Epidemiology & Geographic Distribution

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In the United States, an average of 68 La Crosse encephalitis virus (LACV) neuroinvasive disease cases are reported each year. There is significant under-diagnosis and under-reporting of less severe cases of LACV disease, therefore reported LACV neuroinvasive disease cases are used to compare trends over time and place. To ensure standardization of reporting across the country, CDC recommends that the national surveillance case definition (standardized reporting criteria) be consistently applied by all state health departments.

LACV disease cases occur primarily from late spring through early fall, but in subtropical endemic areas (e.g., the Gulf states), rare cases can occur in winter. Historically, most cases of LACV neuroinvasive disease were reported from the upper Midwestern states (Ohio, Wisconsin, Minnesota, Indiana, Iowa, and Illinois). Recently, more cases have been reported from mid-Atlantic and southeastern states (North Carolina, Tennessee, West Virginia, Georgia, Virginia, and Kentucky).

La Crosse virus neuroinvasive disease cases reported by year, 2009–2018

A line chart depicting La Crosse encephalitis cases by year starting from 2009 to 2018.

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

La Crosse virus neuroinvasive disease cases reported by year, 2007–2016
Year Number of Reported Cases
2009 44
2010 67
2011 116
2012 71
2013 77
2014 76
2015 51
2016 31
2017 63
2018 83

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Data Table: In the United States, the number of La Crosse virus neuroinvasive disease cases reported each year varies. From 2009 through 2018, an average of 68 cases were reported annually (range 31–116).

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La Crosse virus neuroinvasive disease cases reported by state of residence, 2009–2018

A map of the continental United States depicting California Serogroup Virus Neuroinvasive Disease Cases reported by state, 2008 to 2017.

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

La Crosse virus neuroinvasive disease cases reported by state of residence, 2008–2017
State Number of Reported Cases
Alabama 2
Florida 2
Georgia 12
Illinois 1
Indiana 6
Iowa 1
Kansas 1
Kentucky 1
Louisiana 1
Maryland 3
Michigan 3
Minnesota 18
Mississippi 3
Missouri 1
North Carolina 190
Ohio 208
Rhode Island 1
South Carolina 4
Tennessee 107
Texas 5
Virginia 8
West Virginia 77
Wisconsin 24

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Data table: From 2009 through 2018, La Crosse virus neuroinvasive disease cases have been reported in Alabama (2), Florida (2), Georgia (12), Illinois (1), Indiana (6), Iowa (1), Kansas (1), Kentucky (1), Louisiana (1), Maryland (3), Michigan (3), Minnesota (18), Mississippi (3), Missouri (1), North Carolina (190), Ohio (208), Rhode Island (1), South Carolina (4), Tennessee (107), Texas (5), Virginia (8), West Virginia (77) and Wisconsin (24).

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La Crosse virus neuroinvasive disease average annual incidence by county of residence, 2009–2018

A map of the continental United States depicting California Serogroup Virus Neuroinvasive Disease Average Annual Incidence by County, 2009 to 2018.

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

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Data table: This map shows the distribution of La Crosse virus neuroinvasive disease average annual incidence by county of residence from 2009 through 2018. Counties are shaded according to incidences ranging from less than 1.00, 1.00 to 2.49, and greater than 2.50 per 100,000 population. Shaded counties are primarily distributed in eastern half of the country. Most of the highest incidence counties are along the Appalachian range.

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Additional Maps, Charts, and Tables

La Crosse virus disease cases and deaths reported to CDC by year and clinical presentation, 2009-2018

La Crosse 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. (%)
2009 44 1  (2) 6 0  (0) 50 1  (2)
2010 67 1  (1) 7 0  (0) 74 1  (1)
2011 116 1  (1) 14 0  (0) 130 1  (1)
2012 71 1  (1) 7 0  (0) 78 1  (1)
2013 77 2  (3) 8 0  (0) 85 2  (2)
2014 76 3  (4) 4 0  (0) 80 3  (4)
2015 51 0  (0) 4 0  (0) 55 0  (0)
2016 31 0  (0) 4 0  (0) 35 0  (0)
2017 63 0  (0) 0 0  (0) 63 0  (0)
2018 83 0  (0) 3 0  (0) 86 0  (0)
Total 679 9  (1) 57 0  (0) 736 9 (1)

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La Crosse virus disease cases reported to CDC by state of residence, 2009–2018

La Crosse virus disease cases reported to CDC by state of residence and year, 2009–2018
State 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Total
Alabama 0 0 1 0 1 0 0 0 0 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 0 0 0 0 0 0 0 0 0
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 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 1 0 0 1 0 0 0 0 2
Georgia 2 2 2 0 1 2 2 0 2 0 13
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 1 0 0 0 0 0 0 0 1 0 2
Indiana 1 0 2 3 1 0 0 0 0 0 7
Iowa 0 0 0 0 0 0 0 0 1 0 1
Kansas 0 0 0 0 0 0 1 0 0 0 1
Kentucky 0 0 1 0 0 0 0 0 0 0 1
Louisiana 0 0 0 0 0 0 1 0 0 0 1
Maine 0 0 0 0 0 0 0 0 0 0 0
Maryland 0 2 0 0 0 0 0 0 1 0 3
Massachusetts 0 0 0 0 0 0 0 0 0 0 0
Michigan 0 2 1 0 0 0 0 0 0 0 3
Minnesota 0 1 1 4 5 4 1 3 1 0 20
Mississippi 0 0 0 1 3 0 0 0 0 0 4
Missouri 1 0 0 0 0 0 0 0 0 0 1
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 0 0 0
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 1 0 0 0 0 0 0 0 0 1
North Carolina 16 22 26 26 13 23 11 8 21 24 190
North Dakota 0 0 0 0 0 0 0 0 0 0 0
Ohio 5 24 50 13 16 31 24 9 13 39 224
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 0 0 0
Puerto Rico 0 0 0 0 0 0 0 0 0 0 0
Rhode Island 0 0 0 0 0 0 0 0 0 1 1
South Carolina 0 0 1 1 1 0 1 0 0 0 4
South Dakota 0 0 0 0 0 0 0 0 0 0 0
Tennessee 8 11 12 9 23 11 4 3 17 13 111
Texas 0 1 0 3 0 0 0 0 0 1 5
Utah 0 0 0 0 0 0 0 0 0 0 0
Vermont 0 0 0 0 0 0 0 0 0 0 0
Virginia 1 0 1 2 2 2 0 0 0 2 10
Washington 0 0 0 0 0 0 0 0 0 0 0
West Virginia 14 8 26 14 11 2 4 8 4 6 97
Wisconsin 1 0 5 2 8 4 6 4 2 0 32
Wyoming 0 0 0 0 0 0 0 0 0 0 0
Total 50 74 130 78 85 80 55 35 63 86 736

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Page last reviewed: June 24, 2019
Content source: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), Division of Vector-Borne Diseases (DVBD)