If you’re not already a member of the Thoroughbred Health Network, please subscribe FREE TODAY to receive the latest e-news and research reviews.
Research review outcomes will be displayed as 3 progressive layers of information:
1. Key messages – organised into ‘what we have good evidence for’, ‘what we have some evidence for’ and ‘what we don’t have evidence for but would benefit from’.
2. Summarised findings – expand the boxes to view the research that supports that key message.
3. Hyper-links – click the numbers to view the scientific papers themselves*. Our aim is to improve our general understanding of the strength of evidence available on each health issue, and consequently identify knowledge gaps. *Some papers may remain copyright protected according to the journal in which they are published and therefore, may not be accessible in full.
WHAT WE HAVE GOOD EVIDENCE FOR
Post-mortem studies have repeatedly shown that EIPH is associated with extensive and characteristic pulmonary lesions.
Therefore, there are potential serious long term implications for the management of EIPH due to the progressive nature of the lesions.
Research has reported that a single exposure of blood in the airspaces of the lung is not associated with lung damage. As a result the pulmonary lesions described in EIPH must have other causes such as: repetitive bleeds, or the presence of blood in additional parts of the lung causing cumulative and thus, progressive damage (E124).
Consequently, early diagnosis and preventative management should help to minimise the progressive nature of EIPH.
There is good evidence that exercise intensity and speed are important causal factors in the development of EIPH.
EIPH 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 prevalences as high as 75% (E71) and 95% (E43) have been reported.
One study reported that running on an incline increased the likelihood of pulmonary haemorrhage in the Thoroughbred which was thought to be related to an altered respiratory pattern in response to exercise (E142):–
The study proposed that these changes evoke more negative lung pressure swings which increase the incidence/ severity of EIPH.
However, an interesting finding was that the pressure within the pulmonary artery reduced, this indicates that:
whilst pressure within blood vessels (intravascular) is of major importance in the development of EIPH, other mechanisms outside of the vascular system also play an important role (E142).
There is a volume of evidence that supports the idea that EIPH is caused by mechanical failure of the walls of the pulmonary capillaries (small blood vessels in the lungs) when the pressure inside them rises to very high levels during exercise.
However questions remain, as some horses bleed more than others and treatments that lower blood pressure do not consistently eliminate the condition, suggesting there may be associated mechanisms which are not yet understood.
Changes in blood flow occur during high intensity exercise which increases the oxygen-carrying capacity of the blood.
There is significant evidence that EIPH is caused by mechanical stress failure of the walls of the pulmonary capillaries (small blood vessels in the lungs) when the pressure inside them rises to very high levels (E35).
a pulmonary arterial pressure threshold is often exceeded during high speed sprint exercise (as in racehorses) resulting in haemorrhage (bleeding) into the lungs (E163).
However, exercise doesn’t have to be high speed to result in EIPH:
A study of 5 Thoroughbred horses that were trotted to fatigue (approximately 25 min) at 5 meters/second (around 11 miles an hour) on a 10% incline reported signs of EIPH (E129). This study:-
Overall, there is little doubt that capillary failure is the ultimate cause for bleeding.
However, the progressive nature of lung lesions suggest further causal complexities remain to be understood E228.
Remodelling of veins
Repeated bouts of high blood pressure during strenuous exercise cause regional vein wall remodelling via collagen accumulation which leads to capillary tension, failure, bleeding and subsequently, lung damage (E118).
FIGURE 1: Forces exerted by blood flow on microvascular walls. These factors include 1) circumferential stress that causes distention of the vessel, 2) longitudinal stress that tends to elongate the vessel and 3) wall shear stress that is a frictional force. Image courtesy of EVJ: (Weiss et al, 2010) .
suggesting that factors resulting in EIPH may directly or indirectly affect right ventricle remodelling in athletic horses (E19).
EIPH was significantly associated with airway inflammation and evidence of airway fungal material (E25).
Horses with EIPH had inflammatory lesions with evidence of alveolar damage and the study suggested these were the result of viral infections that predisposed the affected regions of the lung to EIPH (E144).
Inflammation increased the risk of developing EIPH suggesting that care should be taken when recommending exercise in horses suspected to be suffering from pulmonary inflammatory disease (E111).
Therapy to reduce pulmonary inflammation may benefit horses prone to the development of EIPH (E111).
Racehorses with LLH (left-sided laryngeal hemiplegia) that undergo surgery are at an increased risk of respiratory conditions (both inflammatory airway disease [IAD] and severe EIPH) (E11).
Racing performance in terms of the number of race starts was significantly less in racehorses that had undergone surgery (E11)
However, for those horses that continued to race and once the reduced number of race starts had been accounted for, stakes money earned was similar between racehorses with and without surgery (E11).
