The Bleeding Edge of EIPH in Horses

Learn about recent research on the causes of EIPH, how it affects horses and their performance, and furosemide use in equine athletes.
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The Bleeding Edge of EIPH in Horses
Very few horses with EIPH actually have epistaxis—blood coming from the nose. | Photo: iStock

Where are we with bleeders, their performance, and furosemide administration

The Thoroughbred racehorse is arguably the poster child for “bleeding.” Study results show, however, that any horse of any breed, age, or discipline participating in any type of strenuous exercise likely suffers some degree of exercise-induced pulmonary hemorrhage (EIPH). Further, researchers say graphic photos of horses with blood streaming from their noses during or after competition do not accurately reflect EIPH’s true nature.

“Very few horses with EIPH actually have epistaxis—blood coming from the nose,” says Scott Hay, DVM, president of the Florida-based racetrack practice Teigland, Franklin and Brokken DVMs, in Fort Lauderdale, and 2021 president of the American Association of Equine Practitioners. “This occurs in only rare instances, an estimated 0.15-0.84% of race starters in Thoroughbreds, for example.”

A more accurate picture of EIPH shows a healthy performance horse breathing hard after exercise without a speck of blood in sight, though some could be present in the lower airways or trachea.

“The only proven medication to help prevent EIPH is to administer the diuretic furosemide (Lasix) prior to exercise—a practice that receives great scrutiny by the public, particularly in racehorses,” Hay relays. 

While most disciplines allow ­furosemide administration, many Thoroughbred racing jurisdictions in the United States have recently adopted a race-day ban of the medication.

In this article we’ll present the most recent research on what causes EIPH and how it affects horses and their performance, and we’ll explore furosemide use in equine athletes.

Failure of the Blood-Gas Barrier

EIPH in Horses: What You Need to Know
EIPH in Horses: What You Need to Know

Horses have experienced EIPH for centuries, dating as far back as the mid-1600s. David Poole, DSc, PhD, director of the cardiorespiratory exercise lab and professor in the department of kinesiology at Kansas State University, in Manhattan, suggests EIPH is a consequence of poor evolutionary design.

“Only a very thin layer of tissue ­measuring approximately 1/50,000 of an inch separates the blood vessels in the walls of the lung tissue and the air inside the deepest regions of the lung,” he says. “That tissue, called the blood-gas barrier, is all that maintains the blood on one side and the air on the other. When a horse is exercising, the cardiac output is very high, which means the blood pressure in the blood vessels inside the lungs is also very high. At the same time, the air pressure in the alveoli (small sacs in the lungs where gas exchange occurs) is quite negative.”

This negative pressure is what draws the air into the lungs, expanding the rib cage. The combination of high blood pressure and negative air pressure across the blood-gas barrier causes it to rupture, allowing blood to flow into the airways.

Hunting for Blood

Because most horses do not have visible blood flowing from their nostrils, veterinarians diagnose bleeders using tracheobronchoscopy (TBE) or bronchoalveolar lavage (BAL, or lung wash). The TBE is a standard “scope” of the upper airways, while a lung wash involves infusing a small volume of fluid into the lungs and collecting a sample of cells from the lower airways for microscopic analysis.

Unlike a BAL, a TBE does not require sedation and is a comparatively easy stallside procedure for equine veterinarians at the track. A standard grading scale (see sidebar above) allows veterinarians to characterize the severity of bleeding in a fairly consistent fashion.

Both TBE and BAL can reveal blood in the airways, but veterinarians have yet to determine which test is better. Jenifer R. Gold, DVM, Dipl. ACVIM, ACVECC, a clinical associate professor at Washington State University, in Pullman, determined that diagnostic technique does influence results. In her study of 149 barrel-racing horses, 54% were diagnosed with EIPH based on TBE. When BAL was performed using a cutoff of > 1,000 red blood cells/microliter of recovered BAL fluid, EIPH prevalence increased to 66%.

While BAL might seem like a clear diagnostic winner, Gold also found that 11% of horses would not have been considered EIPH positive on a BAL fluid red blood cell count > 1,000 cells/microliter alone (those 11% were diagnosed by TBE only).

“Although it may not seem particularly challenging to find blood in a closed, air-filled tube, the best method of diagnosing EIPH, be it TBE, BAL, or both, still needs to be determined,” says Gold.

She adds, “BAL is likely the best way to diagnose horses that have EIPH because the bleeding is occurring in the lower airways. How much blood works its way to the trachea is highly variable, thus the weak to almost no correlation between BAL and TBE in my study.”

She notes that timing of those tests might also affect a veterinarian’s ability to diagnose EIPH, yet researchers still don’t know the best time to perform them.

Poole says standardizing head position between racing and TBE is important: “Because of gravity, horses that have lowered their head to feed will show greater EIPH by TBE than those kept head-up for a given amount of EIPH.”

