Description

Hives are groups of itchy eruptions of localized swelling in the skin of the horse. They often develop and disappear suddenly. They are elevated, round, flat-topped, and any where from 0.5 to 8 inches in diameter and may be slightly depressed in the center. Patches of hair where the hives occur on the horse's body usually stick out.

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.

Hives appear within a few minutes or hours of exposure to the causative agent. In some horses, the hives do not appear to itch.

Sensitive animals, particularly purebred horses, also may exhibit dermographism, a condition in which rubbing or touching the skin of the horse with tack or a whip produces hive-like inflammations. In these cases, the hives are of no medical significance.

Symptoms

• Localized swelling
• Itchy eruptions
• Fever
• Poor appetite
• Dullness
• Restlessness and excitability

Causes

The most common causes of hives in horses are exposure to allergens, insect bites or stings, medications, and sometime, vaccines. Other potential causes include inflammation of the blood vessels of the skin, food allergy, and ringworm. Vaccinations and injected drugs also sometimes cause hives in horses.

Episodes of hives that quickly come and go are usually caused by inhaled allergens such as pollens of various trees and plants, dust, rust, molds, and feathers.

Prevention

In cases of recurrent hives, it is important to identify the allergens that cause the hives and eliminate them from the horse's environment. It is often a challenge to identify the specific antigens to which the horse is allergic, so a detailed patient history of the horse, an assessment of the horse's environment, and a comprehensive clinical examination is usually needed to help in identifying the causative allergens.

Intra dermal skin testing and ELISA blood tests are sometimes necessary to help the veterinarian identify the different allergens causing the hives. Because avoiding airborne pollens is usually not practical, desensitizing injections may be effective.

When hives are caused by a food allergy, the horse will need to be taken off all sweet feeds and supplements. If this does not prevent recurrence, switching to hay pellets or alfalfa hay may be helpful.

Treatment

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.