Increased time in training has been linked to a higher risk of bleeding in the lungs among jumps racing horses in Britain.
The study by Royal Veterinary College researchers Tegan McGilvray and Jacqueline Cardwell centred on 177 racehorses in training for National Hunt races (steeplechase and hurdle events).
Bleeding into the lungs, formally known as exercise-induced pulmonary haemorrhage (EIPH) is an important condition in horses who perform high-intensity exercise.
It is most frequently seen in Thoroughbred and Standardbred racehorses. Its reported prevalence in racehorses, diagnosed by the sight of tracheal blood, is high, ranging from 43% to 75% of horses on a single examination and up to 95% with repeated examinations.
EIPH has been reported most frequently following racing.
The condition may have welfare and performance implications, they said, with conflicting evidence regarding its effect on race-day performance.
McGilvray and Cardwell described EIPH as a complex disease. Causes that have been proposed include pulmonary capillary stress failure or impact‐induced trauma.
Pulmonary capillary stress failure refers to mechanical failure of the thin blood‐gas barrier in the alveoli of the lungs under high capillary pressures and large negative airway pressures, as generated during intense exercise.
Previously identified risk factors for EIPH include age, sex, season, race type, years spent in racing and lower airway inflammation.
For their study, the pair set out to estimate the prevalence of EIPH in British National Hunt racehorses as indicated by two outcome measures: The presence of tracheal blood during an endoscopic examination (a tracheobronchoscopy), and the presence of moderate‐large (significant) proportions of haemosiderophages from a tracheal wash (a quantity a fluid flushed through a portion of the lungs).
Haemosiderophages are cells considered to be a recognised indicator of previous lung bleeding. Their presence in tracheal wash samples has been used in previous studies to indicate EIPH, the authors noted.
They also set out to identify training‐related risk factors for these indicators of EIPH.
Their work involved 177 healthy National Hunt horses, who were examined and sampled monthly while in training, always within a 90-minute window after a workout.
In all, 1184 such observations were undertaken, with tracheal blood seen in 7.2% of endoscopic examinations, and significant haemosiderophages found in the tracheal wash fluid in 36% of samples.
At the individual horse level, tracheal blood was observed in 26% of the horses (33 animals), haemosiderophages in 94% of the horses (97 animals) and significant haemosiderophages in 78% of the horses (84 animals).
No episodes of bleeding from the nose were recorded in any of the examinations.
“Increased time in training was significantly associated with increased odds of EIPH,” the authors reported.
For each additional year spent in training, the odds of tracheal blood and presence of significant proportions haemosiderophages increased approximately 1.5‐fold.
Odds of tracheal blood were almost two times higher in winter and spring than in autumn.
Male horses had 85% lower odds of tracheal blood than females.
The presence of significant haemosiderophages was associated with an approximately five‐fold increased odds of tracheal blood.
“Compared with low‐impact work, medium‐impact work was associated with a near 10‐fold increase and high‐impact work with a 60‐fold increase in odds of tracheal blood,” they reported.
Time in training was associated with a 1.5‐fold increased chance of significant haemosiderophages.
The odds of significant haemosiderophages were approximately 1.9‐fold and 1.6‐fold higher in winter and spring compared to autumn. Odds of significant haemosiderophages increased with increasing neutrophil percentages.
Recent tracheal blood was associated with a 3.5‐fold increased chance of significant haemosiderophages, they said.
The authors found that current inflammation was associated with previous lung bleeding, but not current or future haemorrhage, suggesting that bleeding leads to inflammation but not that inflammation leads to bleeding. Or, put another way, it is the presence of blood in the airways that leads to further airway inflammation.
“These findings support the capillary stress failure theory of EIPH,” they said, “with increased time in training resulting in cumulative remodelling of the pulmonary vasculature, increasing susceptibility to EIPH through capillary stress failure with ongoing training.”
They said cytological examination of the tracheal samples was a more sensitive indicator of EIPH than the endoscopic examination alone, and the development of standardised measures would contribute to improved detection of the condition, particularly during training.
“Although EIPH in racehorses may not be avoidable, identification of horses at risk could contribute towards improved preventive measures in the future,” McGilvray and Cardwell said.
The study was unfunded, but used data from an earlier study funded by the Horserace Betting Levy Board.
Training related risk factors for exercise induced pulmonary haemorrhage in British National Hunt racehorses
Tegan A. McGilvray, Jacqueline M. Cardwell
Equine Veterinary Journal, 12 April 2021 https://doi.org/10.1111/evj.13448