The state Agriculture and Community Health departments said Thursday an 8-year-old horse from Grand Traverse County tested positive for the mosquito-borne virus this month. It was euthanized after it didn't respond to treatment.
The disease affects humans and animals. Symptoms in horses include stumbling, tremors, facial paralysis, impaired vision and seizures.
Officials say there hasn't been a human case of the virus this season.
They say turkeys in a small flock in Ingham County were infected with the disease and were dying at a high rate.
A state veterinarian said people should be diligent about vaccinating horses, using bug repellent and keeping water troughs clean.