What’s causing the hives, hair loss, and scabs on your horse’s body?

One particularly wet spring in Western Pennsylvania, I was grooming my beloved childhood gelding when I felt a series of small bumps along the top of his rump. Being too short to see above 15.2 hands, I fetched a mounting block for a closer look. Sure enough, matted tufts of hair were sticking up like a formation of small, scabby soldiers amidst the partially shedded winter hair on Risco’s rump. As a 10-year-old who cared far more about my horse’s appearance than my own, I took the scabs personally and started furiously picking away at them until they littered the barn aisle and my gelding’s rump was embarrassingly bald.

Luckily (for both of us), my gelding’s first-ever case of rain rot was mild, and my furious eradication of the offending scabs was actually part of the treatment our veterinarian later suggested (along with antimicrobial baths, regular grooming without sharing tools or equipment, and direction to keep Risco in during heavy rain). No harm done. But I’ve learned that it pays to know a bit more about the many lumps, bumps, rashes, and infections that horses are prone to before going on the attack.  

April Showers Bring …

Rain rot (or rain scald) is a bacterial infection also known as dermatophilosis. Anything that compromises the skin’s natural barrier (such as insect bites), particularly in the presence of moisture, can trigger the causative bacterium, Dermatophilus congolensis, to spring into action, spreading the infection throughout the horse’s skin and resulting in the scabbed-over lesions I picked off with such vigor. As its name implies, rain rot is often triggered by wet weather, but high humidity, high temperatures, or biting insects can bring it on, as well. For Risco, a free-range horse with constant access to shelter but little inclination to take advantage of it, springtime showers were the obvious culprit.

While my family’s horses preferred standing out in the rain to coming into the barn, they were mercifully spared from one affliction that plagued horses at a barn down the road from us: scratches, or “greasy heel.” Like clockwork, come March or April, my neighbors’ horses would develop scabby, inflamed heels.

“Scratches isn’t a specific disease; it’s simply the term we use for an inflammation at the end of the leg,” says Stephen White, DVM, Dipl. ACVD, chief of service, Dermatology, at the University of California, Davis, William R. Pritchard Veterinary Medical Teaching Hospital.

“It can be caused by bacteria, dermatophytes (fungi also known as ringworm), chorioptic mange (caused by mites), a viral infection, or a contact reaction. Calling your vet and telling them your horse has scratches is a bit like calling your dermatologist and saying you have a rash on your palm. It’s not a diagnosis or a disease, but it will make your dermatologist start thinking about all of the possible things that might have caused that -reaction.”

So, while the effect might be similar for most horses—red, inflamed heels that look crusty or scabbed-over—the cause varies widely, as does the treatment. Wet, muddy conditions make horses more vulnerable to a bacterial or fungal invasion, but inflammation might just as easily be triggered by biting flies, mites, or leaving the hair on your horse’s lower legs too long, causing it to retain enough moisture to prompt scratcheslike inflammation.

May Flowers Bring …

Allergies. Like people, horses are susceptible to a range of seasonal allergies. And ironically, being a horse provides no guarantee against hay or grass allergies.

“Typically, (affected) horses will exhibit pruritic dermatitis (an itchy inflammation of the skin) or hives, together or separately,” White says. “Horses that are primarily inside during the winter can also have a similar reaction to various grain mites or barn dust.”

Horses can develop both respiratory and skin diseases from allergens. As any allergy-prone person can tell you, spring is typically the season when allergies are at their worst, thanks to the thousands of grasses, flowers, and trees bursting into bloom. The same is true for horses.

Most horses with chronic skin allergies end up in the care of an animal dermatologist, like Rose Miller, DVM, Dipl. ACVD, who is affiliated with the Gilbert, Arizona-based practice Dermatology for Animals. While your usual veterinarian will likely have a good handle on most kinds of common skin gunk, board-certified dermatologists are required to do an additional three-year residency specific to dermatologic issues and are best qualified to help you develop a plan for allergy testing and treatment.

In Miller’s experience, most of the animals she sees are “almost universally itchy,” she says, “with the exception of the occasional autoimmune-related issue. Horses are often allergic to similar things as people—grasses, trees, weeds, dust. Mites and insect hypersensitivity can be an issue, too.”

