By Michael Mahaffey, 2009

Why Do Horses Bleed?

It is a question that has been asked in some form or another for at least the last 300 years. Bleeding in horses was originally spotted when blood ran from a horse’s nostrils after performing hard work or running. This condition, called epistaxis, was considered serious but was not seen as a widespread problem, because most horses simply did not bleed from the nostrils after heavy work.

According to an article titled “Exercise Induced Pulmonary Hemorrhage” by Todd East of the Equine Pulmonary Lab at Michigan State University’s College of Veterinary Medicine, less than 5 percent of horses show signs of bleeding from the nose and mouth after heavy work, which is what many horse owners use as an indicator that there is a problem.

It wasn’t until well into the 20th century that the real reason horses bleed was discovered.

EIPH horseEIPH can surface as a bloody nose, strange behavior during heavy exercise, or excessive coughing after a run, even without blood. BHN file photo.Bleeding occurs when blood enters the air passages of a horse’s lungs, and according to East, EIPH is diagnosed in one of three ways: endoscopy, tracheal washing or bronchoalveolar lavage (BAL). Each method is capable of giving a tentative diagnosis, but many diagnosticians recommend using multiple methods to determine the nature and severity of a bleeding episode.

While there is currently no reliable treatment that prevents EIPH, researchers have determined the reason bleeding is so hard to treat is because there is not one single cause behind the problem.

According to East, most evidence seems to indicate EIPH is caused by the radical increase in pressure in the horse’s natural high-volume, low-pressure pulmonary system, which is involved in the exchange and delivery of oxygen from the lungs to working muscles. The increase in pressure comes as a direct result of strenuous exercise.

“We know that it’s associated with very high vascular pressures in the pulmonary circulation,” said Dr. Howard Erickson, a professor of physiology at Kansas State University, who has conducted many studies into the causes and treatment of EIPH. “They approach values of 100 mm of mercury, which are probably four times normal levels and probably two-fold higher than what you observe in a human athlete when they are exercising. It’s also associated with other factors as well. It may be associated with large changes in airway pressure. It may be associated with impact of the feet on the ground on hard surfaces during racing, so there is not one sole cause.”

Dr. Tammi Epp, a fellow researcher at Kansas State University, says the spleenic contraction of the horse may also play a role.

“The horse has a muscular spleen that contracts,” Epp said. “It stores red blood cells, and probably ejects around 12 liters of red cells into the systemic circulation during exercise, which may alter the thickness and the ease with which the horse can pump blood.”

Epp also stated the case of locomotory impact trauma as a cause. She says with each footfall of the horse, there are forces that come up the leg and across the chest wall and converge on the dorso-caudal (upper back) region of the lung, where the bleeding occurs in a horse’s lungs. Epp says these shockwaves may contribute to some of the capillary ruptures.

Epp adds inflammatory airway disease is another major factor.

“The horse is an obligate nasal breather,” Epp said. “On the airway side, they have to develop tremendously negative pressures to get enough air in and out. They only breathe about 120–140 breaths per minute, which if you do the math, gives them less than a quarter of a second to get the air in and less than a quarter of a second to get it back out, so they develop these tremendously negative pressures.”

Although Thoroughbred and Standardbred horses have been recognized as having the highest rates of EIPH, with 70 to 90 percent or more bleeding at some point during their lives, it seems that there is no distinction among breeds, with even draft and endurance horses showing signs of bleeding after heavy work.

Erickson says scoping horses after a race generally finds 50 to 70 percent will show evidence of blood in the trachea. If you repeatedly scope that same group of horses, probably 80 to 90 percent will show evidence of bleeding.

“It also occurs in Quarter Horses,” Erickson said. “There have been very few studies of bleeding in Quarter Horses, but those that have been done indicate bleeding occurs in about 50 percent of Quarter Horses that race.”RacehorsesBleeding is most likely to occur in Thoroughbreds and Quarter Horses under strenuous exercise such as barrel racing, and especially in racehorses of both breeds. Photo by Danika Kent.

A study conducted by Dr. Kenneth Hinchcliff and associates in Melbourne, Australia, tested 744 2–10-year-old Thoroughbred horses to determine the effect bleeding had on the performance of the horses. They found, using a 5-point scale of bleeding, horses graded 1 or 0 were much more likely to perform better and more consistently than horses with bleeding graded 2-4.

They also found horses older than 5 are at an increased risk for showing EIPH, especially if they are just beginning to race. The presence of excess tracheal mucous or dirt and the concentration of airborne particulate matter was also found to have a significant effect, likely due to inflammation caused in the airway and lungs.

Erickson says the frequency and severity of bleeding episodes tends to increase with age, so if a horse has very minor bleeding, it might get more severe with time. Erickson says there is no evidence that all horses that bleed are destined to become grade 4 bleeders.

The amount of blood in the trachea after intense exercise can vary greatly between horses and even in the same horse over a period of time, so determining the average severity of a horse known to bleed is still not easy to accomplish.

According to an article published by the Animal Health Trust in the United Kingdom, continual bouts of EIPH may cause structural changes in the lungs as a result of the natural repair process, which could explain why bleeding increases with age. The damaged part of the lung becomes harder and more fibrous as it heals.

“Some of those horses, after repeated episodes, will develop exercise intolerance because the lung—the more times it bleeds it tends to scar, so the lung loses the capability for proper airflow and oxygen exchange,” said Tom Hutchins, DVM, a veterinarian at Equine Sports Medicine and Surgery in Weatherford, Texas. “You start to get some fibrosis and scarring of the tissue.”

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