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Hives, also known as urticaria, are fluid-filled, raised swellings or “wheals” on a horse's skin. These bumps or plaque-like eruptions are generally round in shape and flat-topped, ranging from ½-inch in diameter to as large as 8 inches wide. They may or may not be itchy, appear minutes to hours to days after exposure, are often seasonal in nature, and can develop anywhere on a horse’s body, although the most common sites are the neck, back, flank, and legs. A one-time occurrence that goes away on its own may not be a reason to pursue cause or treatment. However, hives that won’t go away, that go away and come back (recurrent urticaria), or that lead to oozing and crusting, secondary bacterial infections, or extreme discomfort to the point of anxiety, excitement, and restlessness should be investigated and treated.
A horse with hives can be made more comfortable with medications like steroids and antihistamines, supplements like omega 3 fatty acids and MSM, and skin care like cool rinses. All attempts should be made to remove or reduce exposure to as many identified allergens as possible. For example, if insects have been found to be the culprit, then fly sheets, boots, and masks can be added to the horse; fans, mesh curtains, and a timed spray system can be added to the barn; and measures to limit the insect population like parasitic wasps, feed-through fly control, manure removal, and control of standing water can all be used.
If the horse still has hives – or the hives are due to one or more allergens that cannot be removed from the horse’s environment like tree or grass pollens or the Culicoides biting midge that causes “sweet itch” – then it may be possible for the veterinary specialist to develop allergen specific immunotherapy or “allergy shots” or the more recent “allergy drops" which are placed in the mouth.