The most frequently occurring neoplasm in the horse is the sarcoid. In fact, sarcoids may account for up to 30% of all tumors in horses. These tumors may be locally aggressive and are non-metastatic, fibroblastic skin tumors of horses, mules, and donkeys.

Although sarcoids may occur in any breed, the Quarter Horse, Arabian, and Appaloosa breeds appear to be more susceptible to their formation. Standardbreds and Thoroughbreds appear to be more resistant to their development. Young adult horses from three to six years of age are the most commonly affected. Sarcoids may develop anywhere on the body but are most commonly located on the head, legs, and ventral trunk. These tumors may occur singly or be multiple in number. Approximately one-third of affected horses have multiple lesions.

A combination of three factors is believed to be involved in the development of these tumors including a viral agent, cutaneous trauma, and a genetic predilection. The viral agent is believed to be the bovine papillomavirus and flies may serve as a vector spreading the virus. Sarcoids frequently develop in areas of the body that were subjected to trauma six to eight months earlier. The genetic predisposition is a suggestion, but the exact genetic association has not yet been determined.

Sarcoids are classified according to their gross appearance. The verrucous type is a small, wart-like growth that typically measures less then 6 cm in diameter and has a rough, dry surface. The fibroblastic form of sarcoid is more variable in appearance and may range from a well differentiated firm nodule with an intact surface, to large masses of greater than 25 cm in diameter having an ulcerated surface prone to bleeding, appearing much like exuberant granulation tissue. The mixed form occurs when a verrucous sarcoid becomes more fibroblastic in appearance following trauma or biopsy. The occult form consists of a slow-growing, thickened area of skin with a roughened surface and alopecia (hair loss) that remains relatively the same in appearance for long periods of time.

Spontaneous remissions may occur after several years in up to 30% of cases. Following surgery, up to 50% of the tumors will recur. Autotransplantation of tumors may occur and it is important that instruments having contacted the tumor during surgical excision do not come in contact with normal skin areas.

The gross appearance of a sarcoid may resemble several other bacteria, fungal, parasitic, and neoplastic diseases and therefore histopathology is usually necessary for a definitive diagnosis. Wide surgical incision is preferred to a biopsy since the trauma associated with the taking of a biopsy may dramatically stimulate the growth rate and aggressiveness of the tumor. Secondary infections often complicate biopsy results and therefore a large sample size is needed to ensure the initiating sarcoid is not overlooked on histopathology.

Adjunctive therapies to surgical excision include cryotherapy, hyperthermia, carbon dioxide, laser photoablation, topical chemotherapy, radiation therapy, radiotherapy, and intralesional chemotherapy and immunotherapy. When adjunctive therapies are combined with wide surgical excision, success rates of 65 to 95% remission may be obtained.


References:

Maxie, M Grant Editor. Pathology of Domestic Animals. Vol 1. Saunders. 2007. Pp. 606.

Brown, Christopher, Joseph Bertone. The 5-Minute Veterinary Consult Equine. Blackwell Publishing. 2005 Pp. 772-773.

Smith, Bradford. Large Animal Internal Medicine. 2nd Edition. 1996. Mosby. Pp. 1432-1435.