Exercise-Induced Pulmonary Hemorrhage (EIPH) is a condition that causes bleeding in the airways, commonly seen in high-performance horses such as racing Thoroughbreds.

Horses that experience EIPH are known as “bleeders.” These horses may experience poor athletic performance, slow recovery after intense exercise and nosebleeds.

EIPH occurs when blood leaks out of the lungs’ capillaries and into the air sacs of the lungs (alveoli) during high-intensity exercise. Only a small percentage of horses with EIPH develop nosebleeds with most bleeding occurring internally.

It is estimated that up to 75% of Thoroughbreds and 26% of Standardbreds have EIPH. It can occur in any horse undergoing strenuous exercise including racing Quarterhorses, Appaloosas, 3-day Event horses and polo ponies. [1][2]

EIPH can be prevented by administering Furosemide (Lasix) before exercise. Using nose strips and supplementing with omega-3 fatty acids and antioxidants may also be beneficial.

Exercise-Induced Pulmonary Hemorrhage

Exercise-Induced Pulmonary Hemorrhage (EIPH) is clinically defined as the presence of blood in the alveoli and airways of horses following exercise. [3]

EIPH is often seen in high-performance horses such as racing Thoroughbreds, Standardbreds, Quarter Horses, barrel racers, polo ponies, and eventers.

EIPH occurs when the very small blood vessels in the lungs, called pulmonary capillaries, are damaged. This results in blood leaking into the alveoli.

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn Equine Nutrition Consultants

Why does Exercise Cause Bleeding?

When oxygen is inhaled, it travels to the lungs and enters the alveoli where gas exchange occurs. The alveoli are tiny sacs in the lungs that allow oxygen to move from the air into the blood flowing through a network of pulmonary capillaries.

These tiny blood vessels have very thin walls that are only one cell thick to facilitate gas exchange, making them fragile and susceptible to damage.

When a horse is performing at extremely high levels during a race, their muscles and organs rapidly use up oxygen. This results in deoxygenation of circulating blood.

The heart muscle must pump harder and faster to increase the amount of oxygenated blood travelling to the rest of the body to ensure the horse can keep up with the demands of high performance. [4]

More air must also be taken into the lungs to offset the reduced oxygenation of the blood, resulting in high tidal volume. [5]

Increased Pressure on Capillaries

This increased cardiac output and high tidal volume produce greater pressure in the blood vessels and alveoli. [5]

Furthermore, horses racing at high speeds land on the ground with incredible force, resulting in locomotory impact-induced trauma. [6]

These forces reverberate throughout the horse’s body and soft tissues including the lungs, causing microscopic tears in the pulmonary capillaries. [3][7]

Blood from the pulmonary capillaries leaks into the alveoli and other pulmonary spaces. This is known as a breach of the blood-gas barrier. [3][8]

In addition, some areas of the lung may be underventilated due to small airway disease or poor ventilation mechanics. Extreme fluctuations in pressure within the alveoli can also cause tearing of the alveoli and surrounding tissue. [2]

Effects of EIPH

In small quantities, blood loss into the alveoli and pulmonary spaces can be absorbed back into the horse’s body. Oxygenation of the horse’s tissues is not significantly impacted and the bleeding may go completely unnoticed.

According to researchers, most high-performance horses experience some degree of EIPH during their career without any ill effect. [2]

However, if the quantity of blood lost is high, symptoms can range from a reduction in performance to bleeding from the nose (epistaxis). [3][8]

Symptoms of EIPH

When most people think of bleeders, they think of horses with nosebleeds after intense exercise.

However, only 5% of bleeders lose blood out of their nose. [3] Most horses with EIPH are asymptomatic with no outward signs of blood loss.

