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.

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.


EIPH is common in high-performance horses, with 65% of racehorses diagnosed through visual examination of the airways and 95% of racehorses diagnosed after bronchoalveolar lavage. [15]

Lasix for EIPH

Most trainers or riders will administer Lasix (Furosemide, Salix) to their horse before a race or competition to prevent bleeding.

Lasix is used preventatively by 90% of racehorses in the US and is recommended by veterinarians to protect the welfare of horses in competition. [16]

It is a short-acting loop diuretic (“water pill”) that reduces the horse’s blood pressure by lowering the overall water content of the horse’s body.

Multiple studies show that Lasix is effective in reducing EIPH. [17][18][19] Permitting the use of Lasix in racehorses resulted in an 80% decrease in reported nosebleeds in one jurisdiction. [16]

How Lasix Works

Approximately 65-75% of a horse’s blood is water. When horses lose water through sweat and urine, overall blood volume is reduced.

This decrease in the volume of blood flowing through the vascular system of the horse results in less pressure exerted on the walls of blood vessels.

EIPH is a direct result of increased blood pressure in the pulmonary capillaries. Reducing blood pressure can limit both the intensity and severity of bleeding in the lungs. [20]

Lasix works by increasing urination and blocking the re-absorption of electrolytes in the kidneys. Lasix increases the rate of sodium, chloride, and hydrogen ion excretion in the urine and alters the magnesium calcium transport system within the kidneys.

As a result, more water is pulled from the blood and excreted through the urine. [21]


Most trainers will administer Lasix intramuscularly 4 hours before competition and withhold water.

The correct dosage to use depends on the horse’s weight and must be discussed with a veterinarian. [21][22]

Side Effects

Lasix is a potent diuretic medication and has a risk of side effects, including dehydration. This drug causes significant water loss from the horse’s body, as reflected by the decrease in body weight following administration.

In one study, hay and water were withheld for 4 hours prior to competition and horses were given either 150, 250 or 500 mg of Lasix. These horses each lost between 28 and 32 lb of weight within the 4-hour period. [23]

Despite the water loss, horses treated with Lasix do not drink more water to replenish hydration status compared to untreated horses. [23] When Lasix is given over multiple days, the effect on dehydration can be profound.

Most veterinarians recommend giving electrolytes post-competition to increase water intake and help prevent dehydration.

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Although rare, Lasix can negatively affect the horse’s muscles and heart, resulting in weak muscle contractions as well as abnormal heart rhythms. [24]

Lasix significantly reduces concentrations of the electrolyte potassium in the blood. Potassium is essential for transmitting electrical signals that tell the muscles to contract.

You can avoid adverse effects on muscle contractions by using the appropriate dosage of Lasix and by administering electrolytes post-competition.

Is Lasix Banned?

Lighter horses need less effort to race at higher speeds. The loss in water weight before a race is seen by some as an unfair advantage in horses that use Lasix.

For this reason, Lasix was banned or restricted in some racing jurisdictions. However, many veterinary organizations campaigned to make Lasix legal to administer, citing the positive impact on equine welfare.

In the US, all racing jurisdictions allow Lasix to be used three or four hours prior to a race. Before giving Lasix to your horse, check the specific policies for your jurisdiction to ensure you are compliant with competition rules.

Preventing EIPH

If you are racing in a jurisdiction that does not allow Lasix to be used, there are alternative measures to prevent EIPH that are race legal and do not carry the same risk of side effects.

Nose Strips

One popular alternative measure is the use of nose strips to increase airflow into the horse’s airways during competition.

These nasal strips are made of sticky and stiff material. They are applied across the nose bridge just below the nasal bone to open up the soft tissue of the upper airway.

Studies indicate that these nose strips reduce how much effort it takes to draw a full, deep breath. Because EIPH is partially caused by greater respiratory effort resulting in increased pressure on capillaries, opening up the nasal passages could reduce instances of pulmonary bleeding. [25]

However, research into the use of nasal strips for EIPH has produced mixed results with some studies showing that nose strips do not affect bleeding severity or prevalence in racehorses. [26]

Nasal strip use in racehorses is relatively new with Kentucky Derby and Preakness Stake winner California Chrome being one of the first horses to wear a nose strip during major races. More studies are needed to validate the efficacy of this alternative treatment.

Concentrated Equine Serum

Concentrated equine serum (CES) is an injectable solution that contains immunoglobins and other proteins. It is derived from blood products of other horses and may reduce inflammation associated with EIPH.

CES inhibits the immune response in the lungs that contributes to bleeding. One study found that administering CES pre-competition resulted in a 53% decrease in red blood cells in the alveoli. [27]

CES is an expensive treatment option and is typically only used by high-level racehorses.

Omega 3 Fatty-Acid and Antioxidant Supplementation

Recent research indicates that supplementing a horse’s diet with the omega-3’s DHA and EPA may reduce inflammation in the lining of the lungs and decrease bleeding. [27]

In a study of 10 Thoroughbred horses fed DHA and EPA for 83 or 145 days, researchers found a reduction in EIPH. [27]

Another study found that EPA and DHA improve the membrane fluidity of red blood cells during exercise. [28] This study included vitamin E in the supplement. Vitamin E is a potent antioxidant that protects against cellular damage from free radicals. [28][29]

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Mad Barn’s w-3 oil contains 1500 mg of DHA and 1400 IU of natural vitamin E per typical serving. This can be used to provide antioxidant benefit, as a cool energy source to support exercise, and as a DHA supplement to reduce the risk of EIPH.

Conjugated Estrogens

Conjugated equine estrogens are drugs that contain a mixture of estrogen hormones obtained from the urine of pregnant mares. Estrogen plays a role in blood clotting by increasing levels of coagulation proteins, which stop bleeding.

Administration of conjugated estrogens has been found to decrease EIPH in horses following high-intensity treadmill training. [27]


Almost all racehorses experience Exercise-Induced Pulmonary Hemorrhage. While most cases of EIPH are mild, repeated episodes of severe bleeding can lead to deterioration of the respiratory capacity and impaired performance.

Call your veterinarian to discuss diagnostic tests and treatment if you think your horse may suffer from EIPH.

Consider omega-3 supplementation to support your horse’s lung health and supplement with electrolytes if you plan to use Lasix.

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.


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  28. Portier K, de Moffarts B, Fellman N, et al. The effects of dietary N-3 and antioxidant supplementation on erythrocyte membrane fatty acid composition and fluidity in exercising horses. Equine Vet J Suppl. 2006.
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