Summary

Dental problems in pet rodents and rabbits are common in clinical practice, and may be caused by many factors, e.g., trauma, infection, neoplasia, congenital deformity, diet deficiencies or lack of wear. Species with continuously growing cheek teeth, e.g. chinchilla, guinea pig, rabbit, are susceptible to both primary and secondary problems involving the roots and/or the crowns, so radiography is often required in order to make an accurate diagnosis.

Malocclusions in murine rodents are most frequently the result of trauma or lack of adequate tooth wear. The incisor teeth of rats (illustrated), mice, gerbils, chipmunks and hamsters tend to overgrow when the husbandry reduces the need for gnawing.

Distortion of the occlusal plane and uneven wear of the molars can produce secondary malocclusion and uneven wear of the incisors, as shown in this rabbit.

Uneven wear of the occlusal surfaces leads to the formation of 'spikes' on the cheek teeth that traumatize the oral soft tissues and result in salivation and anorexia. The chinchilla exhibits excessive salivation associated with advanced cheek teeth disease. Simple smoothing of these small sharp irregularities on the edges of the occlusal planes may be sufficient to alleviate the problem, but recurrence is common.

General anatomy

Rodents

Dental formula 1/1 0/0 0/0 3/3. The incisors are chisel shaped, constantly growing and a pigmented yellow colour. When gnawing the lower jaw moves forward so that the incisors oppose each other but the molars do not. The cheeks are drawn in across the diastema (the gap between the incisors and molars). This enables the rodents to gnaw without wearing the molars or swallowing debris.

Rabbit

The dental formula of the rabbit is 2(I2/1 C0/0 PM 3/2 M 3/3). The total number of teeth is therefore 28. There are no canines present. The gap between the incisors and premolars is known as the diastema. Rabbits possess aradicular hypsodont teeth, which means that they erupt and grow throughout life and do not have true roots. The incisors grow at a rate of 8 –12 cm per year. The continuous growth of the teeth means that the animals can suffer from both primary and secondary problems that may require repeated treatment. The premolars and molars are often grouped together and called the ‘cheek’ teeth. The lips prehend the food, the incisors are used to gnaw and slice the food and the cheek teeth grind the food. The mandible is narrower than the maxilla. Rabbits chew and gnaw by moving the jaws laterally. At rest, the mandibular incisors contact the first and second maxillary incisors. The cheek teeth are out of occlusion when gnawing so that the incisors alone are in wear. The position of the incisors relative to each other and the fact that the enamel is thicker on the labial surface leads to a faster rate of wear of the lingual side. This produces the characteristic chisel shape of the incisors. When chewing is required, the mandible is retracted to separate the incisors and bring the cheek teeth into occlusion.

General Malocclusion

This is the commonest cause of dental disease in the rabbit. There are six main aetiologies, although in some cases more than one factor may contribute to the clinical problem;

Congenital deformity e.g. mandibular prognathism in some dwarf breeds. Mandibular prognathism is thought to be caused by an autosomal recessive trait with incomplete penetrance of 81%. The clinical problem first presents at eight to ten weeks of age although some animals are presented initially at 12-18 months old.
Dietary problems e.g. calcium deficiency leading to alveolar bone resorption and tooth loosening, concentrate based diet leading to lack of wear and tooth overgrowth.
Trauma to tooth e.g. gnawing on bars, falling from a height, tooth clipping
Fracture of mandibular ramus presenting as deviation of mandibular incisors to affected side (may occur iatrogenically during extraction). Other skull or jaw fractures are less common.
Infection of tooth roots, jaw, pulp infection
Neoplasia affecting normal anatomy of jaw / dental occlusion.

