Geneticists at the American Museum of Natural History said the disease, which caused a 1933 epidemic around St. Louis, is transmitted by mosquitoes from birds to people. An analysis of the genome of St. Louis encephalitis completed at the museum's Sackler Institute for Comparative Genomics revealed the evolution of the virus.

The researchers, using a whole-genome approach, determined a single mutation in the coding for an envelope protein, rather than recombination, most likely caused changes that made the virus pathogenic to humans.

Genetic analysis allowed the scientists to trace the evolutionary path of the Flavivirus virus. They determined the older, less-derived strains, or more ancestral strains, are from South America. The North American and Haitian strains were passed from common bird hosts such as finches, robins, blue jays and doves into humans by the Culex mosquito.

The research is reported in the May issue of the journal Molecular Phylogenetics and Evolution.