Approach to Interpretation of the Leukogram in Horses

Reference intervals for equine patients are often wide due to the inclusion of a range of ages and breeds; the benefits of using breed- or age-specific reference intervals must be balanced against the preference for reference intervals generated by the laboratory performing the testing.³⁸ As an approximation, 20,000 leukocytes^L indicates moderate leukocytosis, 30,000/J.L indicates marked leukocytosis, and greater than 30,000/J.L is considered extreme.

Epinephrine-mediated (physiologic) leukocytosis is observed more frequently in horses than in many other species, particu­larly in young warm and hot bloods and horses that are not habituated to transport or handling. This type of leukocytosis is typically characterized by a mild mature neutrophilia and a moderate lymphocytosis that persists for about 20 to 30 minutes. In contrast, the corticosteroid leukogram is less age or breed specific and includes a moderate mature neutrophilia with no left shift or toxic change, lymphopenia, eosinopenia, and inconsistent monocytosis. Corticosteroid leukograms are more persistent than physiologic leukocytosis and frequently can be superimposed on inflammatory leukograms because of the stress of illness.

Leukemias are rare but have been reported in horses. Lym­phocytic leukemia is the most common, frequently associated with high lymphocyte counts and/or immature lymphocytes. Other forms of myeloid leukemia may present with large numbers of atypical cells originating from any myeloid lineage or relatively small numbers of circulating atypical cells. The presence of multiple cytopenias may indicate further assessment of bone marrow in patients with small numbers of circulating atypical cells.² Disruption of the hematopoietic microenviron­ment by proliferating neoplastic cells often suppresses the production of other cell lines.