Fourth Michigan Horse Tests Positive for EEE

The Michigan Department of Agriculture and Rural Development (MDARD) confirmed on Aug. 20 that four horses in three Michigan counties (Barry, Kalamazoo, and St. Joseph) tested positive for Eastern equine encephalomyelitis (EEE).

The Barry County case, an unvaccinated yearling Arabian filly, presented with an elevated temperature on Aug. 13 and was euthanized Aug. 14.

The affected horse in Kalamazoo County, an unvaccinated 5-year-old Quarter Horse mare, showed clinical signs including recumbency (down and unable to rise) and seizures and was euthanized on Aug. 11.

The two cases in St. Joseph County include an unvaccinated 19-year-old Standardbred mare that became ill with fever and recumbency on Aug. 4 and was euthanized Aug. 5. The second affected horse in that county was a 3-year-old unvaccinated Standardbred colt that developed signs of illness consisting of ataxia (incoordination) and circling on Aug. 10 and was euthanized Aug. 11.

“While we typically see a few cases of EEE every year in Michigan, the disease has also been known to have high levels of activity about every nine to 10 years or in years with heavy rain,” said Nora Wineland, DVM, MS, Dipl. ACVPM, state veterinarian and division director at the Michigan Department of Agriculture and Rural Development. “The last major outbreak of EEE in Michigan was in 2010, when there were 56 known cases of EEE in Michigan horses. Additionally, this spring was very wet in Michigan.”

She added that it’s not too late to vaccinate for EEE this year.

EEE 101

Eastern equine encephalomyelitis is caused by the Eastern equine encephalitis virus, for which wild birds are a natural reservoir. Mosquitoes that feed on EEE-infected birds can transmit the virus to humans, horses, and other birds. Horses do not develop high enough levels of these viruses in their blood to be contagious to other animals or humans. Because of the high mortality rate in horses and humans, EEE is regarded as one of the most serious mosquito-borne diseases in the United States.

Tips for preventing mosquito-borne diseases include:

  • Avoid mosquito bites: Use insect repellent when outdoors, especially from dusk to dawn.
  • Look for EPA-labeled products containing active ingredients such as DEET, picaridin (KBR3023), or oil of lemon eucalyptus (p-menthane 3,8-diol).
  • Apply more repellent, according to label instructions, if mosquitoes start to bite.
  • Mosquito-proof homes: Fix or install window and door screens, and cover or eliminate empty containers with standing water where mosquitoes can lay eggs.
  • Protect your horses: Veterinarians recommend commercially available licensed vaccines against EEE for all horses in the U.S. Horses should be vaccinated at least annually (recommendations vary in high-risk areas). It’s not too late this year to vaccinate your horses.
  • Use approved insect repellents to protect horses.
  • If possible, put horses in stables, stalls, or barns during the prime mosquito exposure hours of dusk and dawn.
  • Eliminate standing water, drain water troughs, and empty buckets at least weekly.
  • Stock water tanks with fish that consume mosquito larvae (contact your local mosquito control for assistance), or use mosquito “dunks” (solid “donuts” of Bacillus thuringiensis israelensis [BTi], which are nontoxic to horses) available at hardware stores.