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Exercise induced pulmonary haemorrhage (EIPH), is bleeding that occurs in the lung during exercise and is a common condition thought to be related to poor performance in equine athletes.
The condition is associated with fast bouts of exercise and is seen in equine sports of higher intensity such as, 3-day eventing, polo and show-jumping (E227) however, it is extremely common in racehorses, where prevalence as high as 75% (E71) and 95% (E43) has been reported.
Horses with the condition are often referred to as ‘bleeders’ however, the number who bleed externally via the nostrils (epistaxis) is typically low, with less than 1% (0.15%) being reported in a large study of those with the condition (E159).
It is believed that the majority of racehorses bleed to some extent during exercise, it just isn’t visible without performing an internal examination, such as an endoscopy at rest of during exercise (often referred to as ‘scoping’).
The quantity of blood seen is used to diagnose and grade the severity of the condition (0-4) however, inconsistencies in the severity graded may occur as horses do not bleed to the same extent every time and bleeding may originate from different locations, therefore diagnostic procedures may not be extremely sensitive or quantitative (E55).
Horses can be ‘scoped’ at rest or during exercise to identify whether blood is present in the airways
Increased physical exertion increases the requirement for oxygen, as a result breathing rate increases and the heart beats faster.
The horse’s physiology is capable of significant adaptation in response to exercise which makes them natural athletes (for example their respiratory rate can increase from around 12 breaths per minute at rest to 120 per minute at exercise) however, the extreme demands placed on the respiratory system can result in it becoming a limiting factor at exercise (E227).
Horses are obligatory nasal breathers i.e. they can only breathe through the nostrils and not their mouths.
Any condition that constricts the route from the nostrils to the lungs, impairs the air delivered to the lung, or the amount of oxygen which diffuses into the blood stream, may exacerbate the pressures on the respiratory system, and contribute towards the development of EIPH. This is important when considering common diseases of the larynx (which is already a narrowing of the airway).
Additionally there is evidence that the occurrence of EIPH is a consequence of the extremely high cardiac output required by Thoroughbred racehorses when they are travelling at high speed (E86).
The high prevalence of EIPH in performance horses, and particularly Thoroughbreds which have the capability of outstanding speed, is likely to be related to the high demands placed on their cardio-respiratory systems (heart and lungs).
The THN has collated key findings of research to date to provide the latest understanding on EIPH:-