As part of an epidemiological study of the mosquito vectors of encephalitis in the Yakima Valley, Washington, a large series of blood engorged specimens, collected in domestic habitats, were tested by means of the precipitin method to determine the relative proportions which fed on domestic animal reservoirs of these infections and on man. At the same time, hand collections were made on the horse, cow, and man to determine which of the various mosquito species fed on these hosts.
It was found that Culex tarsalis, the species best fitting the epidemiological picture as a mosquito vector of encephalitis in the Yakima Valley, fed frequently on domestic fowl and included most of the common domestic animals and man in its feeding range. The feeding habits of this species alone could result in the incidence of encephalitis antibodies demonstrated in domestic animals and man in the Yakima Valley.
Species (including Aedes) which were rarely or never found infected in nature appeared to be those which fed almost exclusively on mammalian blood. Culex pipiens was the exception as it fed almost exclusively on fowl. This species, demonstrated to be capable of transmitting only the St. Louis virus, probably plays an important role for this one virus in areas where it occurs in large numbers.
The results of the precipitin tests, and the repeated isolations of virus from Culex tarsalis give strong support to the probability that domestic fowl are an important reservoir of infection in the Yakima Valley.