Six isolates of La Crosse (LAC) virus were obtained from sentinel gray squirrels (Sciuris carolinensis) and four from sentinel chipmunks (Tamias striatus) in an endemic area. Viremia titers were measured by plaquing on Vero cells. Antibody responses of the animals were measured by a microneutralization test employing four California group viruses: LAC, snowshoe hare (SSH), trivittatus, and Jamestown Canyon. In both species LAC antibody titers peaked at approximately 21 days and were still detectable in all animals at 256 days post-viremia. In chipmunks, homologous LAC virus antibody levels were consistently higher than heterologous antibody responses throughout the period recorded. However, in squirrels, homologous LAC virus and heterologous SSH virus antibody responses were initially comparable. This heterologous SSH titer rapidly declined while LAC antibody levels remained relatively high. Data indicate that antibody response persists from one summer season to the next. Viremia titers in both species indicate that these two species are capable of infecting Aedes triseriatus, the principal vector of LAC virus. This is the first reported field isolation of LAC virus from the squirrel.
Captain, U.S. Air Force Veterinary Corps. Present address: Naval Medical Research Unit-2, Taipei, Taiwan. Send reprint requests to Dr. Yuill, Department of Veterinary Science, University of Wisconsin.