Statistics & Maps

Although the geographic range of St. Louis encephalitis (SLE) virus extends from Canada to Argentina, human cases have almost exclusively occurred in the United States. Most 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. In the United States, the annual number of reported SLE virus neuroinvasive disease cases can fluctuate widely, as a result of periodic epidemics. In temperate areas of the United States, SLE disease cases occur primarily in the late summer or early fall. In Southern states, where the climate is milder, cases can occur year-round.

SLE is a nationally notifiable condition. To ensure standardized reporting across the country, CDC recommends that the national surveillance case definition be consistently applied by all state health departments.

All residents of and visitors to areas where SLE virus activity has been identified are at risk of SLE virus infection, particularly people who engage in outdoor work and recreational activities and those living in low-income areas.

St. Louis encephalitis virus neuroinvasive disease cases reported by year, 2012–2021

St. Louis Encephalitis cases reported by year

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

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St. Louis encephalitis virus neuroinvasive disease cases reported by year, 2008–2017 
Year Number of Reported Cases
2012 1
2013 1
2014 6
2015 19
2016 7
2017 6
2018 5
2019 15
2020 14
2021 11

St. Louis encephalitis virus neuroinvasive disease cases reported by state of residence, 2012–2021

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

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St. Louis encephalitis virus neuroinvasive disease cases reported by state, 2007–2016
State Number of Reported Cases
Alabama 2
Arizona 42
California 24
Florida 2
Illinois 1
Louisiana 1
Mississippi 2
Nevada 2
Oklahoma 1
Texas 6
Utah 1
Wisconsin 1

St. Louis encephalitis virus neuroinvasive disease average annual incidence by county of residence, 2012–2021

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

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Data table: This map shows the distribution of St. Louis encephalitis virus neuroinvasive disease average annual incidence by county of residence from 2012 through 2021. 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 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, 2012-2021

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. (%)
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 15 0  (0) 2 0  (0) 17 0  (0)
2020 14 3  (21) 2 0  (0) 16 3  (19)
2021 11 0  (0) 6 0  (0) 17 0  (0)
Total  85 8  (9)   29  0  (0)  114 8  (7) 

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, 2012-2021

St. Louis encephalitis virus disease cases reported to CDC by state of residence, 2009-2018
State 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 Total
Alabama 0 0 1 0 0 1 0 0 0 0 2
Alaska 0 0 0 0 0 0 0 0 0 0 0
Arizona 0 0 1 23 0 6 0 9 6 12 57
Arkansas 0 0 0 0 0 0 0 0 0 0 0
California 0 0 0 0 3 4 5 6 6 4 28
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 0 0 0 0 0 0 0 0
Florida 0 0 2 0 0 0 0 0 0 0 2
Georgia 0 0 0 0 0 0 1 0 0 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 1 0 0 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 1 1
Maine 0 0 0 0 0 0 0 0 0 0 0
Maryland 0 0 0 0 0 0 0 0 0 0 0
Massachusetts 0 0 0 0 0 0 0 0 0 0 0
Michigan 0 0 0 0 0 0 0 0 0 0 0
Minnesota 0 0 0 0 0 0 0 0 0 0 0
Mississippi 0 0 2 0 0 0 0 0 0 0 2
Missouri 0 0 0 0 0 0 0 0 0 0 0
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 3 0 0 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 1 0 0 1
Oregon 0 0 0 0 0 0 0 0 0 0 0
Pennsylvania 0 0 0 0 0 0 1 1 1 0 3
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 1 4 0 0 0 0 0 3 0 11
Utah 0 0 0 0 1 0 0 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 1 0 0 0 1
Wyoming 0 0 0 0 0 0 0 0 0 0 0
Total 3 1 10 23 8 11 8 17 16 17 114

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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