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. Rather it is 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. The basophils contain cytokines, which are protein-signaling molecules that help cells communicate during an immune response.

A Horse’s Allergic Reaction

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. This 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 pressure.

Determining the Cause of Hives

When trying to identify the underlying cause(s) of hives, both Yardley and Harvey acknowledge the triggers are many. They include insect hypersensitivity (e.g., flies, gnats), atopic dermatitis (e.g., pollens, dust, molds), food reactions (e.g., supplements, hays, grains, pasture grasses), drug reactions (e.g., antibiotics, dewormers, anti-inflammatory drugs, narcotics), vaccine reactions, and various chemical contact reactions (e.g., detergents, plants, insecticides, soaps, bedding).

Physical reactions can also cause hives. These are associated with variations in temperature, such as placing a cold compress on a region of the body, exercise, or even bringing the horse into a cool environment following sun exposure. Harvey says infectious (bacterial, viral, fungal, or protozoal) and parasitic disorders have also been reported to cause hives in horses. Even stress can cause hives, adds Yardley, when horses move to a new environment, have a close stablemate removed, or attend an event or activity. Some cases of hives are simply idiopathic, meaning their cause is unknown.

How To Manage Hives

The many causes of equine hives reflect the condition’s complex pathology. So what can we do to treat it? The real aim when treating hives is to remove the cause. The challenge, however, is we often don’t know the cause or don’t identify it until weeks later.

With mild acute (seen within 24 hours of development of the first sign) hives, Yardley recommends simply observing the horse. Individual lesions should disappear within 24 to 48 hours after the trigger event. In the case of severe facial swelling around the eyes, lips, and/or muzzle, Harvey recommends veterinarians administer a single injection of a short-acting glucocorticoid (i.e., dexamethasone). This will usually resolve or improve the appearance of the hives. (Note that glucocorticoid administration might be contraindicated in horses at risk of laminitis.) The veterinarian might also prescribe systemic antihistamines and diuretics, which are typically effective within hours of administration. Cetirizine, more commonly known as Zyrtec, is a drug Harvey says she often uses. It is effective at resolving and preventing new hives from forming and is safe and easy for owners to purchase over the counter.

Harvey says the allergy drug oclacitinib (Apoquel) has come into favor recently. Many equine clinicians report it to be an effective treatment for hives and pruritus (itching) in horses that do not seem to be responding to standard glucocorticoid or antihistamine treatment. Although not labeled for use in horses (it is commonly used in dogs), equine veterinarians can prescribe it in tablet form. Harvey cautions that the drug can be expensive but is another treatment option for severely affected horses.

Persistent Hives in Horses

In the case of persistent (chronic) or recurrent hives, vets typically prescribe long-term oral glucocorticoids (prednisolone), with a gradual reduction in dose. Identifying the offending trigger, however, is important to avoid continued recurrences, says Yardley. This might involve investigating the horse’s management and any recent changes in feed, bedding, medications, or stressors.

Allergy Testing and Immunomodulatory Therapy

Diagnostic testing can help identify secondary problems and rule out other dermatological conditions, says Yardley. If the suspected trigger is an allergic source, intradermal allergy testing (IDAT; which measures cutaneous allergen-specific IgE) is the most effective way to identify allergens, says Harvey. Compared to serum allergy testing (which measures circulating allergen-specific IgE), IDAT is more accurate and provides consistent results. Harvey says the testing process involves clipping a rectangle area on the neck, injecting allergens into the skin, then measuring the size of the response compared to both a positive control (histamine) and a negative control (saline).

The veterinarian uses these results and information about the horse’s environment and exposure to formulate immunotherapy to desensitize the horse to these allergens over time. Ideally, once you’ve identified the offending allergens, removing or avoiding them is the best therapeutic approach. However, this option can be difficult to accomplish because of the horse’s environment or if the horse is already sensitized to multiple antigens.

Harvey and Yardley agree performing IDAT isn’t cheap and costs vary depending on location and whether the veterinarian’s a specialist. Costs range from $900 to more than $1,200 for the initial consultation and testing. This testing is most commonly performed by veterinary dermatologists, a small specialty that’s not easily accessible in every region.

Desensitizing the Horse’s Immune System

While immunotherapy (sometimes called an allergy “vaccine” or shot) can be expensive upfront, it’s the only therapy aimed at treating the allergy itself rather than controlling the clinical signs. These injections are long-term treatment alternatives that expose the immune system to the selected allergens in a gradual and controlled way. Ideally, the ultimate goal is to desensitize the horse’s immune system to the allergen throughout the series of shots, so it no longer reacts when exposed to it.

Harvey says the initial induction protocol most commonly consists of subcutaneous injections administered frequently at first and at a certain frequency for maintenance once the horse is reliably tolerating the maintenance volume and concentration of immunotherapy. The horse might still need accompanying symptomatic treatment, especially during the first few months of starting this therapy. Maintenance injections—which the owner can administer with proper training—typically continue for one year, then the veterinarian adjusts recommendations based on the horse’s response to therapy, which is noted by a reduction in frequency and severity of hives and any associated pruritus.

Preventing Hives Recurrence

Because preventing hives in horses depends on identifying and permanently removing or avoiding underlying causes or triggers, true prevention might be impossible. You can, however, take measures to reduce the chance of hives recurring once you’ve identified certain allergens. These might include:

• Insect allergies Minimize the horse’s exposure to insects by using insect sprays, fly sheets, fly masks, and strong fans in stalls at times when insects are most active (dusk and dawn). Eliminate bodies of standing water where insects breed; and move horses away from the standing water if that isn’t possible.
• Food allergies Remove supplements or treats, use one-grain feeds, use only one type of hay instead of a mixed hay (for example, a grass hay containing only timothy or a legume hay containing only alfalfa). It might take upward of six weeks to determine if a change in the horse’s diet and associated food allergy triggers hives.
• Environmental allergies to dust and mold Clean and disinfect equine living quarters. Provide ventilation in and around the area where the horse lives. Use low-dust bedding; and steam hay.

Take-Home Message

It’s important to understand that hives is a condition that requires ongoing ­management rather than a one-time cure. Have realistic expectations when managing a hivey horse, so you don’t become frustrated and discouraged if the condition persists despite treatment. A key part of treating hives is identifying and managing the triggering cause. If you can’t identify a cause, symptomatic treatment might be your only option.