Acute Leukemia in Horses.

This study pertains to the classification, clinical and pathological features of acute leukemia in horses. Using a sample of twelve horses, the research identified six cases each of acute lymphocytic leukemia (ALL) and acute myeloid leukemia (AML) based on a number of markers and symptoms.

Classification of Leukemia in Horses

• Leukemia involves a division into either acute or chronic lymphocytic and myeloid types. The categorization is contingent upon the proportion of blast cells, the cell morphology, and the expression of specific antigens on the neoplastic cells.
• However, the classification for horses can be a challenging process especially when blasts predominate and due to the limited availability of antibodies to detect different cell types.

Results & Observations

• Out of twelve horses diagnosed with acute leukemia, six had acute lymphocytic leukemia (ALL). This was based on the predominance of blast cells, absence of granulocytic or monocytic differentiation, and the detection of CD3, CD20, and/or CD79a antigens with the use of immunohistochemistry.
• The other six horses were diagnosed with acute myeloid leukemia (AML), with myelomonocytic (four cases), basophilic (one case) and eosinophilic (one case) differentiation. This was based on an excess of 20% bone marrow blasts and partial leukocytic differentiation.
• The antibodies Iba-1/AIF-1, CD172a, and CD163 showed reactivity in all the AML cases.

Systemic Effects and Euthanization

• Common systemic effects seen in these horses included thrombocytopenia (11 horses), neutropenia (10 horses), and anemia (8 horses), with no cases of leukocytosis.
• All of the horses displayed high levels of blood blasts and had significantly increased levels of acute phase proteins in the serum, which are markers of inflammation and often indicate underlying disease.
• The bone marrow cellularity varied from 30% to 100% and the proportion of blast cells ranged hugely, from 80% to 100% in ALL and from 30% to 60% in AML.
• Given the severity of their illnesses, the horses were euthanized after diagnosis within a matter of days or weeks.

Unique Features of Acute Leukemia in Horses

• Distinct features of acute leukemia in horses as compared to other species were the variable lymphocyte antigen expression in ALL and frequent inflammation in both ALL and AML.