Lymphosarcoma with leukemia in a horse.

A 17-year-old Arabian gelding was presented because of CNS depression and weight loss. Physical examination revealed generalized lymphadenopathy, pitting edema, and ascites. The diagnosis of lymphosarcoma with leukemic manifestations was based on cytologic and histopathologic examination of lymph node biopsy samples and a peripheral leukocyte count of 98,300/microliters, 94% of which were mature lymphocytes. These circulating lymphocytes lacked B-lymphocyte markers and did not respond to mitogenic stimulation, indicating functional incompetence. Though ultrastructural findings were equivocal, most morphologic changes indicated these cells were of T-lymphocyte lineage. Virus particles were not observed during transmission electron microscopy of these lymphocytes.