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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, 2007–2016

St. Louis Encephalitis cases reported by year

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

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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 2007 through 2016, 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, 2007–2016

St. Louis Encephalitis average annual incidence by county

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

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Data table: From 2007 through 2016, St. Louis encephalitis virus neuroinvasive disease cases have been reported in Alabama (2), Arkansas (15), Arizona (19), California (3), Florida (2), Illinois (1), Indiana (1), Louisiana (4), Michigan (2), Mississippi (6), Missouri (1), Nevada (3), 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, 2007–2016

St. Louis Encephalitis cases reported by county

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

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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 2007 through 2016. 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.

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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, 2007-2016

Neuroinvasive disease Non-neuroinvasive disease Total
Year Cases
No.		 Deaths
No. (%)		 Cases
No.		 Deaths
No. (%)		 Cases
No.		 Deaths
No. (%)
2007 8 0  (0) 1 0  (0) 9 0  (0)
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)
Total  73 5  (7)   22  0  (0)  95 5  (6) 

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St. Louis encephalitis virus disease cases reported to CDC by state of residence, 2007-2016

State 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Total
Alabama 0 0 0 0 1 0 0 1 0 0 2
Alaska 0 0 0 0 0 0 0 0 0 0 0
Arizona 0 0 0 0 0 0 0 1 23 0 24
Arkansas 2 4 4 2 3 0 0 0 0 0 15
California 0 0 0 0 0 0 0 0 0 3 3
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 0 2 0 0 0 0 0 0 2
Florida 0 0 0 0 0 0 0 2 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 0 1 1
Indiana 0 0 1 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 3 0 0 0 0 0 0 0 0 6
Maine 0 0 0 0 0 0 0 0 0 0 0
Maryland 0 0 0 0 1 0 0 0 0 0 1
Massachusetts 0 0 0 0 0 0 0 0 0 0 0
Michigan 0 0 0 2 0 0 0 0 0 0 2
Minnesota 0 0 0 0 0 0 0 0 0 0 0
Mississippi 2 0 2 0 0 0 0 2 0 0 6
Missouri 1 1 0 1 1 0 0 0 0 0 4
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 1 0 0 0 0 0 0 0 0 3 4
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 3 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 0 1 0 0 0 0 0 0 0 0 1
Pennsylvania 0 1 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 0 4 3 0 3 1 4 0 0 15
Utah 0 0 0 0 0 0 0 0 0 1 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 1 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 9 13 12 10 6 3 1 10 23 8 95

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