Rising temperatures contribute to West Nile virus diversification and increased transmission potential.

West Nile virus (WNV), the most common mosquito-borne disease in the continental United States, is vectored by Culex spp. mosquitoes. Since its introduction to New York State (NYS) in 1999, WNV has become endemic. NYS temperatures have risen by 0.14 °C per decade since 1900, with larger increases linked to increased WNV transmission. In this study, we asked if increases in temperature in NYS influence virus diversification and adaptation, leading to shifts in thermal sensitivity. More specifically, do contemporary WNV strains have increased transmission potential at higher temperatures compared to historic strains? Using surveillance and sequencing data of WNV isolated from mosquitoes in NYS, we found a significant correlation between rising temperatures, increased WNV genetic diversity, and higher prevalence. We then analyzed genetically distinct WNV strains from mosquitoes collected during recent warm summers (2017 and 2018) and cooler historic summers (2003 and 2004). Assessing Culex pipiens dissemination efficiency and calculating the relative R₀ at 20 °C, 24 °C, and 28 °C, we found that contemporary strains exhibit higher transmission potential at increased temperatures. Our results show that contemporary WNV strains possess greater phenotypic and genotypic diversity, suggesting that climate warming in concert with viral adaptation may facilitate the emergence of strains with enhanced transmission potential.