Dental examination

A cursory examination may be made in the conscious rabbit using an otoscope or small vaginal speculum used intra-orally but such an examination in the conscious rabbit is extremely limited and many lesions will not be detected. A full oral examination to inspect the teeth and soft tissue structures in the mouth requires deep sedation or a full general anaesthetic to abolish chewing movement and allow adequate visualisation of the oral cavity. Proprietary oral gags and cheek retractors are available. Alternatively, lengths of bandage hooked over the incisors may be used to open the mouth and allow good visualisation with minimal obstruction. Retraction of the tongue to each side in turn is required to check for spurs that may imbed into the base of the tongue. The entire length of the tongue should be examined for signs of laceration or infection. Similarly, the cheeks should be retracted from the dental arcades and examined for evidence of laceration or infection. The periodontal ligament of each tooth is checked using a blunt probe. The probe is introduced into the gingival sulcus and explored around each tooth to check for periodontal pockets, food impaction, plaque, pus, and areas of inflammation. Bleeding after probing is a sign of inflammation. Each tooth is checked for excessive mobility. The results of the dental examination should be recorded on a dental chart for accurate recording and comparison with subsequent examinations. Radiography will aid in the location of spurs and in the assessment of tooth and jaw pathology.

Radiography

The patient must remain motionless for the exposure of the radiograph. Although some individuals may remain in the desired position, it is often less stressful and easier to sedate or anaesthetise the small mammal patient with isoflurane or a reversible injectable regime. The hair coat should be clean and dry. Water or dirt on the coat will produce artefacts on the radiograph. Plain films must always be taken prior to contrast studies in order to provide comparison, assessment of the exposure factors and to avoid masking a diagnosis.

Exposure settings, film and cassette types must take into account the small size of the patient. Radiography of the small mammal generally involves patients less than 10cm thick and less than 10 Kg weight. The radiographic equipment should be capable of producing 300 mA and obtain a minimum exposure time of 0.008 second. The use of a higher mAs will allow a lower kVp exposure. The kVp used is usually in the range of 35 - 60. The higher setting will be used for thick bone or superimposed bone e.g. rabbit skull radiography. High resolution film screens are essential to obtain diagnostic radiographs in the smaller patients. High detail, rare- earth intensifying screens with appropriate films should be used. Mammography film may be used to give enhanced soft tissue detail. Radiography cassettes and screens designed for human extremities are useful. An enlarged image can be produced using special techniques.

Dental disease is perhaps the most common presenting condition in pet rabbits and rodents. Skull radiography is essential in evaluating for proper diagnosis and evaluation of dental and associated bone pathology. Dental radiography is also required prior to tooth extraction especially if infection or involvement of other teeth is suspected. The dorsoventral view is used to visualise any lateral spurs from the cheek teeth (premolars and molars) and pathology associated with the root of the incisors. The lateral view is also useful in assessing skull conformation. Mandibular prognathism (relative overlong lower jaw or maxillary shortening) has long been known to be an inherited condition in rabbits. Mandibular prognathism as well as brachycephalism is now common in UK pet rabbit dwarf breeds. Incisor malocclusion and overgrowth therefore occurs. Cheek teeth (premolar and molar) elongation may occur in those species that have continuously growing cheek teeth i.e. rabbit, chinchilla and guinea pig. This may be due to lack of wear or osteopaenia. Lysis of the alveolar bone is an early radiographic sign. Dystrophic root growth, further lysis and proliferation of the bone indicate a progression of this disease and the presence of osteomyelitis. Rabbit bone in particular mounts a very aggressive response to infection, radiographically similar to osteosarcoma in the dog. Post mortem studies of rabbit skulls revealed bone of poor quality with deformed teeth having little or no enamel. Radiography of these cases should investigate the following possible sequelae; distorted growth of the roots penetrating the maxillae, mandibles and orbits, osteomyelitis, abscess formation or infections of the lacrimal duct or nasal cavity. Intra-oral radiography is limited in small mammals due to their small oral cavities and opening however a technique for intra-oral radiography in rodents has been described.

