Karla S. Rugh, DVM, Ph.D.
What it is: A skin manifestation of an allergic reaction to various triggers, including insect stings/bites, chemicals, medications or, less commonly, ingested or inhaled substances. In some horses, hives develop or worsen with pressure, heat, sunlight or stress. The exact causative factor is often a mystery.
Symptoms: Multiple round, raised areas on the skin (1/2 inch to 8 inches in diameter), usually with flattened tops and often occurring on the back, flanks, abdomen, neck and chest; variable itchiness; and respiratory difficulty (rare). Hives usually appear and disappear suddenly.
What to do: Hives are not usually an emergency, but you should call your vet immediately if your horse has difficulty breathing or has severe itching (rare). If you know what caused the reaction, eliminate it from your horse’s environment, if possible.
Outlook: Hives will recur if the horse is re-exposed to the allergen that caused them, so allergen avoidance will prevent further episodes. Unfortunately, it’s not always easy to determine the triggering allergen, so most cases of hives are treated symptomatically.
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This article originally appeared in the 2012 issue of Horses USA. Click here to purchase the most recent issue.