Oral T-cell-rich large B-cell lymphoma in a horse.

Lymphoma is the most common tumor of hematopoietic origin in horses. The course of the disease and clinical signs vary greatly, depending on tumor location and extent. The aim of this report is to describe the occurrence of T-cell-rich oral large B-cell lymphoma with marked local infiltration in a 25-year-old Crioula mare. The mare showed an increase in volume on the right side of its face, dyspnea, anorexia, and progressive weight loss. The clinical assessment showed that the lesion was located in the rostral and caudal sinuses and was markedly invasive to adjacent structures. The autopsy revealed a yellow mass with a soft to firm consistency, infiltrating multiple bones in the skull, and extensively invading the hard palate and masseter muscle. Histologically the mass comprised an undifferentiated malignant neoplasm characterized by a densely cellular neoplasm composed of large CD20 + neoplastic B-lymphocytes admixed with sheets of small, CD3 + reactive T-lymphocytes supported by delicate fibrovascular stroma leading to the diagnosis of oral T-cell-rich large B-cell lymphoma.