Significant contribution by Dr Ellie Crispe of Simon Miller Racing.
High intensity exercise is associated with lung haemorrhage (bleeding) in several species and man. The condition has been recognized in horses for more than three centuries. Horses with exercise-induced pulmonary haemorrhage (EIPH) are commonly referred to as ‘bleeders‘. EIPH is a highly prevalent disease of performance horses worldwide. It occurs in the majority of racehorses, and has also been associated with other equine activities, including show jumping, 3-day eventing, polo, and barrel racing.
Blood is detected at the nostril in only a small percentage of affected horses. Furthermore, the appearance of blood at the nostril may not always reflect the severity of a bleed. The detection of blood at the nostril after racing can have a significant impact on the career of Thoroughbreds or Standardbreds in Australia.
The precise cause of EIPH is not fully known, but a well-accepted theory is that pulmonary capillaries rupture in response to the extremely high intravascular pressure and low airway pressure experienced during strenuous exercise. The blood gas barrier is ultra-thin to facilitate the efficient exchange of gases, but this predisposes to breakage. Pulmonary artery pressure increases during exercise (from 20-25 mm Hg to approximately 120 mm Hg); left atrial pressure also increases from 5 mm Hg to ~70 mm Hg. Therefore, pulmonary capillary pressures can be estimated at 80 – 95 mm Hg, a value that exceeds the breaking pressures for equine lung capillaries.
Factors that favour EIPH
Racing and training in cold weather increases the risk of EIPH. Strenuous exercise in cold conditions is associated with airway inflammation and bronchial hyperactivity in human athletes and horses, although the relationship between airway inflammation and EIPH remains speculative. The application of one or more bar shoes was significantly associated with EIPH detection and severity. It was speculated that horses with bar shoes had subclinical foot pain, and this could somehow prematurely increase heart rate and cardiovascular pressures during racing. The effect of race distance is unclear. Some studies identified increased risk for EIPH in races 1600m or longer, while others report increased risk of EIPH in races less than 1400m, and some authors failed to demonstrate an effect of race distance on EIPH. There is a cumulative impact of racing on EIPH or exercise associated epistaxis reflected primarily by lifetime starts rather than age. No studies have identified an association between EIPH and sex, altitude or racetrack surface.
Signs
There are usually no signs associated with EIPH. The signs that are noted typically include epistaxis and excessive swallowing post-exercise. Recent studies have indicated a correlation between EIPH and performance. The risk of sudden death during racing due to EIPH has been overrated, with the association likely very rare. It could be that haemorrhage could occur in response to an acute, fatal cardiac incident, such as mitral valve failure or ventricular arrhythmia. If you see EIPH associated with activities that don’t involve maximal exertion then check for atrial fibrillation.
Does EIPH affect performance?
There is a clear negative association between EIPH and race day performance. While an association exists, it is not applicable to all grades of EIPH. A large Australian study concluded that inferior race-day performance was largely limited to horses with severe (grade 3 and 4) EIPH only; and these categories reflected only 6.3% of all examinations. Horses with the highest grade of EIPH (grade 4) were less likely to finish in the first three positions, finished further from the winner, were less likely to collect race earnings and collected less race earnings, were slower over the final stages of the race, and were more likely to be overtaken by other competitors in the home straight than horses without EIPH.
Horses with moderate to severe EIPH grade 3 or 4 were significantly faster over the early to mid-stages of the race, and significantly more likely to reduce their speed over the final 600 meters, compared to horses without EIPH. A study of barrel racing horses reported that animals with the most severe grade of EIPH were faster than horses without EIPH, a finding that may also reflect rapid acceleration increasing the risk of severe haemorrhage.
Diagnosis
There is no gold standard test for the diagnosis of EIPH, and each diagnostic modality has clear benefits and limitations. The best accepted method for EIPH detection is tracheobronchoscopy conducted 30-120 minutes after exercise, preferably after competitive racing. Scoring systems have been developed to semi-quantitate the volume of blood present. The most widely accepted scoring system for tracheobronchoscopy with good inter-observer reliability uses a 0-4 scoring scale. From grade 0 (no blood seen) through to grade 4 (more than 90% of the tracheal surface covered with blood and pooling of blood at the thoracic inlet). Tracheobronchoscopy is obviously highly specific, but has questionable sensitivity. Failure to detect blood in the trachea can be attributed to an inadequate examination of the airways, examination too soon or too late after exercise, an inadequate or inappropriate level of exercise, or a minimal volume of blood in the distal airway that does not reach the trachea. The reality is that if no blood is seen the horse likely does not have moderate or severe haemorrhage.
Lower airway cytology has been advocated as a sensitive method for detection of EIPH. Acutely the detection of free RBCs, or in sub-acute and chronic cases the detection of haemosiderin-laden macrophages (haemosiderophages), is the hallmark of EIPH diagnosis using bronchoalveolar lavage (BAL) fluid cytology. However, haemosiderophages are detected in BAL fluid from almost all racehorses. Consequently, the presence of haemosiderophages or free RBCs will typically overestimate the number and significance of EIPH positive horses. To further complicate the value of BAL fluid cytology in the diagnosis of EIPH, the persistence of haemosiderophages after a discrete episode of EIPH is not known. Haemosiderophages were present at 28 days and absent at 90 days after intrapulmonary blood inoculation, whereas anecdotally it was reported that haemosiderophages could persist in rested horses for up to a year. Some have used the proportion of macrophages that are haemosiderophages in BAL fluid to distinguish clinically significant from nonsignificant EIPH cases, but given the potential problems described above this could be highly erroneous.
Consistency and progression of EIPH over time
Tracheobronchoscopic EIPH scores can vary markedly from one race start to the next. A high grade of EIPH (grade 3 or 4) diagnosed on one occasion does not guarantee high grades at subsequent race starts. In contrast, horses with lower grades of EIPH are more likely to have consistent EIPH grades at subsequent race starts, but this also is not guaranteed. A one-off diagnosis of EIPH is an unreliable predictor of overall career performance. Based on histological studies there is evidence that on-going lung haemorrhage may induce fibrosis and stiffening of pulmonary veins, resulting in even greater pulmonary vascular pressures during exercise. This would suggest that EIPH worsens over time. In a study of nearly 3000 horses, tracheobronchoscopic EIPH was shown to be mildly progressive in severity over the first 30 race starts.
Treatment and Prevention
In many jurisdictions in North America the identification of pulmonary haemorrhage is used as a justification for pre-race administration of the loop diuretic furosemide in an attempt to prevent EIPH. Approximately 95% of all horses racing in the US are now given the horse on race day. For the last several years, this has been a very contentious issue. Several studies have suggested that furosemide may improve the performance of non-bleeders. Possible mechanisms for enhanced performance include prevention of EIPH, reduction in bodyweight, induction of metabolic alkalosis, and/or bronchodilation. A group of racetrack owners have announced an intention to ban the practice at their tracks and there seems little doubt that race day furosemide will disappear within the next 5 years.
Based on reported risk factors it would be prudent to avoid racing or training horses with a history of repeated moderate to severe EIPH during colder months. Additionally, changing of riding tactics in these horses to settle in the mid or rear of the field off the pace, may also reduce the risk of EIPH. Limiting the number of races in a racing preparation and increasing the days between races may also have a benefit. The consensus statement reported very low-quality evidence that aminocaproic acid, bronchodilators, corticosteroids, NSAIDs, or pentoxifylline reduce EIPH severity. They similarly reported low quality evidence of a benefit of nasal strips in preventing EIPH. Although frequently recommended the effect of rest on EIPH have not been extensively studied.
Tags: Respiratory system