Rabbits are more frequently being seen at veterinary surgeries. One of the most common currently presenting conditions is dental overgrowth. This short article describes the peculiarities of rabbits’ teeth, the reasons why they are predisposed to such dental problems, their early diagnosis and prevention.
A rabbit is not a rodent
Rabbits (lagomorphs) and rodents are the smaller creatures that fall into the order of mammalia alongside carnivores and felines. In very early classification rabbits were originally known as rodents. Indeed they both possessed incisor teeth for gnawing (rodentia is derived from the Latin verb rodere, which means to gnaw), and lacked canine teeth. However, it became apparent that there was some dissimilarity in the dental formula in certain animals. This provided the basis for creating a new order Lagomorpha. Lagomorphs are distinguishable from rodents in that they have 2 pairs of upper incisors (the second pair being small and peg shaped) whilst rodents have only 1 pair. Further more rabbits have extra pre-molars, probably to assist in grinding grass and vegetation, and their mandible (lower jaw) is narrower than the maxilla (upper jaw), the reverse being the case in rodents.
Rabbit Dental Formula and Structure
Between the incisors and pre-molar and molar teeth (collectively referred to as ‘cheek teeth’) there is a large gap (diastema), into which the buccal folds protrude, hindering visualisation of the cheek teeth. All of a rabbit’s teeth are open-rooted (aradicular) and grow continuously, the shapes of the teeth varying (heterodont) with their function. Each tooth has a central core of pulp, surrounded by dentine and enclosed in a layer of enamel. The distribution of enamel around the tooth is uneven.
Enamel is formed, along with the first layers of dentine, in the apical region. Tooth growth here allows for continuous replacement of the tooth as it erupts and is worn away in the mouth. This is different from humans where enamel only covers the crown and root growth stops once the tooth has erupted. In the rabbit’s main incisor teeth the enamel is thicker and the dentine harder at the front surface than behind. Preferential wear from the action of the incisors leads to their chisel-like appearance.
By the same token differential wear of enamel, dentine and cementum at the exposed cheek tooth surfaces results in formation of the normal ridged, highly efficient grinding occlusal surfaces.
Mastication, malocclusion and dental overgrowth
At rest the lower incisors locate between the front upper incisors and peg teeth, and there is a slight gap between the upper and lower molars. As the rabbit starts to eat its jaw opens slightly, grasping and then slicing the food between its upper and lower incisors in a scissor-like action. The tongue moves the food back towards the cheek teeth on one side. A slight retraction and sideways movement of the lower jaw brings the occlusal surfaces of the cheek teeth into contact on that side, the incisors being separated by this action, and the food is ground by a rapid side to side chewing motion. When upper and lower teeth are correctly aligned they are said to be in normal ‘occlusion’. Conversely, ‘malocclusion’ describes the condition where the teeth or jaws fail to align properly. The causes of malocclusion may be:
- Traumatic – physical breakage, dislocation or loss of one or more teeth, or jaw separation of the mandibular symphysis
- Atraumatic – congenitally absent or mal-positioned teeth, hereditary jaw deformity (some dwarf and lop-eared breeds), tooth overgrowth, dietary influences, disease, infection, toxicity or abnormal chewing habits
The teeth continue to grow (being open-rooted) whether or not there is normal wear. If the rate of eruption exceeds wear, the crown gradually elongates. A slight increase in occlusal pressure can tip and rotate the crowns. It may also arrest eruption, forcing the roots to grow backwards into the surrounding tissues (root elongation). Malocclusion of the incisors can prevent closure of the mouth resulting in a secondary malocclusion of the molars. The reverse situation can also occur, whereby overgrowth of the molars prevent the mouth from closing fully causing the incisors to overgrow. Overgrown incisors spiral and twist outwards, and tend to traumatise the opposing lips. Elongation of the maxillary incisor root backwards can lead to obstruction of the lacrimal duct preventing normal tear drainage so that the rabbit appears to be crying. If inflammation or infection is present a creamy discharge may be seen in the eye or nose. The maxillary cheek teeth naturally curve outwards, and the mandibular inwards. As the crown overgrows normal wear is compromised and tooth curvature is enhanced, creating spikes. Spikes on the maxillary cheek teeth traumatise the cheeks, and those on the mandibular teeth lacerate the tongue. Displacement of the growing apices of the cheek tooth roots result in palpable swellings on the ventral surface of the mandibular bone, and protrusion of the maxillary roots into the nasal chamber or behind the eye.
