Overview of equine skin diseases

Once the integrity of a horse's skin is compromised, it is not only the site of the irritation that's a problem, but the general health of the horse since the immune barrier has been breached.

Skin diseases in horses are prevalent throughout the year, although some may be seasonal. They may be due to infectious agents (e.g., bacteria, viruses, fungi, parasites) or environmental irritants. Here are some important skin diseases and conditions encountered in horses.

Dermatophytosis

Dermatophytosis, or ringworm infection, usually manifests as a crusting dermatosis with hair loss and circular lesions on the body.

"Sometimes you see lesions that look almost hivelike," says Christine Rees, DVM, DACVD, a veterinary dermatologist who practices at Hillside Veterinary Clinic, Dallas. "Since there are various ringworm fungal genus species, the best method to diagnose the disease is to do a fungal culture, which also helps to determine the potential source," she says.

For example, it's helpful to know if the horse has contracted ringworm found in the ground (Microsporum gypsum). "If so, you can get more aggressive in bleaching or treating the ground or stall floor," says Rees.

Ringworm infection is common in most animals and is zoonotic, so it's easily passed between individuals. If it's the type usually seen in cats, evaluate any barn cats to determine if one has skin lesions, thereby transmitting it to the horses. Ringworm infection doesn't seem to be species-specific—unlike infections caused by lice or other organisms—so it can be indiscriminant.

"Normally, you'll see it in a stressed animal or one whose immune system is slightly compromised; therefore, it's more common in older or younger horses," says Rees.

Moreover, it can be difficult to treat since the organism can linger. So if horses are sharing tack or grooming aids, ringworm can be spread easily among horses. "One has to treat the environment as a source of infection, which is as important as treating the animal," Rees says.

Rees usually treats with topical therapy. Miconazole works slightly better than ketoconazole, although Rees calls the latter a reasonable alternative. (Note: It's been reported that, in small animals, some fungal infections are resistant to ketoconazole.) If the owner doesn't want to shampoo the horse two to three times a week, a lime-sulfur dip can be used, Rees notes. Although it has a pungent, rotten-egg-like odor, it's effective against fungus, bacteria and parasites at a higher concentration.

Oral griseofulvin also may be used to treat ringworm, although Rees says she hasn't seen a pharmacokinetic study for it's use. Terbinafine is another option, but side effects can include increased liver enzyme activities. Occasionally, fluconazole is used.

Staphylococcus aureus infection

These bacteria may cause warm, painful skin; focal crusts are most common in the pastern region, but similar lesions may occur elsewhere on the skin. Methicillin-resistant S. aureus (MRSA) is a concern in horses.

Rees notes that some horses that get recurring Staphylococcus infections seem to be most at risk for contracting MRSA, especially those repeatedly treated with various antibiotics.

And as the bacteria are zoonotic, horses can contract MRSA infections from their human handlers, particularly those who work in the health field and may carry the bacteria to the stall in their nasal passages. Since the bacteria are zoonotic, people working with an affected horse should wear heavy latex gloves.

Horses commonly get opportunistic secondary infections that result in itchy skin disease, especially those animals with allergic skin conditions. "In an allergic individual, we don't think the barrier function of the skin is exactly the same as with a normal individual," says Rees. "When they're having an allergic flare-up, they may be more predisposed to picking up things that are out in the environment than a normal animal would be."

If the infection is recurrent, do a culture to determine what it is. Many laboratories will test a set of panels for several medication options. "Unfortunately, when a horse contracts a resistant infection, it can get a bit pricey as far as antibiotics and repeated treatments, which can make it more difficult for owner compliance," Rees says.

Owners for whom cost is a problem can bathe the horse with a 2% to 3% chlorhexidine or a benzoyl peroxide shampoo, using it more aggressively (e.g., every other day) before trying a more expensive injectable or oral antibiotic, Rees notes.

Allergic reactions

Rees sees many patients with allergies, for example, insect bite hypersensitivity (Photo 1). Atopic dermatitis and environmental allergies to dust, molds and poor-quality hay are most common. Hives are a common allergic response that can be caused by such things as diet, insect bites, dust, mold and medications. Hives can occur anywhere on the body, but most typically on the face, neck, chest and upper legs (Photo 2). They may or may not be itchy. For horses that get hives (urticaria), Rees has found immunotherapy, such as allergy vaccines, to be effective.

