A survey of equine skin diseases

A survey of equine skin diseases

From ringworm to allergies to skin cancer, a veterinary dermatologist walks you through some updates on the most common conditions you might face in your equine veterinary patients.
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May 23, 2017

The world of equine dermatology is not black and white. Here's a guide to help you face it head on. (Getty Images)Skin diseases in horses are prevalent throughout the year, though some may be seasonal. They may be due to various infectious agents—bacteria, viruses, fungi, parasites or environmental irritants. The need to promptly treat the disease is important not only to alleviate the discomfort experienced, such as itchiness and soreness, but also to improve the horse’s overall health. Remember: The skin is a critical organ, a natural barrier to disease. Here are some commonly encountered equine skin diseases and conditions seen in horses.

Dermatophytosis (ringworm)

Ringworm usually manifests as a crusting dermatosis with hair loss and circular lesions on the body. Christine Rees, DVM, DACVD, of Dallas Veterinary Specialists in Dallas, Texas, says its normally seen in a stressed animal or in one whose immune system is slightly compromised, making it more common in older or younger horses.

Specifics on species. Ringworm is common to most animals, does not seem to be species-specific and is zoonotic, so it is fairly easily passed between individuals, says Rees. “Sometimes you can see lesions that almost look more hivelike,” says Rees. “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.”

Rees says she sees more cases of ringworm caused by Microsporum canis, the type found prevalently in cats, than by Trichophyton equinum, the equine species. “If it is the type usually seen in cats, evaluate any barn cats to determine if one has skin lesions, thereby transmitting it to the horses,” she says.

It’s helpful to know if the horse has contracted the soil-borne ground-type Microsporum gypsum. “In a horse affected by Microsporum gypsum, if there’s a particular area where a horse roots around or rolls, then you want to aggressively treat that area—the ground or stall floor—with some dilute bleach to try to eliminate it,” Rees says.

Tactics on treatment. Various systemic ringworm treatments are available, such as oral griseofulvin, terbinafine and fluconazole. “The only problem with griseofulvin is that I have not seen a pharmokinetic study for it,” Rees says. If you use terbinafine, keep in mind it may cause elevated liver enzyme activities, says Rees. Occasionally, fluconazole is used, though its cost has increased to where it’s not a reasonable option in today’s market.

Rees prefers topical therapies to systemic drugs. “In my opinion, miconazole works a bit better than ketoconazole,” she says, though the latter is a reasonable alternative despite some concerns about resistance, especially for horses infected with Microsporum canis.

If owners don’t want to shampoo their horses two to three times a week, they can also use a lime-sulfur dip. Although it has a fairly pungent, rotten egg-like odor, this treatment is very effective against fungus (and bacteria and parasites) at a higher concentration. A dilute bleach-solution rinse is an alternate treatment option.

Keep in mind that ringworm can be difficult to treat since the organism can linger. “One has to treat the entire environment as a source of infection, which is as important as treating the animal,” Rees says. If more than one animal is affected, make sure to carefully disinfect various items shared by those horses in the barn to try to minimize the spread of the disease.

Product picks. Newer shampoos include BioHex broad-spectrum cleansing shampoo (VetBiotek), which combines a proprietary formulation of 2% chlorhexidine digluconate, 2% miconazole nitrate and MicroSilver BG for enhanced antiseptic activity. “The silver, per se, is antifungal, as well as antibacterial,” says Rees. “It works well in the more difficult ringworm cases.”

EquiShield CK spray (Kinetic Vet), which contains 2% chlorhexidine and 1% ketoconazole, is a topical antiseptic solution formulated for horses, dogs and cats. It can be easily sprayed with a hose on horses.

Staphylococcus aureus infection

Staphylococcos aureus infection can manifest in horses as warm, painful skin, with focal crusts most commonly noted in the pastern region, though similar lesions may occur elsewhere on the skin.

Methicillin-resistant S. aureus (MRSA) is a concern in horses. “Some of the horses that get recurrent infections seem to get MRSA, especially those that have been repeatedly treated with various antibiotics,” Rees says.

Though MRSA is being more commonly seen in horses (Figure 1), it is not as common as in dogs and cats. The way to ensure the diagnosis is by performing a culture. “If you have a nonhealing wound or one that doesn’t look quite right—or one that you never can quite get healed—then you must do a bacterial culture to make sure there is no MRSA,” says Rees.

Figure 1. A resistant staphylococcal infection on the side of a horse's neck. (All images courtesy of Dr. Christine Rees, Dallas Veterinary Specialists)“Usually, when you have a resistant Staphylococcus infection, normally you have to use the antibiotic based on the culture results,” says Rees. “Sometimes it’s most effective to treat it a week longer than you normally treat it, just to make sure it’s completely killed. In an ideal situation, it would be effective to reculture it, especially since it is potentially zoonotic, just to make sure it’s totally gone before you stop the treatment—otherwise it potentially could come back again.”

