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Pet Owner Version

Tumors of the Skin in Horses

By

Alice E. Villalobos

, DVM, Pawspice & Animal Oncology Consultation Service

Reviewed/Revised Oct 2019 | Modified Oct 2022

Tumors are abnormal new growths of cells, also called neoplasms. Neoplasms affecting the skin or the tissue just under the skin are the most commonly seen tumors in horses. Skin tumors are diagnosed more frequently in part because they are the most easily seen and in part because the skin is constantly exposed to the external environment and the many tumor-causing factors in the environment. Chemicals, solar radiation, and viruses are just some of the things that can cause skin tumors. Hormonal abnormalities and genetic factors may also play a role in the development of tumors in and close to the skin.

All of the various layers and components of skin have the potential of developing distinctive tumors. Skin tumors can appear in many forms. Distinguishing a tumor from an inflammatory disease can sometimes be difficult. Tumors are usually small lumps or bumps, but they also can occur as hairless or discolored patches, wheals, or nonhealing ulcers. Because skin tumors are so diverse, identifying them should be left to a veterinarian.

Tumors may be benign or malignant (cancerous). Benign tumors are localized and do not spread to other parts of the body. Malignant tumors can expand into nearby tissues and spread to distant sites. Distinguishing a benign tumor from a malignant tumor requires specialized knowledge and laboratory equipment. A veterinarian can perform a fine needle aspiration of cells or a biopsy (a procedure that removes a small amount of tissue from a tumor) for further examination.

Treatment depends largely on the type of tumor, its location and size, and the overall physical condition of the horse. For benign tumors that are not ulcerated and do not impair the horse’s normal routine, no treatment may be necessary. This may be the most prudent option, especially in aged horses. For malignant tumors or benign tumors that inhibit normal activities or are cosmetically unpleasant, there are several treatment options. For most tumors, surgical removal is the most effective. It is also probably the least costly option and the one with the fewest adverse effects. If malignancy is suspected, a portion of tissue surrounding the tumor will also be removed. For tumors that cannot be completely removed, partial removal may prolong the life of the horse. Radiation treatment or chemotherapy may also be used to provide your horse with a better outcome.

Basal Cell Tumors and Carcinomas

Basal cell tumors and basal cell carcinomas include tumors of the hair roots, cysts on the base of hairs, sweat gland tumors, connective tissue tumors, and more. Even though tumors commonly occur in or near the skin, many of these are either benign (not cancerous) or treatable if found early. Thus, as with human tumors, early treatment offers the best possibility for a successful outcome.

Basal cell tumors are rare in horses. Most basal cell tumors in horses are benign. These tumors generally appear as firm, solitary, hairless or ulcerated lumps and may be darkly pigmented. Although basal cell tumors are benign, their growth may cause extensive ulceration and secondary inflammation. Surgical removal is effective and the treatment most often used for these tumors.

Basal cell carcinomas are the malignant (cancerous) form of these tumors. They are not common in horses. These may appear as ulcers on the head, extremities, or neck. Unlike benign basal cell tumors, these carcinomas generally are not raised up from the skin. Also, they spread, forming new ulcers. Consequently, surgical removal is the treatment of choice. These tumors spread to neighboring skin but seldom spread to other organs. Surgical removal is the best treatment.

Collagenous Nevi

Collagenous nevi are a benign buildup of collagen. They are rare in horses. They generally are found in middle-aged or older animals, most frequently on the legs, head, neck, and areas prone to trauma. They are flat to raised lumps that develop in the dermis skin layer or fat beneath the skin. Surgical removal is generally effective. Infrequently, some may grow too large to be surgically removed. Surgical removal is optional, but a biopsy is recommended to confirm the diagnosis.

Equine Sarcoids

Equine sarcoids are the most frequently recognized tumor in horses. Bovine papillomavirus, now known to be the cause of sarcoids, is transmitted to horses by biting flies. Sarcoids occur commonly on the trunk, the inner thigh region, and the face, particularly around the eyes and ears. They are frequently seen at sites of previous injury and scarring. Sarcoids should be considered tumors of partial malignancy; they may not spread to distant locations but are locally invasive.

Sarcoids can occur as single or multiple lesions in different forms, ranging from small, wart-like lesions to large, ulcerated, fibrous growths.

