Culex molestus was infected by feeding on suspensions of West Nile virus and the virus transmitted to susceptible animals. Strains of virus recently isolated from the blood of West Nile fever patients in Israel were used. When the titer of the suspension fed was 10-4.0 or more, virus was transmitted by mosquitoes to infant mice after an extrinsic incubation period of 7–28 days. When the titer was 10-2.0 or 10-2.8, transmission did not occur, even though virus was present in the mosquitoes. The percentage of mice infected by bite seemed to be a function of the length of the extrinsic incubation period.
The titer of virus in infected mosquitoes varied considerably in different experiments and fluctuated between 10-2.2 and 10-5.7 between the 9th and 28th day of the incubation period. It was found that an infected mosquito pool may be composed of individual mosquitoes having titers of virus ranging from less than 10-1.0 to 10-3.5. Though each mosquito feeding on a suspension having a titer of 10-5.0 or more became infected, only half of those infected were able to transmit the infection by bite after a suitable incubation period. The amount of virus injected into infant mice by the bite of infected mosquitoes was found to correspond to about 1000 ID50.
The authors are indebted to Dr. Joseph L. Melnick, Yale University School of Medicine, New Haven, for helpful advice and criticism. The technical assistance of Miss Miriam Polak, Miss Erna Klein, Miss Pnina Shpigler, and Mr. Joel Medina is gratefully acknowledged.