were detected. Contextual factors included human-animal cohabitation, proximity to wildlife, and operational vulnerabilities (intermittent cold chain, limited logistical resources). District Health Information Software (DHIS2) data suggested infant yellow fever vaccination coverage of ≥90% at the health post and district levels, but adult vaccination status was often undetermined. Conclusion: The detection of a single case without an apparent cluster suggests persistent sylvatic circulation with a risk of spread. A regional yellow fever vaccination campaign, integrated clinical-entomological surveillance (One Health) with regular supervision, and improved routine practices are strongly recommended to prevent the amplification of outbreaks and strengthen the resilience of the regional system.