Skin Problems in Horses
- Feb 1, 2006
There are many problems that can affect a horse's skin--from insect allergies to fungal, viral, or bacterial infections. The skin is the body's largest and most important organ; it protects the inner structures of the body from the outside environment. Although it consists of many layers and is tough and resilient (and generally heals quickly when injured), if the skin is compromised too much, the horse's health is at risk. Skin problems can be minor or serious, but even minor ones should be properly diagnosed and monitored to make sure they don't become major.
There are some general terms to describe a variety of skin conditions that can be caused by several different things. If your veterinarian says your horse has dermatitis, or dandruff, or scratches, this is not a diagnosis; he/she will check further to determine what is actually causing the signs you're seeing.
Christine Rees, DVM, veterinary dermatologist at Texas A&M University, explains that dermatitis simply means inflammation of the skin. "It could be inflammation due to allergy, or infection, or be secondary to trauma and injury," she says. The horse also might be rubbing and itching for some other reason and resultant trauma to the skin could lead to dermatitis.
This is another generic term and means the old skin is shedding in flakes rather than in microscopic particles. The chunks of old skin can be oily or dry. Dry, flaky skin can be due to a nutritional problem, such as not enough fatty acids in the diet, says Rees. "It could also be due to parasites in the gastrointestinal (GI) tract that drain the body of nutrients," she says. "Flaky skin could also be secondary to allergies from insect bites.
"Dandruff in animals is called seborrhea," she adds. "You can find anti-seborrhea shampoos for small animals, which can be used on horses. If the horse has moist (oily) skin and flaking, use a shampoo that has tar as one of the ingredients. If it's small, dry scales (like snowflake dandruff), use a shampoo containing sulfur and salicylic acid, which is milder. These ingredients help break down the scales and normalize the skin so it's not coming off in flakes."
Inflammation of the hair follicles (due to fungal or bacterial infection) can occur in horses and is sometimes called pyoderma, which describes BB-sized pus pockets of bacterial skin infections. These are also called "zits" (pustules).
"Dogs and cats can get true acne (a buildup of material from the oil glands, creating a nodule that sometimes develops secondary infection), but horses don't," says Rees. "In horses the pustules can be secondary to trauma or insect allergies. There may be itching with these."
Hillary Jackson, BVM&S, DVD, MRCVS, Dipl. ACVD, veterinary dermatologist at North Carolina State University, says these infections are not contagious. "The horse might be itchy and breaks the skin (while scratching), allowing bacteria to enter. Or the skin may be moist and softened and easily broken," says Jackson. "This type of infection may also result in patchy hair loss or crusting. A mild bacterial skin infection can be treated topically by washing with an antiseptic like chlorhexidine (Nolvasan). If it's more severe, we might put the horse on systemic antibiotics."
This is just a term that describes the location of the lesion (lower leg). It doesn't tell you whether it's caused by bacteria, fungi, a combination of both, or due to trauma or moisture that compromises the skin. "The term is misleading because a lot of people think it's a diagnosis, but it's just a description of the type of lesion and its location," says Rees.
There are a number of treatments for scratches; unless you get a definitive diagnosis, the most effective treatments are usually the ones that contain ingredients to combat both fungi and bacteria.
Dry, flaky skin around the face is sometimes a sign of anhidrosis, but the most noticeable sign is an inability to sweat. Since he is unable to cool himself, an affected horse is at risk of heat stress. This problem is most common in hot, humid regions, but cases have been reported as far north as Minnesota and Michigan.
"We don't know exactly why the sweat glands stop working," says Jeremy D. Hubert, BVSc, MRCVS, MS, Dipl. ACVS, assistant professor of veterinary clinical science at Louisiana State University. "We think they become overworked and fatigued when the horse has to sweat continually to cool himself. After they shut down, he may have patches of sweat behind his ears, under his mane, and at elbows and flanks, but no moisture over his body."
The horse might have flaky skin and hair falling out (especially around the eyes) because oil from the sebaceous glands is no longer taken out onto the skin by sweat. The dry skin might itch. The horse has overall poor performance, and it takes a long time to cool him out after exercise, says Hubert. If discovered early, however, the condition can be reversed by assisting the horse's cooling system.
"The best way is to remove him from the stressful environment; take him to a cooler climate or try to change the conditions he's living in," says Hubert. "This may mean using deep shade, misting fans, or other ways to cool the horse to bring his
temperature back down to normal, then keeping it low so he remains less heat stressed and doesn't have to sweat."
Keep an affected horse out of the sun, but not in a hot stall. Keep him cool with fans until his sweating reflex recovers. The horse should be exercised only during the coolest times of day. If addressed as soon as it occurs, anhidrosis might resolve in a few days. But if the problem is long-standing, recovery might take weeks, if it ever occurs.
