Bleeding within the lungs
Exercise-induced pulmonary haemorrhage (EIPH) is bleeding within the lungs. This probably results from high blood pressure within tiny blood vessels over extended periods during racing. Some studies indicate that virtually every racehorse suffers some degree of EIPH at some point . As Mundy  notes, “Extreme episodes … can cause massive internal hemorrhage [bleeding] into the lungs … subsequent asphyxia and death.” Instead of decreasing the severity of exercise, e.g. by decreasing race lengths, the standard industry response is to give horses furosemide – a diuretic drug. This lowers blood pressure by increasing urination.
1. Sweeney CR. Exercise-induced pulmonary hemorrhage. (1991). Vet Clin North Am Equine Pract, 7, 93–104.
2. Mundy GD. (2000). Equine welfare. Racing. J Amer Vet Med Assoc, 216(8), 1243-1246.