West Nile Virus: Data
The first cases of West Nile virus (WNV) disease in humans in Wisconsin were reported in 2002. Since its introduction into the state, WNV has become the most common illness spread by mosquitoes in Wisconsin.
On average, 18 cases of WNV disease are reported each year in the state. In 2024, Wisconsin had 32 confirmed and probable cases of WNV disease.
For tips on how to prevent West Nile Virus, visit our Mosquito Bite Prevention web page.
Wisconsin West Nile virus activity
The following dashboard is interactive and shows WNV disease cases since 2019 that occurred in people, horses, and birds as well as detections of the virus in mosquitoes collected from the environment (mosquito pools). It also shows presumptive viremic donors (PVDs), which are healthy people whose donated blood tested positive for WNV during routine screening performed by blood donation agencies. Tracking WNV infections in sick people, healthy blood donors, certain animals, and mosquitoes improves our understanding of where WNV is being spread during the mosquito season.
Additional human WNV case data can be found at the bottom of this page. National WNV data can be found on the CDC (Centers for Disease Control and Prevention) web page.
Data shown are subject to change and may be updated as more information is available.
How to use the data visualization
Users can interact with the data in this visualization by hovering over the counties on the map to see WNV activity for that county. Users can also change the year displayed using the dropdown option.
Each county in the map is colored based on one of four categories of WNV activity listed in the following table. The color key is listed in the dashboard.
| Category of Activity | Criteria |
|---|---|
| Human Activity Only | A resident(s) of this county was reported with a case(s) of WNV that meet the confirmed or probable case definition per the Communicable Disease Case Reporting and Investigation Protocol: Arboviral Infection, P-01930 (PDF) and/or a resident(s) of this county was reported as a PVD. |
| Animal or Mosquito Activity | A report(s) of WNV- positive laboratory test result was received from a horse(s), a bird(s), or other mammal that reside in this county or WNV was detected in mosquitoes in this county. |
| Both Animal/Mosquito and Human Activity | People and either animals or mosquitoes in this county have tested positive for WNV. |
| No Reported Activity | No documented human, animal, or mosquito activity has been reported from this county. A lack of reported WNV activity should not be assumed to mean absence of activity or risk. |
Public health surveillance for WNV does not detect all WNV infections in humans and animals, and testing of mosquitoes from the environment does not occur in all counties in Wisconsin. Human WNV surveillance is biased towards those people with more severe symptoms. People with milder illness are less likely to seek health care or be tested for WNV. WNV may be present in those counties that do not show reported WNV infections in people, animals, or mosquitoes.
Data for the current year will be updated as needed during the months of the year when mosquitoes are active, typically June through November.
Technical data notes and data sources
Data source for human cases: The Wisconsin Electronic Disease Surveillance System (WEDSS). WNV disease cases are reportable in Wisconsin and must be reported within 72 hours of recognition. Information about hospitalizations and deaths associated with these reports are collected as part of routine public health surveillance and follow-up activities. Cases are displayed in this visualization if they meet the probable or confirmed case criteria outlined in the Communicable Disease Case Reporting and Investigation Protocol: Arboviral Infection, P-01930 (PDF).
Data source for PVD: All blood that’s donated in the United States is screened for WNV. Blood donation agencies report PVDs to public health authorities whenever a person’s donated blood screens positive for WNV. Public health officials investigate reported PVDs to confirm that these individuals have not developed any symptoms of WNV disease shortly after donating blood. See Food and Drug Administration Guidance for more information about screening donated blood for WNV.
Data source for animal cases: DHS receives reports of WNV activity in animals from the Wisconsin Departments of Agriculture, Trade and Consumer Protection and Natural Resources, as well as from the Wisconsin Veterinary Diagnostic Laboratory. Not all WNV infections in animals are identified and reported to public health. For example, for a WNV infection to be identified in a horse, the animal has to show signs of disease. The horse's owner must then seek veterinary care, the veterinarian must then order WNV testing, and the WNV test results must be positive.
Data source for mosquitoes: Adult mosquitoes are collected from the environment using mosquito traps set by DHS, local public health agencies, or partners at the Midwest Center of Excellence for Vector-borne Diseases. Once trapped, mosquitoes are identified to species and those mosquito species that can spread WNV are then tested for WNV infection.
County designation: Human cases are reported by county of residence, which may not be the county of exposure. Domestic animal cases, such as horses, are reported by county of residence. WNV infections in wild animals or mosquitoes, are reported by the county where the animal or mosquito was collected.
Additional data resources
For additional details on human WNV disease data by county, see the DHS Environmental Public Health Tracking: West Nile Virus Data webpage.
For WNV activity data from across the United States see the CDC's (Center for Disease Control and Prevention) Data and Maps for West Nile Virus webpage.
Additional human West Nile virus data
Month of illness onset
Age group
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