The Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) confirms that a 4-year-old grade mare in Portage County has tested positive for Eastern Equine Encephalitis (EEE). It is the state’s first confirmed case of EEE in a horse this year. In 2025, DATCP documented one case of EEE in horses; access an online map of the 2025 arbovirus cases.
EEE and West Nile Virus (WNV) are viruses that can cause inflammation of the brain (encephalitis). Symptoms of EEE in horses include altered mentation, impaired vision, head pressing, circling, inability to swallow, lack of coordination, seizures, and death. Affected horses are often found down and unable to rise. EEE is fatal in most horses showing signs of illness. Equine owners should contact their veterinarian if they observe any of these signs.
While people can be infected by EEE, 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 EEE. Since humans and equines get EEE from mosquitoes, the threat is normally highest when mosquitoes are most active, from middle to late summer until the first killing frost. The infections confirm that there are mosquitoes infected with EEE in the state that can transmit the virus to people and animals.
DATCP strongly encourages equine owners to speak with their veterinarian about vaccinating their horses for EEE. The American Association of Equine Practitioners (AAEP) recommends vaccinating for both EEE and West Nile Virus as part of the core vaccine protocol, meaning all horses should be vaccinated for these diseases, regardless of activity level. 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.
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