It’s a typical spring day—the sky is blue, white puffy clouds drift over head, the grass is lush and green. You turn your horse out in the morning, thinking it’s the perfect day for him to spend outside. However, that evening when you bring him in he’s covered with hives. It’s an allergic reaction, but to what?

You treat his hives, but now you have to shed your chaps and helmet and don your Sherlock Holmes outfit to figure out the cause.

Todd C. Holbrook, DVM, of Oklahoma State University, said diagnosing the cause of an allergic reaction might take detective work—and lots of it.

“When a horse has a hypersensitivity problem, it may take record keeping to figure out what might have caused it. The owner may try a process of elimination, or stop using certain feeds or products and then add them back in, one by one, to see which one caused the reaction,” he said.

Your observations are crucial to solving the puzzle, and the veterinarian doing the examination and history taking must try to think of the right questions to ask you. Sometimes keeping a diary or writing things on a calendar regarding what’s done with that horse each day helps track changes in his routine. In some cases, this type of tracking is less expensive and more effective than going through a lot of diagnostic tests.

For a food allergy, diagnosis is often made by exclusion. The hay is changed to a type that’s not been previously fed, and all grains, additives, supplements and treats are discontinued. Once the skin condition improves, the products are reintroduced one at a time, to try to define the cause.

A horse may develop contact dermatitis if his skin has become sensitive to ingredients in a fly spray or shampoo, for instance.

Or he may break out in a rash or hives after walking through nettles.

One clue may be the placement of any skin lesions, such as where a horse has rubbed an itch due to fly bites. Or, the hives may result from something in the environment or could be a response to any previous treatments.

“With skin problems, I’ve seen situations in which an owner is frustrated and tries different shampoos or switches fly sprays, and sometimes horses over-react or develop additional hypersensitiveness beyond the initial cause,” said Holbrook.

Under The Skin

Your veterinarian may take a skin biopsy to rule out other diseases or look for allergic cells from the bump. Intradermal allergy testing is also an option, but it’s not foolproof. And your veterinarian must interpret the test results in combination with the history, clinical signs and environment, said Holbrook.

A skin test may also be difficult to interpret if a horse already has bumps all over his body.

If you do decide on an intradermal skin test, it’s a project. The horse is sedated and a large area of skin is shaved (usually on the neck) so results can be more easily seen. Various antigens are injected into the skin on a grid pattern, with a key representing where each antigen is located. The horse is kept overnight and monitored, and the veterinarian reads the reaction, grading the amount of swelling at different times, to see which ones caused a response.

Holbrook said that many horses respond to skin injection allergens.

“The veterinarian has to be careful in interpretation of test results. But skin testing can be useful in some cases to determine treatment protocols, or to define allergens the horse needs to avoid. The horses we end up using it on usually have a recurrent seasonal problem that hasn’t responded to conventional treatment.

“Management of the horse and his environment—to try to relieve the allergy—often includes steroids and fly sprays and has either been unsuccessful or inadequate. Skin testing and allergy specific treatment is often a last resort,” said Holbrook.

Hilary A. Jackson, a veterinary dermatologist at North Carolina State University, regularly sees seasonal skin allergies in her practice.

“In the southern states we see insect allergies and hives, and we may do skin tests to determine the cause. We use the skin test for environmental allergies and may formulate an allergy vaccine for that particular horse,” she said.

Christine Rees, DVM, veterinary dermatologist at Texas A&M University, said most veterinarians don’t have the training nor qualifications to perform intradermal allergy tests.

“This is why the tests are usually performed by a board certified veterinary dermatologist—who has done a two- or three-year residency similar to human medicine and a rigorous final exam that includes principles in immunology. Cases for skin testing are referred to a university practice or a veterinary specialty practice,” she said.

Some veterinarians may offer to do serum allergy testing from blood samples, but there’s some debate as to whether what goes on in the skin is the same as what goes on in the blood.

“A skin test is thought to be more sensitive than a blood test. In animals with allergies, the clinical signs you see are usually skin signs. Therefore, with the skin test you’re testing the organ of the body that’s affected,” explained Rees.

Prevention Is Key

It turns out that a sudden outbreak of hives is relatively common in horses, although hives can be an early indication of a more serious anaphylactic reaction. Most of the time you don’t need to panic, though.

“I’ve seen dramatic multiple cases at horse shows. A number of horses may develop hives in a certain barn, for instance,” said Holbrook. “In that instance I tend to treat the condition more aggressively—with a small dose of steroids to reduce the swellings—so the owner can show the horse.

“At home, however, I might tell a client to just watch the horse and see if it worsens or goes away. I ask if the horse had new bedding or a new fly spray or shampoo. It may have been an insect bite, or reaction to bedding, or you may not be able to pinpoint the cause, but the hives usually subside on their own,” said Holbrook.

Bathing the horse with a mild shampoo may help if hives were caused by contact with an allergen. Washing a horse frequently may help wash any pollen or other allergens out of the hair coat too.

“Treatment may or may not be needed, depending on severity of the reaction. If there are many wheals and they are painful, or all over the horse’s back so he cannot be ridden, or if there’s evidence of respiratory involvement, the horse needs treatment,” he noted.

The best treatment for insect sensitivities, for instance, is prevention. If a horse is hypersensitive to insect bites, fly spray may give relief. Flysheets should be used at the beginning of the season, before the horse has an itching dermatitis and damaged skin that will be irritated by the sheet.

“Commercial availability of good fly sprays and pour-on products is getting better, and some of these products really help,” said Holbrook.

The gnats that cause problems usually attack horses at dawn and dusk. You can put the horse in a barn (with small mesh screen on windows) at these times, or use a fan in the stall to keep the air moving, which tends to discourage gnats because they’re not strong fliers.

There are two types of drugs used to treat allergies—antihistamine-type drugs, such as hydroxazine, and corticosteroids, such as dexamethasone or prednisone.

“Sometimes antihistamines are helpful in treating seasonal allergies,” said Holbrook. “If we start a horse on antihistamines early in the season, before the reactions [to insects or pollens] get so bad, this is more effective than if given after he already has a problem. Once the reaction starts, we usually have to use a corticosteroid to reverse it,” he said.

The antihistamines used for humans aren’t usually effective for horses, but an antihistamine-type drug called hydroxazine sometimes works well, said Richard A. Mansmann, VMD, PhD, Director of the Equine Health Program, North Carolina State University.

“Hydroxazine works against both type 1 and type 3 allergies in humans and has a little broader spectrum against different types of allergies,” he said.

“Dexamethasone is my drug of choice, to deal with swelling and inflammation,” added Mansmann. “Usually the veterinarian is called when the horse is at its worst, but if the skin problem is seasonal and begins in late April, you should start cortisone treatment the second week in April–before the horse has a problem–and can then keep the horse on a low level of dexamethasone all through the season and the problem won’t get really bad.”

Mansmann said he’s treated horses with 4 mg of dexamethasone every other day, controlling what in earlier years had been significant skin allergies. “The same holds true for seasonal respiratory disease, like summer pasture heaves. Start on it before it becomes advanced. Then you can use the lower levels of dexamethasone and for a longer period of time,” said Mansmann.

Corticosteroids should be used at the lowest effective dose due to possible systemic side effects.