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Table 2 Classification of myelocytic leukemia, main characteristics and cases reported in horses

From: A review of current knowledge of myeloproliferative disorders in the horse

Type of myeloid leukemia Main characteristics and identification Equine cases reported in literature
 AUL Acute undifferentiated leukemia Considered a separate entity of AML-M0. Even up to 100% of the total nucleated cells can be blasts in bone marrow or blood. Highly immature atypic cells. In humans, some AUL are classified as ‘unclassifiable’ leukemia Not reported
Acute myeloid leukemias
 AML-M0 Acute myeloid leukemia Blast cells ≥ 90% of myeloid cells. < 5% of blast cells in general circulation are stained with myeloperoxidase. No Auer blast cells. Ultrastructural detection of lysosomes with myeloperoxidase or immunophenotyping required for characterization Not reported
 AML-M1 Acute myeloblastic leukemia (without maturation)  > 90% of the non-erythroid cells are myeloblasts. Blasts with less than 10% cytoplasmic granulation. At least 5% of the population of blasts stained with Sudan Black B and myeloperoxidase. Auer rods may be present Not reported
 AML-M2 Acute myeloblastic leukemia (with maturation) 30–90% of the non-erythroid cells are myeloblasts. At least 10% of the neoplastic cells shows maturation. At least 50% stained with myeloperoxidase and may have Auer rods Clark et al. [17]
 AML-M3 Promyelocytic Predominance of promyelocytes in blood and bone marrow. Associated with recurrent genetic abnormalities in humans Not reported
 AML-M4 Myelomonocytic Both granulocyte and monocyte differentiation. > 20% blast cells in blood or bone marrow. > 20% of bone marrow cells must be of monocyte lineage Bienzle et al. [19]
Blue et al. [20]
Boudreaux et al. [21]
Brumbaugh et al. [22]
Buechner-Maxwell et al. [23]
Mori et al. [24]
Ringger et al. [25]
Spiers et al. [26]
Miglio et al. [12]
 AML-M5 Monocytic Peripheral monocytosis. Neoplastic monocytes comprise more than 80% of non-erythroid cells in bone marrow. Two subtypes:
AML-5A: predominantly monoblasts (> 80%)
AML-5B: mixture of monoblasts and promonocytes (< 80% blasts)
Burhardt et al. [27]
Monteith and Cole [28]
Latimer and White [29]
 AML-M6 Acute erythroid leukemia
Subtype M6A
Dual lineage with a co-production of myeloblasts and erythroblasts. > 50% of the total nucleated cells in bone marrow are of erythroid lineage; at least 30% myeloblast precursors Not reported
 AML-M6 Acute erythroid leukemia
Subtype M6B
Undifferentiated or pronormoblastic immature cells the erythroid lineage > 80%. M: E ratio < 0.02:1 Forbes et al. [32]
Panziera et al. [32]
 AML M7 Megakaryocytic Blast cells > 20% of the circulating cells or bone marrow and at least 30% of the marrow cells is of megakaryocyte lineage Not reported
 AML-M8 Acute basophilic leukemia Stain with toluidine blue. Metachromatic granules typical of mast cells or basophils Furness et al. [37]
Myeloproliferative neoplasms
 CGL Chronic granulocytic leukemia Proliferation of cell lineages of relatively mature neutrophils, eosinophils or basophils
Relatively mature granulocytes predominate in blood marrow
Searcy and Orr [38]
Johansson et al. [39]
 CEL Chronic eosinophilic leukemia Proliferation of eosinophil precursors with high number of eosinophils in blood, bone marrow or peripheral tissues. Immature or atypical eosinophils Morris et al. [42]
 CBL Chronic basophilic leukemia Increased number of basophils in bone marrow and blood. Blast cells accounted for 17% of all nucleated cells. Positive staining to omega-exonuclease, naphtol AS-D chloroacetate esterase and Sudan Black B Not reported
 Polycythemia vera or primary erythrocytosis It affects all bone marrow cell lines and manifest itself with erythrocytosis. Sometimes accompanied by thrombocytosis and leukocytosis. Erythropoietin values within normal limits Steiger and Feiger [49]
McFarlane et al. [48]
 ET Essential thrombocythemia Overproduction of megakaryocytes in bone marrow Not reported
 CMoL Chronic monocytic leukemia Excessive proliferation of granulocytic and monocytic cells Spiers et al. [26]
 CMMoL Chronic myelomonocytic leukemia Circulating myeloid blast cells can represent 5–10% of the leukocytes. Hypersegmented granulocytes and monocytes Not reported
Myelodysplasic síndromes
 MDS Myelodysplastic syndrome Ineffective hematopoiesis with dysplastic alterations affecting one to multiple cell lineages
Hypercellular marrow, blasts count < 20%, concurrent peripheral cytopenia. AML can be developed after MDS
Durando et al. [10]
Miglio et al. [12]