The Science of Horse Racing: Why Do Some Horses Bleed After a Race?   

February 26th, 2023

The undefeated Eclipse went 18-for-18 in his short 17-month career. This English great was three generations removed from the Darley Arabian, one of the Thoroughbred’s foundation sires, his connection to that forefather through his great-grandsire Bartlett’s Childers. Bred by Leonard Childers and owned by John Bartlett, that stallion was a full brother to the horse considered the first great Thoroughbred, Flying Childers.

Yet Bartlett’s Childers did not race for one good reason: he was a bleeder.  

Alternatively known as Bleeding Childers, the stallion never raced because he bled from his nose after exercise, a phenomenon known as epistaxis. But instead he became a sire of consequence, his line furthered by Eclipse and the multiple sire lines connected to him. From Native Dancer to Mr. Prospector, and from Seattle Slew to Tapit, Bartlett’s Childers is still a part of the breed three centuries later.  

So is the phenomenon that kept him from competition. Even in the 21st century, the sport still wrestles with Exercise-Induced Pulmonary Hemorrhage, or EIPH, of which epistaxis is the most serious example. For this month’s Science of Horse Racing, let us look at EIPH in Thoroughbreds and how it can affect a horse on race day and beyond. 

Defining EIPH 

The term EIPH refers to bleeding in the pulmonary system, or the blood supplying part of a horse’s respiratory system, after exercise. Each time a horse breathes, their lungs take in oxygen and expel carbon dioxide. The lungs then transfer that oxygen and take out carbon dioxide from the blood via the blood/gas barrier, a membrane fifty times thinner than a human hair between the airway and blood vessels. In horse anatomy, the thinner the membrane, the more efficient the transfer of oxygen into the blood, which seems ideal on the surface. However, problems can arise during exercise.  

At rest, a horse’s heart rate is 30 beats per minute; as a result, their blood pressure is lower since their heart is not working hard. However, during exercise, a horse’s heart rate can increase to 240 beats per minute. To keep up with the oxygen demands, the heart must move blood faster through the body, pumping harder. Because the heart is pumping harder, blood pressure increases; that puts even more force on each part of the circulatory system, down to that thin membrane that is the blood/gas barrier.   

Horses are bred to make that blood/gas transfer as effective as possible, but that also means that the barrier has thinned through the generations. Because of this, when the pressure builds up on those membranes in the smallest blood vessels, it can result in blood seeping across that barrier and into the respiratory system. This results in bleeding during exercise, or EIPH.  

Diagnosing EIPH 

That same epistaxis that kept Bartlett’s Childers off the racetrack is relatively rare, occurring in less than 1% of starters in the United States. However, EIPH occurs at an estimated rate of 40-75% of horses, with severity ranging from traces of blood in the trachea or bronchi to multiple streams of blood pooling around where the neck meets the chest. The wide range in the rate of bleeding in Thoroughbreds is a result of the frequency of examinations for potential bleeding.  

If a horse runs poorly, a trainer may elect to have a veterinarian examine the horse with an endoscope, colloquially called ‘scoping.’ The veterinarian would run a flexible endoscope with a camera down the horse’s throat to look for signs of bleeding. However, the timing of such a procedure can affect a diagnosis: a horse may still bleed but that case of EIPH might not be visibly detectable if enough time has passed. While not every case of a horse running poorly is a result of bleeding, trainers may elect to scope a horse to rule that out just in case.  

Recurring bleeding can cause long-term effects that may compromise a horse’s performance over time. The pulmonary veins feeding the lungs can become stiff and develop scarring. Such scarring will cause pressure to continue to build over time, increasing the chances that EIPH will recur and become progressively more serious. 

Trainers have options for preventing EIPH in their horses, with one particular well-known preventative currently under examination as a possible performance enhancer.  

Treating EIPH 

Furosemide, better known as Lasix, is a diuretic commonly prescribed to prevent EIPH. The drug causes horses to urinate more, which reduces the amount of liquid in their system and thus lowers blood pressure. However, Lasix also has potential performance-enhancing properties. Because of this, some jurisdictions restrict Lasix for race-day usage, but will allow it for training. While that drug has been a preferred preventative for EIPH, trainers and other caregivers have other tools at their disposal. 

Nasal strips can help prevent EIPH by supporting a horse’s nasal passages, keeping them open during exertion. Supplements like nitric oxide and omega-3 fatty acids have shown some effectiveness as well. Trainers and other caregivers often work with veterinarians to help find the right solutions, whether via supplements or other treatments, for each horse throughout their careers.  

Because it is connected with any sort of exertion, EIPH is not limited to just Thoroughbreds. Most sport horses from Standardbreds to barrel racers deal with this phenomenon, as do greyhounds and even humans. Understanding why and how it happens and taking steps to prevent EIPH can help a horse stay healthy and optimize their performance throughout their time on the racetrack.