Mycosis fungoides in a horse.

A 17-year-old Quarter Horse mare was examined to determine the cause of a vulvar mass. Differential diagnoses for the swollen, ulcerated tissue included hypersensitivity reaction to insect stings or bites and cutaneous neoplasia. During the next 4 months, the mass enlarged involving the skin of the perineum and ventral aspect of the abdomen with secondary dependent edema of both hind limbs. Histologic examination of biopsy and necropsy specimens revealed changes consistent with a diagnosis of mycosis fungoides (cutaneous T-cell lymphoma). Diagnostic features included invasion of neoplastic lymphocytes into the epidermis and detection of T-cell lineage of neoplastic cells. Location of lesions at a mucocutaneous junction, association with epidermal ulcers, and progressive skin involvement mimic the condition in human beings and other species. Prognosis is poor when the condition is at the tumor stage (invasion of deep tissues).