LABORATORY TRANSMISSION OF WESTERN EQUINE ENCEPHALOMYELITIS VIRUS BY MOSQUITOES OF THE GENERA CULEX AND CULISETA

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Abstract
Western equine virus was transmitted in the laboratory by 3 spp. of mosquitoes from 2 genera not previously reported as laboratory vectors Culex tarsalis, Culiseta inornata, and C. incidens. Though transmission was not demonstrated, survival of the virus for more than a few days was shown to occur in Culex stigmatosoma and Psorophora confinnis. Possibly transmission occurred by the former. In expts. with Culex tarsalis, infection of the mosquitoes occurred from feeding on an inoculated guinea pig, a duck, and virus-blood suspensions. After an incubation period of 10-30 days at a temp. above 25[degree]C. these mosquitoes infected chickens and a guinea pig by their bite and virus was in turn demonstrated in the blood of the chickens and in the brain of the guinea pig. The fact that mosquitoes can be infected from fowl and in turn transmit to fowl, together with much other supporting data from field and laboratory, is interpreted as strengthening evidence that fowl serve as reservoirs of virus in nature. Since C. tarsalis mosquitoes have been repeatedly found infected with Western equine virus and epidemiological evidence supports their incrimination, the vector role of this species is now established, and it may be regarded as fully incriminated. Culiseta inornata has also been found infected in nature and now proven a laboratory vector. This species does not fit the epidemiological picture in the Yakima Valley as well as C. tarsalis, but may play an important role elsewhere.