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.
