Bleeding in the airways of racehorses is common as 90% of horses have some evidence of bleeding after racing.  This bleeding, called exercise-induced pulmonary hemorrhage, or EIPH, has been shown to cause a significant decrease in racehorse performance, depending on the amount of bleeding that occurs. 

Veterinarians thought for years that other performance horses may also be bleeding in the airways, so a Spanish study tried to determine the percentage of EIPH in competing endurance horses.  The study tested 20 endurance horses and found that 65% had some evidence of bleeding either 3-8 or 36-38 days after competing. It was unusual to find evidence of bleeding 30 days after an event, but this was found in 50% of the horses tested.  These samples were collected by passing a scope into the trachea and looking at the trachea and taking samples.  Both techniques are important for the diagnosis. Looking in the trachea for frank blood and the amount of blood can help determine the severity of the bleed, and taking samples and examining cells can detect smaller amounts of bleeding.  The question the study did not answer concerns the possible decrease in performance of these endurance horses due to bleeding.  We know that racehorses with moderate to severe EIPH have a significant decrease in racing times and are not very successful as racehorses.  And racehorses that bleed are routinely given the diuretic furosemide to prevent bleeding.  However, this is not recommended in endurance horses because dehydration is a concern in these horses even without using a diuretic so there is no treatment available at this time to prevent airway bleeding in these endurance horses.