Rees’ solution for owners who find the cost of treatment prohibitive? “They can bathe the horse with a 2% to 3% chlorhexidine or with a benzoyl peroxide shampoo, using it more aggressively every other day before trying a more expensive injectable or oral antiobiotic to treat the infection.”

Environmenal concerns are important as well. “As MRSA is zoonotic, horses can become infected from human handlers, particularly those who work in the human health field, carrying the bacteria to the stall via their nasal passages,” says Rees. People working with an affected horse should wear heavy latex gloves, as they might get infected via scratches on their hands or other skin wounds.

Product picks. Vetericyn Plus VF hydrogel (Innovacyn) is designed to adhere to the site of application, allowing this advanced hypochlorous formula to penetrate the wound bed. The hydrogel does not cause dermal irritation and is safe for use in sensitive areas.

A dilute bleach shampoo shampoo, Command (VetriMax), formulated with sodium hypochlorite and salicylic acid, is an effective monotherapy for the treatment of superficial canine pyoderma associated with MRSA. “I have not used Command that much in horses, so I don’t really know how well it works. In small animals it has been effective in some cases, but not always,” Rees says. “I prefer Biohex rather than Command.”

Insect bite hypersensitivity

Insect bite hypersensitivity, caused by flies and other insects, is a major cause of allergy in horses (Figure 2). Some fly species (black flies, deer flies, horse flies) produce this condition, but there is debate whether it is a true hypersensitivity or allergy or just an irritation, as several fly species deliver painful bites.

Figure 2. Insect allergies with secondary alopecia and mild bacterial skin infection.Although several insects produce hypersensitivity, the one most studied is from the Culicoides genus. It has been shown that certain proteins from the saliva of these insects will induce an allergic response in horses. Some of them are ventral feeders, while others feed along the dorsum.

“If horses are solely allergic to Culicoides species, immunotherapy or allergy shots don’t seem to work as well because the allergen is not only derived 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.

“The treatment mainly involves trying to repel the flies with pyrethrin or similar drugs, though there are studies of varying success of these drugs,” Rees continues. “We probably ideally need to include more horses in the [insect bite hypersensitivity treatment] studies and to identify the species of the fly. It seems that horses that have hypersensitivity to Culicoides also have similar sensitivities to other allergens such as pollens, dust and mold.”

Another key is to reduce the prevalence of insects in the horse’s environment. Horse owners can place fish in stock ponds to reduce mosquitoes, fans in stalls to reduce flies (especially for Culicoides species) or use fly sprays or fly facemasks.

Culicoides tend to be poor fliers and like water sources, so you want to remove the horse from free-standing water areas, if possible, putting it in a pasture without a pond or stream running through it,” Rees explains.

Some cases are difficult to diagnose, looking like typical insect allergy but actually ending up being atopic environmental allergies, Rees says.

Environmental allergies

“Atopic dermatitis and environmental allergies to dust, molds, pollen and poor-quality hay are quite common equine allergic responses,” says Rees. Hives are common and can present anywhere on the body but are typically found on the face, neck, chest and upper legs.

Another consideration is allergic contact dermatitis, which occurs when irritating substances, such as fly sprays, shampoos, liniments or other substances come into direct contact with the skin of hypersensitive horses. Signs may include mild redness, flaky and itchy skin, severe hair loss, skin thickening, pain and occasionally skin sloughing.

Horses often get opportunistic secondary infections with itchy skin, especially horses with allergic skin conditions (Figure 3). “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 are having an allergic flare-up, they may be more predisposed to picking up bacteria or fungus that are out in the environment than a normal animal would be.”

Figure 3. Fungal and bacterial infection around tail and hip area secondary to atopic dermatitis.An owner can try to minimize an inhalant (mold, pollen, dust) allergy by reducing exposure. Strategies include ensuring good-quality clean bedding, changing it often and wetting the bedding down to reduce dust. However, owners should be cautious of excessive wetting, which may encourage mold growth and exacerbate the situation.

With an inhalant-sensitive animal, the hay must be mold-free, and a confined horse must have sufficient air exposure. “Bathing them often to reduce the pollen to skin exposure is useful also,” says Rees. “Unfortunately, some are so allergic that they need antihistamines, allergy shots or corticosteroids such as dexamethasone,” Rees says. “But in horses, one has to be careful with excessive use of steroids due to the concern for hoof issues and laminitis.”

Allergen specifics. Some horses are allergic to alfalfa, so Rees suggests sweet feed, Omolene (Purina) or rolled oats as an alternative to feed pellets containing alfalfa. She says in one unusual case, a horse owner had Guinea hens. As her horse was severely allergic to feathers, it was imperative to keep the horses separated from the hens. Rees says many of her cases respond to immunotherapy better if the allergen load is decreased in the horse’s environment.

