Observations on a natural cycle of La Crosse virus (California group) in Southwestern Wisconsin.

Ecological studies were conducted to document the role of the eastern chipmunk (Tramias striatus) as a vertebrate host for La Crosse (LAC) virus in nature during late summer when Aedes triseriatus mosquitoes are most abundant. Movement, home range and density of chipmunk populations were determined by trap mark and recapture techniques on grid study areas. The temporal distribution of A. triseriatus was estimated by use of oviposition traps. Passive antibodies were found in spring-born juveniles captured prior to mid-July and in summer-born juveniles in September. Active antibodies neutralizing LAC virus were first detected in susceptible chipmunks in mid-July and 68 free-living and 4 sentinel animals developed antibodies during the study. Virus transmission continued at a high rate through August but was not detected in September. Chipmunk habitat was ranked for quality and populations of chipmunks. A. triseriatus were more abundant in study areas with most suitable chipmunk habitat. Populations of A. triseriatus were temporally associated with the elaboration of antibodies in chipmunks. In one study area, antibody prevalence rates in adult and spring-born juveniles reached 100% by September. Findings implicate A. triseriatus as the vector and establish chipmunks as important amplifying hosts in discontinuous foci of virus transmission.