On a farm in southwestern Wisconsin, where a case of California encephalitis had occurred, an 18-month study was made of small, forest-dwelling mammals and the appearance of antibodies to California group virus in them. The population densities of the mammals and their movements were obtained from trapping records and the observation of marked animals in the field. Antibodies to CEV, probably LaCrosse virus, developed in 7 of 44 tree squirrels (Sciurus niger and Sciurus carolinensis) between mid-July and mid-December. Highest antibody rates were found in chipmunks, Tamias striatus (53%), and tree squirrels (39%), which by their habits are closely associated with the mosquito Aedes triseriatus. Lower antibody rates were found in cottontail rabbits, Sylvilagus floridanus (15%), flying squirrels. Glaucomys volans (5%), and white-footed mice, Peromyscus leucopus (0%), which have less ecological overlap with A. triseriatus, from which most isolates of LaCrosse virus have been made. We concluded that our findings were consistent with the hypothesis that A. triseriatus is a common vector of CEV among these mammals in southwestern Wisconsin.
Present address: USARIEM, U. S. Army Natick Labs, Natick, Massachusetts 01760.
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