Exercise-induced pulmonary haemorrhage

What it is

During maximal exercise the horse reaches an enormous cardiac output, and blood pressure in the pulmonary artery, the capillaries and the left atrium rises sharply. At the same time, pressure inside the alveoli becomes very negative with each inspiration. The pressure difference across the wall of the pulmonary capillaries becomes so large that the wall ruptures (capillary stress failure) and blood escapes into the alveoli and airways.

Repeated episodes leave lesions: haemosiderin-laden macrophages, fibrosis, pulmonary vein remodelling and proliferation of the bronchial arteries, which in turn favour further bleeding. It is therefore regarded as a progressive disease.

Who it affects

Horses doing high-intensity exercise: Thoroughbred and Standardbred racehorses, Quarter Horses, polo ponies and horses in other speed disciplines. When examined endoscopically after a race, more than half have some blood in the trachea, and almost all do when examined on several occasions. Epistaxis, by contrast, is rare.

Anatomy involved

• Lungs: bleeding and lesions are concentrated in the caudodorsal regions of the caudal lobes, next to the diaphragm, which receive high blood flow and undergo large pressure changes. At necropsy the pleural surface there shows a bluish-brown discolouration.
• Trachea: blood leaving the bronchi runs down the trachea, where it is seen and graded by tracheobronchoscopy after exercise.
• Heart: the very high cardiac output and heart rates of the racehorse, together with raised left atrial pressure transmitted to the pulmonary veins and capillaries, account for the capillary pressures that cause bleeding.

Clinical signs

• Most horses show no visible signs.
• Poor performance: the horse stops or loses speed at the end of the race.
• Bilateral epistaxis after exercise in a minority, usually with severe bleeding.
• Coughing after exercise and repeated swallowing of the blood that reaches the pharynx.
• Massive pulmonary haemorrhage causing sudden death during exercise is rare and is considered a separate condition.

Diagnosis

• Tracheobronchoscopy 30 to 90 minutes after exercise, the reference test: the amount of blood is graded on a 0 to 4 scale.
• Bronchoalveolar lavage: red cells and haemosiderin-laden macrophages show recent or past bleeding for weeks.
• Thoracic radiography and ultrasonography add little and serve to rule out other diseases.
• Epistaxis must be confirmed by endoscopy, because it can also come from the guttural pouches or the nasal cavities.

Treatment and prevention

• Intravenous furosemide a few hours before racing: it reduces the incidence and severity of bleeding by lowering pulmonary vascular pressures. It is the only treatment with solid evidence. It also makes the horse lose weight, which is associated with better race results, so its use is controversial: in many jurisdictions, such as those in Europe, it is banned on race day, and in others it is regulated.
• Nasal dilator strips and other proposed treatments have scarce or conflicting evidence.
• Rest after an episode of epistaxis; racing rules in many countries impose a period without racing and retire horses that bleed repeatedly.
• Control airway inflammation (equine asthma) and air quality in the stables.
• Drug use in horses is subject to each country's regulations, to withdrawal periods and to racing anti-doping rules. In the EU the horse is a food-producing species, and its identification document (passport) states whether it is excluded from slaughter for human consumption, which determines which drugs may be given.

Key studies

• ACVIM consensus (Hinchcliff et al., 2015): systematic review using the GRADE method by a panel of 7 experts. Much of the evidence was of low or very low quality. There was moderate to high quality evidence that the haemorrhage is progressive, is associated with lung lesions and impairs performance, that the severe form (grade 4) shortens racing careers, and that furosemide reduces its incidence and severity and is associated with better race results. It strongly recommends regarding it as a disease, and weakly recommends the use of furosemide.
• Hinchcliff et al. (2009), randomised, crossover, blinded, placebo-controlled trial in 167 Thoroughbreds that raced twice under real conditions in South Africa, once with furosemide and once with saline. With placebo, the odds of bleeding were 3.3 to 4.4 times higher and the odds of moderate or severe bleeding 6.9 to 11.0 times higher; 67.5% of horses that bled with placebo (81 of 120) dropped at least one grade with furosemide. The races were organised for the study.
• Morley et al. (2015), prospective cross-sectional study of 1,000 Thoroughbreds in South Africa, without furosemide or nasal strips, examined by endoscopy after one race and graded blind. 68% had some degree of haemorrhage. Horses without bleeding were more than twice as likely to win (OR 2.3; 95% CI 1.4–3.7) and finished on average one length ahead, although there was no difference in finishing in the top three. It is a single examination per horse.
• Crispe et al. (2017), prospective observational study in Australia with 3,794 examinations of 1,567 Thoroughbreds. Haemorrhage was detected in 55.1% of examinations. Only severe haemorrhage (grade 4, and in a second model grades 3 and 4) was associated with a worse finishing position, a greater distance behind the winner and lower earnings; mild or moderate haemorrhage did not impair performance on that day. Enrolment was voluntary.
• Takahashi et al. (2001), retrospective study of 251,609 race starts in Japan (1992–1997), with veterinary examination of every horse after racing. Epistaxis from pulmonary haemorrhage occurred in only 0.15% of starts and was more common in steeplechases, in older horses, in short races and in females; 4.64% of horses that bled did so again. It records only epistaxis, not bleeding without blood at the nostrils.