Atypical chronic myeloid leukemia in a German Shepherd Dog

A 4-y-old neutered male German Shepherd Dog was presented with a 3-d duration of lethargy, restlessness, and vomiting. Physical examination revealed generalized lymphadenopathy, pale mucous membranes, systolic heart murmur, dehydration, and fever. Hematologic abnormalities included moderate-to-marked leukocytosis, characterized by neutrophilia with a left shift to progranulocytes and 2% presumptive myeloid blasts, marked anemia that was nonregenerative, and marked thrombocytopenia. Dysplasia was evident in neutrophils and platelets. Bone marrow examination revealed marked myeloid and megakaryocytic hyperplasia with 7% blasts, erythroid hypoplasia, and trilineage dysplasia. Flow cytometric analysis confirmed that bone marrow cells were mostly of neutrophil lineage, with reduced expression of common leukocyte antigens (CD45, CD18) and neutrophil-specific antigen. Bone marrow cells were cytogenetically analyzed for the breakpoint cluster region-Abelson oncogene using multicolor fluorescent in situ hybridization. The genetic aberration was present in 7% of cells, which was a negative result (>10% of cells is considered positive). Euthanasia was elected. Histologic examination showed extensive infiltration of multiple organs by neoplastic myeloid cells, with effacement of lymph node and splenic architecture. The final diagnosis was atypical chronic myeloid leukemia (aCML), an uncommon myeloproliferative disorder with features of myelodysplastic syndromes (dysplasia) and chronic leukemia (neutrophilic leukocytosis with <20% marrow blasts, extramedullary infiltrates). The trilineage dysplasia, lack of monocytosis, and supporting cytogenetics distinguish aCML from CML, chronic neutrophilic leukemia, and chronic myelomonocytic leukemia.