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Variations in biological properties of geographic strains of western equine encephalomyelitis virus before and after passage in Culex tarsalis and Culiseta melanura.

Western equine encephalomyelitis (WEE) virus strains from the eastern and western United States were compared using two biological markers: antigenic properties measured by cross complement fixation (CF) tests and the size of plaques produced in Vero cell cultures. Slight antigenic differences were discernible on a geographic basis, and all of the virus strains from the West produced significantly larger plaques than the eastern isolates. One virus strain from each region was serially passaged in Culex tarsalis and Culiseta melanura by intrathoracic inoculation. No change in plaque size was detected as a result of mosquito passage. After passage of the eastern strain of WEE virus in C. tarsalis, its mouse brain CF antigen titer was increased to resemble the high CF antigen titer produced by the western strain. However, the antisera titers did not change when tested with this antigen. A cross plaque reduction neutralization test (PRNT) with the two virus strains before and after passage in mosquitoes failed to reveal any antigenic changes.