Importance of vertical and horizontal transmission of West Nile virus by Culex pipiens in the northeastern United States

West Nile virus (WNV) has become established in the northeastern United States, where mosquitoes are inactive during winter. There have been no documented studies to explain how this virus survives winter and reinitiates infection in spring. We report that WNV was vertically transmitted to 2 F-1 female Culex pipiens from a naturally infected female collected in Stratford, Connecticut. One vertically infected F-1 female, which was 168 days old, fed on a hamster that died 8 days later of West Nile disease. This suggests that WNV survives winter in unfed, vertically infected C. pipiens with amplification initiated in spring by horizontal transmission.