Key points
- St. Louis encephalitis is a nationally notifiable condition.
- Cases are reported to CDC by state and territorial health departments using standard case definitions.
About St. Louis encephalitis
Although the geographic range of St. Louis encephalitis virus extends from Canada to Argentina, human cases have almost exclusively occurred in the United States. Most cases have occurred in Eastern and Central states, where periodic urban-centered outbreaks have recurred since the 1930s. However, in more recent years sporadic cases and outbreaks have occurred in the Southwest. In the United States, the annual number of reported St. Louis encephalitis virus neuroinvasive disease cases can fluctuate widely. In temperate areas of the United States, St. Louis encephalitis disease cases occur primarily in the late summer or early fall. In Southern states, where the climate is milder, cases can occur year-round.
St. Louis encephalitis is a nationally notifiable condition. To ensure standardized reporting across the country, CDC recommends that the national surveillance case definition be consistently applied by all state health departments.
All residents of and visitors to areas where St. Louis encephalitis virus activity has been identified are at risk of St. Louis encephalitis virus infection, particularly people who work or participate in recreational activities outdoors.