You’ve just gone out to the barn or pasture, only to discover that your horse has developed small, raised bumps along his neck and shoulders. What is it and what caused this to happen? 

The answer: Most likely, your horse has developed hives due to an allergic reaction. But determining the source of that allergic reaction isn’t always easy.

The good news is that hives aren’t typically an emergency situation. However, they are something to take seriously, especially if they happen more than once. Unfortunately, some horses are more prone to developing hives than others due to the state of their immune system. 

What are Allergies?

Allergies occur when the immune system reacts to a foreign substance, such as pollen, certain foods, or chemicals in products. All animals can develop allergies, and horses are no exception. 

The main function of the immune system is to identify foreign substances and attack those that can cause harm. An allergic reaction is a “false alarm” of sorts; in other words, it occurs when the immune system deems something harmless as a threat and mounts a response as a result. 

To explain further, the immune system produces substances called antibodies. When allergies are present, the immune system makes antibodies in response to a substance (allergen), even though that substance may not actually be harmful. Then, when the animal (or person) comes in contact with that allergen, the immune system’s response is to produce inflammation--either in the skin, sinuses, airways, or digestive system. 

The severity of allergies can vary from individual to individual and can also range from minor irritation to anaphylaxis, a potentially life-threatening emergency. 

What are Hives?

Hives, technically known as urticaria, are an outbreak of bumps or welts on the skin that appear suddenly and in response to an allergen. When your horse is exposed to something the body sees as an allergen, it binds the allergen to an antibody known as IgE that circulates in the bloodstream. The IgE, which is now on high alert, then binds to several types of cells, including mast cells in tissues and eosinophils in the bloodstream. These cells react by leaking chemicals, including histamine, that then cause small blood vessels to dilate and leak fluid into surrounding tissues. This fluid accumulation is what makes a “hive”.

It’s important to know that hives are not a disease, in and of themselves, but rather a symptom of an allergic response. As mentioned before, hives are rarely harmful to a horse and fatalities are even rarer. However, if the condition becomes chronic, allergens in the environment should be carefully evaluated and steps should be taken in order to prevent exposure to the allergen if at all possible. 

Causes of Hives

Hives in horses can develop in response to a variety of triggers, including insect stings, certain foods or ingredients, medications, vaccines, or from chemicals in fly sprays or even shampoos. They can also develop due to contact with bedding, tack, or blankets. In rare instances, physical stressors such as cold, heat, or even sunlight may induce hives. 

However, it’s important to know that hives may not necessarily be caused by one thing. Instead, allergies tend to have a cumulative effect, stressing your horse’s immune system over time. If the immune system becomes compromised enough, a horse may finally reach a threshold where symptoms appear. This means that a horse who breaks out in hives after being sprayed by a new fly spray may have just reached that threshold. 

Hypersensitivity reactions can take weeks, months, or even years to develop, so a sudden onset of hives isn’t necessarily the result of a recent change. Because of this, it can be difficult to pin down the actual source of the problem. In some cases, there appears to be a genetic component to hives, with Arabians and Thoroughbreds reportedly at higher risk. 

With that said, the following things have been identified as triggering hives in horses:

  • Antibiotics
  • Antiparasitic drugs
  • Hormones
  • Vaccines
  • Foods and plants
  • Hay
  • Supplements
  • Bacteria
  • Fungus
  • Parasitic infection
  • Insect bites

It should also be noted that, though they are both allergic reactions,  hives and the condition commonly known as sweet itch are two different things. 

Clinical Signs of Hives (Urticaria)

In most cases, hives appear suddenly and disappear within 24-72 hours. They may appear anywhere from a few minutes to several hours after exposure to the causative agent, or in some cases may not occur until a couple of days after exposure. 

Hives appear as raised, round, flat-topped bumps that may vary in size from 0.5 to 8 inches. They may start out as a few small lumps but then multiply and grow into larger welts. Most hive lesions are cool to the touch and may form a pit or “dimple” if finger pressure is applied to them. Occasionally, hives can be itchy, but in most cases, they aren’t. 

Hives can appear anywhere on the horse’s body, but most commonly occur along the neck and shoulders, on the eyelids, along the thorax, or on the buttocks. The swelling that often accompanies hives is known as angioedema. In advanced cases of hives, they may appear on the mucous membranes of the mouth, nose, eyes, or genital areas, and in severe cases, they may be preceded by fever, poor appetite, or dullness. 

What to do if your Horse Develops Hives

If your horse breaks out in hives more than once or if the hives don’t go away within a few days, a visit with your veterinarian is warranted. Your vet can also rule out other allergic responses from something like skin parasites or a bacterial or fungal infection of the skin. 

You should never ride or exercise your horse if he’s currently experiencing hives, as sweat or rubbing from the saddle or tack may make the problem worse. 

Identifying the Triggering Allergen(s)

Though it can be difficult, the first thing you’ll want to do if your horse develops hives, especially on a recurring basis, is to eliminate any new feeds, supplements, or topical products. In some cases, this may mean eliminating everything but hay or pasture. Also consider any recent treatments such as dewormers or vaccinations or even stressors that may have occurred for your horse. Once these items have been removed from your horse’s diet or environment, you can slowly add them back in and watch for any type of reaction.

If eliminating possible allergens doesn’t work, you may want to move on to intradermal allergy testing. Though it can be expensive and time-consuming, this type of testing may be able to isolate the allergic source such as pollen from plants, bushes, or trees on your property, molds, grasses, weeds, dust mites, certain insects, etc. 

Working to identify possible allergens for your chronic allergy horse is important since repeated exposure can increase his risk for the rare but potentially fatal allergic reaction called anaphylaxis. This is a systemic reaction which leads to a sudden drop in blood pressure, difficulty breathing, and possibly, causing the horse to go into shock. If anaphylaxis occurs, the horse’s only hope is getting a dose of epinephrine quickly. 

Remedies for Hives in Horses

Chronic hives are a sign that your horse’s immune system has become over-stressed. Though many people want an immediate treatment for hives, these aren’t always successful and act more as a band-aid to the problem. Treating your horse with a steroid such as dexamethasone can even be risky and lead to other problems such as laminitis. Allergy shots can be helpful for chronic hives cases, but removing possible triggers and managing your horse with nutrition may be your best bet. 

Eliminating sweet and high-protein feeds can be helpful for horses with chronic hives, as these types of feeds can contribute to inflammation. You may also want to eliminate corn or vegetable oils (high in pro-inflammatory omega-6) if those are being fed. 

Adding in certain immune-supportive nutrients may also be helpful for horses prone to allergic reactions, including hives. Some of these include:

  • Flax, hemp, or chia seeds which are high in omega-3;
  • Spirulina (blue green algae), a “super food” high in vitamins, minerals, amino acids, and omegas;
  • Vitamin C, an antioxidant which supports the immune system;
  • Vitamin E, another antioxidant which is especially important to supplement to horses without access to pasture;
  • Chondroitin sulfate, which helps to keep inflammation under control;
  • Magnesium, which is deficient in many equine diets and supports cellular regeneration and repair, among other things; and
  • Trace minerals, also deficient in some equine diets and which help to support immune function.

It can certainly be disarming to discover that your horse has broken out in hives, but understanding why this reaction occurred and working to get to the root of the issue, especially in chronic cases will lead to the best results, not to mention a healthier and happier horse! 

Sources

Allergies (Mayo Clinic) 

Hives in Horses

Hives (Urticaria) in Horses

Myths and Facts about Horse Hives

Managing Equine Allergies

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