The guinea pig has open-rooted (continuously growing) molariform teeth which, physiologically, are very angled: the mandibular teeth are directed towards the tongue, while the maxillary ones are directed towards the cheeks. The occlusal surface is flat and the angle of occlusion is inclined at about 30° (Figs. 1a and b).
Malocclusion of the incisors is relatively rare (Figs. 2 and 3), whereas malocclusion of the molariform teeth is fairly common and difficult to resolve. This problem usually occurs in animals 2-3 years old.
The causes of malocclusion are probably multifactorial, with congenital malformations and dietary errors being implicated; vitamin C deficiency is another possible aetiology in the guinea pig. The administration of commercial foodstuffs (cereal flakes or kibble) and seeds, calorie-rich foods that necessitate only brief mastication, alters the physiology of mastication and the type of attrition of the molariform teeth. Not only is the time used to masticate reduced, but also the movements of mastication are altered from the circular movements of grinding to compression, with consequent trauma to apical tissue. The teeth are not worn down sufficiently and the rate of eruption exceeds that of erosion, resulting in elongated teeth. When contact with the corresponding tooth of the other dental arcade prevents further elongation, eruption ceases, even though the apical tissue continues to grow and intrude at the level of the maxilla and mandible. In the mandible this phenomenon gives rise to hard swellings on the ventrolateral margin of the mandible.
The lengthening of molariform teeth is accompanied by a change in their curvature, such that sharp points are created which damage the soft tissues. The mandibular molariforms grow in a medial direction, finishing up by trapping the tongue and preventing the guinea pig from feeding itself. At this point the signs become evident (anorexia, sialorrhoea, wasting). Elongation of the molariforms also causes a decrease in the mobility of the mandible, which contributes to worsening the formation of the sharp points.
The signs of malocclusion can include loss of weight, anorexia, drooling with saliva smearing the throat, overflow of tears, swellings along the inferior border of the mandible and inability to close the mouth completely. The molariform teeth can be examined in conscious animals by using an otoscope, although the inspection is often hampered by the presence of saliva or food residues. X-rays of the head enable the elongation of the roots to be evaluated as well as the presence of periodontal infections. X-rays often show that the anterior molariforms are longer than the distal ones, a condition that prevents correct closure of the mouth (Figs. 4, 5, 6, 7, 8 and 9).
The incisors can be easily examined and treated with the animal conscious, but the guinea pig needs to be sedated in order to be able to examine the molars and premolars carefully. Anaesthesia is a particularly dangerous procedures for the animal if it has not eaten for a long time; it is, therefore, worth feeding the guinea pig for a while with purée of pellets or Critical Care Oxbow®, administering fluids subcutaneously and providing vitamins. Analgesics should be given for a few days after the treatment.
The treatment consists in levelling the occlusal surface of the teeth under general anaesthesia. A gaseous anaesthesia can be used without intubating the animal, exploiting the fact that respiration occurs almost exclusively via the nares. In fact, by using a small mask covering the nares, it is possible to work freely in the oral cavity. Alternatively, a nasal catheter 2-2.5 cm long can be inserted into one nostril to convey the anaesthetic; the other nostril should be kept closed. The levelling can be carried out with a bur mounted on a micromotor. Great care should be taken to protect the soft tissues as laceration of these could cause fatal haemorrhage. A physiological inclination of the occlusion, that is 30°, must be obtained.
The guinea pig should be treated with analgesics and assisted feeding until spontaneous mastication is restored. Subsequently a physiological diet of hay, grass and abrasive vegetables should be given.
In contrast to the situation in rabbits, the response to treatment in guinea pigs is often very disappointing. Even after careful levelling of the teeth, the animal refuses to eat and lets itself die. Furthermore, it is much more difficult to work on the teeth of the guinea pig because of the small size of the animal and the narrowness of the opening of its oral cavity. Given these problems and considering that the disorder is in any case recurrent, some authors suggest euthanasia for affected animals.