The majority of horses with heart murmurs show no external signs, as the heart is a very efficient pump that is able to compensate. Problems usually only become obvious when the heart fails to distribute enough blood to meet the body's requirements, and the horse develops heart failure as a result.
Signs of heart failure include:
- Exercise intolerance
- Weight loss
- Rapid breathing
- Ventral oedema (a pooling of fluid that is seen as a swinging on the underside of the horse's abdomen).
A horse in heart failure will have a high resting heart rate, usually more than 50 beats per minute in order to compensate for the reduced volume of blood being pumped with each contraction.
Sometimes these signs only become noticeable when extra demands are placed upon the horse, such as a higher level of exercise, or when a mare is put in foal. It must also be remembered that all of the above signs can also have features of other, less serious conditions.
What Causes a Murmur?
The abnormal noise is often caused by a leaking heart valve, but there are many different types of murmur, with a range of underlying causes.
Functional, or flow, murmurs are not associated with a heart problem. These are completely normal and are commonly heard in fit animals, such as racehorses. They are attributed to the large volume of blood moving through the heart that causes audible vibrations similar to water rushing through pipes.
Temporary heart murmurs can happen with any condition that causes an elevation in heart rate. This is sometimes found in horses suffering from colic or other painful conditions, a murmur is detected while the horse is disturbed and its heart rate is elevated, but this later disappears. A common type of murmur occurs as a sult of vibrations as blood leaks back through a valve in the wrong direction.