According to a 2019 review by Crispe and Lester, the currently accepted method of EIPH detection entails performing TBE 30 to 120 minutes after exercise. The authors explain that failure to detect blood via TBE doesn’t necessarily mean the horse is not a bleeder. Instead, the horse might have been scoped too soon or too late after exercise, didn’t exercise at the level required to induce EIPH, or the blood in the lower airways did not reach the trachea. In addition, the authors advised not making judgments regarding a horse’s severity of bleeding based on the findings from a single scoping procedure.

Impact on Performance

Without epistaxis, poor performance is the No. 1 reason vets scope horses looking for blood. In a comprehensive review of EIPH by the American College of Veterinary Internal Medicine (ACVIM, Hinchcliff et al. 2015) that became the organization’s consensus statement on the subject, authors reported evidence that moderate to severe EIPH is associated with an increased likelihood of inferior finishing position in a race. A number of studies have reached that conclusion:

In a study of 744 Thoroughbred racehorses in Australia not medicated with furosemide and not using nasal dilator strips, for example, researchers (Hinchcliff et al., 2005) found horses with Grades 0 and 1 EIPH were four times more likely to win and 1.8 times more likely to place in the top three than horses with Grades 2 through 4. However, they didn’t find an association between EIPH and other career indices, such as lifetime starts, number of wins, and earnings. Additionally, Thoroughbreds with more severe EIPH finished farther behind the winning horse.

A larger and more recent study by Lester et al. (2017) included more than 1,500 horses in Australia (also not medicated with furosemide) undergoing 3,794 postrace TBEs. Overall, 55.1% of horses were diagnosed with EIPH, but veterinarians saw Grade 3 or 4 EIPH in only 6.3% of all examinations, showing how uncommonly severe EIPH occurred. Compared to horses with Grade 0 EIPH, horses with the most severe Grade 4:

  • Were less likely to finish in the top three positions and, therefore, less likely to collect race earnings;
  • Finished farther from the winner;
  • Were slower over the final stages of the race and more likely to be overtaken by competitors in the home stretch.

Interestingly, he noted, horses with Grade 3 and 4 EIPH ran faster over the earlier stages of the race than later. Horses with Grade 1 and 2 were more likely to improve their position or overtake competitors in the final 400 meters (1,300 feet) of a race than horses without EIPH.

Or No Effect on Performance?

Bleeding does not always cause poor performance, as evidenced by data generated by a team of Canadian veterinarians (Leguillette et al. 2016). That study included 170 barrel racers, of which 45.3% were identified as bleeders. The team found a weak positive correlation between EIPH score and average run speed.

One potential explanation for the lack of association between bleeding and performance, says Gold, could be that “EIPH does not affect the airways like IAD (inflammatory airway disease) or heaves where there is excess mucus secretion and (varying degrees of) bronchoconstriction. EIPH does not cause bronchoconstriction, and depending upon the degree, macrophages and the ciliary apparatus rapidly clear blood from the airways. Thus, EIPH may not have performance-limiting pathophysiology like IAD (or) roaring soft palate displacement, for example.”

Preventing EIPH: Where Do We Stand With Furosemide?

Furosemide is a diuretic that lowers blood pressure by removing excess water from the body. It is the only medication known to effectively decrease both the severity and incidence of EIPH. 

But despite furosemide’s efficacy, two camps of opinion exist with regard to its use in Thoroughbred racing. Worldwide, furosemide is largely banned on race day in Thoroughbreds, including in Australia, the United Kingdom, and Japan. In the United States most racing jurisdictions have, starting in 2020, eliminated furosemide in all 2-year-old races and stakes races. In 2021 the Kentucky Derby staged the first ever Lasix-free Run for the Roses.

“Public perception played a large role in eliminating race-day furosemide, as people are against animals having drugs in their systems when they run,” says Hay.

Opinions on race-day medications aside, nearly all strenuously exercising horses do bleed, and the lung damage from bleeding appears to be cumulative.

“Furosemide is the only medication with scientific backing to mitigate bleeding and, therefore, slow ­hemorrhage-related lung damage,” Hay says. 

Since 2020, when tracks began enacting bans on race-day furosemide administration, Hay says he’s been seeing more bleeders in his practice.

“I don’t know if we were seeing more bleeders due to the lack of furosemide or, rather, because we have been scoping more horses since discontinuing race-day administration,” he says.

And although some Thoroughbreds aren’t running on furosemide in races, they still train consistently on it.

“This practice is used worldwide. The goal is to prevent cumulative bleeding events and minimize progressive lung damage,” explains Hay.

In other sectors of the industry, such as barrel racing, horses continue to compete on furosemide.

Take-Home Message

The ACVIM EIPH Consensus Statement says though we should consider EIPH a progressive disease, most affected horses do not experience respiratory distress, behavior changes, or other clinical abnormalities in association with bleeding. Further, there is no evidence it increases a racehorse’s risk of sudden death. While bleeding in severely affected Thoroughbreds appears to negatively impact performance, this effect is not echoed in all sports. Some evidence suggests EIPH Grades 1-3 do not shorten athletic careers.

Exercise-induced pulmonary hemorrhage is widely accepted as progressive, with performance load an important factor in disease progression, which is why furosemide administration remains common.


Written by:

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she’s worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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