Studies on selection of LaCrosse virus variants by natural vertebrate hosts and vector mosquitoes.

The effect of passage in natural hosts on LaCrosse virus plaque size, mouse neurovirulence, and plaquing efficiency was studied. Seven field strains, as previously unpassaged naturally infected Aedes triseriatus suspensions, were inoculated into chipmunks Tamias striatus and thence into colonized Ae. triseriatus. The prototype strain, with seven prior laboratory passages, was also passed through the same natural host species. The field strains changed in neurovirulence and plaque size at various passage levels, but with no consistent pattern. The prototype strain was relatively unvarying in plaque size, but changed moderately in neurovirulence. For all eight strains, chipmunk passage reduced plaquing efficiency beyond the nonspecific inhibition normally found in chipmunk blood. We conclude that natural hosts may select variants in unpredictable patterns from highly variable unpassaged field strains, but that prior laboratory passage reduced the prototype strain to a relatively homogeneous population resistant to change in at least one aspect.