Diet-Induced Allergy
Occasionally, allergies to a specific protein may occur. This may result in the rapid development of round elevated areas on the skin surface over all or small portions of the body. These are referred to as urticaria, wheals, plaques, or protein bumps. They may or may not itch, and they may persist for a few hours to several days, disappearing as rapidly as they developed. Numerous factors other than dietary protein may be responsible, including insect bites, inhalation of chemicals or pollens, internal parasites, vaccines, and reactions following administration of drugs such as penicillins, streptomycin, and tetracyclines. When caused by the diet, the allergy is due to a specific protein in the diet, not to the amount of protein in the diet, and is frequently associated with itching. Severe tail rubbing may occur. Feeds reported to most frequently cause allergic reactions in horses include potatoes and their by-products, distillery wastes, beet pulp, buckwheat, clover, St. John’s wort, wheat, oats, barley, bran, tonic, and chicory. Corticosteroid administration usually results in recovery within 24 hours.
Less commonly, an allergy to an ingested protein, such as clover pasture in bloom or fish meals, may result in the sudden occurrence of subcutaneous edema, which gravitates ventrally. The head is the most commonly affected site. Eyelids may be edematous, the third eyelid may swell and protrude, and profuse lacrimation may occur. Although the condition may be caused by a feed allergy or snakebite, drugs are most often implicated. Spontaneous recovery is common. Treatment consists of emptying the gastrointestinal tract to prevent further absorption of an ingested allergen, and administering corticosteroids, antihistamines, and epinephrine. If the condition is believed to be due to the diet, feed forage only. If it recurs following reintroduction of a particular feed, that feed most likely contains the protein causing the allergy.
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