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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. However, in more recent years sporadic cases and outbreaks have occurred in the southwest.

St. Louis encephalitis virus neuroinvasive disease cases reported by year, 2010–2019*

St. Louis Encephalitis cases reported by year

Source: ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention
*2019 data are provisional and subject to change

St. Louis encephalitis virus neuroinvasive disease cases reported by year, 2008–2017 
Year Number of Reported Cases
2010 8
2011 4
2012 1
2013 1
2014 6
2015 19
2016 7
2017 6
2018 5
2019* 14

*2019 data are provisional and subject to change

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St. Louis encephalitis virus neuroinvasive disease cases reported by state of residence, 2010–2019*

From 2009 through 2018, St. Louis encephalitis virus neuroinvasive disease cases have been reported in Alabama (3), Arkansas (9), Arizona (22), California (9), Florida (2), Illinois (1), Indiana (1), Michigan (2), Mississippi (4), Missouri (1), Nevada (2), Texas (9), Utah (1), Washington (1), and Wisconsin (1).

Source: ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention
*2019 data are provisional and subject to change

St. Louis encephalitis virus neuroinvasive disease cases reported by state, 2007–2016
State Number of Reported Cases
Alabama 3
Arkansas 5
Arizona 30
California 15
Florida 2
Illinois 1
Michigan 2
Mississippi 2
Missouri 1
Nevada 2
Texas 6
Utah 1
Wisconsin 1

*2019 data are provisional and subject to change

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St. Louis encephalitis virus neuroinvasive disease average annual incidence by county of residence, 2010–2019*

This map shows the distribution of St. Louis encephalitis virus neuroinvasive disease (encephalitis and/or meningitis) average annual incidence by county of residence from 2008 through 2017. 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 primary in southern states.

Source: ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention
*2019 data are provisional and subject to change

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Data table: This map shows the distribution of St. Louis encephalitis virus neuroinvasive disease (encephalitis and/or meningitis) average annual incidence by county of residence from 2010 through 2019. 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 primary in southwestern and southern states.

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Additional Maps, Charts, and Tables

St. Louis encephalitis virus disease cases and deaths reported to CDC by year and clinical presentation, 2010-2019*

St. Louis 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. (%)
2010  8  1  (13)  2  0  (0)  10  1  (10)
2011  4 0  (0)  2  0  (0)  6 0  (0)
2012  1 0  (0)  2  0  (0)  3 0  (0)
2013  1 0  (0)  0  0  (0)  1 0  (0)
2014  6 0  (0)  4  0  (0)  10 0  (0)
2015  19 2  (11)  4  0  (0)  23 2  (9)
2016  7 2  (29)  1  0  (0)  8 2  (25)
2017 6 0  (0) 5 0  (0) 11 0  (0)
2018 5 1  (20) 3 0  (0) 8 1  (13)
2019* 14 0  (0) 2 0  (0) 16 0  (0)
Total  71 6  (8)   25  0  (0)  96 6  (6) 

Source: ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention
*2019 data are provisional and subject to change

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St. Louis encephalitis virus disease cases reported to CDC by state of residence, 2010-2019*

St. Louis encephalitis virus disease cases reported to CDC by state of residence, 2009-2018
State 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019* Total
Alabama 0 1 0 0 1 0 0 1 0 0 3
Alaska 0 0 0 0 0 0 0 0 0 0 0
Arizona 0 0 0 0 1 23 0 6 0 9 39
Arkansas 2 3 0 0 0 0 0 0 0 0 5
California 0 0 0 0 0 0 3 4 5 6 18
Colorado 0 0 0 0 0 0 0 0 0 0 0
Connecticut 0 0 0 0 0 0 0 0 0 0 0
Delaware 0 0 0 0 0 0 0 0 0 0 0
Dist. of Columbia 2 0 0 0 0 0 0 0 0 0 2
Florida 0 0 0 0 2 0 0 0 0 0 2
Georgia 0 0 0 0 0 0 0 0 1 0 1
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 1 0 0 0 1
Indiana 0 0 0 0 0 0 0 0 0 0 0
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 0 0 0 0 0 0 0
Maine 0 0 0 0 0 0 0 0 0 0 0
Maryland 0 1 0 0 0 0 0 0 0 0 1
Massachusetts 0 0 0 0 0 0 0 0 0 0 0
Michigan 2 0 0 0 0 0 0 0 0 0 2
Minnesota 0 0 0 0 0 0 0 0 0 0 0
Mississippi 0 0 0 0 2 0 0 0 0 0 2
Missouri 1 1 0 0 0 0 0 0 0 0 2
Montana 0 0 0 0 0 0 0 0 0 0 0
Nebraska 0 0 0 0 0 0 0 0 0 0 0
Nevada 0 0 0 0 0 0 3 0 0 0 3
New Hampshire 0 0 0 0 0 0 0 0 0 0 0
New Jersey 0 0 0 0 0 0 0 0 0 0 0
New Mexico 0 0 0 0 0 0 0 0 0 0 0
New York 0 0 0 0 0 0 0 0 0 0 0
North Carolina 0 0 0 0 0 0 0 0 0 0 0
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 0 1 1 2
Puerto Rico 0 0 0 0 0 0 0 0 0 0 0
Rhode Island 0 0 0 0 0 0 0 0 0 0 0
South Carolina 0 0 0 0 0 0 0 0 0 0 0
South Dakota 0 0 0 0 0 0 0 0 0 0 0
Tennessee 0 0 0 0 0 0 0 0 0 0 0
Texas 3 0 3 1 4 0 0 0 0 0 11
Utah 0 0 0 0 0 0 1 0 0 0 1
Vermont 0 0 0 0 0 0 0 0 0 0 0
Virginia 0 0 0 0 0 0 0 0 0 0 0
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 0 0 0 0 0 0 0 0 1 0 1
Wyoming 0 0 0 0 0 0 0 0 0 0 0
Total 10 6 3 1 10 23 8 11 8 16 96

Source: ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention
*2019 data are provisional and subject to change

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Page last reviewed: August 10, 2020
Content source: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), Division of Vector-Borne Diseases (DVBD)