An alternative EIPH causal theory of locomotory impact-induced trauma was proposed which incorporates the essential elements of both the capillary rupture and mechanical damage of high-speed exercise however, supporting evidence for this theory is minimal.
Lower environmental temperatures and lower altitudes (closer to sea-level) may increase the risk factor for EIPH.
Several studies have reported ambient temperature as a risk factor for EIPH:-
This research suggests horses are at risk of EIPH in colder temperatures. It should be noted that all studies investigated temperature values at the time of racing therefore, it raises the question whether the actual risk could be higher during early morning training sessions when temperatures would be even colder.
Research has also indicated there may be a relationship between EIPH and altitude.
There is minimal research into the relationship of EIPH to season but there is an indication that horses are more vulnerable to developing EIPH during the colder months.
Research has been conducted to investigate several other potential factors but currently evidence is low.
In 2002, 148 UK Thoroughbreds contributed towards 1614 horse-months of data and the prevalence of EIPH was significantly associated with increasing age (E25).
In 2003, out of 747 Thoroughbred racehorses, more horses presented with EIPH-related nasal bleeding (epistaxis) when older than 3 years (E24).
During 1992-1997, 247,564 Thoroughbred and 4,045 Anglo-Arab race starts reported EIPH is more common in older horses than in horses that were 2 years old (E159).
Increasing age has previously been identified as a risk of EIPH but recent research has proposed the causal factor is the cumulative effect of strenuous exercise as the horse’s career progresses, rather than the age of the horse itself.
However, recent research indicates age may not be the causal risk factor of EIPH and proposes these earlier findings may have been caused by the cumulative load of racing.
In 2010, a study of 744 flat racing Thoroughbreds in Australia reported no association between EIPH and age when the number of race-starts were accounted for (E13).
This suggests that EIPH is a progressive condition and damage occurs due to the cumulative load of racing in accordance to career length (E3)
Thus, proposes the opportunity of prevention through early diagnosis and management.
Association between Sex and EIPH
? Relationship did not exist between EIPH and sex (E93).
? EIPH was more common in females than in sexually intact males (E159).
? EIPH-related epistaxis was more common in geldings than in mares or entire males (E24).
Association between Sex and career length of horses with EIPH
? Males had significantly longer average careers when compared with females (E8)
? Geldings have longer careers than females and females have longer careers than entire horses (stallions) (E101)
Due to several conflictions, there is currently insufficient evidence to link EIPH and sex.
There is limited and conflicting evidence investigating the effects of surface / going on EIPH.
Research conducted at UK racecourses between 1996 & 1998 reported a trend of increased risk of epistaxis with increasing ground hardness (E21).
However, this conflicts an Australian study which found no association between the occurrence of EIPH and hard surfaces (E13).
It must be noted, these studies may not be directly comparable due to the differences in surfaces used by these two countries and therefore more research is needed.
|EIPH-positive horses with nasal bleeding
|Steeplechase||67.7% (21/31)||14.3% (3/21),|
|Flat turf||14.3% (2/14)||0% (0/2),|
|Timber races||66.6% (2/3)||100% (2/2)|
Therefore research suggests that the physical exertion of jumping increases the risk of a horse developing EIPH, more than the speed of racing on the flat.
The study suggested that bar shoes may not be the causal factor responsible for the observation and proposed that horses with bar shoes actually continued to experience foot pain causing prolonged heart rate elevation during exercise, increased pulmonary artery pressure and the development of EIPH .
There is no specific evidence re. heritability of EIPH in horses but the genetic composition that influence the physiology and ability of a horse may contribute to the likelihood of it experiencing epistaxis (E101).
Additionally, the use of certain medications like Furosemide (Latex) to mask the expression of Epistaxis (nasal bleeding) may have a detrimental impact on the breed (E101).
Research into markers of inflammation (cytokine and protein expressions) may assist the identification of whether the inflammation observed in EIPH is attributable to pre-existing pulmonary inflammation or whether it is a direct consequence of pulmonary bleeding (E69).
If it was possible to identify a genetic cause of EIPH, then potentially breeding programmes could be used to reduce its frequency.
One study provides support for molecular genetic research targeting specific markers underlying the genetic susceptibility of epistaxis (nasal bleeding) in order to identify specific physiological pathways that may be involved in the development of this condition (E101).
Exercise induced pulmonary haemorrhage – a progressive disease affecting performance ? – Trainer Magazine 25.09.15
Do Bleeders Breed Bleeders? – Trainer Magazine 25.07.11
Increasing Oxygen Concentration in the Blood of the Racehorse – Trainer Magazine 14.02.08