Exercise Induced Pulmonary Haemorrhage in Horses

Bleeding, or Exercise Induced Pulmonary Haemorrhage (EIPH), is one of the most common problems experienced by racehorses and can only really, by clinical evaluation, be identified through loss of blood from the nose, which to most would appear relatively serious. [1] (Figure a.)


(Figure a.)

However, there is evidence to suggest that nearly every racehorse will suffer from some form of EIPH at some point in its career. “In studies of thoroughbreds and standardbreds, where these horses were “scoped” (had their windpipes endoscopically examined) 60-90 minutes after racing, bleeding was detected in 50-60% of horses. In studies where the same horses were examined after three races, almost 90% bled on at least one occasion.”[2] Not only racehorses, but others including 3 day eventers, polo ponies and draught horses are also susceptible to this as a result of prolonged or intense exercise. [3] Although this frequent problem has been known about for around 300 years, there are still huge gaps in our knowledge about the causes, prevention and possible treatment, and a great deal more research must be done if we are to minimise its effects. As a result, with numerous theories about the causation, we will struggle to direct our research or target the correct areas when developing a cure. [4] Surprisingly, this problem has little effect on the health of the animal, but there is evidence to prove that it does and will, have a negative effect on performance, which owners and trainers are constantly trying to maximise. In an older horse the effect of regular bleeding over any period of time may trigger more harmful health conditions. [5]

The original and most widely accepted theory for the causation is known as “pulmonary capillary stress failure”. [6] Within the lungs, small air sacs called alveoli interface with tiny and numerous capillaries, and this is the site of gas exchange between the air and blood. However, between the two there is only a small lining layer consisting of an endothelial and epithelial lining, which is very thin allowing for gas exchange, but this also makes it very weak. A resting horse will retain around one third of its red blood cells within the spleen, however, during exercise this contracts forcing these stored red blood cells into the circulatory system. This allows more oxygen to be carried within the blood and distributed to the various muscle cells, which must respire aerobically to release the required energy sufficient for the current level of exercise. [7] In addition to this increase in pressure, an increase in heart rate from around 36 to 44 bpm also exerts a substantial force on the site of gas exchange. Blood pressure at this site increases drastically during exercise and is in fact around 2 to 3 times greater than other organisms undertaking the equivalent amount of exercise. “The pressure in the small blood vessels, or capillaries, within the lungs of a horse can increase from approximately 25 mm Hg at rest, up to 85-90 mm Hg during strenuous exercise.  Most species reach a pressure of about 35 mm Hg during exercise, making a rise in pressure of this magnitude unique to the horse.” [8] (Figure b.) It is this high positive pressure, when combined with the large negative pressure in the airways that increases the chances of blood vessel rupture. “Research studies show the horse’s heart does not relax quickly enough between beats during intense exercise, when heart rate can approach 230 beats per minute. When the left side of the heart is insufficiently relaxed, greater pressures within the lung are required to facilitate blood flow from the lung to the heart between beats.” [9] It is this pressure which is exerted on the endothelial lining of the capillary which causes them to tear, releasing blood into the lungs. The presence of this blood will reduce the efficiency and rate of gas exchange meaning less oxygen can be provided to respiring muscle cells and as a result performance during exercise is reduced. [10]


(Figure b.)

In contrast, there is another much more recent theory relating to trauma but of which little is known about. As a result, further studies are required to strengthen this theory or even prove/disprove it. It is thought that the continual pounding of the horses’ hooves and forelimbs when in a gallop sends shockwaves up the legs. The shoulder blade (scapula) then acts as a new source and transmits these waves into the thorax, and throughout the lungs. In the latter part of the lungs, or dorsocaudal region, the lungs begin to narrow and as a result these shock waves become stronger, and more highly concentrated. Most of the bleeding occurs in the dorsocaudal region as a result of trauma as this is where the pressure waves are focused. [11] (Figure c.)


(Figure c.)

Unhealthy lungs are also more susceptible to haemorrhaging. For example, if a horse is already affected with bronchitis there will already be inflammation present within the airways. This is the response required to remove the allergen and repair any damage done, which requires an increase in blood supply to the affected area. For this to occur, new capillaries must form and as a result they will be much weaker and therefore more likely to rupture when the pressure within them increases as a result of strenuous exercise. [12] In addition, the blood itself will cause irritation and inflammation within the airways, and over time lead to a build-up of scar tissue which will negatively affect lung function. The majority of the blood in the airways will be removed by the mucocilliary escalator, however, some may be engulfed by macrophages causing further inflammation and prolonging the effects of EIPH. [13] It is this scar tissue from repeated haemorrhaging that will affect the health of an older horse as a result of damaged lungs and poor gas exchange. [14]

