EIPH, or exercise-induced pulmonary hemorrhage, is bleeding that occurs from the lungs of horses during intense exercise. It affects roughly 74% of Thoroughbred racehorses and is one of the most common conditions in equine performance medicine. Despite how widespread it is, most affected horses never show visible signs of bleeding, which is why the condition often goes undetected without a veterinary exam.
How Lung Bleeding Happens During Exercise
When a horse gallops at full speed, its cardiovascular system operates under enormous pressure. The heart pumps massive volumes of blood through the lungs to meet oxygen demands, and the tiny capillaries in lung tissue bear the brunt of that force. Under extreme exertion, the pressure inside these capillaries exceeds what their walls can handle. The result is what researchers call capillary stress failure: the thin membranes separating blood vessels from air spaces physically break open.
Electron microscopy of lungs from recently exercised horses reveals breaks in the capillary walls and basement membrane, along with red blood cells leaking into surrounding tissue and air sacs. This isn’t a single dramatic rupture. It’s widespread microscopic damage across the lung, concentrated in the back and upper portions where blood pressure is highest during a gallop.
Which Horses Are Affected
EIPH is not limited to older, seasoned competitors. In a large study of two-year-old Thoroughbred racehorses, 74% tested positive for EIPH at least once. Even after their very first race start, 67% of horses showed evidence of blood in their airways. The prevalence and severity in these young horses matched what’s typically seen in older racehorses, suggesting the condition begins as soon as horses start working at high intensity.
While Thoroughbred flat racing produces the most data, EIPH occurs across disciplines that demand intense cardiovascular effort, including Standardbred harness racing, barrel racing, polo, and eventing. Any horse pushed to near-maximal exertion can experience it.
Why Most Horses Don’t Visibly Bleed
The classic image of a racehorse with blood streaming from its nostrils (called epistaxis) represents only the tip of the iceberg. Visible nosebleeds occur in just 0.8% to 13.5% of Thoroughbreds, depending on the study. Meanwhile, internal evidence of pulmonary hemorrhage shows up in 26.5% to 74.5% of the same population when examined with a scope. Most horses with EIPH bleed entirely within their lungs and airways, with no external sign whatsoever.
This means owners, trainers, and riders often have no idea their horse is affected. Performance changes may be subtle: a horse that fades in the final stretch, swallows more frequently after exercise, or coughs during cooldown. Some horses show no outward symptoms at all despite significant bleeding inside the lungs.
How Veterinarians Diagnose EIPH
The primary diagnostic tool is tracheobronchoscopy, where a veterinarian passes a small camera through the horse’s nostril and into the trachea after exercise. Blood visible in the airways confirms EIPH, and veterinarians grade what they see on a 0-to-4 scale based on how much blood is present. A grade 0 means no visible blood. Mild cases (grades 1 and 2) show streaks or small amounts of blood, while severe cases (grades 3 and 4) involve blood pooling in the airways. This exam is typically performed within 30 to 90 minutes after exercise for the most accurate results.
For detecting chronic or past bleeding episodes, veterinarians use a technique called bronchoalveolar lavage. A small volume of fluid is flushed into the lungs and then collected for analysis under a microscope. When red blood cells from previous bleeds break down in the lungs, immune cells called macrophages consume the debris and accumulate iron-containing pigments. These iron-laden cells can be scored on a grading system that correlates with the severity of bleeding over time. This method can detect evidence of EIPH days or even weeks after the last episode, making it useful when scoping immediately after exercise isn’t possible.
What Happens to the Lungs Over Time
A single episode of EIPH causes microscopic damage that the lungs can largely repair. The real problem is repetition. Each time blood leaks into lung tissue, iron deposits accumulate and trigger an inflammatory response. Over months and years of training and competition, this cycle of bleeding, inflammation, and healing produces scarring (fibrosis) in the tissue between air sacs, around blood vessels, and along the membranes lining the lungs. The damage concentrates in the back and upper lung regions where bleeding is most severe.
Repeated episodes also cause structural changes in the small veins of the lungs, narrowing them in a process similar to what happens in certain human lung diseases. These narrowed veins raise the back-pressure on capillaries even further, which means less exertion is needed to trigger the next bleed. This creates a worsening cycle: chronic EIPH makes future EIPH more likely and more severe, which is why the condition tends to progress with age and continued training. Horses with severe grades (3 or higher) on endoscopy are more likely to be retired specifically because of EIPH, while horses with milder grades tend to leave racing for other reasons.
Treatment and Prevention Strategies
Furosemide
The most widely studied treatment is furosemide, a diuretic that reduces blood volume and lowers the pressure inside lung capillaries. Administered intravenously before exercise, it decreases the severity of EIPH. Traditionally, furosemide has been given on race day, typically about four hours before competition. However, growing public pressure in the United States and outright bans in some racing jurisdictions have pushed researchers to find alternatives. One approach that shows promise is giving furosemide 24 hours before exercise combined with controlled water intake, which reduced bleeding severity both on treadmill tests and at the racetrack.
Furosemide does not cure EIPH or prevent it entirely. It reduces severity, which for many horses means the difference between manageable and career-threatening bleeding. But it also acts as a diuretic, meaning horses lose body weight before racing, and debates continue about whether this creates an unfair performance advantage separate from its effect on bleeding.
Nasal Strips
External nasal strips, similar in concept to the adhesive strips people wear for snoring, work by pulling open the nasal passages at their narrowest point. In treadmill studies, horses wearing commercially available nasal strips showed significantly lower airway resistance during high-intensity exercise. By reducing the effort required to inhale, the strips may lower the extreme negative pressures generated in the chest during a hard gallop, which are one factor contributing to capillary stress failure. The strips are inexpensive, noninvasive, and permitted in most competition settings, though they address only one piece of the EIPH puzzle.
Environmental Management
Air quality plays an underappreciated role in lung health for performance horses. Horses stabled in barns with dusty hay and bedding face constant low-grade airway irritation that can worsen the inflammatory backdrop against which EIPH occurs. Symptoms of airway disease tend to worsen in winter when horses spend more time indoors, and seasonal allergens like pollen and mold spores add to the burden in warmer months. Practical steps include soaking or steaming hay before feeding, using low-dust bedding like shavings or paper, ensuring adequate barn ventilation, and maximizing turnout time. Reducing airway inflammation won’t eliminate EIPH, but it removes a factor that can make it worse.
Some trainers also adjust workout intensity and frequency for horses known to bleed, spacing hard efforts further apart to give lung tissue more recovery time between episodes. While there is no way to fully prevent EIPH in a horse that competes at high intensity, combining medical treatment with environmental and training adjustments offers the best chance of managing the condition across a horse’s career.