“I do a lot skin testing, though horses are somewhat different from other species in that they can have both delayed as well as immediate hypersensitivity reactions,” says Rees. Thus, she does readings at 15 minutes and 30 minutes and then a delayed reaction reading four to six hours afterward. “I think it does make a difference to do so,” she says.

For both small animals and horses, storage mites are a larger potential cause than previously thought. “I’ve begun to add them to my diagnostic panel and to my immunotherapy based on the allergy test results,” says Rees.

Product picks. Rees says sublingual immunotherapy, a newer treatment form, appears to have helped in several of her allergy cases. “The problem with the sublingual treatment is that one has to do it daily, so it’s a matter of compliance regarding client usage and interest. Most horse owners will accept the allergy vaccine. The injection versus sublingual treatment due to compliance issues makes more sense for horse application.”

Platinum Performance makes a skin and allergy supplement for horses. Adding the powder to the feed seems to be beneficial to reduce itching and the allergy, Rees says.

Common skin cancers: squamous cell carcinoma and melanoma

Squamous cell carcinoma is a malignant tumor of the outmost layer of the skin. It is typified by reddened, roughened or ulcerated skin and is associated with exposure to the sun in horses (unlike melanoma). It’s often seen in sunny areas like Florida, Texas and Arizona on horses’ eyelids, nose, genital region, poorly pigmented areas or areas of lightly haired skin. It’s usually preceded by actinic dermatitis characterized by erosions or ulcers. Most clinicians are familiar with this condition and readily perform a biopsy. It’s important to protect susceptible horses from sun, Rees says—lighter-coated horses such as Palaminos and gray- or white-coated horses are particularly at risk.

With melanoma, surgical removal is the best form of treatment, Rees says. Some breeds, such as Appaloosas, get these tumors at the tail and they tend to be benign. Other lesions are locally aggressive. “Some veterinarians will biopsy them to determine their mitotic rate or only remove them if they are located in a bad spot, such as the penis, the rectum or an area affecting a bodily function,” Rees says. “They are a little different than those seen in people in that there are some horses in which they are not as aggressive a cancer, especially grays. Some have tried a melanoma vaccine or use Tagamet (cimetidine) to try to shrink them. Some claim that it helps, though I have not seen that it does. The only way to properly treat it is surgically.”

Miscellaneous skin diseases

Keep an eye out for these possible causes of dermatologic issues as well:

> Pseudomonas species infection. “I commonly see horses that have hair loss and are itchy,” says Rees—infections are a concern in these cases. “I had one horse with Pseudomonas species infection [in which the bacteria] was getting trapped in the neoprene of the saddle pad blanket (Figure 4). It wasn’t until we started giving the right antibiotic and got more aggressive with cleaning the saddle pad that it resolved.”

Figure 4. Pseudomonas species infection on the side of a horse's chest from bacteria being trapped in the neoprene saddle pad.> Sarcoids. Sarcoids, which frequently occur in areas subject to trauma, are associated with bovine papillomavirus. Some speculate that the virus may be spread by biting flies or fomites. Sarcoids appear as wartlike, ulcerated nodular areas or flat plaque surfaces, usually on the ears, lips, neck and ventral abdomen or around the eyes.

> Fungal infections. “Sometimes we see opportunistic fungal infections when a horse gets a small puncture wound with resultant localized fungal infection,” says Rees.

> Dermatophilosis. Rees says dermatophilosis, or rain rot, occurs if a horse has been out in the rain frequently during a particularly rainy season. Also known as rain scald or mud fever, dermatophilosis is a bacterial infection aggravated by prolonged exposure to moisture coupled with injured skin. Chronically infected animals are usually the source of the infection. Transmission may occur via flies, ticks, grooming equipment or tack. The lower layer of hair is firmly matted in small scabs, which leave a raw surface when removed. The roots of the hair can be seen protruding from the crust and may have a covering of yellowish-orange pus. The affected areas are sore to the touch but are not itchy to the horse.

> Photosensitization. This condition is an abnormal inflammatory skin reaction to the sun’s ultraviolet rays that occurs in areas of white hair or pink skin (Figure 5).

Figure 5. Photosensitization in a paint horse. Note that the skin lesions are limited to the white-haired areas.

The big picture 

A theme echoing in many of these common skin conditions that can develop in horses is the importance of taking in a horse’s environment. Are known allergens prevalent? Is too much sun exposure causing damage? Are irritating bugs buzzing about too readily? Knowing the dangers of what this vital protective layer must fight off each day can help you prevent or reduce the potential for serious skin issues.

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.