Managing dental abscesses in rabbits (Proceedings)

Anatomy and Physiology

Rabbits have 6 incisors (4 upper and 2 lower). The 2 first incisors are large and oppose the lower incisors. The 2 second upper incisors are called 'peg teeth' and are located just caudal to the first incisors. The incisors are relatively long compared with the cheek teeth. The incisors have a beveled cutting edge with both the upper and lower incisors being sharp on the rostral aspect. When the mouth is closed, the lower incisors rest on the peg teeth caudal to the upper incisors. Rabbits use the sharp edges of the incisors to cut off grasses. The roots of the incisors are dramatically curved and very long. They are deep to the roots of the cheek teeth and extend very caudally in the mandible and maxilla.

The cheek teeth consist of the molars and premolars. Rabbits have 3 upper premolars and 3 upper molars and 2 lower premolars and 3 lower molars. There is a lot of action during chewing between the upper and lower cheek teeth grinding fibrous grasses. They move in a rotary manner going both side-to-side and cranial to caudal. This crushes the grasses that the rabbit has cut off with the incisor.

All of the teeth of rabbits grow continuously and do not overgrow because they are worn down by the opposing teeth. They depend on mastication to wear the crowns. New crown is produced below the gingival surface and continually pushed up into the oral cavity. Wild rabbits spend most of the day eating dry grasses, providing the necessary wear on the teeth crowns. Dental crown reduction is not necessary in normal rabbits. Overgrowth occurs if the rabbit does not wear the crowns sufficiently. Additionally, if the crowns are not worn adequately, root ankylosis occurs. The crown is stopped from moving and the root begins to grow into bone.

It seems likely that most rabbit dental problems are related to inadequate amounts of long fiber in the diet. Many rabbits are fed a pelleted diet composed of finely ground alfalfa easily crushed by rabbits. Dry grasses must be ground up before they are swallowed and this grinds the crowns allowing the root to push new crown up keeping the teeth and roots healthy.

Presenting Complaint

Anorexia is a common presenting complaint and dental disease should be considered in any rabbit that presents for anorexia. Rabbits with dental disease are considered to be in pain and do not swallow as much as they normally would. They often have wet fur around the muzzle and ventral cervical fur. Ocular discharge is a presenting complaint that should make the clinician think of dental disease. The upper cheek teeth roots are close to the nasolacrimal duct. If ankylosis occurs, the roots grow away from the mouth and compresses or invades the nasolacrimal duct. Tears no longer drain properly and epiphora results. The discharge may be clear or may contain white, flocculent material. It seems that if the root tip is infected and breaks into the nasolacrimal duct, it causes the discharge to be more purulent in nature. Some rabbits will present for overt malocclusion of the incisor teeth. Many rabbits with primary cheek teeth disease present for incisor malocclusion which is actually secondary to abnormal cheek teeth. Do not just trim the incisors without investigating the cheek teeth. If the cheek teeth crowns overgrow, the mouth no longer closes properly which affects the occlusion of the incisors.

Physical Examination

The incisors are relatively easy to evaluate. It is important to have a general idea of how long rabbit incisors should be so you do not trim teeth that are actually a proper length. The upper lip is split (harelip) but the lower is not. The lips are lifted and the length of the incisors assessed. The lower incisors should oppose the peg teeth and the four main incisors should have a beveled cutting edge. Put pressure on the bone at the base of the incisor crowns and try to express purulent material. There should not be any and if there is it indicates there is a tooth root infection.

Cheek teeth are more difficult to evaluate without general anesthesia. Externally, palpate the ventral line of the mandible. It should be smooth. Any lumps or bumps indicate root disease. Some rabbits' mandible will have a scalloped surface along the entire length indicating that most of the teeth are affected. One method used to visually check the crowns of the cheek teeth is to use an otoscope or vaginoscope inserted into the mouth of the conscious patient. Most rabbits react negatively to this procedure, chew constantly, and try to spit out the scope. This makes it difficult to assess the teeth properly and the clinician usually only gets a brief glimpse of the crowns.

