A rare case of acute erythroid leukemia (AML M6-Er) with pleural metastasis and malignant pleural effusion in a cat.

An 11-year-old castrated male Turkish Angora cat presented with chronic lethargy and inappetence was diagnosed with non-regenerative anemia, increased nucleated red blood cells, and thrombocytopenia. Tests for anemia-related infections were negative. Imaging revealed pleural effusion, hepatomegaly, splenomegaly, and enlarged abdominal lymph nodes, raising suspicion of a round cell tumor in the spleen. His anemia and pleural effusion did not improve despite chemotherapy with cytarabine administration. Erythroid precursor cells were identified in the pleural fluid. His condition deteriorated, progressing to dyspnea and necessitating euthanasia. Necropsy findings revealed extensive infiltration of CD71-positive neoplastic cells and a myeloid-to-erythroid precursor ratio of 0.02, indicating pleural metastasis. Pleural metastasis should be considered a differential diagnosis in feline acute erythroid leukemia cases presenting with pleural effusion.