A focus of activity of Cache Valley virus was located in the Lake Mermet Conservation Area of southern Illinois as a result of field and laboratory studies conducted during 1964–1967. During each of the 4 years, multiple virus strains were isolated from Anopheles quadrimaculatus mosquitoes collected from resting places in the trunks of several trees. A close association apparently existed in this natural habitat between that mosquito and the virus. Although the vertebrate host was not identified, the results of mosquito host-preference study and results of antibody surveys suggest that cattle may be involved in cycles of Cache Valley virus.
School of Public Health, University of California, Berkeley, California, and supported by U. S. Public Health Service Research Career Development Award 5-K03-AI 25,427-03.