There is a wide range of treatment options for sarcoids. Surgical removal, local chemotherapy, cryotherapy, and laser excision are most commonly used to treat sarcoids. Local radiation therapy with implanted radioactive ions is highly effective for tumors for which surgery is not the best option, such as those around the eyes or on limbs. Combination therapy may also be used. Nontreated tumors may regress spontaneously, although more than 50% of equine sarcoids recur after surgery. New treatment methods are being developed.

Keratinized Skin Cysts

Keratin is the protein that forms nails, hooves, hair and horns. In horses, 2 types of keratin-related masses can occur. Keratomas occur within the hoof and are relatively common; dermoid cysts are rare. Both are treated with surgery.

Dermoid cysts are congenital malformations found on the back of the head or along the spine. Thoroughbred horses are the breed most likely to develop dermoid cysts. These cysts are different from other cysts in that they contain fully formed hair shafts.

Keratomas are cystic lesions in the hoof wall of the toe or, less frequently, the quarter or heel in horses. They often occur following a traumatic injury. Although the horse with a keratoma in its early stages may show no signs of the cyst, keratomas often cause lameness and deformity of the hoof wall or sole and may be associated with degradation in bones associated with the hoof.

Lymphoid Skin Tumors

Nonepitheliotropic cutaneous lymphosarcoma is the most frequently recognized form of skin lymphosarcoma in horses. The disease may be recognized at any age but is most common in young and middle-aged animals. Firm lumps are noted in the fat under the skin surface of the lower body surface. There are 2 types of nodular nonepitheliotropic cutaneous lymphosarcoma in horses, and differentiating them is important. The most common is the lymphohistiocytic form, which usually only involves the skin. Affected horses may live for years, although additional nodules are likely to develop and may interfere with breathing at an advanced stage. In contrast, the monomorphic form usually involves internal organs as well, and it progresses rapidly.

Because of the expense of antitumor drugs, treatment is generally limited to corticosteroids administered by mouth or by injection. Remission, if it happens at all, is usually short term.

Mast Cell Tumors

Skin mast cell tumors are a unique tumor form named for the type of skin cell from which they grow. Other names for these tumors are mastocytomas and mast cell sarcomas. In horses, mast cell tumors are uncommon, benign tumors. Most affected horses are young to middle-aged and male.

These tumors may develop anywhere on the body but are most common on the head and legs. Typically, there is a single, solitary mass in the dermis skin layer or in fat under the skin that may expand to involve the underlying muscles. The tumor begins as a lump. As the tumor evolves, the central lump becomes surrounded by dead tissue and pus. In the late stages, the dead tissue hardens and mast cells may be very difficult to identify. Hair loss and skin ulceration can occur. A variant of skin mast cell tumor is seen in newborn foals, in which the tumors may become widespread but disappear over time. Surgical removal is the treatment of choice. These tumors do not spread to other organs and surgical removal usually cures the disease.

Melanomas

A melanoma is a dark-pigmented skin tumor. In human medical terminology, all melanomas are malignant. Among animals, however, melanomas may be either benign (not cancerous) or malignant (cancerous). Most melanomas found in horses occur in those with gray coats, in which the coat turns gray (or white) with age. They are especially common in Lipizzaners, Arabians, and Percherons. It has been found that up to 80% of gray or white horses of these breeds may be affected.

Melanomas are usually in the form of nodules just under or embedded within the skin; multiple small nodules can sometimes form coalescing plaques. They are almost always black in color. Although often solitary, tumors may be multiple, especially in the breeds at risk. They generally occur in older horses but usually begin their development when the animals are 3 to 4 years old. The perineum and the base of the tail are the most common sites of development, but these tumors may develop in any location, including the ears. They increase in size and number over time. They can become quite large and create problems due to their size, such as causing difficulty defecating. Although most are benign, there are malignant variants that spread to other organs.

Melanomas of nongray horses are rare tumors usually found on the trunk and legs of young horses less than 2 years old. Masses appear as solitary lumps. Most are benign. However, congenital malignant melanomas may infrequently develop. Such tumors are invasive, but with little potential to spread to other organs.

Treatment consists of surgical removal or freezing. However, affected animals tend to develop additional tumors. Little is known about the use of radiation or chemotherapy for treatment. If the tumors are benign, the outcome is excellent. For malignant tumors, the outlook is guarded.

Squamous Cell Carcinomas

Squamous cell carcinomas are the most common malignant skin tumor in horses. They are most frequently seen in adult or aged horses with white or part-white coats. The breeds most at risk are Appaloosa, Belgian, American Paint, and Pinto.

