GENOTYPIC AND PHENOTYPIC VARIATION OF SELECTED SAINT-LOUIS-ENCEPHALITIS VIRAL-STRAINS ISOLATED IN CALIFORNIA

The mechanism for long-term maintenance of St. Louis encephalitis (SLE ) virus in California is unknown. Two possibilities are 1) that the vi rus is maintained locally in discrete enzootic foci by one or more res ervoir mechanisms, and/or 2) that the foci are ephemeral in nature and virus is reintroduced periodically from other enzootic areas by migra tory birds or movement of vectors. We have investigated these epidemio logic alternatives by studies of genetic variation within a 277 nucleo tide portion of the envelope-encoding region among 17 strains of SLE v irus isolated since 1952 from different geographic locations in Califo rnia. Three lineages of virus were detected. One lineage, Group A, con sisted of four SLE virus strains isolated in California since 1972 fro m the Coachella, Sacramento, and San Joaquin Valleys. The group A stra ins were closely related to strain MSI-7 of SLE virus isolated in Miss issippi in 1975. The 13 other strains formed the second and third line ages (Groups B1 and B2) that had geographically overlapping distributi ons. Group A (BFN 4585) and Group B2 (BFN 4820) appeared to be sympatr ic in the Sacramento Valley in 1972. Strains from the San Joaquin Vall ey isolated prior to 1989 (Groups B1 and B2) differed markedly from a 1989 isolate from the same location, Kern 373 (Group A). These results suggest that virus introduction(s) led to changes in genotype, or alt ernatively that the enzootic virus was subjected to selective pressure leading to rapid emergence of a new genotype. Nucleotide sequences of the envelope and 5' untranslated region of the viral genome of these virus strains did not correlate with virulence as measured by mortalit y in weanling mice, nor viremia levels and duration in chickens.