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There have been concerns both within and outside the racing industry that erythropoietin (EPO) has been administered to racehorses with the intent of enhancing performance. However, there are now at least two published reports in the scientific literature demonstrating that the administration of EPO has adversely been responsible for an immune mediated anemia and deaths in an unknown percentage of treated horses.
The pharmaceutical form of EPO is currently available as a genetically engineered (recombinant DNA technology) glycoprotein analog of human EPO, the endogenous (naturally occurring in the body) hormone which stimulates the bone marrow to produce red blood cells, thereby increasing oxygen carrying capacity within the circulation.
The drug is designed for use in humans suffering with anemia, usually due to kidney failure and some other non-specific anemia. EPO has also found favor as an aid to performance in athletic endeavors where endurance is involved. Erythropoietin analogues have allegedly found use in athletes to potentially increase red cell mass and therefore improve stamina and performance via increased oxygen carrying capacity (known in human medicine terms as a form of "blood doping"). However, horses already have large splenic reserves of red blood cells which are released into the circulation on exercise.
Performance enhancement in horses has not been demonstrated and adverse reactions are known to occur. In these cases of adverse reaction, the "human" foreign erythropoietin is detected by the horse's immune system as not being of equine origin, and subsequently the horse begins to develop antibodies not only against the foreign erythropoietin, but also against its own erythropoietin. Eventually, the bone marrow no longer has sufficient normal endogenous erythropoietin stimulation to produce red blood cells and the horse ultimately becomes anemic, severely enough in some cases to result in death. Treatment of affected horses is only palliative until inhibition of the adverse immune response has had time to take effect, and endogenous equine erythropoietin can once again have a positive effect on the bone marrow for it to again produce sufficient red cells. More recently, it has been recognized that in both dogs and horses, non-specific immune stimulants such as routine vaccinations can again "kick" the production of anti-erythropoietin antibodies back into gear. This can result in relapses or a refractory anemia that can lead to death. Bone marrow samples from these patients show a complete loss of red cell production (a plastic anemia).
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