CLASSIFICATION OF MYELOPROLIFERATIVE DISORDERS IN CATS USING CRITERIA PROPOSED BY THE ANIMAL LEUKEMIA STUDY-GROUP - A RETROSPECTIVE STUDY OF 181 CASES (1969-1992)

Bone marrow cytology in Wright or Wright-Giemsa stained smears from 181 feline patients were evaluated according to recently published criteria by the animal leukaemia study group for classification of acute myeloid leukaemias (AML) and myelodysplastic syndrome (MDS). AML was recognised in 107 (59.1%) and MDS in 39 (21.5%) cats. The frequency of acute myeloblastic leukaemia (Ml and M2 combined) was high (30.9%), erythroleukaemia (M6 and M6-Er combined) was relatively less frequent (17.1%), and acute undifferentiated leukaemia (AUL), acute monoblastic leukaemia (M5) and acute myelomonocytic leukaemia (M4) were less common, each with an incidence of less than 5.0%. Most of the Ml cats had greater than 70.0% blast cells in bone marrow compared to only a few of the M2 cats. Some blast cells in occasional AUL cats showed cytoplasmic differentiation to either erythroid or myeloid lineage or rarely to both. Type I (classical) myeloblasts predominated in all Ml cats, but occasional type II myeloblasts were seen in some cats and a few cats had some type III myeloblasts. Rubriblasts were prominent in most of the M6-Er cats. Dysplastic changes were more common in M2 than in Ml cats and included primarily dysgranulopoiesis (giant metamyelocytes and bands) and dyserythropoiesis (megaloblastic rubriblasts and rubricytes) and rarely dysmegakaryocytopoiesis (abnormal nuclear morphology). Dysplastic changes involving primarily the erythroid series were seen in M6 and M6-Er cats. Dysmyelopoiesis, in the form of occasional giant metamyelocytes and band neutrophils, was also seen in most cats considered to have myeloid hyperplasia unassociated with leukaemia or MDS. Two M2 and two MDS cats had high (>10.0%) eosinophils in bone marrow and were designated M2-Eos and MDS-Eos, respectively. Cytochemical staining was performed on bone marrow smears from 65 cases to demonstrate myeloid markers such as alkaline phosphatase (ALP), peroxidase (PO), chloroacetate esterase (CAE), and Sudan black B (SBB) and monocytic markers such as alpha-naphthyl acetate esterase (NAE) and alpha-naphthyl butyrate esterase (NBE). NAE and NBE staining reactions were usually complementary. Cytochemical heterogeneity and lineage infidelity were observed in some AML subtypes. Blast cells were positive for myeloid markers in many but not all cases of M1 and M2. Similarly monocytic markers were evident in some but not all cases of M4 and M5. Monocytic markers were also evident in blast cells of one M1 and three M2 cases, myeloid markers were present in two cases of M4, and both myeloid and monocytic markers were demonstrated in one case of AUL. Blast cells were negative for both myeloid and monocytic markers in most cases of AUL and M6. ALP-positive cells were increased distinctly (> 10.0%) only in a small number of cats (15.4%) which included cases of Ml, M2, M4, M6 and MDS. Prominent haematological abnormalities in most of the AML and MDS cases included moderate to severe thrombocytopenia, normocytic normochromic non-regenerative anaemia, normoblastaemia, leukocytosis accompanied by a left shift, and a variable number of circulating blast cells. Circulating blast cell counts of greater-than-or-equal-to 30.0% were seen in only 15.3% cats. Mean leukocyte counts were over 100 000/mul in most of the M5 and M4 cats and in an occasional Ml cat, whereas AUL cats generally had leukocyte counts within the normal range Leukopenia was evident in slightly over half of the MDS-Er cats, in nearly a third of the M2 cats, and in a small number of cats in other categories. Monocytosis was most pronounced in M5 cats and prominent in M4 cats. Other abnormalities seen in a small number of cats included hyperproteinaemia or hypoproteinaemia, hyperfibrinogenaemia, elevated icterus index, and thrombocytosis.