Entomological investigations into yellow fever outbreak in northern Ghana

Arboviruses are pathogens of emerging/re-emerging infectious diseases. Half the world's population is at risk. Recent African epidemics confirmed 19,569 cases with 820 deaths. In Ghana, Savannah Region had yellow fever (YF) outbreak in 2021. Reports revealed 35 deaths, 202 suspected cases (case fatality rate (CFR) = 17%), and 70 confirmed cases (CFR = 50%). This paper presents YF outbreak entomological investigations findings. A team with expertise in YF outbreak response were assembled and tasked to assess, contain and/or control the rapid spread of YF. Immature Aedes aegypti were collected in/around houses, and adult sampled using BG-Sentinel traps, oral aspirators and Prokopack. The chikungunya, dengue fever, YF, and Zika viruses screening using real time reverse-transcription PCR (detection limit <10 RNA copies/reaction) followed mosquitoes morphological identification. Breeding containers (12,264) were examined. Larvae (3,885) and pupae (1,186) were obtained from 173 containers (positivity rate = 1.26%). Larvae were identified with short and stout siphons. Pupae had short but distinct serrated spicules on paddle fringe and branched/fanlike (6-7 branches) seta 9-VIII. Out of 1,001 houses surveyed, 130 were positive for larvae/pupae (positivity rate = 12.99%). Receptacles included plastic [46.95% (6,529)], metallic [43.32% (6,024)], clay jar [5.42% (753)], tire [4.06% (565)], and well [0.24% (34)]. WHO larval indices thresholds (BI, CI, HI, and PDI) were used to assess risk. Exactly 1,571 adults were identified [Aedes aegypti aegypti (35), Aedes aegypti formosus (619), and Culex sp. (917)]. Outdoor collection were higher (86.4%) than indoors (13.6%). No arboviruses were detected. Generally, vector indices/densities suggested high-risk arbovirus transmission although vectors were not arbovirus positive. Aedes aegypti formosus was dominant. They are bridge vectors maintaining sylvatic YF cycles and spillover transmission. High risk for YF poses public health threat that calls for Aedes mosquito surveillance systems and vector control strategies. Ae. ae. formosus might be drivers for outbreak and/or YF transmission.