Mastocytosis

Stephen D. White

In horses, mastocytosis (mast cell tumors, mastocytomas) occur in animals 1 to 18 years old (average, 9 years), with no breed predilection. A predilection for males has been proposed but is not always sustained.^1,2 In addition, multiple mast cell tumors resembling urticaria pigmentosa of humans may occur in newborn foals; these occur spontaneously and regress.^2,3

Equine mastocytosis is usually solitary and occurs most often on the head and trunk, although occasionally multiple tumors may be seen.⁴ Lesions are 0.5 to 20 cm in diameter, well to poorly circumscribed, firm to fluctuant, dermal or subcutaneous, and may or may not be alopecic, ulcerated, and hyperpigmented.

Histology may vary from sheets of mast cells with few eosinophils (presumably early lesions) to sections showing both sheets of mast cells with numerous eosinophils and collagen degranulation. Ultrastructural features are similar to those noted in mastocytomas of other species.⁵

Clinically, most mast cell tumors in horses do not recur after being excised (22 of 25 in one study).⁵ In one anecdotal case of metastasis from a tumor on the muzzle to the regional lymph nodes, the tumor and the nodes were removed, and the horse was clinically sound 3 years later. There is some debate as to whether equine mastocytomas are benign neoplasias or focal dysplasias of mast cells.

Mastocytomas are uncommon in cattle but typically occur in young calves.^6,7 One report describes a cutaneous mast cell tumor in a kid goat.⁸ Although equine mastocytomas are almost always benign, mastocytosis in cattle may be malignant or benign and therefore carries a more guarded prognosis.