Loss of Performance in an Endurance Horse With Erythrocytosis and Colic During Exercise

A 10-year-old endurance horse was presented because of tiredness in training, inability to finish the races, and colic in competition. The horse was continuously supplemented with four different hematinic compounds for more than 8 months. Physical examination showed mild hyperemia, and blood analysis revealed erythrocytosis, increased γ-glutamyl transferase (GGT), and bilirubin with normal total solids (TS) and albumin. Arterial blood gas analysis and ultrasonography were irrelevant. Serum testosterone was within reference range, and increased serum erythropoietin and cobalamin were found. The horse was subjected to a treadmill endurance exercise and after 20 minutes of exercise, blood had a sticky and dark appearance. Exercise was stopped because the horse presented an episode of colic. Packed cell volume (PCV) of 58% and TS of 6.1 g/dL were found at that moment. An inappropriate secondary erythrocytosis because of supplementation was diagnosed. Supplementation was suspended and in the reevaluation 3 months later, PCV reached 49% after 20 minutes of exercise. Bilirubin, GGT, erythropoietin, and cobalamin values were normalized. Seven months later, resting PCV was 36%, and the horse was competing successfully in longer distances. It is concluded that erythrocytosis in an endurance horse might have detrimental effects on performance.