In most patients to be able to adequately evaluate the cheek teeth, general anesthesia is required. Skull radiographs are also vital in assessing rabbit teeth and are best taken with the rabbit under general anesthesia. The mouth of the rabbit does not open very far making it difficult to visualize the cheek teeth crowns. Teeth specula and cheek dilators developed by Dr. David are very helpful in getting a good look at rabbit cheek teeth crowns. A cotton-tipped applicator is used to manipulate the tongue to allow the teeth to be assessed for overgrowth and the presence of spurs. The occlusal surfaces of both upper and lower cheek teeth are concaved. In most cases the upper teeth develop spurs on their lateral aspect while the lower teeth develop spurs medially. These can cut into the cheek or the tongue.

A rigid endoscope can also be used to assess the cheek teeth in an anesthetized rabbit. A conscious rabbit will bite the scope and cause expensive damage. The advantage to using a scope is that it magnifies the image allowing better visualization of subtle changes.

Skull Radiography

As in humans and other animals, the teeth of rabbits cannot be accurately evaluated without radiographs or, better, advanced imaging such as CT scan or MRI. Root disease is very common in rabbits and often occurs before the crown appears grossly abnormal. Skull radiographs are made with high detail techniques and film. General anesthesia is required for proper positioning. The rabbit can be maintained on a mask removing it for brief periods to make the films, though endotracheal intubation is preferred. Four or five views are made – DV, lateral, right lateral oblique and left lateral oblique. A cranial to caudal view can be helpful in evaluating the lateral aspects of the arcades and the angle of occlusion between the upper and lower cheek teeth. The oblique views are very important for assessing the cheek teeth on each side.

Computed tomography (CT) scans has become more available and affordable. They provide very valuable information and are the preferred imaging modality. Many machines can electronically generate 3 dimensional reconstructions of the entire skull or parts of it. These images allow for detailed assessment of the crowns and roots of all the teeth in a short period of time.

Treatment

The treatment indicated for dental disease in rabbits depends on the severity of the disease. A diet high in long fiber is an essential part of managing rabbit dental disease. If possible, all pellets should be eliminated from the diet and the rabbit should be maintained on grass hay. A small amount of pellets can be given as treats. Unless the rabbit chews enough to wear the crowns, dental disease will recur.

In rabbits with simple crown overgrowth, burring the crowns down will often allow the roots to begin pushing new crowns into the oral cavity relieving the root ankylosis. The crowns should be burred down to the level of the gingiva leaving only 1-2 mm of crown exposed. The crowns can be burred with a dental bur, a bone bur, or a motorized rotary tool. I find the cable hand piece works well. It is important to protect the tongue and cheeks. This can be done using a tongue depressor split lengthwise. Dr. Crosley has developed a cheek and tongue protector that works well and is autoclavable; however, it is difficult to hold two of these (one to protect the tongue and one to protect the cheek) and operate the bur.

To trim incisor teeth, a wheel cutting device is best. A diamond wheel is available for the rotary tool and easily cuts through incisor teeth. A tongue depressor is placed caudal to the incisors, the lips are elevated, and the teeth are cut at an angle distal to the gum line to create the cutting beveled edge.

If there is radiographic evidence of root disease (increased lucency at the tips of the roots) but no evidence of infection, crown reduction may be all that is required to reverse the process. This procedure is done and the rabbit is reevaluated in 4-6 weeks both by visual inspection and by radiography. The crowns should grow up into the oral cavity by this time and the radiographs will help you assess if the lucency around the root tips has resolved. If the crown is growing but there is still an increased lucency at the tips of the roots, the procedure is repeated and the rabbit reassessed in another 4-6 weeks. By burring down the crowns you are mimicking the action of chewing to wear down the crowns.

