Nationwide surveillance detects yellow fever and chikungunya viruses in multiple Aedes mosquito species in Nigeria

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BACKGROUND: Since the reemergence of yellow fever in 2017, the Nigeria Centre for Disease Control (NCDC) has been coordinating responses to local outbreaks with the support of the World Health Organization (WHO). The National Arbovirus and Vectors Research Centre (NAVRC) has been implementing targeted vector control interventions to mitigate the occurrence of these outbreaks. This study sought to identify the vectors driving yellow fever (YF) transmission and other public health arboviruses and their distribution across Nigeria. METHODS: Between 2017 and 2020, larvae, pupae, and adult mosquitoes were collected, largely targeting Aedes Stegomyia mosquitoes, in observational, analytical, and cross-sectional surveys conducted in sixteen YF outbreak states of Nigeria. Adult mosquitoes (field-collected or reared from immature stages) were morphologically identified, and arboviruses were detected using reverse transcription quantitative polymerase chain reaction (RT-qPCR). RESULTS: Seven different Aedes mosquito species were found in 11 of the 16 states surveyed, with mosquitoes from nine states found infected with arboviruses. Aedes aegypti was the predominant species (51%), whereas Aedes africanus was the least common species (0.2%). Yellow fever virus (YFV) was detected in 33 (~ 26%) out of the 127 Aedes mosquito pools, with minimum infection rates in the ranges of 0.9 (Ae. circumluteolus) to 62.5 (Ae. luteocephalus) per 1000 mosquitoes. In addition to YFV, the chikungunya virus (CHIKV) was found in nine pools, with minimum infection rates ranging from 1.6 (Ae. aegypti) to 62.5 (Ae. luteocephalus) per 1000 mosquitoes. Except for Ae. africanus, all the Aedes species tested positive for at least one arbovirus. YFV-positive pools were found in six Aedes species (Ae. aegypti, Ae. albopictus, Ae. simpsoni complex, Ae. luteocephalus, Ae. vittatus, and Ae. circumluteolus), while CHIKV-positive pools were recorded in only two Aedes species (Ae. aegypti and Ae. luteocephalus). There was co-detection of YFV and CHIKV in Ae. luteocephalus (Benue State). Edo State had the most positive pools (16), while Nasarawa, Imo, and Anambra states had the least (one positive pool). Breteau and house indices were higher than the standard WHO transmission thresholds in all but one state, suggesting high risk for arbovirus transmission. CONCLUSIONS: In Nigeria, there is substantial risk of arbovirus transmission by Aedes mosquitoes, with YFV posing a large threat. This risk is heightened by the fact that YFV and CHIKV have been detected concurrently in vectors across outbreak locations. There is an urgent need to step up arbovirus surveillance and vector control activities across the country.

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