In horses that do experience symptoms, possible clinical signs of EIPH can include: [3]

  • Poor performance
  • Coughing from deep in the chest
  • Extended cooling out and recovery time
  • Frequent swallowing after exercise
  • Increased respiratory rate
  • Respiratory distress
  • Changes in behavior

Nosebleeds only occur when horses experience significant blood loss in the lungs. Anecdotally, it is said that if a horse is losing blood through their nose, they are losing 10 times that amount into their lungs. [3] However, this has not been measured in any research studies.


Horses are large animals with a high volume of blood. The average 1000-pound horse has approximately 46.5 litres of blood in their body.

A horse can lose 10% of their blood volume before showing signs of shock and 30% of blood volume before their life is in danger. [9]

During an EIPH episode, horses can lose up to 2 litres of blood or approximately 5% of total blood volume. [10] This includes internal blood loss and visible blood loss from the nose.

Although this small amount of blood may not immediately cause ill effects, long-term effects can include permanent damage to the soft tissue of the lungs and reduced respiratory capacity.

EIPH may be associated with sudden death during and after racing. However, it is not clear whether this is a primary cause of death or secondary to other common causes of sudden death such as acute heart failure. [3]

EIPH Diagnosis

Diagnosing EIPH involves verifying the presence of blood in the trachea or lungs following exercise and ruling out other conditions with overlapping symptoms.

Many horse owners assume that a horse that bleeds out of their nose must have EIPH. However, nosebleeds can occur for many other reasons including:

  • Trauma to the mucous membranes – often from debris in hay
  • Trauma to the sinuses – such as a hard hit to the head
  • Sinus infection
  • Abscesses or tumours in the sinus cavity, nostril, or upper airway
  • Fungal infection of the guttural pouch (guttural pouch mycosis)

Endoscopic Examination

To avoid misdiagnosis, your veterinarian will want to perform an endoscopic examination of the upper airway following a bout of exercise.

The veterinarian will have the horse exercise at peak or semi-peak performance, such as a barrel pattern at competition speed or a breeze period for a racehorse.

Your veterinarian will then pass a small camera up the nose, down the trachea and bronchi to look for signs of bleeding. If blood is found, your veterinarian will grade the severity of the bleeding.

The presence of blood in the airways within 30 – 90 minutes of exercise confirms a diagnosis of EIPH.

EIPH Grading Scale

A grading scale from 0 to 4 is used to grade the severity of exercise‐induced pulmonary hemorrhage in horses. [11]

Grade 0: No blood is visible in the trachea or bronchi.

Grade 1: One or more flecks of blood or two or fewer short, narrow streams of blood are visible in the trachea.

Grade 2: One long stream of blood or more than two short streams of blood are visible, covering less than one-third of the trachea.

Grade 3: Several streams of blood are visible, covering more than one-third of the trachea.

Grade 4: Multiple streams of blood are visible, covering more 90% of the surface of the trachea.

Horses diagnosed with Grade 4 EIPH are likely to have their career shortened by this condition. [3]

Horses with Grade 1 – 3 EIPH may not have shorter careers, but they do appear to have diminished race performance and lower accumulated earnings compared to horses with no EIPH or horses with a lower grade of bleeding severity. [3][12]

Bronchoalveolar Lavage

Your veterinarian may also perform a bronchoalveolar lavage to detect the presence of red blood cells (RBCs) in the bronchioli, which are the smaller branches of the respiratory tract.

Bronchoalveolar lavage can help to diagnose cases of EIPH that would otherwise be missed by an endoscope exam. [13][14]

The procedure involves inserting a camera and small catheter through the nose and into the horse’s lungs. Your veterinarian will administer a small amount of fluid (300mL in 50mL portions) through the catheter, wait for the horse to take 2 breaths, and then suction the fluid out.

The fluid will wash through the bronchi branches and alveoli, picking up anything present there, including cells of the immune system, mucous and red blood cells. Your veterinarian will submit this fluid to a laboratory to analyze it for hemosiderin, a by-product of the breakdown of red blood cells.

Based on the level of hemosiderin in the lavage fluid, your veterinarian will determine if the bleeding within the lungs is severe enough to warrant treatment.