Heidi L. Hoefer, DVM, ABVP
Normal Anatomy and Physiology
The rabbit has a large mandible with a deceptively small oral cavity. The dental formula is: I 2/1, C 0/0, P 3/2 M 3/3 with all teeth continually growing and open-rooted (aradicular hypsodont or long-crowned without an anatomical root). The second pair of upper incisors are small "peg teeth" that sit behind the upper incisors and may be difficult to see in the normal dental arcade. Some rabbits do not have this second pair of incisors. Unlike most rodents, rabbits do not have pigmented enamel.
The rabbit uses its chisel-like incisors for lateral slicing movements that cut or crop grasses before mastication by the cheek teeth. The cheek teeth are arranged in parallel rows and have a flat occlusal surface. The jaw can move dorsal-ventral, rostro-caudal, and lateral for chewing. The rostro-caudal action is limited (as compared to rodents) but the lateral action is extensive.
Rodents possess less incisors and cheek teeth than rabbits and tend to have pigmented (yellow) enamel. The murine-type small rodents (rats, mice, hamsters) have a dental formula I 1/1, C 0/0, P 0/1, M 3/3. They have open-rooted incisors but the cheek teeth have anatomical roots. This arrangement reflects their natural dietary habits: a diet of grains, tubers, and roots requires less chewing and is less abrasive to the cheek teeth. The incisors, on the other hand, are worn done from gnawing and need continual replacement.
Guinea pigs and chinchillas have the common formula of: I 1/1, C 0/0, P 1/1, M 3/3. All teeth are open-rooted. They are true herbivores from a somewhat barren, rocky terrain of the South American Andes mountains. Vegetation is tough and fibrous, and its' low energy content requires a high intake and thorough chewing. This natural diet results in the continual wear of the cheek teeth which have a large grinding surface and no anatomical root. Unlike the rabbit which has parallel rows of teeth, the cheek teeth in the chinchilla and the guinea pig diverge from rostral to caudal. The mandibular cheek teeth are set wider apart than the maxillary sets.
Because of the small oral cavity and large tongue of most rabbits and rodents, a thorough oral examination may be difficult without anesthesia. For a quick exam in an awake animal, an otoscope cone with a light source can be inserted alongside the tongue and the cheek teeth. This quick look may be obscured by salivation and food in the oral cavity. A better instrument in the larger species is a nasal (human) speculum with an attached light source (Welch-Allyn bivalve nasal speculum #26030). This resembles a small vaginal speculum and has the added bonus of a light and a rechargeable handle attachment (3.5V). The blades of the speculum are used to push the tongue aside to allow a quick look at the cheek teeth in both the awake and anesthetized animal. Gauze strips placed around the upper and lower incisors aid in visualization by allowing the jaw to be pulled wide open without obscuring the view.
For a complete oral examination, sedation or anesthesia will be needed in most individuals. Isoflurane and a face mask can be used but there is the disadvantage of working around the mask and anesthetic vapors. A short acting injectable can be used; I prefer ketamine and valium given as a small IV bolus in the same syringe (3-5 mg/kg ketamine plus equal or double volumes of Valium to start) or for longer procedures, ketamine can be given IM (15-20 mg/kg for a rabbit and 20-30 mg/kg for a rodent) plus midazolam (Versed® @ 0.5 mg/kg IM) or diazepam (Valium® @ 1-2 mg/kg IM). Other anesthetic protocols are available in the literature. Xylazine (Rompun®), acepromazine, tiletamine-zolazepam (Telazol®), and demetomidine (Dormitor®) have all been used.
To perform a complete dental evaluation, skull radiographs are necessary. This is especially important in those cases that present with clinical signs of malocclusion (drooling, grinding, e.g.) but have no obvious dental points on oral examination. Skull radiographs are important for evaluating the occlusal surfaces as well as the roots. Check for root elongation on both mandibular and maxillary arcades.
Most small mammals will need sedation for skull radiographs. I prefer isoflurane for this procedure because it offers complete immobilization and rapid recovery for out-patient procedures. Four views should be taken if possible: lateral, ventro-dorsal, and 2 obliques. Use of a film-screen combination that is suitable for radiographing extremities is preferrable. Another alternative is mammography film (Min R). "Mammo"" film is a single emulsion film that provides ultra-detail films, especially of boney structures. A used cassette can be purchased from most distributors. On a typical Bennett machine, use an mAS of 10 (100mA) and KVP of 60-64 in the chinchilla, for example. The cassette is used in the tabletop position.
Clinical signs of dental disease is what one might expect: ptyalism, grinding, selective appetite or complete anorexia. Facial fur can become matted from salivation, and the forepaws are often matted from facial grooming. Halitosis can be a problem in cases with abscessation. Epiphora may be present, representing maxillary root extension into the nasolacrimal duct area. In severe maxillary root problems, there can be upper respiratory compromise, especially in obligate nasal breathers like the rabbit or prairie dog.
Patients with primary dental disease are still bright and alert initially and appear to be hungry. In long-standing cases, however, these individuals can become very debilitated and suffer from secondary problems like dehydration, cachexia, hepatic lipidosis, and in guinea pigs, the effects of hypovitaminosis C.
Common Dental Diseases
Malocclusion is the most common dental condition seen in small mammals. The incisors and/or cheek teeth can be involved. Incisor malocclusion in rabbits is often the result of brachygnathism or abnormal crown wearing in the cheek teeth. Maloccluded incisors can become "tusk-like" in severely overgrown cases (rabbits). Rodent incisor malocclusion tends to be more subtle. Some individuals are adapt at breaking off these teeth on the cage bars. Improper alignment results in uneven wear of the cheek teeth and sharp spikes can form in the enamel with subsequent oral pain. Incisor malocclusion is not always accompanied by points on the premolars and molars, but all teeth should be checked.
