Acute Leukemia in Horses.
Authors: Cooper Carina J, Keller Stefan M, Arroyo Luis G, Hewson Joanne, Kenney Daniel, Bienzle Dorothee
Journal: Veterinary pathology
Summary
Acute leukemia in horses presents diagnostic and clinical challenges that differ markedly from other species, prompting a detailed characterisation of 12 cases by Cooper and colleagues. Using morphological assessment and immunohistochemical staining with cell-type-specific antibodies, the authors classified six cases as acute lymphocytic leukaemia (ALL) and six as acute myeloid leukaemia (AML), revealing that equine acute leukaemia frequently exhibits variable lymphocyte antigen expression and concurrent inflammatory changes not typically observed in human or small animal patients. Clinical presentation was consistently severe: 11 of 12 horses presented with thrombocytopenia, 10 with neutropenia, 8 with anaemia, and notably all demonstrated circulating blasts without leukaemoid responses (no leucocytosis), alongside elevated acute phase proteins in 10 cases, though bone marrow cellularity was variably high (30–100%) with blast proportions of 80–100% in ALL and 30–60% in AML. Prognosis was uniformly poor, with horses euthanised within days to weeks of diagnosis. For equine practitioners, these findings underscore the importance of bone marrow evaluation in horses presenting with cytopenias and blast cells, whilst acknowledging that immunophenotyping remains essential for accurate classification and that the absence of leukaemoid response should not exclude acute leukaemia from the differential diagnosis.
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Practical Takeaways
- •Acute leukemia in horses presents as a rapidly fatal disease; clinical signs include thrombocytopenia, neutropenia, and anemia without leukocytosis—prognosis is grave and euthanasia is typically the outcome
- •Diagnosis requires bone marrow examination and immunohistochemistry given variable antigen expression in equine cases; ancillary testing with CD3, CD20, CD79a, and myeloid markers helps differentiate ALL from AML
- •This is a rare condition with few treatment options; early recognition of the clinical pattern (severe cytopenias + circulating blasts + elevated acute phase proteins) is important for humane case management
Key Findings
- •12 horses with acute leukemia (6 ALL, 6 AML) presented with thrombocytopenia (n=11), neutropenia (n=10), anemia (n=8), and circulating blasts in all cases but no leukocytosis
- •ALL cases showed variable lymphocyte antigen expression (CD3, CD20, CD79a) while AML cases demonstrated myelomonocytic (n=4), basophilic (n=1), or eosinophilic (n=1) differentiation with >20% bone marrow blasts
- •All horses were severely ill at diagnosis with 10/12 showing elevated acute phase proteins; all were euthanized within days to weeks
- •Equine acute leukemia differs from other species by variable lymphocyte antigen expression and frequent concurrent inflammation in both ALL and AML cases