Laboratory Transmission of St. Louis Encephalitis Virus by Three Genera of Mosquitoes

1. St. Louis virus has been successfully transmitted in the laboratory by the following 9 species of mosquitoes from 3 genera: Culex tarsalis, Culex pipiens, Culex coronator, Aedes lateralis, Aedes taeniorhynchus, Aedes vexans, Aedes nigromaculis, Theobaldia incidens, and Theobaldia inornata. 2. Though transmission has not been demonstrated, survival of the virus for more than a few days was shown to occur in Culex quinquefasciatus, Culex stigmatosoma, Psorophora ciliata, and Anopheles maculipennis freeborni. 3. In experiments with Culex tarsalis, infection occurred from feeding on chickens and ducks which had been previously inoculated by the subcutaneous route. After an incubation period these mosquitoes infected other chickens and virus was in turn demonstrated in the blood of these. This is interpreted as proof that fowl may serve as reservoirs of virus in nature. Since mosquitoes have been repeatedly found naturally infected with St. Louis virus and epidemiologic evidence supports their incrimination, their rôle as vectors is now established. The fully incriminated species is Culex tarsalis.