Case Study: Diagnosing and Managing a Horse With Allergies

An equine dermatology expert highlights the importance of a multipronged approach to managing equine allergies.
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What you need to know about steroids, antibiotics, and pest management

Krymsun Kid's hives, allergic reaction
Many different allergies caused hives in Krymsun Kid that were so severe he could not be ridden. | Courtesy Dr. Rosanna Marsella

Rosanna Marsella, DVM, Dipl. ACVD, professor of dermatology at the University of Florida, in Gainesville, sees a lot of skin allergy cases.

One case, that of a chestnut Arabian gelding with multiple allergies, highlights the importance of a multipronged approach to managing allergies.

You must limit exposure to the allergen. You need to treat the problem the allergy created, and you must devise a plan to treat the allergy itself—all while considering regional climate, antibiotic resistance, and many competition rules.

Let’s meet Krymsun Kid, a horse for which a nuanced plan proved to be the ticket. The mix involved finding the right antibiotic, the right non-steroidal anti-inflammatory drug (NSAID), the right insect repellent, and custom immunotherapy.

‘A 10 Out of 10 Itchy’

Krymsun Kid, then a 16-year-old, came to Marsella in 2021 with a history of severe allergies. He was “10 out of 10 itchy,” Marsella explains. He couldn’t wear a saddle, and he couldn’t work.

Marsella starts with testing. For Krymsun Kid that included skin cytology, biopsy, culture, and testing. Steroids had stopped working for the gelding. He had an infection and, as it turned out, multiple allergies to pollen and insects.

“One of the manifestations of insect allergy … is that some horses can also—besides being itchy and getting hives—they can also develop solid lesions that we call eosinophilic granulomas (EG),” Marsella says. These lesions are calcium accumulations; you can tell the difference between them and hives by pressing on the bump. A hive will disappear, but an EG won’t. It will feel hard.

allergy skin testing
Skin testing is one way to determine what sensitivities a horse has. | Courtesy Dr. Rosanna Marsella

Most horses get just one or two lesions, but Krymsun Kid “had a ton of them.” Cases involving EG “are very difficult to treat,” Marsella says, because you have to treat the inflammation, minimize the development of new lesions, and make the granulomas go away.

Treatment options for EG include surgical removal or topical steroids. But it’s not enough to remove the lesions.

“I always encourage people not to just do symptomatic treatment with steroids, but to find out exactly what the horse is allergic to,” Marsella says, adding, “If you don’t control the underlying allergy, eventually there is no steroid that is going to save you. And particularly when you have a severe (resulting) bacterial infection.”

In addition to the EG, a biopsy showed the gelding also had vasculitis, which can happen to horses with severe allergies, Marsella explains. In vasculitis, inflammatory cells attack and damage blood vessel walls, leading to tissue swelling.

Marsella prescribed pentoxifylline, an anti-inflammatory and antifibrotic medication that hadn’t been tried on Krymsun Kid previously. Pentoxifylline decreases tumor necrosis factor alpha (TNFα) and interleukin 1, pro-inflammatory chemical messenger proteins. Because it worked, he remains on the drug now.

Secondary Infections and Antibiotic Resistance

An itchy horse isn’t just an allergic horse. It’s also probably an infected horse.

“The itch is multifactorial,” Marsella explains. “Part of it is the allergy, and part of it is the secondary infection that develops as a result of the uncontrolled allergy.”

When people focus on treating the allergy/inflammation with steroids but fail to address the secondary infection, indeed, steroid failure can follow.

“That’s why the very first thing that is done with these cases, including him, is to investigate what kind of infection you have,” Marsella says. “Because inflamed skin always gets a secondary staph infection.”

Horses have sensitive systems, and there are fewer antibiotics available to treat them than other species such as humans or dogs. With antibiotic resistance rising, Marsella chooses to be targeted with her approach.

“Rather than saying, ‘Okay, well, this one did not work, let me pick another one,’ I do a culture to figure out exactly what is available (that will be effective for treating this pathogen),” she says.

