Abstract
Summary
Four strains of St. Louis virus, 2 from the Midwest with many mouse brain passages and 2 from Florida in low mouse brain passage, were tested for CPE and plaque formation in cultures of chick embryo tissue. CPE was readily observed, thus differing from results reported by a number of other workers, Plaque formation also was readily obtained where most others had reported negative results. The 2 newly isolated strains from Florida formed the clearest and most easily counted plaques but both of these strains gave plaques of 2 distinct sizes. Virus lines from the two P-15 strain plaque sizes were clone "purified" and they appeared to represent separate genotypes of St. Louis virus, closely related antigenically if not identical. They appeared to differ significantly, however, in their relative cytopathogenicity for HKTC, in plaque counts in CETC, and in their virulence for mice by the intracerebral route. The line from the large plaque of P-15 gave promise of being very useful in plaque reduction tests with human sera, a test which may be more sensitive than the standard mouse neutralization method.
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