Multicentric Mastocytoma in a Horse

Abstract

A 9-year-old 610-kg Paint horse gelding presented for evaluation of dyspnea of 5 days’ duration. The gelding had been retired for at least 2 years after an episode of laminitis and had no history of illness in that time until the preceding 4 weeks. The owner initially noted slight lethargy and decreased appetite, followed 1 week later by the appearance of multiple, small (,1 cm diameter and height), cutaneous circular nodules initially on the face and subsequently on the neck, lateral thorax, lateral abdomen, and upper limbs. The referring veterinarian examined the horse 12 days before presentation and did not report any abnormalities on physical examination except the multiple cutaneous lesions. Lesions on the neck and lateral thorax were biopsied and the horse administered 4 doses of procaine penicillin (20,000 IU/kg, IM, q12 h). The horse developed an increased respiratory rate and effort 5 days before presentation and was treated once daily orally for 5 days with pyrilamine maleate (300 mg), pseudoephedrine hydrochloride (300 mg), trimethoprim sulfadiazine (30 mg/kg), and phenylbutazone (1 g).