Abstract
Acute monocytic leukaemia (M5a) was diagnosed in a 17-year-old Standardbred gelding with lethargy, intermittent pyrexia, oedema of the limbs, harsh lung sounds and submandibular lymphadeopathy. Haematological findings included moderately severe anaemia, thrombocytopenia and a leucocyte count within the reference interval, but characterised by neutropenia and numerous blast cells. Monocytic lineage of the cell population was suggested by examination of Wright-Leishman-stained blood and bone marrow smears. A panel of cytochemical stains disclosed diffuse cytoplasmic α-naphthyl-acetate esterase activity which could be markedly inhibited or abolished in all leukaemic cells by pretreatment with sodium fluoride. In ultrastructural preparations of buffy coat, neoplastic monoblasts had one to two nucleoli, dispersed chromatin, elongated mitochondria, scattered profiles of rough endoplasmic reticulum, bundles of microfilaments and pseudopodia. More differentiated monocytoid cells had infrequent lysosomal granules.
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