Subleukaemic T-Lymphoblastic Leukaemia in a Horse

A 20-year-old Dutch Warmblood gelding was referred with clinical signs of anorexia, weight loss, intermittent fever, cough, subcutaneous oedema and exercise intolerance. Haematological examination revealed the presence of blast cells, decreased lymphocytes, mild thrombocytopenia and anaemia but no leucocytosis. Serum analyses detected elevated aspartate aminotransferase and gamma-glutamyl transferase activities and triglyceride concentrations. Twenty-two days after the initial visit, the horse died after showing clinical signs of decreased appetite, increased body temperature, tachypnoea and tachycardia. At necropsy, there was mild splenomegaly but enlarged lymph nodes, masses or nodules were not seen in any organ. Histologically, neoplastic cells were seen in the subcapsular and medullary lymph sinus of the mediastinal, axillary, mesenteric and renal lymph nodes. The bone marrow was densely cellular with numerous large round neoplastic cells that had round nuclei with clear nucleoli and scant cytoplasm. The neoplastic cells were immunopositive for CD3 but negative for CD20, BLA36, CD204, Iba-1, CD204 and granzyme B. Based on these findings, the neoplasm was diagnosed as subleukaemic T-lymphoblastic leukaemia, which, to the best of our knowledge, is the first report of this neoplasm in horses.