Hives In Horses: What They Are And When To Worry

When your horse greets you with large areas of their body suddenly covered in clusters of bumps, you might wonder if they're experiencing a medical emergency. Urticaria, or hives, are flat, raised areas of skin that tend to form localized clusters. They can vary in size and appearance, and they may or may not itch. Hives are not uncommon in horses and generally not a cause for alarm. Here’s how you can figure out what your horse’s skin is trying to tell you.

Why Do Horses Get Hives?
Hives occur when your horse’s immune system falsely detect a threat, be it a food, plant, or other substance. The body produces histamine, a chemical that signals the body to try to get rid of the allergen. Capillaries dilate and leak fluid, creating bumps close to the surface of the skin. In mild cases, hives appear within minutes of exposure and vanish as quickly as they appeared.
Some cases of hives are sudden and seemingly random, never appearing again. You may never figure out what triggered them. In other cases, though, they may reappear due to repeated exposure to an allergen present in your horse’s diet or environment. Repeated exposures tend to lead to increasingly severe allergic reactions, sometimes resulting in anaphylactic shock. So, if your horse has recurring episodes, you’ll need to work with your veterinarian to figure out the cause.

Identifying Allergens That Cause Hives
If your horse develops hives on a trail ride or immediately after spending time at pasture, the cause might be a plant that they’ve come in contact with. It’s also possible for bug bites to trigger this reaction. The hives are not bug bites, rather, they can be a reaction to a substance in an insect’s saliva.
Take note of any recent changes you’ve made to your horse’s diet, or any new supplements or medications. Antibiotics can cause hives, as well as vaccines. Call your veterinarian if your horse may be having a reaction to their medication or vaccine.
Fly sprays, soaps, and shampoos can also cause hives, as well as detergents used to wash equipment. You might not see them the first time you use them, as the allergic reaction may start to appear after multiple exposures.
New beddings or new tack made up of unfamiliar materials can also cause hives. In many cases, horses can get accustomed to new materials over time.
If you suspect the culprit is a component of your horse’s diet, remove any supplements, grains, and additives, one by one, with a few weeks between each change. As horses have a delicate digestive system, they can develop colic if you suddenly remove everything from their diet besides hay.
Not all hives are caused by an allergen. They can also be triggered by stress, heat, exercise, or excessive pressure from their tack or over-vigorous grooming.

What To Do If Your Horse Has Hives
Hives may go away without treatment and never return, even if you never figure out the cause. If they persist, though, and you’re having trouble ruling out the cause, it’s best to call your veterinarian. Your vet can conduct an intradermal allergy test in which they inject different allergens into the skin on your horse’s neck. You can also try the at-home 5Strands Pet Intolerance Test to check for over 300 potential allergens in your horse’s diet or environment.
It’s usually not necessary or effective to administer an antihistamine. If your horse’s hives are itchy, you can help relieve the discomfort by compressing the affected area with a rag soaked in cool water or diluted apple cider vinegar.
Repeated incidences of hives can escalate to angioedema, which presents as severe swelling that can affect the muzzle or eyelids. Let your veterinarian know if your horse’s episodes seem to be increasing in severity, or if they seem to have trouble breathing.