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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 11 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.

From 2011-2020, most cases of EEE have been reported Massachusetts, Michigan, Florida, Georgia, New York, and North Carolina. EEEV transmission is most common in and around freshwater hardwood swamps in the Atlantic and Gulf Coast states and the Great Lakes region.

Eastern equine encephalitis virus neuroinvasive disease cases reported by year, 2011–2020

A line chart depicting Eastern Equine encephalitis cases by year starting from 2010 to 2019.

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

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Eastern equine encephalitis virus neuroinvasive disease cases reported by year, 2009–2018
Year Number of Reported Cases
2011 4
2012 15
2013 8
2014 8
2015 6
2016 7
2017 5
2018 6
2019 38
2020 13
Eastern equine encephalitis virus neuroinvasive disease cases reported by state of residence, 2011–2020

A map of the continental United States depicting Eastern Equine encephalitis Neuroinvasive Disease Cases reported by state, 2010 to 2019.

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

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Eastern equine encephalitis virus neuroinvasive disease cases reported by state of residence, 2009–2018
State Number of Reported Cases
Alabama 2
Arkansas 1
Connecticut 5
Florida 9
Georgia 7
Indiana 2
Louisiana 1
Maine 2
Maryland 1
Massachusetts 26
Michigan 18
Missouri 1
Montana 1
New Hampshire 3
New Jersey 5
New York 6
North Carolina 7
Pennsylvania 1
Rhode Island 3
South Carolina 1
Tennessee 1
Vermont 2
Virginia 1
Wisconsin 4
Eastern equine encephalitis virus neuroinvasive disease average annual incidence by county of residence, 2011–2020

A map of the continental United States depicting Eastern Equine encephalitis Virus Neuroinvasive Disease Average Annual Incidence by County, 2010 to 2019.

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

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Data Table: This map shows the distribution of Eastern equine encephalitis virus neuroinvasive disease (encephalitis and/or meningitis) average annual incidence by county of residence from 2011 through 2020; countiesare 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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Additional Maps, Charts, and Tables

Eastern equine encephalitis virus disease cases and deaths reported to CDC by year and clinical presentation, 2011-2020

Eastern equine encephalitis virus disease cases and deaths reported to CDC by year and clinical presentation, 2009-2018
Neuroinvasive disease Non-neuroinvasive disease Total
Year Cases
No.		 Deaths
No. (%)		 Cases
No.		 Deaths
No. (%)		 Cases
No.		 Deaths
No. (%)
2011  4  3  (75)  0  0  (0)  4 3  (75)
2012  15 5  (33)  0  0  (0)  15 5  (33)
2013  8 4  (50)  0  0  (0)  8 4  (50)
2014  8 2  (25)  0  0  (0)  8 2  (25)
2015  6 4  (67)  0  0  (0)  6 4  (67)
2016  7 3  (43)  0  0  (0)  7 3  (43)
2017 5 2  (40) 0 0  (0) 5 2  (40)
2018 6 1  (17) 0 0  (0) 6 1  (17)
2019 38 19  (50) 0 0  (0) 38 19  (50)
2020 13 4  (31) 0 0  (0) 13 4  (31)
Total  110 47  (43)   0  0  (0)  110 47  (43) 

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

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Eastern equine encephalitis virus disease cases reported to CDC by state of residence, 2011-2020

Eastern equine encephalitis virus disease cases reported to CDC by state of residence, 2009-2018
State 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 Total
Alabama 0 0 0 1 0 0 0 0 1 0 2
Alaska 0 0 0 0 0 0 0 0 0 0 0
Arizona 0 0 0 0 0 0 0 0 0 0 0
Arkansas 0 0 1 0 0 0 0 0 0 0 1
California 0 0 0 0 0 0 0 0 0 0 0
Colorado 0 0 0 0 0 0 0 0 0 0 0
Connecticut 0 0 1 0 0 0 0 0 4 0 5
Delaware 0 0 0 0 0 0 0 0 0 0 0
Dist. of Columbia 0 0 0 0 0 0 0 0 0 0 0
Florida 0 2 3 0 0 0 1 3 0 0 9
Georgia 0 1 1 0 0 1 2 1 1 0 7
Hawaii 0 0 0 0 0 0 0 0 0 0 0
Idaho 0 0 0 0 0 0 0 0 0 0 0
Illinois 0 0 0 0 0 0 0 0 0 0 0
Indiana 0 0 0 0 0 0 0 0 1 1 2
Iowa 0 0 0 0 0 0 0 0 0 0 0
Kansas 0 0 0 0 0 0 0 0 0 0 0
Kentucky 0 0 0 0 0 0 0 0 0 0 0
Louisiana 0 0 0 0 1 0 0 0 0 0 1
Maine 0 0 0 1 1 0 0 0 0 0 2
Maryland 0 0 0 0 0 0 1 0 0 0 1
Massachusetts 1 7 1 0 0 0 0 0 12 5 26
Michigan 0 0 0 1 0 2 0 1 10 4 18
Minnesota 0 0 0 0 0 0 0 0 0 0 0
Mississippi 0 0 0 0 0 0 0 0 0 0 0
Missouri 1 0 0 0 0 0 0 0 0 0 1
Montana 0 0 0 0 0 1 0 0 0 0 1
Nebraska 0 0 0 0 0 0 0 0 0 0 0
Nevada 0 0 0 0 0 0 0 0 0 0 0
New Hampshire 0 0 0 3 0 0 0 0 0 0 3
New Jersey 0 0 0 0 0 1 0 0 4 0 5
New Mexico 0 0 0 0 0 0 0 0 0 0 0
New York 1 0 0 2 3 0 0 0 0 0 6
North Carolina 0 2 1 0 1 2 0 0 1 0 7
North Dakota 0 0 0 0 0 0 0 0 0 0 0
Ohio 0 0 0 0 0 0 0 0 0 0 0
Oklahoma 0 0 0 0 0 0 0 0 0 0 0
Oregon 0 0 0 0 0 0 0 0 0 0 0
Pennsylvania 0 0 0 0 0 0 0 1 0 0 1
Puerto Rico 0 0 0 0 0 0 0 0 0 0 0
Rhode Island 0 0 0 0 0 0 0 0 3 0 3
South Carolina 0 0 0 0 0 0 0 0 0 1 1
South Dakota 0 0 0 0 0 0 0 0 0 0 0
Tennessee 0 0 0 0 0 0 0 0 1 0 1
Texas 0 0 0 0 0 0 0 0 0 0 0
Utah 0 0 0 0 0 0 0 0 0 0 0
Vermont 0 2 0 0 0 0 0 0 0 0 2
Virginia 0 1 0 0 0 0 0 0 0 0 1
Washington 0 0 0 0 0 0 0 0 0 0 0
West Virginia 0 0 0 0 0 0 0 0 0 0 0
Wisconsin 1 0 0 0 0 0 1 0 0 2 4
Wyoming 0 0 0 0 0 0 0 0 0 0 0
Total 4 15 8 8 6 7 5 6 38 13 110

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

To request data for previous years, please email dvbid2@cdc.gov.

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Page last reviewed: December 14, 2021
Content source: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), Division of Vector-Borne Diseases (DVBD)