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Epidemiology & Geographic Distribution

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. In the rural west, transmission has followed a more endemic pattern. The largest epidemic of SLEV neuroinvasive disease ever recognized occurred in the United States in 1975, with nearly 2,000 cases reported, primarily from the central states in the Ohio-Mississippi River Basin.

St. Louis encephalitis virus neuroinvasive disease cases reported by year, 2008–2017

St. Louis Encephalitis cases reported by year

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

St. Louis encephalitis virus neuroinvasive disease cases reported by year, 2008–2017 
Year Number of Reported Cases
2008 8
2009 11
2010 8
2011 4
2012 1
2013 1
2014 6
2015 19
2016 7
2017 6

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Data Table: In the United States, the number of St. Louis encephalitis virus neuroinvasive disease cases reported each year varies. From 2008 through 2017, an average of 7 cases were reported annually (range 1–19).

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St. Louis encephalitis virus neuroinvasive disease cases reported by state of residence, 2008–2017

From 2008 through 2017, St. Louis encephalitis virus neuroinvasive disease cases have been reported in Alabama (3), Arkansas (13), Arizona (22), California (5), Florida (2), Illinois (1), Indiana (1), Louisiana (1), Michigan (2), Mississippi (4), Missouri (1), Nevada (2), North Carolina (3), Texas (9), Utah (1), and Washington (1).

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

St. Louis encephalitis virus neuroinvasive disease cases reported by state, 2007–2016
State Number of Reported Cases
Alabama 3
Arkansas 13
Arizona 22
California 5
Florida 2
Illinois 1
Indiana 1
Louisiana 1
Michigan 2
Mississippi 4
Missouri 1
Nevada 2
North Carolina 3
Texas 9
Utah 1
Washington 1

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Data table: From 2008 through 2017, St. Louis encephalitis virus neuroinvasive disease cases have been reported in Alabama (3), Arkansas (13), Arizona (22), California (5), Florida (2), Illinois (1), Indiana (1), Louisiana (1), Michigan (2), Mississippi (4), Missouri (1), Nevada (2), North Carolina (3), Texas (9), Utah (1), and Washington (1).

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St. Louis encephalitis virus neuroinvasive disease average annual incidence by county of residence, 2008–2017

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.

St. Louis encephalitis virus neuroinvasive disease average annual incidence by county, 2008–2017
State Number of Reported Cases
Alabama 2
Florida 2
Georgia 14
Illinois 1
Indiana 6
Iowa 1
Kansas 1
Kentucky 2
Louisiana 2
Maryland 3
Michigan 3
Minnesota 19
Mississippi 4
Missouri 1
North Carolina 175
Ohio 179
South Carolina 4
Tennessee 101
Texas 4
Virginia 8
West Virginia 86
Wisconsin 27

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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 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 primarily in 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, 2008-2017

St. Louis encephalitis virus disease cases and deaths reported to CDC by year and clinical presentation, 2008-2017
Neuroinvasive disease Non-neuroinvasive disease Total
Year Cases
No.
Deaths
No. (%)
Cases
No.
Deaths
No. (%)
Cases
No.
Deaths
No. (%)
2008  8 0  (0)  5  0  (0)  13 0  (0)
2009  11 0  (0)  1  0  (0)  12 0  (0)
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)
Total  71 5  (7)   26  0  (0)  97 5  (5) 

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

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St. Louis encephalitis virus disease cases reported to CDC by state of residence, 2008-2017

St. Louis encephalitis virus disease cases reported to CDC by state of residence, 2008-2017
State 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 Total
Alabama 0 0 0 1 0 0 1 0 0 1 3
Alaska 0 0 0 0 0 0 0 0 0 0 0
Arizona 0 0 0 0 0 0 1 23 0 6 30
Arkansas 4 4 2 3 0 0 0 0 0 0 13
California 0 0 0 0 0 0 0 0 3 4 7
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 0 0 2 0 0 0 0 0 0 0 2
Florida 0 0 0 0 0 0 2 0 0 0 2
Georgia 0 0 0 0 0 0 0 0 0 0 0
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 1 0 1
Indiana 0 1 0 0 0 0 0 0 0 0 1
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 3 0 0 0 0 0 0 0 0 0 3
Maine 0 0 0 0 0 0 0 0 0 0 0
Maryland 0 0 0 1 0 0 0 0 0 0 1
Massachusetts 0 0 0 0 0 0 0 0 0 0 0
Michigan 0 0 2 0 0 0 0 0 0 0 2
Minnesota 0 0 0 0 0 0 0 0 0 0 0
Mississippi 0 2 0 0 0 0 2 0 0 0 4
Missouri 1 0 1 1 0 0 0 0 0 0 3
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 0 0 3 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 3 0 0 0 0 0 0 0 0 0 3
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 1 0 0 0 0 0 0 0 0 0 1
Pennsylvania 1 0 0 0 0 0 0 0 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 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 0 4 3 0 3 1 4 0 0 0 15
Utah 0 0 0 0 0 0 0 0 1 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 1 0 0 0 0 0 0 0 0 1
West Virginia 0 0 0 0 0 0 0 0 0 0 0
Wisconsin 0 0 0 0 0 0 0 0 0 0 0
Wyoming 0 0 0 0 0 0 0 0 0 0 0
Total 13 12 10 6 3 1 10 23 8 11 97

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

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