Erythroid hypoplasia and anemia following administration of recombinant human erythropoietin to two horses

A Standardbred gelding and a colt were examined because of poor performance and anemia. Each horse had been given recombinant human erythropoietin (rhEPO; 4,000 IU) at least twice within the preceding 2 to 4 months. The horses had an Hct of 16 and 24%, serum iron concentrations of 210 and 304 mu g/dl (reference range, 73 to 140 mu g/dl), total iron binding capacities of 239 and 321 mu g/dl (reference range, 266 to 364 mu g/dl), values for the percentage saturation of transferrin by iron of 87.9 and 94% (reference range, 20 to 52%), and serum ferritin concentrations of 255 and 355 ng/ml (reference range, 43 to 261 ng/ml), respectively. There was no clinical or laboratory evidence of immune-mediated hemolysis or an infectious or inflammatory cause of the anemia. Examination of sternebral marrow biopsy specimens revealed generalized bone marrow hypoplasia; myeloid-to-erythroid ratios were 6.7 and 3.2. Moderate-to-marked erythroid hypoplasia was diagnosed in both horses. Compared with serum from a healthy control horse, serum from the affected horses inhibited rhEPO-induced proliferation of erythroid progenitors in vitro. Results suggested that the horses had developed anti-rhEPO antibodies that cross-reacted with endogenous erythropoietin, thereby inhibiting erythropoiesis. Horses were discharged with instructions that rhEPO administration be discontinued and that dexamethasone be administered. Five months later, both horses were back in training. For 1 horse, Wet had increased to 35%, and the other horse was not available for examination.