[Case report: polycythemia in a horse]

A 13 year old Thoroughbred gelding was presented with a history of a single episode of collapse during mild exercise. Clinical examination revealed a high packed cell volume (PCV) of 72%, a haemoglobin concentration of 24.9 g/l and 15.2 millions erythrocytes/microliters. Despite continuous intravenous infusion therapy with large volumes, the PCV never decreased to a physiological level. The animal showed a normal appetite and no signs of discomfort or syncope. Arterial blood gas values were in the normal range as well as the concentration of erythropoietin (measured by radioimmunoassay, RIA). A test for neoplasms (carcino-embryonic antigen, CEA) was negative. The liver enzymes of the animal were extremely elevated and a liver biopsy showed a severe fibrosis. Examination of sternal bone marrow aspirate revealed no abnormalities. Based on these findings, the presumptive diagnosis was "absolute polycythaemia". The animal was treated for 7 days with repeated phlebotomy. During this time, the PCV never decreased below 50%, despite no obvious signs of discomfort from the animal. Because of the poor prognosis based on the liver biopsy result, the animal was euthanized 11 days after hospitalization. Post mortem findings were: a granular cell myoblastoma with a diameter of approximately 5 cm in the lungs, severe fibrosis of the liver, mild acute tubular nephrosis in the kidneys, activation of the erythropoietic cells in the bone marrow and thrombosis of the abdominal aorta. The possibility of secondary polycythaemia due to the lung neoplasia was not entirely excluded, but considered to be unlikely. Therefore, the definite diagnosis was polycythemia vera.(ABSTRACT TRUNCATED AT 250 WORDS)