Laboratory Studies on the Transmission of West Nile Virus by Culex (Culex) Univittatus Theobald; Factors Influencing the Transmission Rate1

Mosquitoes from laboratory colonies were allowed to feed on chicks circulating virus at various levels or, in 1 experiment, on 2 blood-virus suspensions. Subsequently the transmission of virus by individual mosquitoes to individual chicks was tested, the mosquitoes having been held at 26°C or, in 1 case, 18°C. With mosquitoes infected on an optimal viral dose and held at 26°C, 100% transmission rates were obtained on the 7–8th, 41st and 49th day after the infective feed and mosquito infectivity was still 100% after 81 days. A reduction in temperature for the post-infection period from 26 to 18°C caused a drop in the transmission rate from 97 to 48%. This indicates the influence weather conditions would be expected to have on transmission by wild populations of C. univittatus. A reduction in the infecting titer from 5.0 to 2.6 logs in mosquitoes held at 26°C caused a decrease in transmission rate from 89–33%. This underlines the importance of the level of infecting dose chosen for transmission tests.