Signs and Symptoms of Dental Disorders and Disease
Rabbits presenting at surgery can show a variety of signs, ranging in severity, and depending on the extent of progression of the overgrowth or disease. In the initial stages: weight loss, unkempt and matted or dirty coat, caecotrophs adhering around the anus and perineal fold, smaller, fewer or even an absence of faeces. In addition the rabbit appears depressed, isolated, possibly in pain (tooth grinding) and aggressive, and it’s appetite is reduced or is anorectic. Alternatively it’s appetite may be normal but the rabbit appears to be having difficulty in or is unable to eat and drink (dysphagia i.e. wants to eat but can’t, as opposed to anorexia) which exacerbates the problem. On closer examination, there may be asymmetry, deformity, prognathism, swelling (periapical), or wounds and facial abscesses on the head. The eyes may appear to bulge or there may be watery lacrimation or discharge, and bleeding or rhinitis and discharge from the nose. Excess salivation, halitosis, stomatitis, gingivitis, ulceration of lip, cheek, tongue, palate, and visibly long, broken or displaced, discoloured teeth are all indicative of a dental disorder. A physical examination of the rabbit is necessary, although the rabbit may be reluctant to be handled, and be touched especially around the face, if it is in pain. The buccal folds make visualisation of the cheek teeth almost impossible, but a good look is essential to aid diagnosis, and the rabbit may require sedation or anaesthesia.
Factors contributing to dental disorders
Contrary to current belief diet is not the sole factor responsible for dental overgrowth. There are a number of other factors that contribute to the development of dental overgrowth and or disease including genetics, trauma, disease and poisons. There is a genetic predisposition for dental overgrowth – more common in dwarfs (brachycephalism) and lop-eared breeds. When incisor malocclusion is detectable at an early age it is most likely to be a genetic or developmental problem. In a mature rabbit not previously showing signs as a youngster it is likely to be secondary to cheek tooth overgrowth or injury (fractures / dislocations of teeth, mandibular symphysis). Disease, whether due to bacterial infection or even hormone imbalances can also adversely affect the development of dentine and enamel, as too can toxic compounds.
Rabbits have evolved with continuously growing open-rooted teeth as a result of the fibrous nature of their diet. Low energy density, high fibre diets lengthen the eating (grinding) period and assist the wearing process (due to the abrasive nature of fibres, silicates and minerals within the plants) and prevent overgrowth. As well as the physical nature of the diet, the chemical nature is important too. Foods rich in sugars and starch can cause dental caries leading to cavities on the surface of the tooth and weakening of the tooth structure. Calcium deficiency is often mistakenly blamed for dental disorders (poor mineralisation, horizontal ribbing). In rabbits, calcium absorption is not well regulated and appears to be proportional to dietary calcium levels. Thus calcium absorption is highly efficient, and true calcium deficiency is extremely rare. Vitamins A and D, Phosphorus and Magnesium are also required for tooth and bone formation, and so the general nutritional status of the animal should be viewed as more important than calcium status alone.
It is easy to understand how rapidly dental problems develop, when tooth growth rates are in the order of 2 to 3mm per week. Although dental techniques have advanced significantly in the last few years, cures are still not common so it is essential to think in terms of prevention. Although nutrition is not the sole factor responsible for pre-disposing an animal to dental overgrowth, it can certainly play a vital part in promoting recovery post operatively and preventing or reducing the likelihood of recurrence. With the latter in mind it is essential that a complete and balanced diet is fed, one which is a low energy, high fibre diet (coarse fibres such as alfalfa, hay, dried grass). Rabbits should have access to grass and the occasional fresh vegetable should be provided. Sugary foods and treats should be avoided, and dental exercise should be provided in the form of safe materials to chew on e.g. twigs of non-toxic trees, toilet-roll innards. Recommend that owners record the rabbit’s weight weekly, although dramatic changes may be apparent to the owner it is not unusual for gradual weight loss to go unnoticed. Weighing not only provides an opportunity to determine the weight but also to examine the rabbit’s front incisors but also to cast a critical eye over the rest of the body for other signs of illness. Encourage owners to bring the rabbit to the vets at the first sign of trouble, and stress the importance of regular and thorough dental checks. If you suspect that a rabbit may have a genetic problem, discourage the owner from breeding that animal, and recommend neutering to be on the safe side.
Rabbits possess an unusual dentition, and when problems arise, the rabbit can suffer an enormous amount of discomfort and pain. Small dental abnormalities often go undetected in the early stages, and quickly develop into major problems (due to their continuously growing nature). It is essential to promote awareness of dental issues in rabbits and provide advice on the best approach to preventing such problems from arising in the first place.
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Author : Supreme Pet Foods