Hives (urticaria) are groups of itchy eruptions of localized swelling in the dermis. They often develop and disappear suddenly. The most common causes of hives in horses are insect bites or stings, medications, and exposure to allergens. Other potential causes include vasculitis (inflammation of the blood vessels of the skin), food allergy, ringworm Ringworm (Dermatophytosis) in Horses Ringworm is an infection of the skin or hair caused by a type of fungus. In horses, Trichophyton equinum and Trichophyton mentagrophytes are the primary causes of ringworm, although... read more , and pemphigus foliaceus.
Hives appear within a few minutes or hours of exposure to the causative agent. They are raised, round, flat-topped, and 0.5 to 8 inches (1 to 20 centimeters) in diameter; they may be slightly depressed in the center. Hives can develop on any part of the body but occur mainly on the back, flanks, neck, eyelids, and legs. In advanced cases, they may be found on the mucous membranes of the mouth, nose, eyes, rectum, and vagina. In severe cases, the skin eruptions are preceded by fever, poor appetite, or dullness. Horses often become excited and restless.
In most cases, the hives disappear as rapidly as they arise, often within a few hours. Hives are very seldom harmful to the horse. Fatalities are even rarer. Antihistamines are of questionable value. If hives are chronic, allergens in an environment should be considered potential causes, and steps taken to prevent exposure to the allergen, if possible. The hives promptly disappear but return rapidly if the allergen is not eliminated. Usually, local treatment of the lesions is not necessary.
Sensitive animals, particularly purebred horses, also may exhibit dermographism, a phenomenon in which rubbing or whipping produces hive-like inflammations. These are of no medical significance.