ACUTE MONOCYTIC LEUKEMIA (M5A) IN A HORSE

Acute monocytic leukaemia (M5a) was diagnosed in a 17-year-old Standar dbred gelding with lethargy, intermittent pyrexia, oedema of the limbs , harsh lung sounds and submandibular lymphadeopathy. Haematological f indings included moderately severe anaemia, thrombocytopenia and a leu cocyte count within the reference interval, but characterised by neutr openia and numerous blast cells. Monocytic lineage of the cell populat ion was suggested by examination of Wright-Leishman-stained blood and bone marrow smears. A panel of cytochemical stains disclosed diffuse c ytoplasmic alpha-naphthyl-acetate esterase activity which could be mar kedly inhibited or abolished in all leukaemic cells by pretreatment wi th sodium fluoride. In ultrastructural preparations of buffy coat, neo plastic monoblasts had one to two nucleoli, dispersed chromatin, elong ated mitochondria, scattered profiles of rough endoplasmic reticulum, bundles of microfilaments and pseudopodia. More differentiated monocyt oid cells had infrequent lysosomal granules.