Epidemiology & Geographic Distribution

St. Louis encephalitis virus (SLEV) is transmitted to humans through the bite of an infected mosquito. All residents of and visitors to areas where SLEV activity has been identified are at risk of SLEV infection, particularly persons who engage in outdoor work and recreational activities, and those living in low-income areas. Most SLEV infections are silent; clinical infections range in severity from mild nonspecific febrile illnesses to meningitis or encephalitis. The risk of severe disease and fatality increases with age. SLEV infection is thought to confer life-long immunity against re-infection.

In the United States, the annual number of reported SLEV neuroinvasive disease cases reported fluctuates widely, as result of periodic epidemics. To ensure standardization of reporting across the country, CDC recommends that the national surveillance case definition be consistently applied by all state health departments.

In temperate areas of the United States, SLEV disease cases occur primarily in the late summer or early fall. In the southern states, where the climate is milder, cases can occur year round. Although the geographic range of the virus extends from Canada to Argentina, human cases have almost exclusively occurred in the United States. The majority of cases have occurred in eastern and central states, where episodic urban-centered outbreaks have recurred since the 1930s. In the rural west, transmission has followed a more endemic pattern. The largest epidemic of SLEV neuroinvasive disease ever recognized occurred in the United States in 1975, with nearly 2,000 cases reported, primarily from the central states in the Ohio-Mississippi River Basin.

Additional Maps, Charts, and Tables

St. Louis encephalitis virus disease cases and deaths reported to CDC by year and clinical presentation, 2008-2017

Source: ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention

Print only version [PDF – 1 page]

St. Louis encephalitis virus disease cases reported to CDC by state of residence, 2008-2017

Source: ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention

Print only version [PDF – 1 page]

• National & state maps (including county-level data) from 2003 – present

 Top of Page