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 6 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.

Top of Page

Cases Reported by Year [PDF - 1 page]

Data Table: In the United States, the annual number of reported Eastern equine encephalitis virus neuroinvasive disease cases reported varies. From 1964 through 2010, an average of 6 cases were reported annually (range 0-21). This graph demonstrates how the number of cases can vary markedly from year to year.

Top of Page

Cases Reported by State [PDF - 1 page]

Data table: From 1964 through 2010, EEEV neuroinvasive disease cases have been reported in Alabama (7), Delaware (3), Florida (70), Georgia (28), Indiana (3), Louisiana (17), Maryland (4), Massachusetts (37), Michigan (16), Mississippi (6), New Hampshire (10), New Jersey (20), New York (4), North Carolina (17), Pennsylvania (2), Rhode Island (6), South Carolina (13), Texas (2), Virginia (4), and Wisconsin (1).

Top of Page

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 from 1996 through 2010. Counties are shaded according to incidences ranging from less than 0.05, 0.05 to 0.19, and greater than 0.2 per 100,000 population. Shaded counties are distributed along the Gulf Coast, Eastern seaboard, and the Great Lakes. Most of the highest incidence counties are in Florida, southeastern Georgia, southern Alabama, and the Carolinas.

Top of Page

Additional Maps, Charts, and Tables

• National & state maps (including county-level data) from the United States Geological Survey (USGS) from 2004 to present
• Eastern equine encephalitis cases reported to CDC by state and year [PDF - 3 pages]