Endemic human mosquito-borne disease in Wisconsin residents, 2002-2006

West Nile virus (WNV) and La Crosse virus (LAC) are the primary mosquito-borne arboviruses associated with human disease in Wisconsin. We examined WNV and LAC human illness surveillance data collected during 2002 through 2006.

ELISA-based tests developed by the Centers for Disease Control and Prevention (CDC) were used to detect acute WNV and LAC infection in patient sera or cerebral spinal fluid. Public health personnel conducted patient follow-up using standard arbovirus reporting forms. CDC/Council of State and Territorial Epidemiologists definitions were used to determine cases.

From 2002 through 2006, 114 confirmed human cases of WNV illness were reported in Wisconsin residents; 82% of illness onsets occurred during August or September. Median age of WNV case patients was 51 years, 49% reported neuroinvasive disease, 56% were hospitalized, and 7 cases were fatal. Confirmed LAC illnesses declined from a high of 27 cases during 2003 to a low of 3 cases during 2005 and 2006. Most LAC illnesses occurred in residents of Western Wisconsin; median age of LAC cases was 9 years. Mean annual incidences of reported confirmed WNV illnesses calculated for high, medium, and low population density groupings were very similar (range: 0.40-0.46 cases/100,000 population).

Humans are at risk for mosquito-borne diseases in Wisconsin. Protection and prevention measures are important statewide, especially during July through September when the risk is greatest.