Chinchillas presenting with ocular discomfort, e.g. epiphora, should have skull radiography. This epiphora may be in response to pain from root elongation, tooth root penetration of the orbit or obstruction of the lacrimal duct. Retrobulbar abscessation is relatively common in the rabbit and presents as exophthalmos. Radiography of the skull is required to check for signs of osteomyelitis. Radiography of the thorax is also indicated as some cases of exophthalmos are related to thymoma or other thoracic masses.

Dacryocystitis (infection of the nasolacrimal duct) is a common finding in the rabbit in clinical practice and should be investigated with radiography. The nasolacrimal duct runs from the single punctum in the lower eyelid through the bony naso-lacrimal canal in the maxilla, over the roots of the cheek teeth and down over root of the first incisor. The nasolacrimal ducts empties into the nose at a position rostro-medial to the apex of the incisor root. Therefore pathology of these structures will impinge on the duct and promote blockage and infection. The nasolacrimal duct may be cannulated and radio-opaque contrast material instilled into the duct to allow investigation of abnormalities. Marked unilateral dilatation of the duct proximal to a dorsal flexure at the caudal limit of the incisor tooth root has been reported. Occlusion of the nasolacrimal duct has been attributed to fat droplets, scar formation or blockage with purulent or inflammatory debris.

Facial masses should be investigated using radiography. The differential diagnoses include bacterial or fungal infection of soft tissues or bone, trauma, bone cysts or neoplasia. An osteogenic sarcoma has been reported affecting the mandible of a rabbit.

Tooth trimming

The use of nail clippers or wire cutters is not recommended for trimming teeth for the following reasons;

The excessive force applied to the crown as it is cut may damage the periapical germinal tissues adversely affecting future tooth growth
Longitudinal fissures in the tooth may be produced. These may lead to periodontal problems and pulp infection.
Accurate reshaping and smoothing of the teeth is difficult if not impossible
The sharp edges created may lacerate the tongue and cheeks or contribute to oral discomfort

A bur in a dental hand piece should be used for accurate and humane tooth trimming and shaping. The pulp cavity normally extends to approximately half the length of the normal occlusal height of the tooth i.e. midway between normal height and the gingival margin. Trimming and reshaping to normal height should not therefore expose the pulp cavity. Many rabbits will tolerate trimming of the incisors whilst conscious and manually restrained. A tongue depressor or syringe case of appropriate size should be placed behind the incisors to protect the soft tissues whilst trimming. In order to fully examine the oral cavity for corrective trimming of the molars a general anaesthetic is necessary to abolish chewing and tongue movements and allow manipulation of the mouth and tongue. An appropriate sized guard is used to retract and protect the soft tissues either side of the tooth to be burred. The affected molar is burred down and reshaped. Each tooth should not be subjected to long periods of burring that may produce excessive heat and damage the pulp tissue. The debris should be wiped away frequently. Ideally the animal should have an endotracheal tube placed for the supply of oxygen and/or anaesthetic gases and to prevent inhalation of the tooth debris. Damage to the soft tissues should be treated appropriately by flushing or cleaning with dilute chlorhexidine and systemic antibiotic therapy instigated if the lesions are infected. The short-term use of an oral topical anaesthetic gel may give some relief from oral discomfort while the soft tissues heal.

Summary

It is wise to radiograph the skull and perform a dental examination in any small mammal presenting with anorexia or signs of pain. Chronic cases of molar disease can be successfully managed lon g term if careful attention is paid to ensuring regular dental work and case assessment is performed and the animal is maintained in a pain-free state using appropriate analgesia.

References and further reading (some material taken from the following)

Redrobe S. 1999. Incisor disorders in mammals Exotic DVM 1(5):36-37;

Redrobe S. 1999. Resolving molar teeth disorder of herbivorous mammals. Exotic DVM 1(4):15-16

Redrobe S. Surgical procedures and dentistry’ in BSAVA Manual of Rabbit Medicine and Surgery. Ed P Flecknell. Pp117-134

Redrobe S. 2001. Imaging techniques in small mammals. Seminars in Avian and Exotic Pet Medicine. 10(4):187-197