Studies on Mites as Vectors of Western Equine and St. Louis Encephalitis Viruses in California

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  • George Williams Hooper Foundation for Medical Research, and the School of Public Health, University of California, Communicable Disease Center, Public Health Service, San Francisco and Berkeley, California


Virus tests of over 130,000 Bdellonyssus sylviarum and Dermanyssus americanus collected from wild bird nests in Kern County, California, from 1946 through 1949, resulted in 9 isolations of Western equine encephalomyelitis virus, 3 of St. Louis encephalitis virus, and in 1 instance isolations of both viruses from a single sample pool. The infected mites, principally B. sylviarum, were collected from nests of four different bird species: yellow-headed blackbird, Brewer blackbird, tricolored blackbird and English sparrow. Virus isolations did not correlate with the bird species from which the largest mite samples were collected, nor with the distribution of neutralizing antibodies in six common species of birds.

Tests of the ability of Dermanyssus gallinae and B. sylviarum to become infected with and to transmit St. Louis encephalitis or Western equine encephalomyelitis viruses gave no conclusive evidence of successful long-term infection of these mites with virus or of biologic transmission.

The importance of these two mite species as reservoirs or vectors of the two encephalitis viruses studied in California and other western areas must be seriously questioned.

Author Notes

Present address: Graduate School of Public Health, University of Pittsburgh, Pittsburgh, Pennsylvania.

Present address: 406th Medical General Laboratory, A.P.O. 500, San Francisco, Calif.