Treatment involves trimming of the sharp edges or overgrown teeth. Care should be taken with incisor cutting. Splitting and shattering is common with certain instruments and can lead to pain and less likely, root infection. Dental drills are probably best but are more likely to lacerate soft tissue in the un-sedated patient. The cheek teeth need to be done under sedation. A small bone rongeur (Lempert) or drill (dental or Dremel) can be used to file points. Antibiotics are indicated for root abscesses and carries a very guarded prognosis for full return to function. Owners should be aware of the likelihood of recurrence and the need for regular dental care (every 6-8 weeks).
In some chronic cases of incisor malocclusion in the rabbit, extraction may be indicated. The technique has been described by Dr. Susan Brown and Dr. Jeff Jenkins (see references). Incisor extraction is performed under sedation (e.g., ketamine/valium) using a straight-tipped 1mm periosteal elevator to break down the interdental ligaments. A large gauge hypodermic needle can also be used. All 6 incisors are removed. Care must be taken not to pull too hard or against the natural curvature of the tooth to prevent breaks. The pocket is cultured if infected, flushed and left open. The bunnies are given analgesics before recovery and oral antibiotics for 10 days post extraction or longer if tooth root infection is present.
Tooth root elongation and crown extension is seen sometimes in rabbits and is a common problem in chinchillas and guinea pigs on a low roughage diet. Pellets, grains, and most vegetation do not provide enough chewing to wear down the cheek teeth and root extension can occur. The roots form palpable "bumps" on the mandible and are readily seen on radiographs and CT scans. Maxillary root extension is difficult to palpate but may be associated with ocular problems (epiphora, proptosis, e.g.) or upper respiratory signs. Oral examination may show a somewhat normal occlusal surface and lack of points, or the crowns may appear elongated. This condition is generally irreversible once diagnosed and may result in chronic weight loss and painful mastications. These individuals are usually on syringe feedings and analgesics, which exacerabates the lack of wear on the crowns. Filing all cheek teeth crowns down to the gingival surface every 6-8 weeks may help. Euthanasia may be indicated for chronically painful individuals.
Tooth root abscesses are a serious problem in any rabbit or rodent. Most of these abscesses cannot be totally excised and recurrence is possible. Treatment consists of extraction of affected teeth and aggressive surgical debridement. There is usually long-term follow-up care in these patients with second and third surgical procedures common. Radiographs must be taken before surgery to assess the extent of tooth root involvement and evaluate for osteomyelitis of the mandible or maxilla.
Once the infected area has been surgically debrided, several therapeutic options are available. One technique involves packing the pocket with calcium hydroxide, a dental preparation that has a pH of 12 and essentially "nukes" everything in it's wake. This can cause severe necrosis of normal tissues if leakage or improper placement occurs. Placement of bone matrix materials like hydroxyapatite or bioglas (Consil® Nutrimax Labs) may enhance healing of the affected bone following thorough debridement. A newer technique involves the use of antibiotic-impregnated polymethylmethacrylate beads into the site. Dr. Avery Bennett has provided us with information on the preparation and preliminary pharmacokinetics of this procedure (see reference).
Prairie dogs can have incisor tooth root aberrations that can lead to upper respiratory (nasal) obstructions. Odontomas and other related space occupying masses can form at the tooth roots leading to eventual encroachment on the nasal airways. These obligate nasal breathers present with non-responsive dyspnea. Diagnosis is with radiographs or CT scan. Corrective treatment is unlikely.
Analgesics and Antibiotics
The long term use of antibiotics may be indicated in some dental conditions like root abscessation or osteomyelitis of the mandible or maxilla. Common aerobic bacterial isolates include Pasteurella, and Staphylococcus. Antibiotic choice based on culture and sensitivity testing is recommended whenever possible, keeping in mind, of course, those drugs that can be safely used in rabbits and rodents. Safe antibiotics include the quinolones (enro-/ciprofloxacin), the aminoglycosides, trimethoprim-sulfa, chloramphenicol, and in rabbits, injectable penicillin. Because of the potential suppression of normal enteric flora, avoid any antibiotic that has a selective gram-positive spectrum. This includes the beta-lactams (penicillins and cephalosporins), clindamycin, lincomycin, and erythromycins.
Analgesia is an important part of dental disease management, especially involving the tooth roots where pain can be chronic. Injectable agents like butorphanol (Torbugesic® @ .5 -1 mg/kg IM q 6-8 h in rabbits and large rodents) or buprenorphine (Buprenex® @ 0.05 mg/kg IM q 12h in rabbits and large rodents) are good choices. Oral analgesics have not been extensively studied but some clinicians use a compounded carprofen suspension (Rimadyl® @ 2 mg/kg PO q 12h) to manage pain short term.
References and Recommended Reading
1. Bennett RA: Management of abscesses of the head in rabbits. Proceedings, The North American Veterinary Conference, Orlando, Jan. 1999.
2. Brown SA: Surgical removal of the incisors in the rabbit. J Small Exot Anim Med 1992, 1; 150-15
3. Crossley, DA: Clinical aspects of rodent dental anatomy. J Vet Dent, 12(4):131-135, 1995
4. Crossley, DA: Clinical aspects of lagomorph dental anatomy: the rabbit. J Vet Dent, 12(4):137-140, 1995.
5. Donnelly TM, Schaeffer DO: Disease problems of guinea pigs and chinchillas. In Hillyer EV, Quesenberry KQ (eds.): Ferrets, Rabbits, and Rodents, Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 270-281.
6. Jenkins JR: Soft tissue surgery and dental procedures. In Hillyer EV, Quesenberry KQ (eds.): Ferrets, Rabbits, and Rodents, Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 227-239.