Photo 1: A horse with marked insect bite hypersensitivity.

"I do a lot skin testing, though horses are a bit different in that they can have both delayed as well as immediate hypersensitivity reactions," Rees says. "So I usually do readings at 15 minutes and 30 minutes and a delayed-reaction reading at four to six hours afterward."

Photo 2: Generalized hives, a common allergic response, in a horse.

Owners can be advised to minimize an inhalant allergy by reducing exposure and ensuring good-quality, clean bedding is provided and changed often. Wetting the bedding can reduce dust, although clients should be cautious of creating greater mold growth. In a horse with inhalant allergies, it's also important to ensure that the hay is mold-free and that a confined horse has sufficient air exposure.

Bathing horses often to reduce the pollen on skin also is helpful, Rees advises. "Unfortunately, some horses are so allergic—so bad off—that they need antihistamines, allergy shots or corticosteroids such as dexamethasone. In horses, one has to be careful with excessive use of steroids due to the concern for hoof issues and laminitis," Rees says.

Allergic contact dermatitis

A particular type of allergic reaction, allergic contact dermatitis occurs when irritating substances, such as fly sprays, shampoos, liniments or other substances, come into direct contact with the skin of hypersensitive horses. Almost anything used in the barn may be suspect, even substances in saddle pads. Their chemical composition may provide an irritant leading to allergy. If a fly spray is applied, such an offending chemical may become trapped between equipment and sweaty skin, exacerbating the situation. Certain plants may act as contact allergens, affecting the skin of the tongue or muzzle (Photo 3). Signs may include mild redness or flaky, itchy skin, severe hair loss, skin thickening, pain and, occasionally, skin sloughing.

Photo 3: Contact allergy on a horse's tongue.

Culicoides infestation

This genus of biting midges can produce a very itchy insect bite hypersensitivity in horses, says Stephen White, DVM, a professor at the University of California-Davis College of Veterinary Medicine. The Culicoides species from North America is different from those found in other parts of the world. "Certain proteins from the saliva of these insects will induce an allergic response in horses," White says.

Some of the midges are ventral feeders, while others feed along the dorsum. Treatment involves repelling the flies with pyrethrin or similar substances, though there are studies of varying success of these drugs, White says. Fly masks may be helpful. "It seems horses with hypersensitivity to Culicoides also have similar sensitivities to other allergens, such as pollens, dust and mold," he says.

Regarding some of the other fly species (e.g., black flies, deer flies, horse flies), there's debate about whether it's a true hypersensitivity or allergy or just an irritation, since several of these fly species deliver painful bites, says White.

"If the horse is solely allergic to Culicoides, immunotherapy or allergy shots don't seem to work as well, because the allergen isn't derived only from the insect saliva but the entire insect ground within it, making it less effective," says Rees. "Usually steroids or antihistamines are helpful. Some use methylsulfonylmethane, an anti-inflammatory medication, or omega-3 fatty acids."

Pemphigus foliaceus

Because pemphigus foliaceus is an autoimmune disease, "presumably there are antibodies that are directed against some of the substances that keep the epidermal cells attached to each other," says White. "While the target proteins have been pretty well elucidated, we don't really know what it is in the horse." He says they see just one or two cases a year of pemphigus foliaceus at UC Davis.

Pemphigus foliaceus can become severe and affect much of the body. Its acute crusting can be painful. Signs include blistering skin, most commonly affecting areas of the head and lower extremities, although other areas may be involved.

Perform a biopsy in a horse with acute crusts that isn't responding well to other treatments. The biopsy specimen should include a crusted area, White recommends. "Usually pemphigus foliaceus presents as a crusted disease. At first, the horse might have a little edema on the ventrum, or their legs may stock up, or they may have a very mild anemia. Sometimes they'll start out with urticaria, but fairly rapidly it progresses to a very crusty type of disease. It's important, prior to biopsy, to not prep the area since the superficial crust provides the answer."

Pamela Ginn, DVM, DACVP, associate professor at the University of Florida's College of Veterinary Medicine, says she only occasionally sees pemphigus foliaceus in her equine patients. "We see it more often in dogs and cats. We don't really know what triggers it; sometimes it's just spontaneous. Some cases could be triggered by drug therapy, but that hasn't been documented in the horse."

