We used to call it the "creeping crud"--a colorful, youthful label applicable to any gross-out skin disease the horse happened to have (and not to be confused with the cold-and-flu-like symptoms in humans that earned the same appellation). Besides the crud, there were also those weird little bumps that could show up now and then on the horse--not quite as disgusting, but unpleasant nevertheless.

Chances are if you own a horse long enough, your animal is apt to develop a skin disorder. Veterinarians Rosario Cerundolo, Cert. VD, Dipl. ECVD, MRCVS, RCVS, a specialist in veterinary dermatology and assistant professor of veterinary dermatology at the University of Pennsylvania; and Daniela Bedenice, VetMed, clinical instructor and clinician at Tufts University; discuss the most common (and thus most likely) skin diseases your horse might encounter.

Crud, Scabs, and Warts, Oh My!

Rain rot, rain-scald, dermatophilosis, streptothricosis. By whatever name you call it, this common bacterial infection causes crusty, scabby skin, matted hair, and big clumps of hair loss, primarily across the back and trunk. Removing the crust might reveal a pink skin surface. A thick, yellowish or greenish pus can also be present.

The two most important risk factors for rain rot are trauma to the skin and moisture. According to a Client Information Handout from the Virginia-Maryland Regional College of Veterinary Medicine, "Skin damage can be related to fly bite trauma, increased exposure to rain, poor hygiene (lack of grooming), or mechanical trauma. Any horse that spends time outside where it is exposed to biting insects, prickly vegetation, or moisture is at risk of developing the disease." Horses in hot, humid areas are especially susceptible. Many believe the bacteria can be spread through shared equipment, blankets, and grooming tools, and through contact with infected horses. Diagnosis is based on clinical signs, direct smears, and culturing of crust material.

Affected horses should be protected from rain, biting insects, and prickly vegetation. Topical therapy with a 1% povidone iodine shampoo for 10 days might aid healing. The UC Davis Book of Horses: A Complete Medical Reference Guide For Horses and Foals also suggests vigorous grooming to remove crusts. Sometimes, rain rot will regress spontaneously with drier weather. Severe cases may require systemic antibiotics.

To reduce risk of rain rot, limit the horse's exposure to rain, provide shelter for outside horses, maintain a fly control program, and regularly groom your horse with clean grooming equipment.

Hives (urticaria) is a hypersensitive (allergic) reaction leading to pitting and localized, transient swellings (wheals) in the skin or mucous membranes. "Wheals most frequently cover the neck and side of the chest, although generalized distribution is common," says Bedenice. "Lesions usually range from 1-10 cm in diameter, are pitting (leave a dent after compression), and may or may not induce itching. Some areas may also show serum (clear fluid) leakage."

History and clinical signs are usually enough for a diagnosis. "Most of the time it is possible to correlate the eruption with the use of a particular drug such as dewormers, feed supplements, vaccines, feeding a type of food, or exercise," notes Cerundolo.

For sure, it's important to identify the source of the allergy, as recurrence is a real possibility if the causative agents aren't eliminated. Allergic triggers are many--food or substances in the environment (pollens, mold spores, aerosols, etc.), infections, certain drugs (especially penicillin), insect bites, physical stimulants (heat, light, exercise, etc.), and various feed components (including feed additives or supplements). Says Bedenice, "Drugs and ingested materials are the most common causes."

If the cause is not apparent, clinical trials involving exposure to possible offenders should be performed sequentially to determine if insects, feed, or environmental factors induce hives, Bedenice says. Intradermal skin testing (sometimes called scratch tests) might be helpful in identifying inhaled causative agents.

For a horse which gets hives, avoiding or limiting exposure to offending agents is the best way to avoid future outbreaks. For the horse which is already affected and doesn't show spontaneous resolution when the cause is removed, Bedenice recommends symptomatic treatment of antihistamines (hydroxyzine); however, others don't feel that antihistamines are useful. Bedenice also suggests slowly tapering doses of corticosteroids (dexamethasone, prednisolone) to the lowest possible maintenance dose that prevents recurrence. Doxepin (a tricyclic antidepressant) might also be helpful in selected cases, based on its antihistaminic effects, she adds. Hyposensitization (desensitization) therapy may be useful for inhaled allergens; however, this is a very difficult treatment.