When Miller examines a horse she believes might be suffering from allergies, she has the option of conducting a serologic (blood) test and/or an intradermal allergen skin test (IDST). Veterinarians frequently use serology to determine what environmental allergens might be at play, but intradermal testing is typically where most veterinary dermatologists begin their investigations.

“For an intradermal allergen test, we start by injecting about 70 to 75 different allergens into the skin of the horse’s neck, using a positive and negative control to test their reaction,” Miller says. “Once you’ve determined what a horse is allergic to, you can start formulating a desensitization plan.”

Miller will contact the lab and request a vial containing a concentrated dose of those allergens. She’ll then formulate a series of increasingly higher doses of the allergen to be administered in a shot, which might cost around $40, on a weekly or monthly basis to desensitize the horse.  

“You’ve got to teach the horse’s body that the allergens are not actually a problem,” she says. “The amount of time that takes varies from horse to horse. We generally warn our clients that it may be a lifelong commitment.”   

Burning Sun and Biting Insects

A few months after Risco’s first run-in with rain rot, I brought him into the barn sporting a bright-pink nose that was peeling around his nostrils. A quick call to the veterinarian confirmed that Risco was suffering from a nasty sunburn, a common enough occurrence for horses with light-colored coats or on any horse with a broad blaze or snip like Risco’s. Before advising that I apply SPF regularly to my horse’s nose, though, my veterinarian ran through a series of questions that I later realized were to check for photosensitization, which can have an appearance similar to sunburn.

“Photosensitization is caused when horses come into contact or ingest certain plants that are activated by light to cause a chemical reaction in the skin,” says Toby Pinn, DVM, an associate at Vermont Large Animal Clinic, just downt he road from me in Milton. “It can cause what looks like a horrible chemical burn, mostly in nonpigmented areas.”

While it’s worth checking your pastures for the plants that can cause contact photosensitivity, such as red and alsike clover, St. John’s Wort, and wild carrot, your horse is far more likely to suffer the effects of a typical summer insect invasion or common sunburn. Depending on where you live, your horses can encounter everything from deerflies and no-see-ums to black flies and horse flies, as well as mosquitoes and barn flies. Fly masks, lightweight fly sheets, and fly sprays can all help, but as White notes, “most fly repellents don’t last nearly as long as the label claims. Most are only good for about 48 hours, if that, and always be sure to follow the directions. You have to be sure it’s diluted properly and that you’re applying it as directed.” And sometimes it’s just easier to bring a horse in from the field when bugs are at their worst.

The same goes for sun protection. Because horses are also prone to squamous cell carcinoma (skin cancer), it’s best to bring them in during the midday hours. At the least, slather sunscreen on burn-prone areas as frequently as you’d put it on yourself during a trip to the beach.

Creepy Crawlies and Ringworm  

Flying insects aren’t the only bugs that can cause your horse to suffer. Ticks, mites, fleas, and lice can all take up residence in manes, tails, the hair coat on the belly and ears, or in the fetlock feathers, causing irritation, hives, or bumps as they feed on their equine hosts. If your horse exhibits any of these signs, ask your veterinarian to take and test a skin sample so you know what you’re dealing with.

Your veterinarian will follow a similar procedure if you spot the signature circle of ringworm on your horse’s coat. While ringworm—an infection of the skin or hair caused by a type of fungus—isn’t common here in Vermont, or in California where White practices, a raging case of ringworm once spread through the Pennsylvania barn where I occasionally boarded my horse during the winter. It wasn’t picky; it spread from the horses to the barn cats to my friend, who was unlucky enough to pick up one of the cute (and infected) kittens, and all affected species developed the scaly patches that typify the infection. It didn’t help that horses shared grooming tools and saddle blankets, which causes the infection to spread quickly, but luckily for all, using medicated shampoos and dedicated grooming tools helped curb the infection.  

Take-Home Message

“Skin issues are generally pretty equal-opportunity,” says Pinn. For that reason, it’s important to stay on top of any changes you see in your horse’s skin and to alert your veterinarian if any areas become inflamed or infected. “Sometimes we’ll see a (Staphylococcus) infection coming secondarily after a case of allergies or biting flies,” White says. “Anything that diminishes the natural barrier of the skin can lead to an infection, particularly if the horse is in poor health.”

So while Risco was in excellent health when he showed up with rain rot, I’ve learned my lesson: My horse’s skin is his largest organ, and it deserves as much attention and care as the rest of him. So next time I’ll ask questions first, and only attack once I know my enemy.