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Epidemiology & Geographic Distribution

Eastern equine encephalitis virus (EEEV) is transmitted to humans through the bite of an infected mosquito. Human EEEV cases occur relatively infrequently, largely because the primary transmission cycle takes place in and around swampy areas where human populations tend to be limited. All residents of and visitors to areas where EEEV activity has been identified are at risk of infection. People who engage in outdoor work and recreational activities in endemic areas are at increased risk of infection. Persons over age 50 and under age 15 seem to be at greatest risk for developing severe disease when infected with EEEV. Overall, only about 4-5% of human EEEV infections result in EEE. EEEV infection is thought to confer life-long immunity against re-infection. It does not confer significant cross-immunity against other alphaviruses (e.g., western equine encephalitis virus), and it confers no cross-immunity against flaviviruses (e.g., West Nile virus) or bunyaviruses (e.g., La Crosse virus)

In the United States, an average of 8 human cases of EEE are reported annually. To ensure standardization of reporting across the country, CDC recommends that the national surveillance case definition be consistently applied by all state health departments.

Most cases of EEE have been reported from Florida, Georgia, Massachusetts, and New Jersey. EEEV transmission is most common in and around freshwater hardwood swamps in the Atlantic and Gulf Coast states and the Great Lakes region.

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Eastern equine encephalitis virus neuroinvasive disease cases reported by year, 2004–2013

A line chart depicting Eastern Equine encephalitis cases by year starting from 2004 to 2013.

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

Cases Reported by Year [PDF - 1 page]

Data Table: In the United States, the number of Eastern equine encephalitis virus neuroinvasive disease cases reported each year varies. From 2004 through 2013, an average of 8 cases were reported annually (range 3–21).

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Eastern equine encephalitis virus neuroinvasive disease cases reported by state, 2004–2013

A map of the continental United States depicting Eastern Equine encephalitis Neuroinvasive Disease Cases reported by state, 2004 to 2013.

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

Cases Reported by State [PDF - 1 page]

Data table: From 2004 through 2013, Eastern equine encephalitis virus neuroinvasive disease cases have been reported in Alabama (4), Arkansas (1), Connecticut (1), Florida (15), Georgia (4), Louisiana (3), Massachusetts (24), Michigan (3), Missouri (1), New Hampshire (9), New York (3), North Carolina (7), Rhode Island (1), South Carolina (2), Vermont (2), Virginia (1), and Wisconsin (1).

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Eastern equine encephalitis virus neuroinvasive disease average annual incidence by county, 2004–2013

A map of the continental United States depicting Eastern Equine encephalitis Virus Neuroinvasive Disease Average Annual Incidence by County, 2004 to 2013.

Average Annual Incidence by County [PDF - 1 page]

Data Table: This map shows the distribution of Eastern equine encephalitis virus neuroinvasive disease (encephalitis and/or meningitis) average annual incidence by county from 2004 through 2013. Counties are shaded according to incidences ranging from less than 0.20, 0.20 to 0.49, and greater than 0.50 per 100,000 population. Shaded counties are primarily distributed along the Gulf Coast, Eastern seaboard, and the Great Lakes.

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