It is not uncommon to hear that a racehorse has “burst a blood vessel” during a race, and horses that do this on a regular basis are often referred to as “bleeders”. But what is the problem — which blood vessels are broken and what are the effects?

It has been recognised for centuries that a small number of racehorses have nosebleeds during or shortly after races. Only recently has it been realised that the source of this bleeding is not the head, but the lungs.

It is important to differentiate between nosebleeds due to exercise-induced pulmonary haemorrhage (EIPH) and those that occur in horses at rest and are unrelated to exercise. With the latter, it is likely that the bleeding is originating in the upper airways. Since there are one or two causes of nosebleeds that can be serious and even fatal, it is advisable to ask your vet to examine horses who have nosebleeds at rest to rule these out.

How many horses are affected?

The number of horses that suffer nosebleeds associated with racing is very small, but in the 1980s a number of post-race surveys conducted around the world found that 40-75% of horses have blood in the trachea (windpipe) following racing.

The term “exercise-induced pulmonary haemorrhage” was coined to describe this condition. Although this name looks scary, it is in fact quite simple — “pulmonary” merely means “pertaining to the lungs”.

In the late 1980s, Dr Chris Hillidge and I conducted an endoscopic survey of flat racehorses in the UK to determine whether EIPH occurs as frequently here as overseas. Our results were similar to those of other countries: 55% of the horses had blood in their lungs after racing.

The frequency of EIPH was age-related: blood was observed in the airways of 42% of two-years-olds, 65% of three-year-olds and 82% of older horses. The severity of bleeding also increased with age. These figures refer to the likelihood of EIPH being seen on any single occasion; other studies suggest that probably virtually all racehorses have EIPH at least once during their careers.

EIPH occurs also in racing Standardbreds, Arabs and Appaloosas, as well as eventers, show jumpers and polo ponies, although it appears to be rare in endurance horses. Fortunately, in the majority of cases of EIPH, affected horses are probably unaware that the condition has occurred and the volume of blood loss is small.

Why does it happen?

The cause of EIPH is uncertain. One might suspect that some form of respiratory disease would be necessary for lung haemorrhage to occur. However, the most prominent current ideas suggest that bleeding is inevitable, even in healthy lungs, during strenuous exercise and it has been proposed that EIPH occurs due to a phenomenon called “pulmonary capillary stress failure”.

In the exercising horse, the blood pressure in the pulmonary circulation — which provides the capillary vessels that pass through the lungs for gas exchange — becomes extremely high, something that does not happen in healthy humans. As a result, the capillaries in the lungs are subject to tremendously high internal pressures. At the same time, there are massive pressure changes in the airways and for some of the time there is a “negative pressure” that adds still further to the total pressure across the capillary wall.

It appears that the pressures across the pulmonary capillary walls during strenuous exercise are so high that stress failure occurs. This theory has not been definitively proven, but there is some experimental evidence to support it and it is regarded as the likely cause of EIPH by most workers in the field.

It is unclear why some horses bleed more severely, but perhaps factors such as current or previous respiratory disease or excessively high pulmonary blood pressures during exercise are necessary for the condition to become severe enough to cause visible bleeding from the nostrils.

One would imagine that bleeding into the lungs must be detrimental to performance, but there is little evidence of this. Severely affected racehorses that finish with a nosebleed do appear to show reduced performance (although the occasional one wins). If, however, there is no visible bleeding and the haemorrhage is only discernible by endoscopy, the picture is far less clear
and most studies have not shown this lower level of bleeding to be clearly associated with poor performance.

Intriguingly, in a study of horses endoscoped because of disappointing performance, we have so far found that not only are poor performers no more likely to bleed than a random group of racehorses, but there is actually a slight trend for them to bleed less. This finding is supported by a study performed in the USA. This does not mean that EIPH improves performance, but it is more likely that horses who perform poorly place slightly less strain on their pulmonary blood vessels.

Trainers often request that horses are examined endoscopically after racing to determine whether they have bled, but I have concerns about the value of this. Since the prevalence of EIPH is so high, especially in older racehorses, discovering that a horse has had a mild to moderate bleed is of little value. In our racecourse survey, around four out of five racehorses aged four or more had EIPH, mostly graded three or four on the five-point scale.

Discovering, therefore, that a four-year-old horse has grade four EIPH after finishing last is of little value — there is a good chance that the winner will have bled as severely!

How can it be treated?

How should we treat a horse when we believe its performance is affected by EIPH? First, we have to decide whether the condition really is impairing performance. And the evidence from the racecourse surveys suggests that it is only worth treating the most severe bleeders — principally, the ones who show visible haemorrhage and/or a grade five bleed on endoscopy.

Treatment of a horse who has only ever shown low grades of haemorrhage is probably a waste of time. Furthermore, it diverts you from investigating the real reason for that horse’s poor performance.

Of the many treatments used to try to prevent EIPH, the best known is probably the diuretic frusemide, more generally known by its trade name Lasix. In North America, huge numbers of horses are treated with Lasix prior to racing. In fact, there is convincing evidence that frusemide does not prevent EIPH, although it does reduce the volume of bleeding a little. Frusemide has a number of effects on the body and it is unproven which of these affects EIPH, but the product does cause a reduction in blood pressure in the pulmonary circulation. This may have the effect of lessening the likelihood of pulmonary capillary stress failure.

In the UK, medication rules for racehorses are stricter and horses cannot race with frusemide present in their bodies. Some clinicians treat horses perceived to have EIPH problems with frusemide during training, but there is no hard evidence that this reduces the risk of haemorrhage during races. Many other treatments have been used for EIPH, but none is consistently effective.

Some trainers alter the training regime of horses they believe have problems due to EIPH, but there is no general consensus as to the best way to train these horses. There is some evidence that if horses accelerate slowly, their pulmonary blood pressures do not show such high peaks as when they gain speed more rapidly, suggesting that longer, slower gallops may help a little in reducing EIPH at home. This tactic, however, would be of no value on the racecourse!

The prognosis for horses suffering from frequent bouts of severe EIPH is very guarded. Occasional horses show severe bleeding once or twice and then never again, while bleeding may temporarily occur more frequently when a respiratory infection affects a yard. However, many “repeat bleeders” have to be retired from racing, although they may be perfectly happy as pleasure horses or when competing in a different discipline.

  • This veterinary feature was first published in Horse & Hound