New Research Investigates Physiologic Cause of Bleeding in Horses

Studies have shown that as many as 95% of racehorses have some amount of bleeding in their respiratory tracts during hard exercise. Known as exercise-induced pulmonary hemorrhage (EIPH), this bleeding may be severe enough to require treatment in about 50 to 75% of racehorses.

In spite of years of research, no one knows exactly why this bleeding occurs. Ongoing studies at Michigan State University have helped to construct a theory of vein remodeling in response to increased blood pressure during strenuous exercise.

According to the researchers, greater circulatory pressure during exercise causes stretching of the blood vessels. Over time, repeated stretching leads to scarring, thickening, and a decreased capacity to carry blood. Because blood can’t be handled in some areas, circulation would be shunted to nearby veins, increasing their load and beginning the process of remodeling in these new areas.

When researchers examined the lungs of horses with severe EIPH, they noted that the small veins showed increased fibrous tissue and a smaller opening for the passage of blood. This supports the theory that scarring and remodeling with narrowing of pulmonary veins causes the rupture of capillaries and the release of blood into the lungs. After capillaries rupture, blood irritates tissues within the lungs, leading to fibrosis (scarring).

In the past, bleeders have been treated with furosemide (Lasix/Salix), a diuretic that also tends to relax blood vessels, allowing them to accept a larger volume of blood. With new regulations that limit the use of this treatment, the research team is investigating vein remodeling in hopes of finding other medications or different training protocols to reduce the tissue changes that lead to bleeding.