The agent described in this paper is considered to be a newly-recognized arthropod-borne virus. Infection with this virus seems to be fairly prevalent in C. tarsalis in California, the recovery rate of the virus being approximately 3 per cent of all the pools tested. Infection is not restricted to C. tarsalis, however, since three strains were recovered in 1956 from Culex stigmatosoma. With respect to C. tarsalis, evidence of infection in this species appears during June or early July, the infection rate becomes maximal during July and thereafter diminishes, to fall to non-demonstrable levels by October.
It appears probable that the Turlock virus has been present in the Central Valley of California for some time but was not detected until the chick embryo was employed routinely for testing field materials. Since the adult mouse has been almost uniformly employed for the examination of arthropod specimens in epidemiologic studies on the viral encephalitides, agents such as the Turlock virus, which have very little if any pathogenicity for the adult mouse, would be missed. Following the identification of the 46 agents isolated from C. tarsalis in 1954 as strains of a single agent, the immune sera prepared against these strains were used in the identification of two agents which had been isolated in previous years but had remained unidentified. One virus had been isolated from a pool of C. tarsalis captured in Kern County in 1953, and a second had been isolated from a pool of C. tarsalis collected at Turlock (Stanislaus County), California on July 9, 1947. Both isolates were found to be identical with the 46 agents isolated from C. tarsalis in 1954. The 1947 virus, identified retrospectively eight years after its isolation, was used as a basis for naming the new virus; since the mosquitoes which yielded the 1947 strain had been captured at Turlock, the new agent was named after this area.
The role of the Turlock virus, if any, in the causation of human or animal disease has yet to be determined. Information thus far acquired, and based on limited material, suggests that human infection with the agent is not common. Likewise, no evidence was obtained that this virus is etiologically involved in the human encephalitides.