A mini-outbreak of Eastern equine encephalitis (EEE) infections is killing horses in Michigan and Indiana, resulting in warnings from animal and public health officials who say the virus can also be lethal to humans. 

Michigan agriculture officials reported on September 4 the state’s first case of Eastern Equine Encephalitis (EEE) for 2022 in a three-year-old unvaccinated Standardbred filly from St. Joseph County. The horse later perished, according to Michigan Dept. of Agriculture and Rural Development (MDARD) State Veterinarian Dr. Nora Wineland, DVM.

“On September 8, 2022, a gelding became ill with neurologic signs—including leaning, wobbling, and stiffness/trying not to move. The horse was unvaccinated against EEE and was humanely euthanized due to the severity of his condition,” she said.   

According to the EDCC Health Watch, EEE is caused by the Eastern equine encephalitis virus, for which wild birds are a natural vector. Mosquitoes that feed on EEE-infected birds can transmit the virus to humans, horses and other birds, though 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, with a 90 percent fatality rate among horses that become ill and a 33 percent fatality rate among humans who become ill.

After Michigan’s first reported EEE case of 2022 was reported in early September, more suspected cases followed. On Sept. 14, MDARD confirmed an unvaccinated 14-year-old Percheron mare in Roscommon County has tested positive for Eastern equine encephalitis. The horse began showing signs of muscle tremors, dribbling urine, colic and “dog sitting” before collapsing and ultimately being euthanized.

The reports were not limited to Michigan. On Sept. 23, the Indiana State Board of Animal Health confirmed that an unvaccinated Standardbred mare in LaGrange County tested positive for EEE. The horse began showing signs of mild head-pressing, hind-limb ataxia (incoordination), lethargy and fever on Sept. 2 and died despite receiving supportive treatment. Another horse on the property had recently died after presenting with similar signs, the Board reported. 

Most recently, on Sept. 26, Michigan’s third equine fatality from EEE infection was reported, this time concerning a two year-old Paint cross gelding from Eaton County. 

 “The gelding became ill with neurologic signs—including leaning, wobbling, and stiffness/trying not to move. The horse was unvaccinated against EEE and was humanely euthanized due to the severity of his condition,” Wineland said in an MDARD news release.

“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.”

The EEE virus has not been restricted to the upper Midwest; at least 11 horses have perished from the disease in Florida thus far in 2022, according to EDCC Health Watch. In Illinois, State Veterinarian Mark Ernst told Farm World that there are currently no reported instances of EEE, and none have been documented for several years. Nationwide, there are currently no reported current cases of EEE in humans, though EEE will remain a threat until there has been at least one hard freeze, MDARD said.

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.