"There are some things that seem to help," says Hubert. "Some horsemen feel that if they give their horses a commercial electrolyte supplement (such as One AC) or add Lyte salt (a combination of potassium chloride and sodium chloride) to the feed before the hot season begins, these horses do better. If a horse has anhidrosis, he will be affected every year during the heat, so you try to prepare ahead of time."
Allergy is a condition in which the body reacts adversely to a certain substance. The reaction might appear locally in the skin (swelling, redness, or itching) or can involve other body systems. Diagnosing the cause of the skin reaction might be simple (if the horse breaks out in hives after an injection or when new bedding is put in his stall) or might require testing or biopsy to pinpoint the cause. Skin allergies are some of the most common allergic conditions in horses, and many of these are caused by hypersensitivity to insect bites. The resulting itchy swelling is often called sweet itch or Queensland itch.
Insect allergies can be triggered by almost any type of fly bite, but most troublesome are bites of Culicoides (gnats, or "no-see-ums"), according to Stephen White, DVM, Dipl. ACVO, a dermatologist at the University of California, Davis.
"Most species affect the underline or back of the horse, or both," says Davis. "Some feed mainly around the horse's neck. The best thing to do is prevent it by using a fly repellent daily containing permethrin. In severe cases, a veterinarian may treat the horse with corticosteroids to shut down the allergic response."
Jackson says gnats usually feed on the horse at dawn and dusk. A lot of horse owners aren't aware of this and put their horses out when they get home in the evenings. She recommends keeping them in the barn at dawn and dusk (with protective small-mesh netting on the windows so the tiny gnats can't crawl through) and using fans in the stalls. Air movement keeps gnats away from horses since these small insects are not strong fliers.
If a horse has a serious problem with insect allergies, he can be given allergy shots. "We may formulate an allergy vaccine for that particular horse," says Jackson. "This is a reasonably safe form of treatment and generally must be continued throughout the life of the horse."
The maintenance shots are usually given every two weeks after an induction phase in which the horse is given small amounts of allergen that are slowly increased. If the horse is only affected during summer, you might not have to give the shots during winter. In the South, however, horses are generally given shots year-round to keep up their defenses.
Trying to prevent the problem by protecting the horse early in the season (with fly sheets, repellents, or allergy shots) is better than having to deal with it after the horse develops a reaction. Once the horse has skin problems and itching dermatitis, fly sheets will be very irritating on the damaged skin.
Another skin reaction to insect bites is eosinophilic granuloma, which is usually a benign raised, firm nodule above skin level. "This is best diagnosed with biopsy (removal and examination of a tissue sample)," says Rees. "It's thought to be secondary to a hypersensitivity reaction or insect bites, but is much more firm and raised than hives and more permanent. It won't flatten out when you press on it. It looks like a tumor, but isn't."
An eosinophil is a type of cell in the blood that is present during an allergic or hypersensitivity reaction, and a biopsy of the lump will show a lot of these cells and sometimes other inflammatory cells.
Older lesions can develop calcium deposits, which make them even harder. Surgery is the only way to get rid of them. If it's not bone-hard, however, the lump can be injected with steroids to shrink it. You might have to use systemic steroids if there are lumps all over the body, says Rees. If the problem is triggered by insects, it could happen again in following years, and you should keep flies away from that horse. "You might want to use fly masks and fly sheets along with repellents and sprays to keep the flies completely away," she says.
Another form of allergy is contact dermatitis (sensitivities to things that contact the skin). The horse might be sensitive to ingredients in a fly spray or shampoo, or break out in a rash after contact with certain plants (walking through nettles, for instance) that do not bother other horses.
Richard Mansmann, VMD, PhD, director of the Equine Health Program at North Carolina State University, says observations by the horse owner (such as keeping a diary of what is done with the horse and when) are crucial in helping the veterinarian pinpoint the cause of an allergy problem. Once the things that trigger an allergic reaction are determined, those can be avoided.
"In these odd cases, a complete history of the horse is a lot less expensive, and usually more effective, than going through a lot of diagnostic tests," says Mansmann.
Hives are a common allergy problem in horses. These lumps might be triggered by reaction to a certain plant (ingested or contacted by the skin), fly bites, ingredient in a fly spray or shampoo, or reaction to the carrier in a vaccine. Some horses develop chronic hives, and it's difficult to pinpoint the cause.
"A skin biopsy can tell you it's an allergic reaction, but won't tell you the cause," says Mansmann. "Skin testing may help, but not if the horse already has bumps all over the body; it's hard to find a place to put the injections for the test," says Mansmann.