Insect hypersensitivity is an allergic reaction to the saliva of biting insects such as Culicoides spp. (biting midges) or other flies (horn, stable, horse, or black flies). "This disease can be seasonal in cold climates and non-seasonal in warm climates," says Cerundolo. "It can affect any age, sex, or breed and is believed to have a genetic predisposition. It may be more severe each year."

Itchiness is the most common clinical sign of insect hypersensitivity. "Affected horses may have dorsal and/or ventral lesions such as crusts, exudation, and skin thickening caused by rubbing their ears, face, mane, ventral midline, groin, and the tail base," Cerundolo says. "The self-trauma may also result in mane and tail hair loss."

Diagnosis is based on a compatible history (usually a summer problem), clinical signs, and the presence of insects. Allergy tests might be helpful in confirming the Culicoides hypersensitivity.

Prevent or minimize insect problems by controlling the insect population, spraying the horse with a residual insecticide, and stabling the horses during the local insect population's feeding time. Install small-mesh screens on stall windows and spray with parasiticide.

"Installing fans in stalls can be helpful as Culicoides are poor fliers," Cerundolo says. "Masks can protect affected areas of the body. Eliminating larval development areas (compost, standing water, and decaying vegetation) is strongly recommended."

Address clinical signs with antihistamines or corticosteroids. "Hyposensitization (in the form of allergy shots) may be done, but the efficacy is still to be confirmed," notes Cerundolo. "This approach is indicated when avoidance of the allergens is impossible and systemic drugs are contraindicated."

Aural Flat Warts (aural plaques) are warts that look like gray-to-white keratinous crusts on the inner surface of the ear, says Bedenice. "These tend to coalesce, but generally remain asymptomatic, causing minimal itch in some animals," she said. The probable cause is a papilloma virus; black flies can serve as vectors.

Diagnosis is based on clinical signs. These warts tend to persist indefinitely and there is no effective treatment. However, insect control and fly protection might reduce the risk. Although the crusts can be dislodged, revealing a pink non-ulcerated base, this should not be done as it causes the horse irritation.

Dermatophytosis (ringworm) is a fungal infection of the skin most commonly caused by the fungi Trichophyton equinum and T. mentagrophytes. "Initially, affected animals may have localized lesions resembling urticaria," Cerundolo says. "Then areas of focal hair loss with scaling and crusting develop. Reddened skin may be visible in non-pigmented skin. Hairs are typically raised."

Commonly affected areas of the body include the saddle, girth, and halter regions, localized where tack contacts the skin. Lesions extend (outward from a center area) centrifugally and can be itchy.

Ringworm is usually transmitted by direct contact with an infected host (horse, mouse, cat, dog, or cow), indirect contact with contaminated objects (such as shared tack, blankets, or grooming tools), or the environment. "Illness, poor nutrition, overcrowding, age (young or immune-suppressed horses), chronic moisture from sweating and stressful environments (hot and humid weather), or trauma may predispose horses to the infection," Cerundolo warns. "It may occur sporadically or as outbreaks."

Diagnosis is based on the history of transmission to and/or from other animals or humans, clinical signs, and laboratory tests.

Spontaneous recovery can occur in five or six weeks, although lesions might be prolonged if secondary infection is present. However, to avoid transmission to other animals and humans and to reduce the recovery period and severity of the disease, Cerundolo recommends anti-fungal topical treatments of enilconazole or miconazole.

"Systemic therapy is not usually done," he says, "as no specific oral medication is available for horses and most of the human drugs are expensive."

The environment might also need to be treated. Pressure-hose or disinfect all stalls and woodwork that might be infected by fungi using bleach; also disinfect tack, grooming tools, and other objects that have touched the infected horse. Isolate affected animals to prevent spread to other animals or people. In Europe a vaccine is available.

Photosensitization is a skin disease associated with exposure to sunlight. "Initial clinical signs consist of skin areas of inflammation with redness that progress to painful ulceration covered by crusts and scales," says Cerundolo. "Lesions are seen in those parts of the body which are usually depigmented, such as the face and the limbs. They may resemble, although are more severe than, lesions occurring from sunburn."

