Vertical transmission of West Nile Virus by three California Culex (Diptera: Culicidae) species.

Three California Culex species previously identified as efficient laboratory vectors of West Nile (WN) virus were tested for their capability to vertically transmit WN virus. Wild-caught Culex pipiens pipiens L., Culex pipiens quinquefasciatus Say, and two populations of Culex tarsalis Coquillett females were inoculated intrathoracically with 10(2.7 +/- 0.1) plaque-forming units of WN virus. F1 progeny were reared at 18 degrees C and subsequently tested as adults for infectious virus on Vero cell culture. Virus was not detected in 197 pools comprising 4,884 Cx. p. pipiens. The minimum filial infection rate (MFIR) for Cx. p. quinquefasciatus was approximately 3.0/1,000 for 665 progeny tested in 28 pools. There was no virus detected in 102 pools of 2,453 progeny from Cx. tarsalis collected in Riverside County. The MFIR for Cx. tarsalis collected in Yolo County was approximately 6.9/1,000 for 2,165 progeny tested in 86 pools. Mosquito progeny infected vertically during the fall could potentially serve as a mechanism for WN virus to overwinter and initiate horizontal transmission the following spring.