MADISON — The Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) confirms that an unvaccinated six-year-old Quarter Horse mare in Brown County has tested positive for West Nile Virus (WNV). It is the state’s first confirmed case of WNV in a horse this year. In 2025, DATCP documented five cases of WNV in horses; access an online map of the 2025 cases.
West Nile Virus (WNV) can cause inflammation of the brain (encephalitis). Symptoms of WNV in horses include fever, loss of coordination, hind-end weakness, depression, loss of appetite, muscle tremors, teeth grinding, inability to swallow, head pressing, excessive sweating, and going down with an inability to rise. The disease can cause brain inflammation in horses and people, and it is fatal in 30-40% of horses showing signs of illness.
While people can be infected by WNV, the virus does not pass directly between humans and horses. The only route of transmission is from a mosquito bite. Mosquitoes transmit the virus from birds, which serve as natural reservoirs for WNV. Since humans and equines get WNV from mosquitoes, the threat is normally highest when mosquitoes are most active, from middle to late summer until the first killing frost. More information on preventing mosquito bites can be found on the DHS website.
DATCP strongly encourages equine owners to speak with their veterinarians about vaccinating their horses for WNV. The American Association of Equine Practitioners (AAEP) recommends vaccinating for both WNV and Eastern Equine Encephalitis as part of the core vaccine protocol, meaning all horses should be vaccinated for these diseases. Adult horses that have never been vaccinated require two doses of the vaccine initially, followed by annual boosters.
In addition to vaccination, DATCP offers these tips to limit horses’ exposure to mosquitoes:
- Remove items that could collect stagnant water (old tires, buckets, unused troughs, etc.).
- Keep rain gutters draining properly; turn wading pools and wheelbarrows upside down.
- Clean and chlorinate pools, outdoor saunas, and hot tubs; drain water from pool covers.
- Empty and replace water in birdbaths at least once a week.
- Consider keeping horses in the barn from dusk to dawn, when mosquitoes are most active.
More information on WNV can be found at datcp.wi.gov.
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