Dengue outbreak investigation in Mfandena I Health Area, Cameroon, 2025: Evidence of Aedes vectors and routine surveillance gaps
Introduction: In December 2024, a sudden increase in dengue cases was reported in Mfandena I health area (HA), Yaounde, Cameroon. Two confirmed cases that escaped routine surveillance were notified, and little data were available. A multidisciplinary team was deployed to conduct active case-finding, identify potential vectors and their larval habitats, and evaluate the dengue surveillance system. Methods: We conducted a cross-sectional study in January 2025 (dry season) in Mfandena I HA. Cases were identified through healthcare records reviews and community screening. Suspected cases were people living in Mfandena I HA with fever >38.5°C lasting 2-7 days and at least two of the following symptoms: headache, retro-orbital pain, myalgia, arthralgia, skin rash, hemorrhagic manifestations since November 2024. Immunoglobulin M (IgM) and non-structural protein 1 (NS1) tests were carried out on consenting suspected cases and their contacts. An entomological survey was conducted within a 1 km radius around the case residences. We followed the CDC framework combined with the WHO IDSR approach for surveillance system evaluation. Results: We identified 46 dengue cases (44 suspected cases and 02 confirmed cases) and 16 contacts, but no additional cases were confirmed. Dengue cases were mostly women aged 21 years or older, with fever, arthralgia, and headache as main symptoms. The confirmed cases were male adults: one residing in Cameroon and one arriving from France, both presenting with fever and a red-spot rash. Among 143 Aedes mosquitoes collected, Ae. albopictus accounted for 88% (126/143). We detected a discarded tyre containing Ae. albopictus larvae as the likely source of infection. The surveillance system exhibited gaps in detection and reporting and was neither simple, acceptable, nor stable. Only 16% (4/25) of staff were familiar with the case definition, no sites had notification forms, and no cases had ever been reported. Conclusion: This investigation suggests potential dengue transmission in Mfandena I HA and highlights important surveillance gaps that limited timely detection and reporting. Strengthening surveillance capacity and reinforcing routine entomological investigations are essential to improve outbreak detection, response and prevention.