Epidemiology & Geographic Distribution

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. LAC virus disease is a nationally notifiable condition. Cases are reported to CDC by state and local health departments using the national surveillance case definition. 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, Illinois, and Iowa). Recently, more cases have been reported from northeastern, mid-Atlantic and southeastern states (North Carolina, Tennessee, West Virginia, Georgia, Virginia, Kentucky, and Rhode Island).