from the Vet Corner Archives

Part of Horse Previews Magazine website. Posted on 12/03/99; 2:00:00PM.

Veterinary Corner 12/99: Common Skin Tumors of the Horse

by Scott Habegger, DVM
Edgecliff Equine Hospital
S. 1322 Park Road, Spokane, WA 99212 * 509/924-6069

Several skin masses of the horse may have a similar gross appearance but may vary widely in their cause and behavior. These masses may occasionally be neoplastic and further diagnostic procedures should be considered. Strictly speaking, the word tumor refers to a swelling that could result from a wide range of inciting causes. Today, the term tumor is usually applied to neoplastic masses that may cause swellings on the body. Neoplasia is generally defined as an abnormal mass of tissue that continues to grow and is unresponsive to the regulatory influences that control normal cell growth. Three of these common skin tumors of horses are reviewed in this month's article for your consideration.

The most common neoplasm diagnosed in horses is the equine sarcoid. Sarcoids account for up to one third of diagnosed tumors of horses and tend to be locally aggressive, fibroblastic, and nonmetastatic. Sarcoids can occur on any part of the body either singly or in clusters. The most common locations tend to be the head, ventral abdomen, and the limbs. Sarcoids can also assume a variety of appearances and are generally classified into four broad categories: verrucous (wart-like), fibroblastic (proud flesh-like), mixed verrucous and fibroblastic, and flat or occult (flat, crusty, scaling, non-haired). Sarcoids are generally not life threatening but can be extremely unsightly and depending upon their location can impair the performance of the horse. Sarcoids are generally first observed between the ages of 3 and 6 years of age but have been reported in horses as young as 1 year of age. Please refer to the December 1998 issue of Horse Previews for a more in depth discussion of sarcoids and treatment options.

Squamous cell carcinoma has been reported to be the second most commonly diagnosed tumor of the horse, representing up to twenty percent of diagnosed neoplasms. The average age of diagnosis has been reported to be between 8.6 and 14.6 years, but has been reported from as young as 1 year of age to 29 years. UV radiation has been implicated as a contributing factor for development of this tumor and lightly pigmented horses tend to be at an increased risk. Cutaneous squamous cell carcinomas arise from epidermal cells and are often locally invasive but tend to be slow to metastasize. However, it has been reported that the frequency of metastasis is as high as 18.6%. When metastasis occurs, local lymph nodes are generally the affected sites. Squamous cell carcinoma can develop in virtually any location in the integument but tends to occur more commonly in nonpigmented areas of skin at mucocutaneous junctions such as the eyelids, lips, nose, vulva, prepuce, and penis. These tumors commonly arise as solitary lesions, but adjacent skin may be at risk for development of the tumor, especially in the prepuce. The gross appearance of squamous cell carcinoma is quite variable. Early lesions tend to be small superficial nodules that may be covered with normal skin. As the tumor advances, the overlying epidermal layers are destroyed and ulceration, necrosis and a foul odor may be observed as the lesion gradually enlarges.

Equine melanocytic tumors (melanomas) are common skin tumors and have a reported incidence rate of 3.8% to 15% of diagnosed skin tumors in horses. Melanomas may occur anywhere on the body and although they can arise in horses and mules with any coat color, they are generally most frequently reported in gray and white horses. At least 80% of old gray horses develop melanocytic tumors. It has been reported that approximately 95% of melanocytic tumors are benign in horses at the time they are identified. It has been reported that at least 66% of equine melanocytic tumors eventually develop malignant behavior.

The most common locations at which these tumors originate include the skin underneath the tail and around the rectum and external genitalia. These tumors less commonly occur on the head, limbs, neck, ears, eyelids, and internal locations. Melanomas commonly develop as firm, flat, solitary or multiple, nonulcerated masses underneath the skin. They are commonly black or dark brown, but some may occasionally lack pigment.

Treatment options for skin tumors of horses are quite variable and first depend on obtaining an accurate diagnosis of the type of tumor involved. Skin masses in horses often originate from non-neoplastic causes but may share a common appearance at the early stages with several of the more aggressive neoplastic masses. Because of the common initial appearance of many of these skin masses it is important to have them evaluated by your veterinarian. In many of these cases your veterinarian may decide that a biopsy of the affected skin is indicated to determine the cause and treatment options of any unusual skin masses that you may have observed on you horse.

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