Culture results take a few days to come back so, in the meantime, Marsella prescribes a topical therapy and conducts a skin test exposing the horse to different allergens. Those results are faster; about 15 minutes, she says, and dictate the immunotherapy she prescribes.

In addition to the antibiotic, Marsella sent Krymsun Kid home with an essential-fatty-acid shampoo to soothe his skin and instructed his owner to add the corticosteroid dexamethasone to his fly spray.

“I found over the years that it is effective and cheap to spike fly spray with a little bit of dexamethasone … to kind of give him a break” while waiting for immunotherapy to take hold and the infection to subside, Marsella says.

Designing the Right Immunotherapy

Krymsun Kid’s report showed dozens of allergies, including various grasses, trees, weeds, and insects.

Designing his immunotherapy meant considering his history. Which seasons were the worst for him? Marsella says that matters because the vaccine can’t have more than 14 ingredients. She might choose to vaccinate against a seemingly milder allergy if the allergen is active year-round.

Once Marsella customizes the vaccine, the owner injects the doses subcutaneously (just under the skin) on a schedule that begins with frequent, low-concentration doses and moves to higher-concentration doses every few weeks.

“Most horses are maintained once every three weeks,” she says. “But sometimes it’s necessary to do it every two weeks if they’re having a flare or in the worst of the season.”

Krymsun Kid’s case took “a little bit of adjustment and frequent communication to figure out the best schedule for him, which is not cookie-cutter,” Marsella says, adding that it’s normal to stay in frequent communication with her allergy clients.

Over time the horse’s sensitivities could change, and the vaccine recipe could evolve.

A custom drug might sound expensive, but Marsella estimates the cost at about $400 for the year, though the initial testing and first appointment can easily be $1,000.

In addition to pursuing immunotherapy, you can help horses’ pollen allergies by making informed choices about hay.

Some horses, like Krymsun Kid, are allergic to common forage grasses. When that’s the case, your first choice is to feed them a hay to which they aren’t allergic. If there isn’t one, then you pick a grass to which the horse’s allergy isn’t as severe and desensitize him to it using immunotherapy.

“His situation is, sadly, not rare,” she says.

Krymsun Kid after treatment
With different treatments specific to Krymsum Kid’s allergies, his skin healed well and he was comfortable enough to go back to under saddle work. | Courtesy Dr. Rosanna Marsella

Insect Repellent and Avoidance

Unlike pollen, you can’t build a horse’s tolerance to insect bites through immunotherapy. You need to prevent him from being bitten, Marsella says.

“The type of fly control that you need to have on allergic horses, particularly in a place like Florida, is completely different from what will be sufficient for a regular horse,” she explains.

Whether you choose a chemical (such as permethrin) or natural (such as neem oil) fly repellent or insecticide is up to you. As for botanical ingredients, Marsella doesn’t recommend citronella or garlic, saying they haven’t worked for her.

Be prepared to apply fly spray often—more than the bottle suggests, she says.

“Products that claim to kill and repel for up to 14 days? That is not true. Certainly not true in Florida,” Marsella says. Regular rain and sweat mean more frequent applications. Consider applying repellents or insecticides at dawn and dusk when mosquitoes are the most active.

Besides fly spray, horse owners can also consider environmental or property management changes to avoid bites. You can move horses to a paddock farther away from standing water; Culicoides (biting midges) don’t travel far and are also weak flyers.

You could add fans to stalls to deter insects from landing on horses, and install traps around the farm to draw mosquitoes and other insects away from horses. Avoid turning horses out during the most active times for insects and use gear such as fly masks, sheets, and wraps to protect them when they are outside.

Take-Home Message

In Krymsun Kid’s case, once his treatment was determined and administered, his skin healed well, and he was comfortable enough to return to ridden work. With an itchy horse, have testing done by a vet. Consider a potential secondary infection, control the itch, and implement insect control strategies. It takes a multipronged approach to help your sensitive horse.


Written by:

Karen Hopper Usher has a Master’s degree in journalism from Michigan State University, where she reported for Great Lakes Echo. She previously worked in local news and is a lifelong equestrian.

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