Dealing With Hives in Horses

Allergens and Allergies – How Hives Form

• Your horse’s immune system protects your horse from disease invaders, but sometimes the messaging gets confused. Horses may have an overreaction by the immune system to something non-infectious, which creates an allergic reaction. The range of reactions varies from mild itching to anaphylaxis, a dangerous systemic response that inflames the entire body, making breathing difficult, among other things. Anything that triggers a reaction is called an allergen.

• The characteristics of hives are a specific cutaneous reaction pattern that can help your vet determine what’s happening in the skin and how to treat it moving forward.

• Urticaria can happen anywhere on a horse, from the nose, eyelids, shoulders, withers, flanks, and even under the tail.

What’s the difference between an antigen and an allergen in horses?

• Antigens are any substance that the body’s immune system recognizes as a foreign protein, virus, bacteria, toxin, or any other invader.

• An allergen is a type of antigen that triggers an allergic response. All allergens are antigens, but not all antigens are allergens.

Hives as an Allergic Response

• The first exposure usually doesn’t happen when your horse first encounters an allergen. It could be days, weeks, or years for a reaction to occur. For urticaria, it starts in the bloodstream and may happen within 15 minutes or longer, sometimes a day later.

• The reaction begins when mast cells in the skin and tissues are triggered. During mast cell degranulation, the mast cells release histamine and other inflammatory mediators like cytokines and leukotrienes. These inflammatory mediators then signal the blood vessels to widen.

• As the blood vessels dilate (called angioedema), the blood vessel walls become leaky. As fluid moves from these small vessels, it collects under the skin, and hives are born. The raised areas that you see are called wheals.

• Hives are usually not itchy, but that’s not always the case, especially with insect bites or plants that sting. Your horse may be itchy without urticaria, and you may notice excessive scratching or mystery inflammation in the skin that doesn’t look like a hive.

• The annoying bumps may have varying diameters, with some looking like dimples. These skin lesions may even open up with scratching, creating sores.

• In some cases of urticaria, the hives in horses you see may not be the only ones. Angioedema can also create problems in the digestive tract.

Atopic dermatitis

• Horses can also have an eczema-like skin condition called atopic dermatitis. This condition is an allergic reaction that includes itching and hives and may also include hair loss and angry red skin. Sometimes, the area will start to ooze.

When do hives go away?

• Most horses quickly recover from hives, although in rare cases, they can interfere with breathing.

• Once a hive starts to appear, it grows for approximately 30 minutes before disappearing after an hour. You may find your horse has waves of urticaria bumps, which appear and disappear all over their body.

• The severity of urticaria indicates how extreme the reaction is. Of course, this varies from horse to horse.

• If your horse is continuously exposed to the allergen, your horse could have hives until you discover and remove the allergen.

• For isolated incidences, most horses recover fully by the next day. More severe cases may continue longer, and any case involving the airway or digestive system needs swift medical attention.

• It’s always a good idea to involve your veterinarian if your horse shows up with hives.

Causes of Urticaria

• Horse hives will return if your horse is continuously exposed to the underlying cause. There are dozens of causes of urticaria, so you’ll spend time ruling out everything your horse eats, touches, rolls in, or is otherwise exposed to. Hives in horses are caused by an allergic reaction, perhaps from one of these potential causes:

Ingestion of these things can trigger an adverse food reaction:

• Pasture grasses
• Hay
• Weeds
• Supplements
• Commercial feeds
• Or anything else they get their little mouths on

Inhalation of these things can create adverse reactions:

• Dust mites
• Mold and mildew
• Hay dust
• Pollen

Topical things that trigger the appearance of hives:

• Insects
• Sunlight
• Bedding
• Fly sprays
• Grooming products like shampoo and sheen products
• Tack soap or leather conditioner
• Laundry detergents

Medicinal triggers

• Medications – oral, IV, IM, any delivery, really
• Stressors and autoimmune conditions weaken the immune system, making a horse more susceptible to developing an immune response
• Vaccinations
• Bacterial or viral infections
• Topical ointments and sprays

Finding the Cause of Horse Hives With Your Vet

• Allergy testing to check for allergies is questionable, and some veterinarians prefer skin tests to check for possible allergens. A blood draw is a starting point for identifying possible triggers.

• Intradermal testing is another option and involves injecting many allergens into the skin individually and observing any reactions.

• You can also figure things out by eliminating possible sources of irritation via elimination testing.

• For elimination testing, strip your horse’s diet down to hay only. Then, wait 4-8 weeks for any allergies to subside. Slowly add one ingredient to the diet, looking for any clinical signs of a reaction to narrow down the allergen. You may have to add one ingredient per month.

• Do elimination testing on everything touching your horse, from saddle pads (check the detergents), fly sprays, grooming potions, and topical medications.

• Some allergic reactions develop over time, so a seemingly routine part of your horse’s life may have reached the tipping point, and now they’re a hot mess of hives.

Treatments for Hives in Horses

• Your veterinarian can ultimately give you a treatment plan, which starts with identifying possible allergens and minimizing exposure to them. This is easy for a fly spray ingredient and more difficult for grass or pollen allergies.

• Steroids like prednisolone or dexamethasone can provide immediate relief but are not suitable for all horses.

• Antihistamines like hydroxyzine help soothe urticaria, too.

• It is possible to use allergy shots to hyposensitize your horse if specific allergens are found.

• Topical itch relief medications or sprays can help, too.

• A cool shower soothes some of the hives, or perhaps your horse wants a little scratch. Horse owners should avoid riding a horse with urticaria. They already feel bad, and tack may irritate the bumps even more.

Frequently Asked Questions

Can you prevent hives in horses?

Hives in horses are often hard to predict, but once you notice possible triggers like insects, specific feeds, or topical grooming products as a cause, you can work to avoid exposing your horse in the future.

Should I call the vet if my horse has hives?

Yes, call your vet if you notice hives. Urticaria bumps are often itchy and uncomfortable and may also be in your horse’s mouth and digestive tract, causing complications. Modern veterinary medicine has many treatment options for fast relief.

Are certain breeds more likely to get hives?

Yes! Thoroughbreds and Arabians are more likely to have sensitive skin, especially when it comes to hives. These breeds are more likely to inherit a condition called atopy, which makes allergic reactions more likely, including reactions creating urticaria.

What bugs cause hives in horses?

Almost any bug can create hives, but biting midges will likely create problems for horses. These tiny gnats are responsible for sweet itch. Other insects, like black flies, wasps, and mosquitoes, can trigger insect bite hypersensitivity and hives.

Don’t ignore these possibly itchy bumps! Give your horse some relief with the help of your vet and work toward finding possible allergens your horse is eating or living around.