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State Veterinarian Statement on Eastern Equine Encephalitis-Positive Roscommon County Horse

Preventing mosquito bites is the first step to protecting your animals, yourself, and your family

LANSING, MI — Today, State Veterinarian Nora Wineland, DVM, released the following statement due to the discovery of Eastern Equine Encephalitis (EEE) in 14-year-old Percheron mare from Roscommon County. This is the first case of EEE in Roscommon County this year.

“On September 2, 2022, the mare became ill with colic-like symptoms, which progressed to neurologic signs—including muscle tremors, dog sitting, and being down with an inability to rise. The horse was unvaccinated against EEE and was humanely euthanized due to the severity of her condition.

This case shows the virus is present in the area’s mosquitoes and highlights the need to take precautions. Protect animals against mosquitoes by placing livestock in a barn under fans during peak mosquito activity (from dusk to dawn), eliminating standing water on one’s property, using an insect repellant on animals that is approved for the species, and contacting a veterinarian to vaccinate horses against EEE and other mosquito-borne diseases.

Also, please contact a veterinarian if a horse shows signs of the illness: mild fever and stumbling, which can progress to being down and struggling to stand.”

EEE is a zoonotic, viral disease transmitted through the bites of mosquitoes to both animals and people; it is typically seen in late summer to early fall each year in Michigan. The disease is not spread by horse-to-horse or horse-to-human contact. Overall, EEE and other mosquito-borne illnesses will continue to pose a risk to both animals and humans until temperatures consistently fall below freezing.

EEE is one of the most dangerous mosquito-borne diseases in the U.S. with a 90 percent fatality rate among horses that become ill and a 33 percent fatality rate among humans who become ill.

For 2022, the mare is Michigan’s second case of EEE in a domestic animal, and there are no reported cases of EEE in humans. 

For more information about EEE, please visit