Technically called urticaria, hives are soft, distinct, raised skin wheals created when capillaries below the skin’s surface leak serum. Hives begin as small areas of swelling but may grow to merge into larger patches.

Hives can be a reaction to a topical irritant, such as a bug bite, or a systemic response to food or medicine the horse has ingested. Topical irritants tend to cause reactions limited to one area of the body, whereas systemically induced hives appear over the entire body. In addition to the appearance of wheals, hives may also cause itchiness, particularly in the earliest stages of the reaction.

Make sure the horse’s nostrils are clear.

If the hives are the result of a systemic allergic reaction, his airway may swell shut. Look for signs of puffiness around the nose and mouth, listen for strained, raspy inhalation, and watch the horse’s sides to see if he is taking even, deep breaths.

Rinse the affected area.

Many cases of hives are a reaction to something on the surface of the skin (contact dermatitis). If the wheals are localized, douse the affected area with cool water. Or cleanse the area with a mild soap and rinse thoroughly.

Consider possible causes

Hives are an immune response to a topical or systemic allergen. Investigate whether something in your horse’s environment may have led to the reaction. The first exposure to a new allergen won’t trigger a reaction, but the second or even third might. Consider any recent management changes you may have made.

• Have you started using a new fly spray?

• Is the horse on a new medication?

• Are you turning out your horse in a new pasture?

• Is he being fed a new grain or hay?     

Look for other clues as well. If the hives are limited to the saddle area, for instance, the trigger may be the detergent used to wash the saddle pad. Jot down your ideas to share with the veterinarian when she arrives.