For skin testing, usually the neck is shaved and various allergens are injected intradermally in a grid pattern. Then the vet observes these to see which ones create swelling.
In most instances, hives subside on their own. Bathing the horse with a mild shampoo might help if the bumps were caused by contact with an allergen. But if there are many lumps (or they are on the horse's back so he can't be ridden) or respiratory problems along with the hives, the horse needs treatment. Hives can be an early indication of a more serious reaction.
Mansmann says the best solution for a horse with chronic hives is treatment with a low dose of corticosteroid. "I also use an antihistamine type of drug, hydroxyzine," he says. "Typical antihistamines are usually ineffective in horses, but hydroxyzine has broader-spectrum activity against different types of allergies."
Ingestion of certain plants can cause serious skin reactions in horses. Photodynamic agents in the plant are absorbed from the gut into the bloodstream and travel to the skin, where they react with ultraviolet rays from the sun. The chemical changes that take place damage and kill skin cells, causing the skin to slough off. Usually only the unpigmented areas of skin are affected, such as a white marking on the face (or any white areas on a pinto or Paint horse), white stockings, etc. Dark skin has more protection from UV rays.
Horses that develop liver disease might suffer from photosensitization even if they don't eat the photosensitizing plants, just because the liver is no longer filtering out the normal chlorophyll products of plants. These products build up in the bloodstream and are deposited in the skin, causing the skin cells to die. The swelling and redness might be mistaken for other skin conditions, but the pattern of the lesions (usually limited to white markings) can be a clue, says Rees.
Treatment of the skin irritation involves getting the horse out of direct sunlight, using topical ointments to ease pain and inflammation, and good wound management if the skin is sloughing off. If it's secondary to a liver problem, your veterinarian must determine the cause to treat the problem. The cause could be anything from a mild bout of hepatitis to a more serious condition from which your horse might not be able to recover.
Viral Skin Problems: Warts
The most common warts in horses are those around the muzzle, which are probably caused by the papilloma virus. These are generally seen on young horses, and they spontaneously resolve and disappear after the animal develops immunity to the virus. In some cases (if they interfere with eating or for cosmetic reasons), people try to get rid of them quicker by using an autologous (derived from the horse's own tissue) vaccine made from some of the wart tissue, says Jackson.
Another skin problem caused by the papilloma virus sometimes appear on the inner surface of the ears. These flat, white areas are called aural plaques and are sometimes scaly. "These are spread from horse to horse by fly bites. They don't seem to bother the horse unless they become infected," says Jackson. Insect bites can irritate these and make them worse.
Rees says, "Treatment is usually not effective. I had a mare with these, and no matter what I tried, they didn't go away. It's usually best to just leave them alone. The main thing is to keep flies out of the ears so they are not irritated and don't grow larger. If you are showing the horse and it's a cosmetic issue, you can color them with a marking pen so they are not so obvious."
"Sarcoids are supposedly the most common equine skin tumors, and certain horses seem more predisposed to getting these," says Rees. "There are several types, including a flat, circular, scaly type that looks like ringworm. If you try to biopsy that one, it will become worse."
One type looks nodular (firm, raised above the skin, and looking like a tumor) and is usually 5-20 millimeters wide. Another type resembles ulcerated tissue and is called a fibroblastic sarcoid. Yet another kind looks warty (a verrucous type of sarcoid).
White says the exact cause of sarcoid tumors is not understood, but might have something to do with the papilloma virus that causes warts. Some sarcoids seem to occur at sites of injury, which might indicate a virus as the cause, he says.
"The malevolent type usually has multiple nodules and is very hard and thickened," says Rees. "Some of those can infiltrate the lymph nodes and produce spreading, cordlike lesions. The problem with sarcoids is that when you surgically remove them, they tend to recur; the growth may appear somewhere else on the body later. Surgery is one of the more common treatments, however, and is the treatment of choice for fibroblastic sarcoids. The veterinarian may also freeze the tissue to debulk it and follow up with an injection of bacille Calmette-Guerin (BCG) vaccine, especially if the growth is near the eye."
The BCG vaccine is used in human cancer chemotherapy, but it produces a lot of inflammation, says Rees. "The sarcoid becomes ulcerated and falls off," she explains. "Because of the swelling, the horse is usually put on drugs to help control that secondary inflammation."
Radiation therapy is used on some lesions and is more beneficial for smaller sarcoids that do not penetrate the deeper layers of the skin.
