The number of cases in the United States varies year by year with periodic outbreaks. Most cases occur in the southeastern and central US, with some in the western states. Cases are most prevalent in the summer and early fall but may occur year-round in the South. Cases are rarely seen outside the US.
People who work or play outdoors are most at risk. Older persons are most at risk for death. The case fatality rate ranges from 5 to 15%.
In 2024, there was only 1 reported case of St. Louis encephalitis. This case was not neuroinvasive.
