The 'Skinny' on Skin Conditions

Sarcoids are skin tumors linked to papillomavirus infection.

Photo: Anne M. Eberhardt/The Horse

Dermatologic conditions in horses are notoriously difficult to diagnose and treat; thankfully, veterinary researchers are continually learning more about equine skin.

Diagnosing skin diseases is probably one of the greatest challenges to handlers and veterinarians alike. With such a variety of conditions all looking so similar, it's no wonder common names like "scratches," "greasy heel," and "girth itch" have emerged. These catch-all terms sound like diagnoses, but they actually only describe the clinical signs. Determining which disease is the real cause of that "greasy heel" requires a specially trained eye and, usually, a laboratory analysis.

"Equine skin is particularly sensitive, much like human skin, so there's just no room for experimenting," says Patrick Bourdeau, DVM, PhD, Dipl.EVPC, professor and head of the parasitology, dermatology, mycology, and zoology unit at the Veterinary School of Nantes, in France. "Dermatologic problems require the right treatment from the start, and that generally means a veterinary examination."

The following are descriptions of some of the more common equine dermatologic problems, grouped according to cause.

Allergies and Immune-Mediated Problems

Allergies are a kind of immune-mediated condition, meaning that the horse's immune system overreacts to something (an "agent") in his environment. Skin inflammation, called dermatitis, can cause itching, hives, pain, and open sores--not to mention a great deal of frustration for owners seeking the source of the problem. When home-based efforts yield no clues, a trip to the veterinary dermatologist for diagnostic testing can help.

"The number one reason for referral into our equine dermatology clinic is the investigation of allergies," says Janet Littlewood, PhD, MA, BVSc(Hons), DVR, DVD, MRCVS, an RCVS-recognized specialist in veterinary dermatology and practicing veterinary dermatologist at three equine referral clinics in the U.K.

The offending agent might be something directly contacting the skin, such as fly spray, insects, weeds, or plant pollens. But it could also be something ingested: particular grains or forage, for example, or medicines (especially antibiotics and non-steroidal anti-inflammatories).

The most common equine skin allergy is a reaction to the saliva of Culicoides midges (gnats). This sweet itch, or summer eczema, frequently leads to massive scratching, primarily of the mane and tail, causing hair loss, redness, and sometimes ulceration. Small papules or wheals might also develop.

Medicines fight the clinical signs, but prevention is the real key in battling sweet itch. "Cover them in fast-acting insecticides or fly blankets," Bourdeau says. "Those are the only things that could really work."

Nodular lesions with grayish-white centers, particularly along the saddle area or the sides of the body or neck, could represent what is known as equine eosinophilic granuloma. These immune-mediated manifestations seem to be the result of insect bites, trauma, and/or additional factors. Veterinarians generally treat them with local corticosteroid injections, systemic antibiotics, and occasionally surgery.

Another immune-mediated disease, vasculitis, is characterized by inflamed blood vessels within the skin that cause skin irritation. Vasculitis is most commonly seen in the pasterns, but it can also occur on the lips and in the eyes (with corneal edema, or fluid swel-ling, for example). It starts with swelling and redness, and raw lesions soon follow, along with increased swelling. Photoactivated vasculitis occurs predominately in white legs. Swelling is minimal, but the outermost layer of skin in the lesions might turn black from necrosis. It's important to protect affected legs from sunlight.

In all these situations removing the agent is the preferred treatment. When this is not possible, or when the clinical signs continue, corticosteroids are often required.

However, improper or excessive use of corticosteroids could lead to other serious problems, including laminitis. "A horse with con-firmed vasculitis needs steroids," Littlewood says. "But the problems happen when people treat symptomatically without proper diagnosis." Corticosteroids also cause performance horses to test positive for drugs, even if the agent is applied topically at normal doses.

Parasites

Lice are visible to the naked eye on the horse's coat or in the mane and tail, depending on the species. Typically, horses with lice are in poor condition or already have a weakened immune system, according to Bourdeau. "We practically never see lice on healthy individuals," he says. These parasites are extremely easy to treat with repeat doses of insecticides to ensure extermination of newly hatched insects.

Chorioptic mites (which cause chorioptic mange, also called leg mange) tend to live in the hair of the legs and pasterns, especially of horses with "feathers," but the disease can also move up to the lower body. These mites are microscopic in size, which makes them harder to diagnose. Clinical signs include leg-biting, rubbing against objects, and stomping feet. Topical prescription medication is usually very effective if applied according to your veterinarian's instructions.

Bacterial Infections

Dermatophilosis, more commonly known as rain rot or mud fever, causes streaks of hair loss that can resemble paint brush strokes, sometimes with matted hair, oozing, and crusting. White head and leg markings are most commonly affected, especially if horses are constantly exposed to wet grazing areas. This can happen in any climate, but it's more common in tropical and subtropical regions and often worsens in the legs with age, Littlewood says.

Move affected horses to a dry environment. Treatment includes strong antiseptics and, in severe cases, systemic antibiotics. On rare occasions, dermatophilosis has spread to human handlers.

Pyoderma (or folliculitis)--frequently referred to as saddle dermatitis, tail dermatitis, or pastern dermatitis--develops when excessive sweating causes a shift in the pH balance that favors bacterial growth. Clinical signs usually include crusting, scaling, and hair loss, frequently on the chest and legs and across the back and croup.

