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Normal mastication, or chewing, in the horse is performed by the cheek teeth — which consist of three premolars and three molars in each quarter of the mouth. The occlusal (chewing) surfaces of these cheek teeth are tightly compressed together and function as a single grinding unit.

The presence of any space between the cheek teeth is abnormal and is known as a diastema, or diastemata when plural. Once a space is present, food can become trapped and impacted. Bacteria then proliferate in the decaying food, causing gingivitis (gum disease), and a breakdown of the periodontal ligament that anchors the cheek teeth in the jaw.

The resulting periodontal disease, known as periodontitis, can cause great pain, particularly when the horse is chewing long fibre, such as hay or haylage.

There are two types of diastemata: valve and open. The valve type, where the gap is narrower at the occlusal surface than at the level of the gum, is more likely to trap large amounts of food and lead to severe periodontitis. Open describes a gap that is the same width from occlusal surface to gum.

Developmental or primary diastemata occur when there is insufficient angulation of the cheek teeth, so they are not tightly compressed together. Developmental diastemata may also result because teeth have grown too far apart — typically in young horses aged between three-and-a-half and six years, as they develop a mature mouth.

The condition can also be described as secondary, where a supernumerary (extra) cheek tooth grows and disrupts the tight pattern of the existing teeth. A tooth that becomes displaced to the inside or outside, or rotates, can also lead to an abnormal space and additionally serves to funnel food into the resulting gap. Sometimes the large overgrowth of a cheek tooth can cause displacement and the creation of a larger gap.

Geriatric animals can develop senile diastemata, as they lose angulation of their cheek teeth and the teeth themselves become narrower at the roots.

Chewing it over

The most common sign of diastemata is quidding, where the horse drops balls of semi-chewed food while eating. Affected horses typically quid their hay or haylage, but eat concentrate feeds normally.

Weight loss can also occur if associated gum and periodontal disease are severe and the disorder has been present for some time. Halitosis, or smelly breath, may be noticeable, and the horse may become inconsistent on the bridle or headshake when ridden.

Diastemata can only be diagnosed by a detailed oral exam, using a headlight and either a dental mirror or a viewing instrument called an oral endoscope. This is best performed while the horse is sedated, as he is likely to be in considerable discomfort and may object to examination. Sedation also makes it easier to view the most common sites for diastemata — typically at the back of the mouth between the lower cheek teeth, obscured by the large bulk of the tongue.

Initial diagnosis should establish the site of the diastema and whether it is of valve or open type. The next stage is the removal of food from between the tooth and the periodontal pocket, the gap that develops around the gum when healthy bone and tissue has been destroyed.

Again, this is extremely painful for the horse and should be carried out under sedation — and preferably local anaesthesia. Every visible piece of food should be removed, using a combination of instruments including picks, forceps and sharp-edged Hedstrom files, and high-pressure water lavage (flushing).

Removal of the food is both diagnostic and therapeutic. The depth of each periodontal pocket should be recorded and the teeth adjacent to deeper pockets checked for loosening. Radiography is useful in developmental cases or if there is suspicion of disease or infection of the surrounding bone or sinuses.

The mouth should then be routinely rasped, removing all overgrowths from teeth opposite diastemata, often called exaggerated transverse ridges. Any mildly displaced teeth or large overgrowths should be reduced. Balancing the mouth to a high standard, coupled with removal of food from the periodontal pockets, will make a big difference to the majority of horses.

Filling the periodontal pockets beneath the gum with certain materials can prevent food accumulating again. The most common material is vinyl polysiloxane putty, known as dental impression compound, which sets to a rubbery consistency. It’s important to remove all food, however, before filling the periodontal pocket, as infection can occur beneath it and lead to continued pain when chewing.

Another option with valve-type diastemata is widening, where the occlusal portion of the tooth is converted to an open diastema to prevent food from becoming trapped so readily. This treatment can only be performed by a vet and should take place in a clinic environment under heavy sedation, due to the risk of permanent damage to the teeth either side.

Extraction of teeth, particularly displaced, rotated or supernumerary cheek teeth, can also solve the problem.

A moderate to severely affected horse will need a carefully managed diet. Avoid feeding him long fibre and even short-chopped fibre, replacing this with either grass or grass replacement pellets. Horses with multiple diastemata can be managed throughout the winter months by repeated flushing of periodontal pockets at six- to eight-week intervals.

What’s the prognosis?

The earlier a diastema is identified and dealt with, the more successful treatment is likely to be.

Prognosis can be good if diastemata adjacent to displaced, rotated or supernumerary cheek teeth are treated before any secondary structures are involved.

Young horses with developmental diastemata are likely to improve, particularly if there is good angulation of the cheek teeth that will encourage them to drift together as they continue to erupt, although they may need intensive management for a number of years.

In severe cases, the outlook may be less positive. Periodontal disease can cause an abscess to develop around the root of the tooth, due to infection, which can lead to loosening of the teeth. The supporting bones can then become infected, a condition called osteomyelitis, which can be extremely painful and may result in euthanasia. Sinusitis can also result from advanced periodontal disease affecting the caudal (rear) upper cheek teeth.

The most effective means of reducing the risk of diastemata is to arrange regular examination of your horse’s mouth by a vet, or a dental technician who is qualified with the British Association of Equine Dental Technicians (BAEDT) or the WorldWide Association of Equine Dentistry (WWAED). This should take place at least once a year from the age of three, so that potential problems can be identified and treated appropriately.

Ref Horse & Hound; 15 February 2018