Comparison of biological properties of St. Louis encephalitis and Rio bravo viruses

St. Louis encephalitis (SLE) virus, an arbovirus, and Rio Bravo (RB) virus, a non-arthropod-borne virus, are flaviviruses which cross-react in neutralization tests. Several of their biological properties were compared. Viral growth curves revealed that greater than 99% of infectious SLE (Parton) virus remained cell-associated in Vero cells but was released slowly into the medium of infected BHK-21 cells. In contrast, RB (M-64) virus was released upon maturation into the fluids of Vero and BHK-21 cell cultures. Fortyfold more SLE virus than RB virus adsorbed to monolayer cultures of Aedes dorsalis cells. SLE but not RB virus replicated and reached high titers (10(8) pfu/ml) in Ae. dorsalis cell cultures maintained in media which were expected to enhance recovery of RB virus. Further, SLE but not RB virus replicated in cultured bat epithelial cells and primary duck embryo cells incubated at 42 degrees C. Clearly, the prototype strains of RB and SLE viruses are distinct viruses. Both the classification of RB virus in the genus flavivirus and the antigenic relationship between RB and SLE viruses require further clarification.