Key points
- Eastern equine encephalitis is a nationally notifiable condition.
- Cases are reported to CDC by state and territorial health departments using standard case definitions.
About eastern equine encephalitis
Eastern equine encephalitis virus is transmitted to humans through the bite of an infected mosquito. Human eastern equine encephalitis cases occur relatively infrequently, largely because the primary transmission cycle takes place in and around swampy areas where people are less likely to go. Eastern equine encephalitis virus transmission is most common in and around freshwater hardwood swamps in the Atlantic and Gulf Coast states and the Great Lakes region. All residents of and visitors to areas where eastern equine encephalitis virus activity has been identified are at risk of infection.
People who work and participate in recreational activities outdoors in endemic areas are at increased risk of infection. Persons over 50 years of age and under 15 years of age seem to be at greatest risk for developing severe disease when infected with eastern equine encephalitis virus. Overall, only about 4–5% of human eastern equine encephalitis virus infections result in eastern equine encephalitis.
Eastern equine encephalitis virus infection is thought to provide life-long immunity against re-infection. However, the immunity does not protect against other alphaviruses (e.g., western equine encephalitis virus), flaviviruses (e.g., West Nile virus), or bunyaviruses (e.g., La Crosse virus).
In the United States, an average of 11 human cases of eastern equine encephalitis 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.