California encephalitis virus, a newly described agent.

In three cases of encephalitis in humans that occurred in the area where the newly described California virus was isolated from mosquitoes, serological evidence seemed to indict the California virus as the etiological agent. In the case of an infant with very severe disease, the serological evidence was convincing; the evidence was almost as strong in the case of a seven-year-old boy; the results in an adult were equivocal. Inapparent infection in man is quite common as indicated by neutralization tests on the sera of nearly 600 residents of California, but encephalitic manifestations of infection are extremely rare. In Kern County, California, where the virus was discovered, approximately 11 per cent of the population has been infected. Infection rates are higher in adults than in very young children. Absence of neutralizing antibodies from 64 specimens of blood from persons in Japan, Washington, and other states supports the specificity of the neutralization test in man and suggests that this virus is absent or is not being similarly transmitted in some areas. Serological evidence from serial bleedings of two sick horses suggested, but did not definitely establish, that this virus leads to a naturally acquired encephalomyelitis in horses. Serological tests with the viruses of western equine and St. Louis encephalitis did not lead to any other etiological diagnosis in the sick animals studied. Results of neutralization tests on the sera of eight horses and three cows in Kern County suggested extremely high infection rates, and an immunity rate of 18 per cent was noted in rabbits and ground squirrels. In the natural biological cycle rabbits and ground squirrels are suspected as the possible counterpart of birds in the St. Louis and western equine virus cycles. There is no evidence from field or laboratory to indicate that birds become infected with the California virus. Sera from 33 mammals other than man were collected from Northern California and Washington. All were free from neutralizing antibodies, again supporting the specificity of positive findings from Kern County.