What is the significance of blood in a horse’s nostrils after hard work? Dr Colin Roberts discusses exercise-induced bleeding and its effect on athletic performance

It has been recognised for centuries that bleeding from the nostrils — known as epistaxis — is seen in some horses after strenuous exercise.

Famous thoroughbred racehorses from the past that were reported to have experienced this during races include the highly influential stallions Bartlett’s Childers and Herod. More recently, the occurrence of the condition in elite steeplechasers Best Mate and Sprinter Sacre has hit the headlines.

For many years it was assumed that the site of the bleeding in horses with epistaxis associated with racing was the upper airways. However, in a seminal scientific paper published in 1974, Professor Bob Cook put forward the notion that the source of the
blood was in fact the lungs.

The condition is now known as exercise-induced pulmonary haemorrhage (EIPH) and we know that it occurs primarily in the upper, rearmost part of the lungs

Only a small proportion of racehorses bleed visibly from the nostrils. Upon examination of the respiratory tract with an endoscope shortly after racing, however, a very large proportion are found to have blood in the trachea (the windpipe). This demonstrates that they have bled into their lungs during the race.

When endoscopic surveys of the prevalence of EIPH have been conducted around the world, 40-75% of horses have been found to have bled during racing. A survey carried out by the author in Flat racehorses in the UK found an overall prevalence of 55% with an increase in frequency between the ages of two and four years.

These surveys demonstrate that EIPH occurs commonly in racehorses, but in fact, analysis of respiratory secretions obtained endoscopically from horses in training (by the technique of tracheal wash sampling) yields evidence that virtually all racehorses experience EIPH to some degree once they commence fast work.

Epistaxis during racing is usually associated with the highest grade of EIPH and it is likely to be accompanied by impaired performance. In horses that show epistaxis only upon lowering the head after racing, the EIPH grade can be lower. In these cases, it may not affect performance.

Reports of the relationship between EIPH that is detectable only by endoscopic examination and race finish position are conflicting, with positive and negative associations, as well as no relationship being reported in various studies around the world.

In some countries — for example, Hong Kong and Australia — horses with recurrent nosebleeds due to EIPH are banned from racing.

Why do horses bleed?

While the cause of EIPH remains uncertain, the most widely accepted explanation is that it usually occurs due to stress failure of the pulmonary capillaries.

This causes bleeding into the alveoli (air sacs), as a result of the unusually high blood pressures seen in the pulmonary circulation of the exercising horse. Additionally, the negative pressure that occurs in the alveoli during inspiration probably adds to the pressure across the capillary walls, making them even more likely to burst.

The roles of upper and lower airway disease as causes of EIPH are still under debate, but it seems likely these factors may exacerbate the “normal” level of bleeding in some animals.

The blood-gas barrier (the division between the gas in the alveoli and the blood in the pulmonary capillaries) must be strong enough to resist the stresses placed upon it during strenuous exercise. Yet it must also be extremely thin to facilitate gas exchange, as gases pass much more efficiently between the alveoli and the capillaries if the distance that they must travel is small.

It is probably the need for the blood-gas barrier to strike a compromise between thinness and strength that makes it so susceptible to the occurrence of EIPH.

It is also noteworthy that since the equine lung does not respond to training, the increased maximal cardiac output which the horse is capable of as a result of conditioning is directed through a pulmonary vascular system that will not improve during the training process. This being the case, it may be that it is the fitter, highly performing horse that is more likely to experience EIPH.

To treat — or not?

Many different medications have been used to treat EIPH, the vast majority of them without scientifically demonstrable effects.

The drug most frequently used is furosemide, a potent diuretic (a drug that increases the production of urine) that is permitted for the treatment of EIPH in racehorses in some countries, primarily in North America. It is more commonly known by its trade name Lasix.

Furosemide does not prevent EIPH, but it does reduce its severity — probably by lowering pulmonary arterial and hence pulmonary capillary pressure. Furosemide is not permitted in racing in Europe, but it is used during training gallops for horses perceived to perform poorly due to EIPH, although it is unclear whether this has tangible benefits when the horse then goes on to race without it.

When considering whether a horse requires treatment for EIPH, it should be remembered that unless the condition is severe, it is probably not significant.

Besides furosemide, numerous other medications have been used in an attempt to prevent EIPH. No pharmacological agent is licensed for this use in horses, so treatments are generally empirical, with little firm evidence of their efficacy.

There is some debate about the use of commercially available adhesive nasal strips, which can reduce upper airway resistance and may reduce EIPH to some degree. They are permitted in eventing in the UK, but not for racing.

Different disorders

There are many other reasons for nosebleeds. Although horses showing epistaxis during, or shortly after, exercise usually have EIPH, when nosebleeds occur at rest they are more likely to be due to one of a range of upper airway disorders — some of which may be life-threatening and need immediate veterinary attention.

Although it is relatively uncommon, sudden death does occur after intense exercise in a small number of horses. In many cases, bleeding in the lungs is found on postmortem examination. These fatal haemorrhages may well represent a different condition to proper EIPH, involving as they do, the rupture of larger blood vessels rather than capillary bleeding.

H&H 18 June, 2015