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 (see Skin Disorders of Horses: Ringworm (Dermatophytosis) in Horses), and pemphigus foliaceus.
Hives appear within a few minutes or hours of exposure to the causative agent. They are elevated, 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.
The usual treatment for hives is antihistamines; however, antihistamines are often ineffective and unnecessary. 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. 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. Topical medication may be prescribed to control itching and reduce the chance of further skin damage.
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.
Last full review/revision July 2011 by Karen A. Moriello, DVM, DACVD; Carol S. Foil, DVM, MS, DACVD; John E. Lloyd, BS, PhD; Bertrand J. Losson, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice Villalobos, DVM, DPNAP; Patricia D. White, DVM, MS, DACVD; Thomas R. Klei, PhD; David Stiller, MS, PhD; Stephen D. White, DVM, DACVD