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Evaluation of food allergies in horses can be difficult. There are several caveats that horse owners and veterinarians must keep in mind when interpreting results of allergy testing. It is not unusual for normal horses to react to at least some of the allergens on the allergy panel. In other words, normal horses can have positive reactions to allergens that they are actually not allergic to.

The panel of allergens used on the test may not contain the allergens that the horse is most allergic to, which means that whatever the horse is allergic to may not be tested against. It is very important to consult a board-certified veterinary dermatologist to interpret the allergy test, to determine if hyposensitization, in the form of allergy shots, is necessary, and to formulate which allergens should be included in hyposensitization. Allergy panels should be tailored for specific geographic areas, as veterinary panels are usually based on the most common human allergens in the region. Therefore, horses should be tested by veterinary dermatologists who practice in the area where the horses live to ensure that horses are tested for the most commonly encountered allergens.

Studies have shown that the most reliable diagnostic method for equine allergy is the intradermal allergy test (IDAT). This test involves injection of allergens such as mold and pollen into the horse’s skin. The dermatologist looks for skin reactions (wheals or hives) and measures them for several hours after injection. The largest wheals indicate which allergens should be used in hyposensitization injections. Usually this test is performed on the side of the horse’s neck, and the hair is clipped in an area about the same size as a regular sheet of paper. Horses to be tested should not be treated with corticosteroids or antihistamines for several weeks prior to allow for more reliable results.

Allergy testing using serum has not been shown to be as reliable as IDAT. This is unfortunate because these blood tests are very convenient. The most significant problem with serum allergy testing is that the antibodies (IgE) that are most important for allergic reactions are more abundant in the skin than in the blood. IgE antibodies in the skin are found on mast cells that burst and cause the redness, pain, swelling, and itching that occurs when the antibody encounters its allergen. These are the reactions that are evaluated during an IDAT, and the test is based on this principle.

Food allergy in horses is rare and extremely difficult to diagnose. This is confounded by allergy tests that often show hypersensitivity to something the horse commonly eats. Clinical signs of food allergy include hives, itchiness, and possible self-trauma due to scratching. The only dependable method to test for food allergy is to completely remove the suspected food allergen from the horse’s diet for 4-8 weeks and to observe for a decrease in itching or hives.

If a specific food allergen cannot be pinpointed, a practical way to begin a food-allergy trial is to remove concentrates (grain or pellets) and all supplements from the horse’s diet. Ideally, if the horse responds well, then the suspected allergenic feed is fed again to see if clinical signs return. However, few owners are willing to challenge the horse, especially if it has improved with dietary change. Allergic horses are usually also sensitive to insect bites and inhaled allergens such as mold spores and pollen. Insect control and avoidance are essential.

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