Selection of La Crosse virus variants by sentinel squirrels (Sciuris carolinensis) and chipmunks (Tamius striatus)

Comparisons of La Crosse (LAC) virus strains, obtained from sentinel squirrels and chipmunks, were made using three viral markers: plaque size on Vero cells, virulence in 8 days old laboratory mice, and antigenic characteristics as measured by the plaque reduction neutralization test. All strains were in their first suckling mouse brain passage. The mean plaque size of viruses isolated from squirrels was slightly larger than the mean plaque size of viruses isolated from chipmunks. There were no differences in virulence and antigenic characteristics of LAC strains isolated from chipmunks compared to those from squirrels. However, significant differences in these characteristics between individual virus strains did occur, irrespective of the vertebrate species from which the strain was isolated. First suckling mouse brain passage of viremic blood apparently selected for a smaller mean plaque size than was present in the blood. These results indicate that the squirrel and chipmunk were not rapidly selecting for greatly divergent subpopulations of the three measured markers in nature. There were some indications, however, that even one suckling mouse brain passage of field LAC virus apparently decreased mean plaque size.