If there is evidence of osteomyelitis – lysis and proliferation of bone surrounding a tooth – the tooth should be removed. The root has probably undergone necrosis and the tooth is acting as a foreign body. If the infection is mild, the tooth can be removed per os. Dr. Crosley has developed an incisor luxator to aid in removing incisors of rabbits. It is flat and has a long curved end to allow the root to be elevated all the way to its tip. The periodontal ligament is broken down first. Once this is loosened circumferentially, the incisor luxator is inserted alongside the root and used to free the root from that adjacent bone circumferentially. The tooth root is elevated until the entire tooth is loose. It is then pulled out in a curved motion to prevent the tooth from breaking. Cheek teeth are more difficult to remove per os. Various dental elevators are used to free the tooth from the surrounding alveolar bone. If the root is ankylosed, it is harder to free the tooth from bone. If there is infection it is usually easier to remove the tooth as the osteomyelitis breaks down most of the attachments. If there is minimal infection, the defect is allowed to heal by second intention. There may be an apparently large defect but these typically heal within 1-2 weeks.

Many questions arise as to what happens to opposing teeth when teeth are removed. I typically do not do anything with opposing teeth at the time of tooth removal. The rabbit's teeth are reevaluated by oral exam in 4-6 weeks. Because of the rotary action of the teeth, teeth on the opposite arcade that have not been removed may wear the opposing teeth. In these cases, no further action is indicated. Sometimes the teeth do not grow anymore. The reason for this is unclear. And sometimes the opposing teeth continue to grow and have the potential to damage structure if they are allowed to over grow. In these rabbits the owners have a choice of having you remove the opposing teeth or returning to have the crowns burred every 4-6 weeks. Additionally, it should be noted that if the germinal tissue is healthy and not removed with the rest of the tooth, the tooth can grow back.

A variety of surgical therapies have been used in treating dental abscesses in rabbit with variable results. In general, the prognosis for complete recovery without recurrence is guarded. Multiple procedures may be required making client education prior to surgery is vital. Frequent visit to the veterinarian and a significant financial commitment are required as well.

Regardless of the treatment method selected, it is important to remove any teeth that are involved either from tooth root infection or as a result of osteomyelitis. Long term antibiotic therapy is also essential. Two weeks of antibiotics is usually inadequate because of the great potential for recurrence and some recommend low dose, life-long antibiotic therapy to prevent recurrence. The traditional treatment of abscesses in mammals (lancing and draining) is not effective in rabbits. The pus is too thick to drain adequately and they usually have fibrous tracts containing bacteria making recurrence common.

Because of the high recurrence rate of these abscesses, it appears that excising the abscess as one would remove a tumor is more likely to result in a cure without recurrence. Most abscesses of the head involve bone or teeth making it very difficult to remove them without rupturing them. It is best to dissect the abscess out down to bone, quickly remove the abscess at the level of the bone, curette the bone, remove any teeth that are involved, and irrigate copiously to decrease contamination from the abscess. It may not be possible to completely excise abscesses associated with teeth and bone; however, it is vital to remove as much abnormal tissues - soft and hard – as possible.

Antibiotic impregnated polymethylmethacrylate (AIPMMA) beads release relatively high concentration of antibiotic locally with little systemic absorption. The defect is loosely filled with AIPMMA beads and soft tissues are closed over the beads routinely. There is no open wound and no wound irrigation is needed. The antibiotic chosen is best based on preoperative culture and sensitivity results. Most isolates are sensitive to cefazolin or amikacin. If the beads are not removed, they do not cause clinical problems being biologically inert. Indications for removal are the presence of a fistulous tract, recurrence of the abscess, poor cosmetic results, and interference with normal function. The rabbit is still placed on systemic antibiotics for 2 weeks because of the potential for bacteremia resulting from the surgery, but long term therapy is not necessary as the beads release antibiotic for many months.

