Presumptive Histiocytic Neoplasm with Unusual Immunophenotype in a Cat

True histiocytic proliferative diseases have been infrequently documented in the cat, with progressive histiocytosis and histiocytic sarcoma representing the bulk of the reported cases. A 5-year-old, neutered male domestic shorthair cat presented with a 1-month history of anorexia, depression, mucosal pallor and persistent non regenerative anemia. On admission, the cat appeared depressed, icteric, dyspneic and had moderate splenomegaly. The cat had a severe, temporarily regenerative anemia, and a blood smear evaluation revealed numerous neutrophil- and monocyte-phagocytosed erythrocytes. Splenic and bone marrow aspirate smears were predominated by a monomorphic population of individual or aggregated round-to-oval cells, likely of histiocytic origin that often phagocytosed erythrocytes and leukocytes. A comprehensive immunocytochemical staining panel failed to definitively identify the neoplastic cells. This report emphasizes the clinical course, the sequential hematological abnormalities, the cytology and the unusual immunophenotype of a presumptive histiocytic sarcoma in a cat.