Diagnosis of this disease is normally extremely difficult, because unless there are very obvious and severe signs such as bleeding from the nose, the damage is internal and therefore it may be overlooked or undetected. Signs of less severe cases include coughing, but normally its presence can be indicated through poor performance, for example in a race. One study carried out in 2005, showed horses with diagnosed EIPH finished on average 6 metres behind those without the disease. Currently there are three methods of detection for this particular condition, however, each alone cannot conclude whether or not a horse is suffering from this disease or another, such as guttural pouch mycosis and ethmoid hematoma. As a consequence, two or three are usually used in combination. Firstly, endoscopy can be used which involves passing a small camera through the nostril and into the trachea which must be performed 60-90 minutes after exercise, and this can detect the presence of blood in the airways. [15]

A tracheal wash is also commonly used which involves collecting samples of cells from within the trachea. If macrophages with a pigment from engulfed red blood cells are present then this can be used as evidence of previous haemorrhaging. (Figure d.) However, this does have its drawbacks because this does not give a time scale of the event, nor does it show the extent of the bleeding as the cell count obtained will vary greatly from the actual location of the haemorrhage. [16]


(Figure d.)

The final, more accurate method of detection is known as brochoalveolar lavage, which again involves passing a tube through the nostril into the trachea, whereas this time it continues going down further air passages deeper into the lungs where it obtains another sample of cells. Veterinarians will use this sample to find out the number and types of cells present that lie deep within the lungs, which will allow them to see if there is any inflammation present and also any abnormally large numbers of red blood cells which will show the degree of haemorrhaging. [17] To study this in further detail, horses are often run on treadmills in order to induce bleeding. [18] (Figure e.) “Three thoroughbreds with known EIPH were galloped on a treadmill, and after the horses were killed with intravenous barbiturate the lungs were removed, inflated, and fixed for electron microscopy.” [19]



(Figure e.)

Unfortunately no cure has yet been found to treat horses suffering from EIPH, and although there is one treatment known as Lasix used in the USA it is at present banned in most other countries. [20] In fact, Lasix is a diuretic and used commonly in human medicine to treat high blood pressure/hypertension as it induces urination. [21] This plays a key role in the treatment of EIPH as it is the high blood pressure within the pulmonary circulatory system which is believed to cause the symptoms. “Lasix is administered intravenously about four hours before exercise, and acts rapidly on the kidneys, resulting in an increase in urine production.  When given Lasix, horses can lose up to 2.4% of their body weight in urine excretion.  By filtering this large amount of water and electrolytes from the blood, the kidneys reduce the blood plasma volume and blood volume by approximately 8.4% and 11% respectively.  By reducing the volume of blood flowing through the vascular system, while retaining the same number of red blood cells necessary for oxygen transport, the blood pressure within the lungs has been shown to drop approximately 7-10 mm Hg without compromising oxygen transport to the muscles.” [22] (Figure f.) This treatment should therefore in theory reduce the amount and extent of haemorrhaging whilst undertaking strenuous exercise, however, it will not prevent it completely. Research is still being done into the effectiveness of this drug as some horses respond really well, whereas others show there to be no less blood within the trachea after exercise. [23] On the other hand, there is also evidence to suggest that Lasix actually improves race performance as those that have been treated show a faster race time than those without. This is thought to be as a result of weight loss due to excessive urination, which will therefore lead to a lighter horse and in essence one which is thought to be much faster. This is because an equal amount of energy can be used in moving a lower mass around the track. All of the outcomes mentioned above are the reason why there is controversy surrounding this drug and why it is illegal in some countries. [24]


(Figure f.)

Another form of treatment which is currently being researched into is nitric oxide (NO). This acts as a vasodilator which allows the lung capillaries to accommodate the large increase in both blood volume and pressure flowing through them, and from counteracting these changes the effects of EIPH should decrease. [25]

There is evidently a lot more research which must be done in this field, but due to its time of occurrence and internal location this is extremely difficult. Although it may seem cruel to euthanize animals for research purposes, many more will be saved in the long term if this disease is further understood and a cure developed.

[3] [15-17] Anon “Exercise induced pulmonary haemorrhage” (2014)

[2-3] [6-9] [13] Ray Geor “EIPH: Exercise induced pulmonary haemorrhage” (2001)

[15-18] Bonnie. R. Rusk “Exercise induced pulmonary haemorrhage in horses” (2014)

[1] [3-8] [10-18] [20-25] (Figures b, c, d, f.) Todd. D. East and equine pulmonary lab “Exercise induced pulmonary haemorrhage”

[5] [15] [17] “Exercise induced pulmonary haemorrhage” (2013)

[19] Journal: J. B. West “stress failure of pulmonary capillaries in racehorses with exercise induced pulmonary haemorrhage” (1993)

(Figure a.)

(Figure e.)

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