The terms ‘whistler’ and ‘roarer’ are used to describe horses which make an abnormal respiratory noise during exercise. The noise is heard during inspiration (i.e. breathing in) and may be anything from a high pitched soft whistle to a harsh ‘roar’.

With each breath, air is taken in through the nostrils and passes via the nasal passages to the throat (pharynx), which is a dynamic and muscular tube. From here it passes through a cartilaginous valve, the larynx, before entering the windpipe (trachea) and lungs. During exercise, the nostrils dilate and the horse extends its head and neck, further opening the pharynx and larynx to take in more air. Anything which interferes with the smooth passage of this increased air flow may result in the horse making an audible noise.

Laryngeal hemiplegia (one-sided paralysis) is the most common cause of horses making abnormal noises during inspiration at fast exercise. The noise is caused by partial or total paralysis of one (usually the left) side of the larynx. It occurs mainly in larger horses with long necks, because it is thought that this conformation can predispose to injury to the long nerve (recurrent laryngeal nerve) which motivates the left side of their larynx. Nevertheless, it can be seen in smaller horses and ponies, either spontaneously or with a history of treatment or injury to their necks.

In mild cases the noise may only be heard during strenuous exercise but in severe cases the noise might be evident during trotting. Exercise intolerance (i.e. getting tired quickly) can be a problem as the horse has difficulty getting enough air through the incompetent larynx. The disease is progressive and gets worse with time. In a severely paralysed larynx, the airway actually gets smaller rather than bigger during strenuous exercise as the paralysed side collapses inwards.

We will listen to your horse exercising on the lunge and, if necessary, at fast exercise, to detect either normality or an abnormal inspiratory noise. Next, we will perform an endoscopic examination, i.e. a flexible ‘periscope-type instrument is passed through the nose, into the pharynx, to look at the larynx as the horse breaths. Both sides of the larynx should open and close in synchrony and almost symmetrically and completely. In the horse with laryngeal hemiplegia, usually the left side of the larynx moves sluggishly and incompletely, ‘hanging’ into the larynx and obstructing air flow during inspiration. In some difficult cases, it may be necessary to have an endoscopic examination performed while the horse is exercising on a high speed treadmill, in order to make the diagnosis.

Horses used for hacking or less strenuous jobs can cope without treatment. It is important to keep their respiratory tract healthy from infections and allergies with good management (i.e. low dust, good ventilation, proper vaccination regime etc.).

In moderately severe cases, it may help to do a ‘Hobday’ operation in which a piece of laryngeal tissue (laryngeal saccule) is surgically removed to encourage a scar to form to ‘tighten’ the larynx in a more open position. This can be done with the horse sedated and restrained in the standing position, using laser surgery, or more conventionally under general anaesthetic. In more severe cases a ‘tie-back’ operation is often recommended. This more elaborate operation places nylon sutures in the paralysed side to pull and hold it open. The ‘Hobday’ operation is often performed at the same time so that the resulting scar also helps to hold the paralysed side open, even if the sutures fail to retain their strength over time. In horses where a tie back has failed or is not an option for other reasons, a brass or plastic tracheotomy tube may be inserted into the windpipe to allow air to bypass the larynx completely. The tracheotomy tube and wound must be carefully managed to prevent infection and secondary complications and this method is very invasive and many people understandably find it unacceptable for aesthetic reasons.

There are several other conditions which may cause a horse to make an abnormal inspiratory noise.

1. Lymphoid hyperplasia is a term used to describe a condition where lymphoid (immune) tissue lining the pharynx becomes inflamed and nodules form. It is a condition affecting young horses and most cases improves with age.

2. Cysts (fluid-filled sacs) may form beneath the epiglottis. The epiglottis is a triangular cartilage at the base of the larynx which usually sits on the floor of the pharynx during breathing. Its job is to prevent food material from going down ‘the wrong way’ i.e. down the wind pipe. If a cyst forms under it, the epiglottis is pushed up and it obstructs the opening of the larynx causing a noise. Large cysts require surgical treatment to remove them.

3. Infections, tumours etc. in the nostrils or nasal passages may result in the horse making an abnormal noise.

4. Epiglottic entrapment is a term used to describe a condition where the epiglottis is trapped under an abnormal fold of tissue and cannot move normally. Treatment consists of cutting the abnormal tissue, for which a surgical procedure is required, and is performed with the horse either sedated and restrained in the standing position or under general anaesthetic.

5. Congenital problems are conditions of the pharynx and larynx which are present from birth. An abnormally narrow pharynx may narrow the airway and may result in a horse making an abnormal noise. Abnormalities causing displacement of supporting structures of the pharynx may have a similar effect. In general terms, these conditions cannot be treated.

If your horse has started to make an abnormal noise while breathing, you should ask your vet to examine him/her. Your veterinary surgeon will need to hear the noise so it will be necessary to ride or lunge the horse. After performing a general clinical examination, to help rule out other illness, an endoscopic examination should reveal most of the conditions described above and is the best way to diagnose diseases of the nasal passages, larynx, pharynx and windpipe. In cases that are marginally affected or where the diagnosis remains unclear, it may be necessary to have the horse scoped during fast exercise on a treadmill. This can only be performed at a small number of specialised centres but may be very useful in racing or eventing horses which are suffering from marginal loss of performance.

There is little doubt that laryngeal hemiplegia, or at least the conformational predisposition to it, is a heritable condition.