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Epidemiology & Geographic Distribution

In the United States, approximately 80-100 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 (Minnesota, Wisconsin, Iowa, Illinois, Indiana, and Ohio). Recently, more cases have been reported from mid-Atlantic and southeastern states (West Virginia, Virginia, Kentucky, North Carolina, and Tennessee).

La Crosse virus neuroinvasive disease cases reported by year, 2004–2013

A line chart depicting La Crosse encephalitis cases by year starting from 2004 to 2013.

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

Downloadable version of above graph [PDF - 1 page]

Data Table: In the United States, the number of La Crosse virus neuroinvasive disease cases reported each year varies. From 2004 through 2013, an average of 72 cases were reported annually (range 44–116).

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La Crosse virus neuroinvasive disease cases reported by state, 2004–2013

A map of the continental United States depicting California Serogroup Virus Neuroinvasive Disease Cases reported by state, 1964 to 2010.

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

Downloadable version of above map [PDF - 1 page]

Data table: From 2004 through 2013, La Crosse virus neuroinvasive disease cases have been reported in Alabama (3), Florida (4), Georgia (16), Indiana (11), Illinois (9), Iowa (4), Kentucky (2), Louisiana (7), Maryland (2), Michigan (5), Minnesota (17), Mississippi (5), Missouri (1), North Carolina (182), Ohio (164), South Carolina (4), Tennessee (102), Texas (4), Virginia (10), West Virginia (145) and Wisconsin (24).

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La Crosse virus neuroinvasive disease average annual incidence by county, 2004–2013

A map of the continental United States depicting California Serogroup Virus Neuroinvasive Disease Average Annual Incidence by County, 1996 to 2010.

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

Downloadable version of above map [PDF - 1 page]

Data table: This map shows the distribution of La Crosse virus neuroinvasive disease (encephalitis and/or meningitis) average annual incidence by county from 2004 through 2013. 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 primary 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

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