Factors contributing to the development of skin lesions are the presence of a photoactivating substance in the skin (the horse eats photosensitizing plants, such as St. John's wort and perennial rye grass, after which photosensitizing agents are absorbed and reach the skin), exposure to ultraviolet light, lack of pigment in the skin, and in rare cases liver disease, which sometimes inhibits proper metabolization of photosensitizing agents. Notes Cerundolo, "Horses with depigmented skin of any age or sex are predisposed to this condition."

History of lesions appearing during the sunny months of the year and affecting only the white areas of the body surface are strongly suggestive of this disease.

"Treatment should be directed to eliminate the photosensitizing agent, if possible, and to stable the horse during the day to avoid the sunlight," Cerundolo suggests. Hepatic disease creates a poor prognosis. "Anti-inflammatory drugs may be used to reduce the inflammation," he continues. "Systemic antibiotics and topical treatments to reduce the secondary infections are also indicated." Predisposed horses should not eat any photosensitizing agents and should have limited exposure to sunlight by being kept indoors during the day or switched to a shaded pasture or paddock.

Pastern dermatitis, also called dew poisoning, greasy heel, scratches, or mud fever, is a moist exudative dermatitis affecting horses at the caudal heel and pastern area. It's not a condition itself, Cerundolo states, but describes a clinical presentation that might be caused by a long list of skin conditions, including infection (viral, bacterial, fungal, or parasitic), allergies, irritants, autoimmunity, and photosensitization.

"Initially the heel is affected, presenting with swelling, exudation, and loss of hair," says Cerundolo. "Lesions may progress to involving the surrounding areas with matting of the hair, crusts, ulcers, fetid odor, and sometimes lameness." The lesion might be due to primary or secondary infection. Horses which have long fetlock hair and/or are kept in muddy environments are particularly prone to this condition.

Numerous diagnostic tests might be necessary in order to achieve an accurate diagnosis; these might include microscopic examinations to rule out parasites as well as fungal and bacteriological cultures.

"Treatment should be directed to the underlying cause," Cerundolo advises. "Owners often start to treat their horses with various products bought over the counter that may be not effective, delay the resolution, or even cause a skin irritation--thus complicating the clinical picture. Clip affected areas of horses with long fetlock hairs to allow proper inspection and utilize a topical treatment with appropriate drugs. Anti-inflammatory drugs may be necessary to reduce the inflammation."

With prompt, proper treatment, most cases of greasy heel can be easily treated following the veterinarian's advice.

Maintaining a clean environment and lower limbs along with routine examinations can reduce the risk of this condition and help the horse heal more quicky.

Bumps and Lumps

Nodular necrobiosis (equine collagenolytic granuloma) is the most common nodular skin disease in horses. You'll see one or more firm nodules (0.2-1.2 inches, or 0.5-3.0 cm, wide) located in the deep skin layer, primarily on the withers, neck, and back, although they also can appear elsewhere.

Says Bedenice, "The overlying skin and hair coat usually appear normal, although some larger lesions may occasionally ulcerate. There is usually no pain or pruritus (itching) associated with the disease."

There are no known predisposing factors, nor is the cause known, although some sort of hypersensitivity reaction related to insect bites and mosquitoes is likely. Incidence is higher during the warm seasons due to higher insect activity.

Diagnosis is usually based on clinical signs, with skin biopsies or fine needle aspirates providing a definitive diagnosis in questionable cases. Care should be taken to differentiate nodules from hives and other bumps of similar appearance.

Treatment is usually not necessary because the nodules are frequently no more than a blemish. In addition, they can come back. For larger nodules at inconvenient sites such as the saddle area, therapy consists of corticosteroid treatment and/or surgical removal of the nodules. Nodules with excessive mineralization are slower to respond to corticosteroids, Bedenice notes. There is also a risk of infection.

"Corticosteroids such as triamcinolone or methylprednisolone can be injected directly into the lesion or perilesionally (around the lesion) since the nodules are very dense," says Bedenice. "This procedure may be repeated at two-week intervals, if necessary. If large numbers of nodules are present, systemic corticosteroid treatment is used. Single, small nodules that do not interfere with the saddle area are frequently left untreated." Avoiding turnout during times of high mosquito incidence (dawn and dusk) reduces risk.

