Hives are commonly associated with immunologic reactions against allergens a horse has inhaled, ingested, had injected, or that have come in contact with the skin. | Photo Courtesy Dr. Susan White

Learn the causes of and treatment options and prevention strategies for this uncomfortable skin reaction

They might seem benign, those raised welts on your horse’s body. But they can be itchy—even painful under tack and equipment—unsightly, and challenging to manage.

Hives, or urticaria, can be a most frustrating condition for both horse owners and veterinarians. To properly understand how to treat hives, it is important to realize the condition is not a dermatological disease on its own but a cutaneous reaction pattern that is not specifically associated with one etiology, says Jonathan Yardley, DVM, an associate professor in Equine Field Services at The Ohio State University College of Veterinary Medicine, in Columbus. He stresses that hives are a symptom of an underlying problem.

Understanding the Allergic Response

Hives are commonly associated with immunologic reactions against allergens a horse has inhaled, ingested, had injected, or that have come in contact with the skin. The immunologic hypersensitivity reaction leading to urticarial lesions is a type 1 hypersensitivity reaction that occurs because of exposure to an antigen (any substance that triggers the body to prompt an immune response against it). This response to the antigen occurs in two stages: sensitization and elicitation.

During the sensitization stage, the horse’s immune system produces specific antibodies (type E immunoglobulins or IgE) at first exposure to the antigen. This initial response, however, does not yet lead to a symptomatic or allergic response. The horse’s immune system has only been “sensitized” and is now primed and ready to attack upon subsequent exposure to the antigen. It is during the elicitation stage (reexposure to the antigen) that the horse experiences a triggered type 1 hypersensitivity reaction. When this occurs, the allergen-specific IgE antibodies attach to the surface of mast cells (resident cells present in connective tissues throughout the body) and basophils (white blood cells that work with the immune system). Mast cells contain large granules that store inflammatory mediators, and the basophils contain cytokines, which are protein-signaling molecules that help cells communicate during an immune response.

When a horse has an allergic reaction, says Yardley, it is because the sensitized antibodies have come in contact with the antigen, causing the mast cells and basophils to release their contents. The ultimate result of this hypersensitivity reaction is the release of a storm of inflammatory mediators, which can cause blood vessels to become more permeable—thus, allowing plasma to leak out of the vessels and into the surrounding tissues.

Clinically, this inflammatory wave causes the formation of raised bumps or patches on the skin, called edematous (fluid swelling) papules or plaques. If the fluid leaks out of deeper vessels where tissue pressures are lower, more fluid can escape, potentially pressing on surrounding tissues. This type of swelling of the deeper tissues is called angioedema.

Diagnosing Hives

Aja Harvey, DVM, an associate veterinarian and internal medicine specialist at B.W. Furlong & Associates, in Oldwick, New Jersey, says the dermatological challenge is not in diagnosing hives; in most cases diagnosis is straightforward because of the obvious clinical signs. She explains that hives can develop on any part of the horse’s body, although they tend to show up on the neck and shoulder, along the rib cage, and on the buttocks. A typical hive bump can develop in the skin as quickly as 15 minutes to up to 24 hours following exposure to the trigger. Hives differ in their pattern of presentation, ranging from localized or generalized papules or plaques to giant plaques of fluid that can cover the horse’s entire body. Veterinarians commonly note pitting edema (when pressure is applied to the excess fluid under the skin, it forms a long-lasting indentation or “pit”) and blanching of the skin when they apply