Longitudinal Surveillance of West Nile Virus in Mosquitoes in One Virginian County (2012-2025)

Abstract

West Nile Virus (WNV) is the leading arboviral disease in the United States, making long-term vector surveillance important for understanding WNV transmission and guiding mosquito control strategies. 519 counties (in 2024) have had longitudinal active mosquito surveillance and WNV testing programs, including Prince William County (PWC) in Virginia. The mosquito surveillance program started in 2012 and has covered most of PWC. Mosquito traps are placed throughout the county from April to October. Mosquitoes are collected weekly from each site, entomologists determine the species, and then put 10-60 mosquitoes of the same species in a single pool. That pool is then tested for WNV. We analyzed the resulting dataset of 25,608 mosquito pools tested from 2012-2025. We calculated the annual and weekly positivity rate and compared the rate by species and trap type. The positivity rates were calculated as the proportion of tested pools that were WNV-positive. The analysis identified substantial annual variation, with positivity ranging from 0.43% (2019) to 8.36% (2017). From 2012 to 2019 a cyclical trend was observed with peaks occurring every two years. That was followed by a multi-year decline and a gradual resurgence to 8.27% in 2025. There was a consistent seasonal peak in August, coinciding with peak Culex mosquito abundance. Gravid traps captured significantly more WNV mosquitoes compared to other traps (up to 15.65% vs. under 5.50%). These findings show an increase in WNV in mosquitoes. Further research is needed to understand the reasons for this increase.