The rationale behind the use of AIPMMA is to provide high local concentration of antibiotic with low systemic absorption and, therefore, less toxicity. It is useful in infections where long-term use or the systemic use of the antibiotic of choice would be contraindicated as is often the case with rabbit abscesses. The ideal antibiotic for use in AIPMMA beads is one that is bacteriocidal, broad spectrum, effective in low concentration, heat stable (up to 100° C), and has high water solubility and low tissue toxicity. Gentamicin has been studied extensively and has been shown to elute concentrations above the break point susceptibility concentration for over 80 days with levels detectable for over 5 yrs. The average serum concentration (1 g gentamicin/20 g PMMA) was only 0.5 mcg/ml and the wound fluid concentration was 80 mcg/ml. The elution properties of the injectable gentamicin are the same as the powered. The equine product (100 mg/ml) is preferred to reduce the amount of liquid required.

Other antibiotics commonly used where elution information is available include the following: tobramycin (1g/20g PMMA), cephalothin (2g/20g PMMA), cefazolin (2g/20g PMMA), amikacin (1.25g/20g PMMA) (the liquid form eluted slower than the powder), and ceftiofur (2g/20g PMMA). In an in vitro study with ceftiofur, antimicrobial concentrations were only maintained for 7 days. Clindamycin has been studied at doses of 1.5-3.0 g/20 g PMMA and shows good elution properties when compared with other antibiotics. In a recent study the in vitro elution properties of gentamicin and metronidazole were studied individually and combined in the same beads. Metronidazole doses of 0.5, 0.75 and 1.0g/20g PMMA were evaluated, gentamicin at 1 g/20g PMMA, and the combination contained 1 g each in 20g PMMA. When combined in the same beads, elution of both was more rapid than when they were alone. In a study from my laboratory, we demonstrated similar elution characteristics from beads made of amikacin, cefazolin, and a combination. The combination eluted effective concentrations of both antibiotics for only 3 days.

The elution of the antibiotic is bimodal with a rapid release in the first few days followed by a slow, long term release of antibiotic over weeks to months. The beads become encapsulated with fibrous tissue within a few weeks and then only tissues within about 3 mm receive the high concentration of antibiotic. Because of this it is essential to remove the abscess as completely as possible before placing the beads. They should not be placed within the abscess capsule. The rate of elution is affected by various factors. The amount of fluid flowing past the beads influences the rate such that highly vascular areas elute the antibiotic more rapidly with more systemic absorption and more rapid depletion of the antibiotic within the beads. The diffusion properties of the antibiotic and its heat stability also affect elution rates. The exothermic polymerization reaction can denature antibiotics such as penicillins making them less effective. The previously mentioned antibiotics are heat stable. Until elution studies have been conducted it is not recommend that new antibiotics be used clinically. Enrofloxacin does not mix into the cement. During the polymerization, the liquid antibiotic separates from the polymerizing cement and will not mix.

The shelf life for these beads is unknown but assumed to be the normal expiration date of the antibiotic. The beads can be gas sterilized which does not affect their elution or antimicrobial properties. The package containing the beads should be labeled with the expiration date of the antibiotic as well as the sterilization date. Because the fumes are annoying and potentially damaging to contact lenses, many prefer to make the beads in a hood prior to surgery. The cement comes in 20 and 40 g packets one of which is enough to treat several rabbit abscess. Any unused beads are gas sterilized and used in future patients. The antibiotic is mixed with the copolymer powder prior to adding the liquid monomer. Once the polymerization begins, the cement hardens within 10 minutes. Refrigerating the reagents prior to use will extend this time. It can be challenging to make all the beads that quickly and the aid of an assistant is very helpful. Beads may be rolled into spheres. The size of bead that can physically be made with the fingers is often too large for use in rabbit abscesses. As an alternative, the mixture is placed in a syringe (catheter tip for larger beads and regular tip for small beads) and squirted out onto a plastic or metal surface (such as a table drape intraoperative). A scalpel is used to cut the tube of cement into small pieces creating small to function as beads. When placing the beads, count the number implanted and record it in the patient record.