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Do heart murmurs really affect performance horses?

Should you risk buying a horse with a heart problem and what are the chances of a murmur being debilitating? Recent research has given some answers.

A study I carried out last year that was published in the Journal of Veterinary Internal Medicine concluded that race-fit thoroughbred horses with heart murmurs did not appear to perform any differently from those who were unaffected.

Hard evidence

As the results of this study showed no consistent associations between racing performance and the presence — or grade — of heart murmur, its authors concluded that unless the heart itself fails, there should be no adverse effects on performance.

Different murmurs

A murmur is a noise that can be heard with a stethoscope at a time when the heart should normally be silent, for example between the normal heart sounds.

Most healthy sport horses have a heart murmur of some sort and only occasionally will the murmur mean there is a serious heart problem. But just knowing that a horse has a murmur tells you nothing.

The vet must decide whether the problem is caused by normal blood flow or whether a heart valve might be leaking. If a leak is suspected, then the specific valve must be identified because some are more likely to cause problems than others.

The significance of valve regurgitation depends on its severity and the likelihood of valve function deteriorating and the leak worsening over time.

Modern technology

Specialised ultrasound equipment can be used to visualise normal and abnormal blood flow in the heart, allowing an accurate diagnosis of the source of the murmur to be made.

In the vast majority of horses, diagnosis of valvular regurgitation certainly does not mean the end of athletic activity and a significant proportion of affected horses are successfully sold on.

What are the implications?

Heart murmurs are very common in sport horses, as is poor performance. But, conversely, cardiac disease is a very uncommon cause of reduced performance.

When it is, however, it is almost always an abnormal cardiac rhythm that is to blame, not a murmur. So be careful that a horse with a cardiac murmur is not written off until you have all the relevant facts.

For the full article on heart murmurs, see the current issue of Horse & Hound (28 May, ’09)

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