Three horses in Michigan recently contracted mosquito-borne diseases. The horses lived in Kalamazoo, St. Joseph, and Allegan counties. 

In Kalamazoo County, a 2-year-old filly became dually infected with both Eastern equine encephalitis (EEE) and West Nile virus (WNV). The horse was vaccinated against both diseases in April 2024 and had no known underlying health issues. She became recumbent on September 2 and was euthanized. 

In St. Joseph County, an unvaccinated yearling Standardbred colt tested positive for WNV on September 17 after developing clinical signs on August 30, including fever, leaning, recumbency, and inability to rise. He was euthanized.

In Allegan County, a 5-year-old Quarter Horse gelding tested positive for EEE on September 17 after developing clinical signs on August 26, including ataxia, circling, head pressing, and seizures. The horse had reportedly been vaccinated in the spring by his prior owner, but no other vaccinations had been administered since. He was euthanized. 

EDCC Health Watch is an Equine Network marketing program that utilizes information from the Equine Disease Communication Center (EDCC) to create and disseminate verified equine disease reports. The EDCC is an independent nonprofit organization that is supported by industry donations in order to provide open access to infectious disease information.

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, which are nontoxic to horses) available at hardware stores.

WNV 101

West Nile virus is transmitted to horses via bites from infected mosquitoes. Not all infected horses show clinical signs, but those that do can exhibit:

• Flulike signs, where the horse seems mildly anorexic and depressed;
• Fine and coarse muscle and skin fasciculation (involuntary twitching);
• Hyperesthesia (hypersensitivity to touch and sound);
• Changes in mentation (mental activity), when horses look like they’re daydreaming or “just not with it”;
• Occasional drowsiness;
• Propulsive walking (driving or pushing forward, often without control); and
• Spinal signs, including asymmetrical weakness; and
• Asymmetrical or symmetrical ataxia.

West Nile virus has no cure. However, some horses can recover with supportive care. Equine mortality rates can reach 30-40%.

Studies have shown that vaccines can be effective WNV prevention tools. Horses vaccinated in past years need an annual booster shot, but veterinarians might recommend two boosters annually—one in the spring and another in the fall—in areas with prolonged mosquito seasons. In contrast, previously unvaccinated horses require a two-shot vaccination series in a three- to six-week period. It takes several weeks for horses to develop protection against the disease following complete vaccination or booster administration.

In addition to vaccinations, owners should work to reduce mosquito population and breeding areas and limit horses’ mosquito exposure by:

• Removing stagnant water sources;
• Dumping, cleaning, and refilling water buckets and troughs regularly;
• Keeping animals inside during insect feeding times (typically early in the morning and evening); and
• Applying mosquito repellents approved for equine use.