Mastocytosis and mastocytoma
Mast-cell tumors are generally benign, solitary lesions of the skin and subcutis characterized histologically by sheets of
mast cells. Mastocytoma is a benign neoplasm of horses between age 1 and age 18 with a mean age of 9.5 years. It is most often
see in Arabians, with no sex predisposition. Tumors are commonest on the trunk and head, less so on the neck and extremities.
Gross lesions range from 3 mm to 8 cm in diameter, average 3.5 cm. The majority are firm, tan and raised cutaneous nodules.
Surface ulceration and foci of caseous debris are seen. After surgical removal, there is no recurrence in about 90 percent
of the cases, followed for six years or more. Metastasis is uncommon.
Clinically they occur predominantly in males at a mean age of 7.7 years. Nodules develop over weeks to months. There is no
breed predisposition. Most tumors are on the head, neck and chest, and they range from 1 cm to 20 cm. In most cases, they
are solitary nodules with aggregates of mast cells in the dermis and subcutis and occasional extensions into underlying musculature.
Keratoma has been seen in the hooves of horses for many years. Microscopically they consist of cores of mature keratin and
debris surrounded by a rim of neoplastic squamous epithelium. Inflammation, secondary infection and necrosis are common. Surgical
excision is curative with a one-year follow-up.
Cutaneous hemangioma and hemangiosarcoma are often found in horses 1 year of age and younger, but they also have been seen
in horses 8.5 to 13 years of age. Lesions are most common on the head, neck and extremities. Grossly, lesions range from 1.5
cm to 31 cm, and most are ulcerated and bleed easily. These cancers are seen primarily in Arabians, but also in Thoroughbreds,
Quarter horses and Appaloosas, among others.
Basal-cell tumors are benign cutaneous neoplasms of basal epidermal layers and adnexa (hair follicles, cutaneous glands).
Although such skin tumors are common in dogs and cats, they are rare in horses. The age range of affected horses is 4 to 20
years with a mean age of 10.6 years. There is no breed predisposition.