The horse's throat region (the pharynx and larynx) can be affected by conditions that physically restrict the amount of air entering the lungs. Probably the best known of these is laryngeal hemiplegia, also known as whistling or roaring.
The larynx closes to prevent food entering the windpipe during swallowing and opens up fully during peak exercise to allow air to enter the lungs. Provided that the muscles controlling the larynx are functioning properly, it opens to form a diamond-shaped structure.
However, it is relatively common for the left side of the larynx to become paralysed, which results in only half of the larynx opening and causing reduced airflow into the lungs.
Diagnosis of whistling or laryngeal hemiplegia has traditionally been by listening for a characteristic whistle or roar at exercise, combined with endoscopy of the upper airway at rest to confirm the diagnosis.
The degree of laziness or paralysis is graded by vets on a scale from one to five. Grades one and two are considered within the normal range, but grade three and higher are abnormal.
"However, there are a significant number of horses in which the diagnosis is not clear cut," says Dr Dillon. "This means there are many affected horses that scope normally, but make a whistle at exercise, and alternatively there are horses that fail on scope but make no noise at exercise.
In fact, 25pc of horses with upper respiratory obstructions will make no outward noise at exercise.
In order to detect these cases, a high-speed treadmill video-endoscopy was introduced in specialist equine hospitals such as Troytown. It allowed a horse's larynx to be observed while at fast exercise. The first high-speed treadmill in Ireland was installed by Troytown in 2002 and was a huge step forward in accurate diagnosis of whistling.
However, this was quickly followed by a new method -- the overland scope -- which has been in use for the past three years.
The overland scope is a lightweight diagnostic machine that can be worn by a horse while carrying out his usual work, such as galloping on the track. Video images of what happens to the larynx while the horse is galloping are recorded by the machine and saved so that the vet can analyse the images at a later time.
"The overland scope means we can diagnose problems with the larynx without having to train the horse to run on the treadmill and he can remain in his usual yard and usual work," says Dr Dillon.
Whistlers or roarers are commonly treated by one of two surgeries, depending on the severity of the paralysis.
• Hobday: Most horses affected by whistling are found to be within the grade three category, where a procedure known as a 'Hobday', or ventriculocordectomy, is likely to be very effective.
The Hobday procedure, named after veterinary surgeon Frederick Hobday, who first invented the operation, is a relatively simple procedure that is performed under general anaesthetic.
It involves making a surgical incision into the larynx under the throat to remove both ventricles and the left vocal cord. While the technique has remained unchanged for the past 60 years, it has been proven to be successful in selected cases.
However, with the introduction of exercise endoscopy and the improvement in laser technology, it is now possible to do the same surgery standing under mild sedation.
Laser surgery offers a number of benefits over the traditional surgery, particularly because it can be performed without a general anaesthetic, meaning the horse can return to work more quickly. The laser procedure also leaves no surgical scar under the throat and it is less traumatic for the horse.
• Tieback: In horses that have got grade four or five paralysis, a procedure called a laryngeal tieback is used. In this procedure, a suture is placed on the left side of the larynx to permanently hold it open.
The tieback is a more complicated surgical procedure and, therefore, horses are potentially prone to more complications such as failure of the suture, a persistent cough at feeding time, and sometimes the collapse of other areas of the pharynx and larynx.
Tiebacks are less problematical for horses working at slower speeds, such as showjumpers and eventers than it can be in racehorses operating at maximum breathing capacity.
"In recent years, the surgeons at Troytown have begun to use a new technique for tieback surgery, which, in our opinion, is giving consistently better results," says Dr Dillon.
"The surgery is done in combination with ventriculectomies by laser surgery rather than the traditional method, which avoids a second scar and possible infection of the tieback site."
Laser surgery allows the vets to remove the soft tissue structures that can collapse after tieback, such as the aryepiglottic folds.
"It is still worth doing this surgery on good horses as there have been many success stories. Without surgery, the horses usually are retired from racing," adds the vet.
Dorsal displacement of the soft palate, also known as choking or gurgling, is another common condition affecting the upper respiratory tract.
In years gone by, the condition was often described as "swallowing the tongue", although this is now known to be incorrect.
"It is sometimes diagnosed when a jockey describes how a horse, in the last few furlongs of a race, suddenly hears a loud flapping or choking noise," says Dr Dillon.
Soft palate displacement is sometimes described as being similar to what happens when a person snores at night. Needless to say, the effect of this displacement on a galloping horse is far more dramatic than what happens to a person sleeping in bed.
"There are a myriad reasons for dorsal displacement of the soft palate," says the vet. "Weak muscles, immaturity, unfitness, infection, anatomy and even the ground conditions can cause displacement."
Once again, onboard endoscopy is the tool of choice for diagnosis of the condition.
Many horses will choke up temporarily due to other conditions, such as infection, bleeding or immaturity. In these cases, it could be wrong to operate on some of these horses without first trying other methods of treatment.
Other horses may benefit from minimal surgical intervention such as thermocautery of the soft palate to stiffen it. Other horses may need surgery to shorten the soft palate or alternatively to do a tie-forward procedure.
However, there are numerous surgical solutions to this problem, which can be discussed with your veterinary surgeon.