Megakaryoblastic Leukemia in a Dog

A 4-year-old, intact, male Cocker Spaniel was lethargic, anorectic and had mild diarrhea of two weeks duration. Physical examination revealed weight loss, pale mucous membranes, lymphadenopathy, tense abdomen, hepatomegaly and splenomegaly. Results of a CBC included severe anemia (HCT 12.5%), thrombocytopenia (platelets 7 x 109/L) and leukopenia (2.2 x 109/L). Approximately 1/3 of the peripheral blood leukocytes were blast cells with small pseudopods and occasional binucleated cells, which suggested they were megakaryoblasts. Deparaffinization, antigen retrieval, immunohistochemical staining and counterstaining was performed on bone marrow smears and on histologic sections of the liver. The slides were incubated with a mouse monoclonal anti-CD79a antibody, a mouse monoclonal anti-CD61 antibody and a rabbit polyclonal anti-factor VIIIRA antibody. CD61 was clearly positive in mainly the multinucleated cells, confirming the diagnosis of megakaryoblast leukemia. Factor VIIIRA was weakly positive whereas CD79a appeared negative