Vector Competence for West Nile Virus Lineage 1 and 2: Temperature Dependency in Temperate Culex pipiens Mosquitoes

West Nile virus (WNV) is a mosquito-borne pathogen predominantly transmitted by Culex mosquitoes. Although birds are the primary hosts, other vertebrates, such as humans and horses, can also be affected. WNV incidence is increasing globally, expanding to northerly latitudes. This study investigated the vector competence of UK Culex pipiens exposed to WNV lineage 1 (Lin1) and lineage 2 (Lin2) at temperatures frequently experienced during northern European summers. Colonised Cx. pipiens exposed to blood meals containing WNV Lin1 and Lin2 were maintained at 20 °C or 25 °C and sampled at 7, 14, and 21 days post-infection (DPI) to assess infection, dissemination, and transmission potential. Differences between lineages were observed; at 20 °C, dissemination and transmission potential were greater for Lin1, whereas at 25 °C both lineages disseminated and showed transmission potential, with consistently higher rates for Lin2. These findings indicate that Cx. pipiens demonstrate transmission potential for both WNV Lin1 and Lin2 at temperatures representative of current northern European summers, with Lin2 showing greater transmission efficiency, particularly at 25 °C, while Lin1 was less efficient but remained active across a broader temperature range. These data highlight the potential risk of this virus in temperate areas currently free of this mosquito-borne virus.