MYCOSIS-FUNGOIDES IN A HORSE

A 17-year-old Quarter Horse mare was examined to determine the cause o f a vulvar mass. Differential diagnoses for the swollen, ulcerated tis sue included hypersensitivity reaction to insect stings or bites and c utaneous neoplasia. During the next 4 months, the mass enlarged involv ing the skin of the perineum and ventral aspect of the abdomen with se condary dependent edema of both hind limbs. Histologic examination of biopsy and necropsy specimens revealed changes consistent with a diagn osis of mycosis fungoides (cutaneous T-cell lymphoma). Diagnostic feat ures included invasion of neoplastic lymphocytes into the epidermis an d 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 o ther species. Prognosis is poor when the condition is at the tumor sta ge (invasion of deep tissues).