Nationwide surveillance identifies yellow fever and chikungunya viruses transmitted by various species of mosquitoes in Nigeria.

Background: Since its reemergence in 2017, yellow fever (YF) has been active in Nigeria. The Nigeria Centre for Disease Control (NCDC) has coordinated responses to the outbreaks with the support of the World Health Organization (WHO). The National Arbovirus and Vectors Research Centre (NAVRC) handles the vector component of these responses. This study sought to identify the vectors driving YF transmission and any of the targeted arboviruses and their distribution across states.

Methods: Eggs, larvae and pupae as well as adult mosquitoes were collected in observational, analytical, and cross-sectional surveys conducted in sixteen YF outbreak states between 2017 and 2020. Adult mosquitoes (field-collected or reared from immature stages) were morphologically identified, and arboviruses were detected using RT-qPCR at the African Centre of Excellence for Genomics of Infectious Diseases (ACEGID).

Results: mosquitoes were collected in eleven of the sixteen states surveyed and the mosquitoes in nine states were found infected with arboviruses. A total of seven species were collected from different parts of the country. was the most dominant (51%) species, whereas was the least (0.2%). Yellow fever virus (YFV) was discovered in 33 (~26%) out of the 127 mosquito pools. In addition to YFV, the Chikungunya virus (CHIKV) was found in nine pools. Except for , all the species tested positive for at least one arbovirus. YFV-positive pools were found in six (6) species while CHIKV-positive pools were only recorded in two species. Edo State had the most positive pools (16), while Nasarawa, Imo, and Anambra states had the least (1 positive pool). Breteau and house indices were higher than normal transmission thresholds in all but one state.

Conclusion: In Nigeria, there is a substantial risk of arbovirus transmission by mosquitoes, with YFV posing the largest threat at the moment. This risk is heightened by the fact that YFV and CHIKV have been detected in vectors across outbreak locations. Hence, there is an urgent need to step up arbovirus surveillance and control activities in the country.