Isolation of St. Louis Encephalitis Virus from a Fatal Human Case in California.*

Abstract

Isolation of the St. Louis encephalitis virus from a fatal human case in California is described. This represents the first isolation of this virus from any vertebrate host outside the endemic St. Louis area, and the 2d isolation of this virus in California. Epidemiologic data indicate that the patient probably acquired his infection in Kern County, where Hammon and Reeves have found naturally-infected Aedes dorsalis. The pathogenic properties of the freshly-isolated virus are described. The human brain tissue did not produce overt infections in hamsters as young as 2-3 days of age; after 3 mouse brain passages, however, the virus uniformly killed hamsters up to 10-13 days of age, about 50% of animals 15-16 days of age, and only rarely an adult animal. The pathogenicity of this strain in its early passages, therefore, is quite different from that of the standard laboratory passage strains, and the pitfalls attendant on attempts to identify a virus on the basis of animal pathogenicity are emphasized. Comparative data are presented to show the advantages of using young mice and combined routes of inoculation for the isolation of viruses.