Chemotherapy is also useful in treating equine sarcoids. The drug reported to have the most success is Platinol or cisplatin, but it requires repeat injections and has some handling precautions for humans (gloves, protective clothing, special disposal), so not all veterinarians use it. It should not be used in breeding animals, says Rees. The veterinarian will decide what's best to use in each case.
Bacterial Skin Problems: Rain Rot
Dermatophilosis (rain rot) can be spread from horse to horse by direct contact, by a carrier animal, or by carrier "crusts" in the environment, says White. The bacteria can live in these crusts for long periods of time, he explains. During a rainy period, a horse might develop these crusts along its back or legs (whatever gets wet). The crusts are not itchy, but they are painful if you try to pull them off.
Diagnosis can be made by culturing organisms from the crusts or taking a biopsy from the skin lesions. "The infection responds well to several different antibiotics," says White. Horses that are physically stressed with poor nutrition are more susceptible to rain rot; part of the treatment plan for those horses in an improved diet. "Sometimes a combination of penicillin and streptomycin is used. The antibiotics are given systemically. Mild cases generally respond to topical treatments, such as washing the horse with an iodine shampoo," says White.
Fungi are primitive plants that are hardier than bacteria or viruses and can survive and multiply in a wider range of temperature. Some are parasites, causing skin lesions on humans and animals and multiplying by sending out microscopic spores. The spores can live in the environment a long time.
Ringworm is a common fungal skin problem, often appearing in a crusted, circular shape with hair loss. "Lesions may be either scaly or crusty," says Rees. It can be spread if horses share tack or blankets, and it is contagious to humans.
One soil-borne fungus (Microsporum gypseum) causes skin inflammation that looks like hives, says Rees. This is confusing because you might not think to do a fungal culture when you see hives. The fungus can be cultured and typed using plucked hairs from around the hairless areas. If it's M. gypseum, it's from soil, she advises.
Treatment generally involves a body wash with a good fungicide (prescribed by your veterinarian). Even though the horse will recover in a few months without treatment, it's best to treat ringworm to keep it from spreading to other horses, cats, dogs, or humans.
Girth itch is also caused by fungus, but it appears mainly at the girth area where there is friction and abrasion from a cinch. Unless halted, it can lead to ever-widening raw spots, causing the horse to eventually become so sore a saddle can't be used. The fungus can be spread from horse to horse via a contaminated cinch or brushes. To avoid spreading the infection, change cinches between horses (if you use the same saddle) or wash the cinch in diluted bleach or another fungicide.
Some horses seem more susceptible to fungal infections than others. Even if you clear up girth itch, it is likely to return during the next riding season unless you make sure the cinch fits perfectly and does not rub the horse (no friction sores through which fungi can enter the skin) and that all tack and grooming equipment is clean and free of fungal spores. To clear up an existing case of girth itch, treat it as soon as it appears with a topical fungicide product prescribed by your vet.
Called swamp cancer, kunkers, Gulf Coast fungus, Florida horse leeches, bursatti, or phycomycosis, equine pythiosis is a subcutaneous (beneath the skin surface) inflammation caused by a fungal organism (Pythium insidiosum) that becomes encysted in damaged skin. It affects animals and people in tropical and subtropical climates and is occasionally seen in the southeastern part of the United States. In horses, spores of the organism probably enter through a lesion in the skin, says Hubert. The spores might be present in a warm water environment such as a swamp or muddy pasture, and they stick to damaged skin or animal hair. The lesions usually occur on the lower legs, lower abdomen or chest, or on the face if the horse is grazing swampy areas. This can be a problem in hurricane areas.
The resulting inflammation appears as a large, fast-growing ulceration characterized by lumps, pus, and raw tissue, and it can be very itchy. The lesions often contain coral-like accumulations of fungus filaments and necrotic inflammatory cells. These clumps are called kunkers. Diagnosis might be difficult.
"It looks a lot like proud flesh, sarcoid, or even a summer sore (inflammation caused by migrating larvae of a parasitic worm), or may be mistaken for certain types of fungal lesions. But the pythiosis lesion grows very fast," says Hubert.
Diagnosis can be made by culturing the kunkers or observing the fungal filaments within them (under a microscope). "Your vet can take a sample of the abnormal tissue and send it to a lab that is equipped to evaluate this type of organism, such as our lab at LSU, where Dr. Amy Grooters (DVM, Dipl. ACVIM), associate professor of veterinary clinical sciences, would try to culture the organism. The earlier it can be recognized and treated, the better,"
"We recommend a combination of immunotherapy and surgery," says Hubert. "A vaccine has been developed by Leonel Mendoza, PhD, a professor of clinical laboratory science/medical technology at Michigan State University. It's experimental, and a veterinarian must get a USDA permit on a case-by-case basis to use it."
