British scientists on April 27 released a paper with a new theory which they think explains why horses bleed during exercise. The authors, who include Professor Bob Schroter of Imperial College, London, and Dr. David Marlin of the Animal Health Trust, Newmarket, believe that exercised induced pulmonary hemorrhage (EIPH) is caused by the force of impact transmitted through a horse’s forelimbs as they strike the ground.

The theory explains a key feature of EIPH which is the localization of the bleeding in a horse’s lungs in the dorso-caudal region, i.e. toward the rear of the lungs located nearest the spine. The theory says the impact of the forelimbs on the ground is transmitted via the shoulder to the chest wall from where a complex series of wave forms in the lung is generated.

These wave forms are then transmitted from the front to the back of the lung, being reflected by the ribs and spine and amplified in various parts of the lung, ultimately causing fracture of capillaries and therefore bleeding.

As well as being found in flat performers, EIPH is evident in steeplechasers, hurdlers, polo ponies, and eventers, as well as racing camels and possibly Greyhounds