Monocytic leukemia in a horse

On clinical examination, a six-year-old Hassian gray gelding with a history of impaired performance, slight cough, colic, and edema of the ventral abdomen, prepuce and the legs had reduced skin turgor, pale mucous membranes, forced costoabdominal breathing, reduced venous return, enlarged lymph nodes, and splenomegaly. Hematologic findings revealed anemia, leukocytosis and a high percentage of monocytoid leukemic cells. Generalized lymphadenopathy, splenomegaly, ascites, hydrothorax, and a diffusely thickened gut wall were found at necropsy. Massive infiltration with monocytoid leukemic cells was detected in lymph nodes, spleen, bone marrow, liver, gut wall, kidneys, and choroid plexus. Incubation of living cells obtained from a leukocyte concentrate with latex particles revealed phagocytosis in the leukemic cells on light and electron microscopy. The leukemic cells also had a marked alpha-naphthyl-acetate and naphthol-AS-acetate esterase activity, but were only weakly positive to naphthol-AS-D-chloroacetate esterase. A very weak alkaline phosphatase activity only was demonstrated in a few leukemic cells. On scanning electron microscopy, the leukemic cells had prominent ruffles and ridge-like profiles. These features of the leukemic cells excluded lymphocytic and granulocytic leukemia, and monocytic leukemia was diagnosed.