Perhaps the most widely recognized of all disorders affecting racehorses is “bleeding,” or exercise-induced pulmonary hemorrhage (EIPH). We now know that most racehorses bleed at some time during their careers. In fact, many horses might bleed every time they undertake intense exercise, such as breezing and racing. We also know that bleeding can occur in situations other than flat racing. For example, EIPH has been detected in three-day eventers, steeplechasers, and polo ponies, among others. Bleeding has even been observed in draft horses pulling heavy loads. The common denominator is strenuous exercise.
Although EIPH has been recognized for more than 300 years, we still have more questions than answers when it comes to the cause and prevention of this problem. Research in the last 20 or so years has shed some light on why horses bleed during strenuous exercise. But even today, there is no consensus regarding the true cause of EIPH.
How to treat and prevent bleeding is no less contentious, in large part because of controversy concerning the potential performance-enhancing effects of furosemide (Salix, formerly known as Lasix), the drug administered to racehorses with a history of bleeding.
How Common is EIPH?
Just how common is EIPH? To answer this question, we need to define bleeding. Years ago, before the advent of endoscopes, a horse was called a “bleeder” when it shed blood from the nose after a race (epistaxis). However, research studies have shown that epistaxis occurs in only a very small percentage of racehorses. For example, in a Japanese study, epistaxis related to EIPH was recorded in only 369 of 251,609 race starts, or about 0.15% (Takahashi et al. 2001). In other studies, a slightly higher percentage of racehorses–between 1% and 2%–bled from a nostril after racing.
However, if bleeding is defined as the presence of blood in the windpipe or trachea after hard