Please, DO NOT use the contents of this blog post as veterinary advice. The purpose of this blog is to help me revise for my exams. If your animal has any symptoms, consult with a registered veterinary surgeon as to what is the best course of treatment.
EXERCISE-INDUCED PULMONARY HAEMORRHAGE (EIPH) IN HORSES
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| Epistaxis in horse (1) |
A.K.A. Bleeder horses
EIPH is a condition seen in many high performance horses, especially racehorses and horses used in other equine sports - therefore, it is often observed in thoroughbreds and standardbreds. As the name suggests, horses affected by this condition have internal bleeding in the lungs during high intensity exercise and this bleeding can be so severe to the point of causing epistaxis (bleeding through the nostrils). Usually, bleeding originates from the dorsocaudal area of the diaphragmatic lobes of the lungs.
Up to 75% of racehorses presented some degree of blood in the trachea and bronchi after exercise when examined via endoscopy, and haemorrhage was observed in over 90% of racehorses who had had the cytology of their bronchoalveolar lavage analysed. Epistaxis itself occurs only in up to 5% of racehorses.
According to Mair et al. (2013) there are different theories as to what causes this condition:
"Stress failure of pulmonary capillaries: pulmonary capillaries have extremely thin walls to allow rapid exchange of respiratory gases across them. Mechanical failure of the walls of the capillaries may occur when the pressure inside them rises to high levels during exercise.
Haemorrhage occurs from areas of lung neovascularization, where there has been proliferation of the bronchial arterial system as a response to low-grade lower airway disease, e.g. recurrent airway obstruction.
Haemorrhage occurs in areas of lung subjected to high-pressure swings and stress during exercise.
Haemorrhage occurs in the lung secondary to partial asphyxia due to an upper airway obstruction such as laryngeal hemiplegia.
Haemostatic abnormalities.
Changes in blood viscosity."
Some studies have looked into the hereditary genetic component of epistaxis in racehorses and found that although genes are not the only factors involved in the condition, they have strong influence on its development.
"Regardless of the cause, rupture of pulmonary capillaries and subsequent haemorrhage into the airways causes inflammation and damage (fibrosis) to the interstitium in the affected area." (Mair et al., 2013)
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| Detail shows the interaction between capillaries and alveoli inside a horse's lung when blood pressure inside the vessels are high, which allows RBC to move to inside the alveoli (2) |
Did you know?
"At the canter and gallop, the horse synchronises stride and respiratory frequency in a 1:1 ratio, which is known as respiratory-locomotor coupling. That is, they take one breath perfectly in time with one stride.
The amount of air moved in and out of the lungs increases in direct proportion to how fast the horse is running. If a horse runs twice as fast, it must move twice as much air in and out.
At rest, the horse will inhale approximately 67L of air per minute. During exercise, this can increase to 1800L per minute."
Source - http://www.kelato.com.au/eiph/
Clinical signs
Most horses have no obvious clinical signs, i.e. EIPH is usually subclinical. Nevertheless, some horses show a degree of exercise intolerance or poor performance, they may stop during exercise, present with respiratory distress, coughing, repeated swallowing, and may appear dull or lethargic; and haematology may show progressive anaemia. Of course, a minority will present epistaxis after exercise and occasionally, a massive fatal pulmonary haemorrhage may occur.
Diagnosis
Endoscopic evaluation of the trachea 30 - 120 min after intense exercise can provide definitive diagnosis if blood is found (be careful to rule out other sources of bleeding in the upper airway such as guttural pouch mycosis and ethmoid haematoma);
The amount of blood seen in the airways can be used to classify the EIPH:
- Grade 0: no blood
- Grade 1: one of a few flecks of blood, or one to two short and narrow streams of blood in the trachea or mainstem bronchi
- Grade 2: one long stream of blood (greater than 50% of the length of the trachea) or more than two streams of blood covering less than a third of the tracheal circumference
- Grade 3: multiple streams of blood covering more than a third of the tracheal circumference with NO blood pooling at the thoracic inlet
- Grade 4: multiple coalescing streams of blood covering >90% of the trachea and blood pooling at the thoracic inlet
If endoscopy is not available, cytology of tracheal aspirate or bronchoalveolar lavage can be performed and the presence of haemosiderophages (macrophages that have phagocytised red blood cells and now contain haemosiderin in their cytoplasm) can be assessed with the use of stains that highlight iron-containing pigments, such as Prussian blue.
Another option is radiography - where opacity in the dorsocaudal lung field may be observed within 7 to 10 days of intense exercise. However, the other tests are much more efficient at reaching a final diagnosis.
Treatment
There is no specific treatment for EIPH. Treatment usually consists in solving any primary airway disease; managing the environment to prevent dust or other possible airway irritating agents; and, resting horses well after exercise, especially the ones believed to be affected by EIPH.
Furosemide has been used and it reduces the increase in pulmonary artery and capillary pressures, thus decreasing the incidence and severity of EIPH in Thoroughbred racehorses. However, horses not affected by EIPH also have improved performance after administration of furosemide, which probably means that furosemide enhances performance via mechanisms unrelated to EIPH. Also, the use of this and other performance enhancing drugs (such as the ones listed below) are regulated by the local racing/sports authorities.
Flair nasal strips reduce the increase in pressure associated with resistance created by the collapse of the soft tissue in the nasal passages. According to the Merck veterinary manual (2010), application of nasal dilator bands reduces RBC counts in bronchoalveolar fluid from affected horses running on a treadmill by 33%.
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| Demonstration of how the Flair nasal strips work (3) |
Oestrogen, disodium cromoglycate (used for the treatment of asthma), clenbuterol (bronchodilator) and supplements such as PulmonAID are commonly used, but they have no proven therapeutic benefit.
Sources:
The Merck veterinary manual - 10th edition (2010)
Equine medicine, surgery and reproduction - Mair et al. - 2nd edition (2013)
(1) https://thehorse.com/148586/study-epistaxis-has-complex-hereditary-basis/
(2) http://www.kelato.com.au/eiph/
(3) http://www.eponaconsultinginc.com/index_files/Flair.htm
https://veteriankey.com/the-respiratory-system-anatomy-physiology-and-adaptations-to-exercise-and-training/
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