West Nile Virus Dissemination Among Individual Wild Mosquito Vectors in Northern Colorado, 2023.

West Nile virus (WNV) transmission risk is typically estimated from pooled whole-mosquito infection data, which may overestimate the proportion of mosquitoes capable of transmission. To assess natural viral dissemination in field-collected Culex tarsalis, we tested infection rates in tissues of 1,793 individual mosquitoes collected from northern Colorado in August 2023. Abdomens were screened for WNV ribonucleic acid (RNA), and corresponding thorax and head tissues from positive mosquitoes were tested. Fifteen mosquitoes had detectable abdominal infections, but WNV RNA was detected in only 53% (8 of 15) of both the thorax and head tissues, whereas another 27% (4 of 15) had WNV RNA detected in either the thorax or the head alone. Logistic regression suggests an inconsistent relationship between abdominal viral RNA load and virus dissemination, whereas receiver operating characteristic analysis identifies a threshold of ∼59,000 RNA copies in the abdomen predictive of dissemination (area under the curve 0.80; 95% confidence interval, 0.55-1.00). These results suggest whole-body RNA detection may overestimate transmission potential from field-captured mosquitoes and that incorporating infection data could refine surveillance-based risk indices for WNV.