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Allergies are not as common in horses as they are in small animals, but diagnosing and treating them is tricky.

“Probably the most common skin-complaint call we get is an owner saying, ‘My horse was fine last night, and this morning has so many hives its skin looks like corn on the cob,’ ” says Alfredo Sanchez Londoño, an equine veterinarian with Tufts’ Ambulatory Service in Woodstock, Conn. “A case like that is kind of a nightmare, because there are so many factors to try to control.”

Dogs and cat owners can try a variety of foods to eliminate potential allergens, whether it’s wet food or kibble made from venison, rabbit or duck. With horses, “you can try a different source of hay or grain,” says Londoño, “but the ingredients are all pretty much the same.”

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Environmental allergies in horses are quite common, caused by dust, pollen and molds, which thrive in the fields and stables where they spend most of their time.

To diagnose this kind of allergy, horses have to undergo both blood and skin-prick tests, as is the case with dogs and cats. Londoño recommends the testing be done in a hospital environment. “It’s hard to do this testing outside in a field,” he says. “If you inject a horse with an allergen, and a severe reaction occurs three hours after you’re gone, that can be a disaster. The horse needs to be monitored continuously.”

In addition, “a lot of horses are extremely sensitive to insect bites,” says Londoño.

There are a number of steps owners can take to lessen animals’ exposure to bugs, says Linda Frank, V85, a veterinary dermatologist at the University of Tennessee’s Veterinary Teaching Hospital.

The most common cause of insect allergy in horses is Culicoides biting midges, which feed at night, Frank says. She recommends that owners keep their horses inside from an hour before dusk an to hour after dawn and spray them nightly with insect repellant. Installing a fan in the barn is also helpful, because these insects are less likely to hang around if there’s a breeze. And be sure to eliminate pools of standing water, where the bugs breed.