Pyoderma might be preventable through good management. "If you rinse down your horse after exercise, you're probably not going to see pyoderma," says Bourdeau. Local antiseptics are the primary treatment choice, but advanced cases frequently require systemic antibiotics to prevent further complications.

Fungal Infections

Dermatophytosis ("ringworm") is probably the most common fungal skin disease in horses worldwide, according to Samantha Crothers, BVMS, Dipl. ACVD, a veterinary dermatologist at the Veterinary Medical Teaching Hospital at the University of California, Davis.

The appearance of ringworm can vary dramatically, which means it is also one of the most commonly misdiagnosed equine skin diseases. In classic situations, ringworm starts as hair loss in a localized area that slowly expands larger and larger. However, many cases are clinically difficult to distinguish from other diseases, especially bacterial infections and sarcoids, according to Stephen White, DVM, Dipl. ACVD, also a UC Davis veterinary dermatologist. Accurate diagnosis requires laboratory culture of the lesion, and it might take up to a month to grow the organism.

Ringworm generally heals within a few months without treatment; however, it is uncomfortable for the horse and highly contagious to other horses and humans. Worse, horses can remain carriers even when clinical signs have disappeared. Therefore, vets recommend treating all infected horses with antifungal medications.

Tack and grooming supplies should not be shared, and all equipment and even stall walls should be disinfected to prevent spreading the fungus. Careful attention to detail is required, as fungi can remain alive on surfaces for years, Bourdeau says.

Pythiosis, while technically not a fungus, is treated as such (it is technically termed a "parasitic oomycete"). Veterinarians typically see the condition only in warm or tropical climates and quite frequently the southern United States, according to Crothers. Like many skin infections, it appears as an ulcerated swelling or nodule. Pythiosis occurs deep in the skin and can lead to severe debilitation and death, White says.

Treatment usually involves combining immunotherapy and surgery. Although not as contagious as ringworm, pythiosis is occasionally transferred from horse to handler via open sores. "We're still not certain about how to prevent pythiosis," White says, "but it's probably best to keep open sores clean and avoid standing water."

Viral Infections

Equine and bovine papillomaviruses are the most frequently spread viruses in horses. Causing both warts and tumors, they are transmitted via direct contact and perhaps through vectors, such as flies.

Equine viral papillomatosis causes small, bumpy nodules, very similar to human warts, on soft parts of the horse's skin, such as the muzzle. These harmless warts are better left alone and will resolve spontaneously within a few months.

 

Skin Tumors

Sarcoids are skin tumors linked to papillomavirus infection. They show up particularly in areas where horses have very little hair and thin skin and where they are most likely to sweat. The six recognized kinds of sarcoids can resemble other skin diseases, including ringworm and warts. Biopsies offer the only sure diagnosis.

While sarcoids are not life-threatening, the affected animal could eventually require euthanasia because of complications associated with the tumors, according to Josh Slater, BVSc, BVM&S, PhD, Dipl. ECEIM, MRCVS, former president of the British Equine Veterinary Association and professor of equine clinical studies at the Royal Veterinary College in the U.K. Treatment options include surgery, local chemotherapy, and local radiation, all with varying financial costs and success rates.

Melanomas primarily affect gray horses, likely because their skin has more pigment- (melanin) bearing cells. They are most frequently found under and around the tail or behind the head. Melanomas tend to spread aggressively along and under the skin and increase in bulk if not treated.

Unlike melanomas in most species, equine melanomas rarely metastasize (spread). Even so, they can become life-threatening when their mass begins to hinder normal body functioning, blocking off the anus or the esophagus, for example. Melanomas also frequently ulcerate as they increase in size.

"There really is no good treatment for melanomas," says Slater, who is the director of the college's equine referral hospital. "Really the best hope is just to catch these tumors when they're still small, as that makes them much easier to deal with." Horses with advanced melanoma are usually humanely euthanized for welfare reasons.

Squamous cell carcinoma tumors affect the skin and mucous membranes. The inner eyelids and the penis and sheath are common places to find these tumors, which might begin as small bumps or sores.

Unless the vet begins treatment at a very early stage, he or she might have to remove a portion or all affected tissue. If left untreated, euthanasia is usually required as the tumors begin to compress nerves and blood vessels in the surrounding areas.

New Dermatologic Research

Equine dermatology research is under way, advancing veterinary understanding of skin conditions and their treatment and prevention. Fungus researchers at UC Davis are investigating a yeastlike organism that might be responsible for causing horses to rub their tails, Crothers says. Meanwhile, virus researchers are studying how to send "silencing" messages into viruses to halt gene activity and, thus, stop the spread of disease, according to Lubna Nasir, MSc, PhD, professor of comparative oncology in the Faculty of Veterinary Medicine at the University of Glasgow, Scotland. Vaccine development for papillomavirus is also well under way, in hopes of preventing sarcoids and possibly even squamous cell carcinomas, according to Slater. And improved sarcoid treatment methods, including the use of imiquimod to boost the local immune system, are showing great promise in research laboratories, Littlewood says.

Take-Home Message

A horse owner should work with their veterinarian to choose an appropriate treatment plan for his or her horse's skin problems. In the meantime, there is plenty owners can do to improve their horses' dermatologic health: Feeding good quality grains and hay, properly grooming before and after exercise, using well-fitting/comfortable saddles and pads, and hosing a horse down after a workout (especially the legs if the horse wore boots) will go a long way in keeping his skin in good shape. And above all, a good handler will be observant. If a skin condition doesn't resolve within two to three days, a call to the veterinarian is not only justified, it's essential.

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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