This investigation confirmed and extended the work of Hammon and Reeves (1943) which demonstrated that the mosquito Culex tarsalis is capable of transmitting Western equine encephalitis virus from bird to bird. Virus was transmitted from chicken to chicken, from canary to canary, and from canary to chicken; 131 transmissions of virus were obtained in the study, and 29 (22 per cent) of the agents transmitted were re-identified by neutralization test.
Infectivity was acquired by these mosquitoes only when they fed on a bird with a threshold, or greater, level of viremia. This threshold level, of viremia was shown to be not more than an LD50 titer of 3.2. The minimal extrinsic incubation period was found to be four days, and it was further demonstrated that the rate of transmission steadily increased from 10 per cent on the fourth day to 84 per cent after the thirteenth day following the infective blood-meal. Transmission was readily accomplished by the bite of a single infective mosquito, and virus was transmitted by such a mosquito as often as four times in eleven days. Virus transmission was obtained from mosquitoes as late as sixty days after they had had an infective blood-meal, and from these results it is presumed that mosquitoes can remain infective for life.
Negative results were obtained in all attempts to demonstrate transovarian passage of western equine encephalitis virus in C. tarsalis. One mechanical transmission of virus by C. tarsalis was obtained in 24 attempts, suggesting that this phenomenon may also occur in nature, but the epidemiologic significance is not known.
Address: Walter Reed Army Institute of Research, Washington, D. C.
This work was carried out at the Rocky Mountain Laboratory, National Microbiological Institute, U. S. Public Health Service, Hamilton, Montana.