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Next > 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. |
| Brush up on a dermatologic diagnosis: Make sure you can identify and treat common skin diseases in your patients and help them get back that healthy coat. John Slater/Getty Images
Dermatophytosis “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. |
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Simulated methicillin-resistant Staphylococcus aureus (MRSA) bacteria. Photo Science Library/Getty Images Staphylococcus aureus infection 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. |
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Allergic reactions |
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Photo 1: A horse with marked insect bite hypersensitivity.
Photo courtesy Christine Rees, DVM, DACVD, Hillside Veterinary Clinic, Dallas. 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. |
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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.
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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. |
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Allergic contact dermatitis |
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Photo 3: Contact allergy on a horse’s tongue.
Culicoides infestation 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.” |
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Pemphigus foliaceus 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 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 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 Sarcoids
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| Photo 4: A close-up view of dermatophilosis.
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| Photo 5: A cytologic sample of Dermatophilus congolensis, a gram-positive bacterium causes dermatophilosis.
Rain rot |
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Photosensitization Conclusion |
