The Michigan Department of Agriculture and Rural Development said Thursday that the 12-year-old mare from Van Buren County is the state's first case of the disease this year. In 2012, the state's only case was in a puppy from the same county.
"Horse owners in Michigan's southwestern counties should be especially aware of the risk and take extra measures to protect their animals," Dr. James Averill, the state veterinarian, said in a statement.
The horse wasn't vaccinated against the potentially deadly disease and died after developing severe swelling of the brain, officials said.
Eastern equine encephalitis is rare but can also be deadly among humans. Spread by mosquitoes, the disease typically affects people, horses and poultry, but also can occur in other mammals, reptiles and amphibians. Precautions for horses include vaccination.
In Michigan, the highest concentration of the disease has been in the southwestern portion of the state. Since 1980, the state said Barry County had 27 cases, Calhoun County had 22, Cass County had 46, Kalamazoo County had 44, St. Joseph County had 59 and Van Buren County had 26.
"In 2010, statewide there were 56 cases of EEE, and since then the cases have steadily declined — more than likely because Michigan veterinarians encourage EEE vaccinations as part of the spring horse vaccination protocol," Averill said. "Vaccinating at any time against the virus is encouraged, even this late in the year."
People also should take steps to avoid being bitten by mosquitoes, health officials said.