Although squamous cell carcinomas can arise on any part of the body, in horses they are seen most frequently in nonpigmented, poorly haired areas near mucous membranes. Thus, the tumors can be seen most frequently around the eyes, lips, nose, anus, and external genitalia (especially the sheath around the penis). Most squamous cell carcinomas are solitary tumors. They appear as raised, irregular masses with either ulcers or pimples. Solitary tumors grow slowly. Thus, these tumors are often overlooked until defects appear on the ear tips, openings of the nose, or eyelids.

Treatment options for squamous cell carcinoma include surgery, radiation therapy, and topical chemotherapy. These tumors commonly invade deep tissue and many are not curable.

Limiting exposure to ultraviolet radiation may help prevent squamous cell carcinomas, especially in horses with white or partially white coats. This may be accomplished by providing face shades, providing shade in pastures and other open areas where the horse spends time, or keeping the horse stabled during hours of peak sunlight.

Vascular Tumors

Vascular tumors of the skin and soft tissues are benign growths that closely resemble blood vessels. These tumors are usually found on the head, legs, and abdomen. In horses, they are most common on the lower legs of animals less than 1 year old. Hemangiomas are single to multiple, circular, often compressible, red to black lumps that may resemble a “blood blister.” Hemangiomas are benign, but they tend to develop ulcers and grow quite large. Surgical removal is the recommended treatment. However, the tumors may be large and involve the lower legs, making removal difficult. In these cases, freezing or radiation treatment may be needed. More tumors do not usually develop at new sites after complete surgical removal.

Angiosarcomas can vary greatly in appearance. Most commonly, they appear as 1 or more red lumps in the skin or underlying soft tissues. Less frequently, they appear as a poorly defined bruise. Tumors that appear in the underlying soft tissues tend to be more aggressive. Surgical removal of both the tumor and a margin of surrounding tissue is the recommended treatment. The tumors in the skin do not seem to spread or invade local tissues, and recurrence rather than spread to distant locations is more common after surgical removal. These tumors seem to arise spontaneously, but chronic injury due to sun exposure may play a role in their development, especially in white-coated animals. Surface tumors are easily controlled with freezing (cryosurgery) as needed. Avoidance of further sun exposure may reduce the development of new tumors.

Warts (Papillomas)

Warts are caused by papillomaviruses. The virus is transmitted by direct contact or by contact with infected items such as bedding, blankets, saddles and other tack, and hard surfaces in the horse’s environment. It is also possible that insects may spread papillomaviruses. There are several distinct papillomaviruses. Among companion animals, papillomas are most common in horses and dogs.

Multiple warts of skin or mucus membranes generally are seen in younger animals. Single warts are more frequent in older animals, but they may not always be caused by viral infection. The period between the initial infection and the development of visible warts varies but normally takes several months.

Most warts appear as bumps with a hardened surface resembling a cauliflower. When multiple warts are present they may be sufficiently characteristic to make a diagnosis. However, there are many things that look like warts and a definitive diagnosis may require identification of the virus or its effects on individual cells.

In horses, small, scattered papillomas develop on the nose, lips, eyelids, bottoms of legs, penis, vulva, mammary glands, and inner surfaces of the ears. Warts are often seen following mild skin abrasions. They can be a problem in a herd, especially when young horses are kept together. Warts on foals may shrink or disappear in a few months as the foal’s immune system matures. When warts develop in mature horses, they often persist for over a year. Equine warts are disfiguring but benign. They should, however, be distinguished from sarcoids (see above). Warts will usually go away on their own, although the duration of warts varies considerably. A variety of treatments have been advocated with uncertain results. Surgical removal is recommended if the warts are sufficiently objectionable. However, because surgery in the early growing stage of warts may lead to recurrence and stimulation of growth, the warts should be removed when near their maximum size or when regressing. Affected animals may be isolated from susceptible ones, but with the long incubation period (months), many are likely to have been exposed before the problem is recognized.

Congenital papillomas of foals are rare and are probably a developmental defect rather than a result of papilloma virus infection. They are found anywhere on the body but most commonly on the head. Thoroughbreds may be predisposed. Present at birth, the lesions are often several centimeters in diameter, hairless, highly raised, with an uneven surface reminiscent of a cauliflower. They are benign, and surgical removal cures the condition.

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