Skin disease of the lower legs and pasterns

Pastern dermatitis can be caused by myriad conditions, says White, "We tend to put a name on it, such as greasy heels or scratches, but it's not one disease. It can be caused by a number of things, such as bacterial or fungal infection, parasite infestation, vasculitis or contact irritants. So those need to be investigated."

Draft horses with a lot of feathering on their legs seem to be particularly susceptible. Signs include inflammation and foul-smelling crusts on the rear half of the pasterns and matted hair.

Skin cancer

Two common forms of skin cancer in horses are squamous cell carcinoma and melanoma. Squamous cell carcinoma is a malignant skin tumor of the outmost layer of the skin. It's typified by reddened, roughened or ulcerated skin.

Melanoma in horses isn't associated with exposure to the sun, but squamous cell carcinoma is, says Ginn. "Squamous cell carcinoma is something we see often here [in Florida] on the eyelids, nose and genital region and in areas of poorly pigmented, lightly haired skin. It's usually preceded by actinic dermatitis characterized by erosions or ulcers. Most clinicians are tuned in to what it might be and often biopsy it. You basically have to protect the horses from sun. We also see lesions of habronemiasis, pythiosis and actinic dermatitis, which precedes squamous cell carcinoma."

In Texas, Rees sees patients with melanoma, but squamous cell carcinoma is more common, especially in lighter-coated horses such as Palominos or grey or white-coated horses. In fact, it tends to be a chronic problem. Squamous cell carcinoma affects horses with greater UVA exposure to intense sun in southern climates such as in Texas, New Mexico, Arizona and Florida.

Some horse breeds, such as Appaloosa, get it at the tail. These tend to be more benign, Rees notes. But some are more aggressive. "Some people will biopsy them to determine their mitotic rate or only remove them if they're located in a bad spot, such as the penis, the rectum or an area affecting a bodily function. They're a little different from those seen in people in that in some horses, especially grey horses, they aren't as aggressive a cancer."

As for treatment, Rees says some veterinarians have tried a melanoma vaccine, or they treat with cimetidine and trimediazine to try to shrink the lesions. "Some claim it helps, though I haven't seen that it does," she says. "The only way to properly treat melanoma is surgically."

Injection site reaction

While uncommon, some horses do show sensitivity to injections, such as from the silicon coating on the needle, and develop a small nodule at the injection site, White says.

Sarcoids

Ginn says she frequently sees vasculitis in horses. "In Florida, we see a lot of sarcoids." Sarcoids, which frequently occur in areas subject to trauma, are associated with bovine papillomavirus. It's speculated that the virus may be spread by biting flies or fomites. Sarcoids manifest as wartlike, ulcerated, nodular areas or flat plaque surfaces, usually on the ears, lips, neck and ventral abdomen, or around the eyes.

Rain rot

Dermatophilosis is most likely to develop after a particularly rainy season on a horse has been left out in the moisture. Also known as rain scald or mud fever, it's a bacterial infection aggravated by prolonged exposure to moisture, coupled with injured skin. Chronically infected animals are usually the source of the infection for other animals; transmission may occur by flies, ticks, grooming equipment and tack. The lower layer of hair is firmly matted in small scabs that leave a raw surface when removed (Photos 4 and 5). The hair roots can protrude from the crust and may have a covering of yellowish-orange pus. The affected areas are sore to the touch but are not pruritic.

Photo 4: A close-up view of dermatophilosis.

Photosensitization

This is an abnormal inflammatory skin reaction to the sun's UV rays that occurs in areas of white hair or pink skin. The condition is different from simple sunburn. It may occur due to ingestion of photodynamic agents (e.g., St. John's wort, buckwheat) from certain plants or chemicals. When the horse consumes specific pigments, they may circulate to the skin where they're exposed to UV light and cause oxidative damage to skin cells.

Photo 5: A cytologic sample of Dermatophilus congolensis, a gram-positive bacterium causes dermatophilosis.

Conclusion

By knowing what may be triggering a horse's skin condition, you can determine the best treatment option to help alleviate discomfort and bring the horse back to optimal health.

Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.