The horse should be vaccinated as soon as diagnosis is made to reduce the size of the lesion so surgical removal is easier and possibly more successful. "When you cut it with a scalpel, you can't see how far the organism has gone into the tissues, so you make a calculated guess of how much to remove," he says. Then the remaining tissue bed is treated with a laser (photoablation) to kill any fungal filaments that might have infiltrated the surrounding tissues. Prognosis is guarded, since the lesion has usually been there awhile by the time it is diagnosed and treated."
In experimental work with the vaccine, Mendoza found that response to immunotherapy is best (cure rate 100%) in horses with lesions that had been present for 15 days or less when treatment begins. No cures were obtained, however, in horses where the lesions had been present for more than two months, says Hubert.
There are a number of external parasites that feed on horses. Some of these create skin irritation or hair loss due to the horse itching and rubbing.
Lice spend their entire life cycle on the host. They can be spread from horse to horse by direct contact or by tack or grooming equipment used on more than one horse. There are two species that infect horses--biting lice and sucking lice. They are brown and usually down next to the skin. To see them, spread the hair apart so you can see the skin. On a dark-colored horse, it might be easiest to see them with the help of a flashlight. You can generally see them around the mane, neck, and shoulders.
"The blood-sucking louse is a little smaller than the biting louse," says Sandy Gagnon, extension horse specialist at Montana State University. "Both types move around a lot, but hang onto the base of a hair as they feed."
If you use ivermectin for deworming, it will kill blood-sucking lice, but won't protect against the chewing/biting lice. These cause itching and the horse might rub out patches of hair. Where the hair is missing, the lice move into the hairy areas. They don't like sunlight and need the hairs to hang on to as they feed. Lice prefer a long hair coat, and their population increases during winter, says Gagnon.
You can treat the horse with a topical product to kill all the adult lice, but the eggs glued to the hairs can keep hatching for up to two weeks. "So in two weeks you need to treat again to get the freshly hatched lice that are not yet mature enough to lay eggs," advises Gagnon. "The best thing to use is a liquid organophosphate product because lice are down at the base of the hair and the product must come into contact with the skin. Hair penetration is better and you can get it all over the horse easier than you can a powder. Powders can be used in winter, however, if you don't want to get the whole horse wet during cold weather." You must make sure you rub it down through the hair and onto the skin.
Mites (mange) can also be a problem in horses. Jackson says the most common type is chorioptic mange, especially in draft horses with abundant feathering (long hairs on the lower legs). "The mites like to live at the base of the long hairs," says Jackson. "These horses get very itchy and will stamp their feet or rub the lower legs on objects. Mites are contagious between horses by direct contact or grooming tools. Treatment (a prescription from your veterinarian) is usually topical. To be most effective, you should first clip all the hair off the lower legs before applying the medication."
Several types of skin cancer affect horses. Squamous cell carcinoma is the most visible and readily detected. It often occurs on the eyelid or around the vulva or sheath if the skin is unpigmented (skin pigment helps shield the skin cells from UV rays). Most growths occur on the third eyelid or the lower eyelid's inner surface, appearing as a bump or raw area. It will continue to spread unless removed. These growths can be removed surgically, frozen or burned off, or treated with radiation or immunotherapy (vaccine injected into the tumor).
Melanoma is a tumor in cells that produce skin pigment and is most common in gray horses. The dark lumps appear singly or in groups, and they might develop slowly or rapidly. Most stay on the exterior of the body and are not life-threatening. The dark nodules under the skin might appear under the base of the tail, near the anus, vulva, or sheath, or around the head or shoulders. Only a few will progress to killing the horse.
Not all dark skin lumps are melanomas, but you should have any suspicious bump checked by a vet. Melanomas are usually smooth, raised areas on the skin, without scabs or ulcers. They might start appearing any time after age four. Growths that appear early in life are more likely to become malignant (spreading to internal tissues via bloodstream or lymph system) than those appearing after age 15. Slow-growing external lumps are usually best left alone, but they should be monitored to see whether they are starting to spread.
"Proper diagnosis (and treatment) of any skin problem depends on being able to give your veterinarian a good history of how long it's been there, whether there are any other animals affected in the barn or pasture, etc.," says White. "Presumptive treatments (such as topical antimicrobial products) may be used for awhile on some skin conditions, or the veterinarian may do fungal or bacterial cultures or a biopsy to see exactly what you are dealing with."
There are many things that can affect the skin of horses. Since skin is so important to the health and well-being of your animal, if you notice any unusual lumps or bumps, have them accurately diagnosed by your veterinarian.
About the Author
Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at http://insidestorey.blogspot.com that comes out on Tuesdays.
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