Bleeding in the lung, called exercise induced pulmonary hemorrhage or EIPH for short, is a common problem in race and other performance horses. It has been shown that up to 90 percent of racehorses in the U.S. bleed to some degree when racing. This is the reason furosemide, or Lasix, is so commonly used on the racetrack as it has been shown to reduce bleeding. However, a group of researchers in Australia studied 1600 horses over a 3-year period and found no differences in racing results between horses with mild to moderate bleeding in the lungs versus horses that did not bleed. The horses in the study were not treated with furosemide before racing. However, horses with severe bleeding were significantly more likely to place lower in the race than horses that did not bleed and seemed to slow down in the final stretch of the race. The study showed that 55 percent of the horses bled in the lungs to some degree and this is not surprising and actually is less than most studies performed in the U.S.
Dr. Guy Lester indicated that bleeding is to be expected as we breed animals to maximize performance and a byproduct of this is a thin membrane between the pulmonary blood vessels and the air sacs in the lungs which allows for highly efficient uptake of oxygen. So bleeding is expected in these equine athletes and EIPH does not just occur in horses as it occurs in other species including humans. It is believed the more serious the bleeding, the less gas exchange can take place. Also, horses that bleed, don’t always bleed the same amount each race so it is difficult to determine if a horse will bleed a significant amount during a race to affect the results or not. This is the reason so many racehorses receive furosemide to limit this bleeding if it occurs.