Melanoma is a potentially malignant tumor of pigmented skin cells. Notes Bedenice, "Tumor formation occurs in the deep and superficial skin layers. Of variable size, the tumor is mostly pigmented (gray to black), nodular, hairless, and is typically located underneath the tail, around the anus, external genitalia (vulva, sheath, penis), udder, parotid gland (a salivary gland), or eye."

Although very common in aging gray horses, melanoma can be seen in any color horse, Bedenice states. Occurrence is higher in Arabians, Percherons, and Lipizzans, with most affected horses being six years old or older. There is no known prevention.

Diagnosis is usually via clinical signs. Biopsies are generally not necessary or recommended in typical cases, Bedenice warns, as any incomplete resection might increase the tumor's malignant potential.

Treatment varies: "Older gray horses are usually left untreated as the location of these tumors seldom allows complete surgical excision, the rate of metastasis is low, and the recurrence rate is high," Bedenice says. "Freezing (cryotherapy), an intralesional injection with the immunomodulator BCG (Bacille Calmette-Guerin), or cisplatin (a chemotherapeutic) has yielded variable success. Treatment with cimetidine, a drug with immunomodulatory potential, may be tried and has shown some success in limiting the progression of the disease and decreasing tumor size in some cases. However, the tumor persists indefinitely, sometimes only as a cosmetic blemish, while larger masses around the perineum or penis may eventually cause difficulties with urination and defecation."

If metastasis to internal organs doesn't occur, then the life expectancy is good for horses with typical melanomas. However, seemingly benign growths can become invasive and metastasize years later, and some atypical melanomas can be more malignant.

Sarcoids are benign, but locally aggressive skin tumors unique to horses, mules, and donkeys; they reign as the most common equine tumors.

"Sarcoids are usually classified into three different types--warty, fibroblastic (proud flesh), and occult tumors," explains Bedenice. "They can occur anywhere on the body, but are most commonly seen on the head, legs, and lower abdomen. The warty type is usually small (under two inches, or five centimeters) and sessile (attached at the base) or pedunculated (attached by a stalk). The fibroblastic tumor appearance can range from a nodal lesion covered by intact skin to tumors with ulcerated surfaces. Firm areas of skin thickening may also involve the eyelid and periocular (near the eye) region."

The bovine papilloma virus has been implicated in the etiology, Bedenice states. "Genetic susceptibility and skin trauma may play a role in tumor development," she adds. "Flies may serve as vectors."

Diagnosis is by clinical appearance, location, and elimination of other nodular or tumorous lesion causes. "Bacterial granulomas, deep fungal or parasitic infections, as well as other tumor types should be considered in the differential diagnosis," suggests Bedenice. "Diagnostic biopsies may be performed, although partial biopsies of warty sarcoids may be contraindicated as trauma to the lesion may initiate transformation to more aggressive, locally invasive growths."

Usually, only the more aggressive fibroblastic tumor type is treated, while benign neglect may be indicated for static, warty sarcoids. Says Bedenice, "Surgical excision is frequently impractical due to inaccessible tumor location and high recurrence rates. Radiation therapy, freezing, as well as intralesional injection of chemotherapeutic (cisplatin) or immunomodulating agents have been used successfully."

Prognosis for survival is generally good for animals with single tumors, as they do not metastasize. However, the degree of local invasiveness, tumor size, number, and location affects the prognosis. In addition, there is a high rate of recurrence. Fly control can reduce the tumor incidence.

Maximizing Success

Fortunately, most of the usual conditions are treatable or manageable and are seldom life-threatening. But as with most veterinary problems, accurate diagnosis and prompt, proper treatment enhances the outcome. So whether it's a weird bump or something creepy and cruddy, don't leave it to nature to deal with; seek veterinary counsel for a better resolution.


Sloet, M. The Practitioners Guide to Equine Dermatology. Leeuwarden, The Netherlands: Uitgeverij Libre BV, 2001.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More