Small Mammal Oral and Dental Diseases

Small Mammal Oral and Dental Diseases

Loïc Legendre

Northwest Veterinary Dental Services Ltd, North Vancouver, British Columbia, Canada

21.1 Introduction

With the growing popularity of some of the “pocket‐pets” and rabbits, astute practitioners must be aware of the challenges facing them due to the unique characteristics of rodents’ and lagomorphs’ oral cavity and dental structures. Being able to recognize normal variations and to assess any changes will help to adequately treat many conditions commonly encountered. Between the two groups, there are similarities and differences, as there are some differences among the various rodents. Where possible, the similarities will be discussed as a whole, while the differences will be pointed out as they are mentioned.

21.2 Patient Oral Assessment

As with any veterinary patient presented, an accurate history will cover the scope of past medical information and include specific items related to oral and dental disease. Any facts on past and present dental problems including treatment, dietary information, and chewing or eating habits can be beneficial. With the owner’s assistance, an initial oral assessment can be made in the examination room, but the actual information gathered can vary remarkably depending on the patient’s attitude, size, and species. External evidence of swelling, head symmetry, exudate drainage, and rostral malocclusion will be the first signs noted, if present. By gently lifting the lips, the gingivae and mucous membranes can be checked for color, hydration, capillary refill, or any evidence of swelling, discoloration, hemorrhage, ulceration, or recession. Any changes in the teeth, such as fractures, discoloration, plaque, calculus, caries, erosions, malocclusion, mobility, or developmental defects should be recorded. Although sometimes difficult to visualize, the examination should include the palate, floor of the mouth, and all tongue surfaces to check for ulcerations, injuries, tumors, and foreign bodies. Any evidence of epistaxis, halitosis, rhinitis, oral hemorrhage, and masticatory muscle problems should be recorded accurately, along with charting of all oral and dental findings (Figure 21.1) [1]. Charts for most of the common rodent and lagomorph patients are available in the text of Drs. Capello, Gracis, and Lennox [2].

Rabbit dental chart displaying structures of a rabbit’s head in lateral left and right views with dental areas shaded. Below is a front view of teeth with 3 lateral views of head with different dental structures.

Figure 21.1 Rabbit dental chart.

Source: Courtesy of David A. Crossley. Reprinted with permission.

In addition to the information collected during the initial examination and from the history, other oral diagnostics such as biopsies, cultures, transillumination, and radiographs may be utilized. All those will require the patient to be anesthetized or at least sedated. Extraoral radiographic surveys using No. 4 or No. 5 occlusal intraoral dental sensors or films may sometimes be supplemented by the intraoral use of size 0, 1, 2, or specific rodent films, such as modified size 3 film, in sedated patients using the bisecting angle techniques [3] (Figure 21.2). Other data, such as urine, blood, and fecal assays, may not be as helpful as in other species due to the sparseness of significant data to be gathered [4], as well as the difficulty of specimen collection, although urine and feces can sometimes be collected following examination from the table surface.

Image described by caption.

Figure 21.2 Examples of phosphoric sensor plates designed to fit intraorally in rabbits to allow detailed imaging of maxillary and mandibular cheek teeth. With permission of Drs. Regalado and Legendre JVD 2017.

Complete physical examination of the oral cavity is therefore very important, though at times challenging. Restraint of smaller patients in towels or tubes [57], or by carefully grasping a rabbit by the scruff and supporting the hind legs, can help facilitate the assessment [5, 6, 8, 9]. The oral cavity is typically long and narrow, so the rostral portion is the only part readily accessible in the awake patient. Sedation or anesthesia is often necessary for complete examination [7 911], although otoscopes with large cones, small vaginal speculum, and coffee stir sticks are helpful in the conscious patient [12].

21.3 Anesthesia

In rodents, ketamine hydrochloride is probably the most common injectable anesthetic used, although it requires some special considerations. In lagomorphs and other species, there is usually insufficient relaxation of the oral cavity with ketamine alone, so supplements of additional injectable or inhalant anesthesia can be used. Anesthetic depth is best monitored by respiration and jaw tension. Some baseline physiological parameters for a variety of small mammals is presented in Table 21.1 [6, 7, 13].

Table 21.1 Baseline physiological data [6, 7, 11, 13].

Life span mean (years) Respiration (per minute) Heart rate (per minute) Body temperature (Celsius) PVC (%) BUN (mg/ dl)
Chinchilla 8–10 40–65  40–100 36.1–37.8 27–54 17–45
Gerbil 2–4 70–120 260–600 38.1–38.4 35–45 17–27
Guinea pig 4–5 42–104 230–380 37.2–39.5 35–45 9.0–31.5
Hamster 1 1/2‐3 35–135 250–500 37.0–38.4 45–49.8 12–26
Mouse 1 1/2‐3 94–163 325–780 35.5–38.0 35–40 17–28
Rabbit 5–6 32–60 130–325 38.0–39.6 30–50 17–23.5
Rat 2–4 70–115 250–450 35.9–37.5 35–45 15–21

Care should be taken using injectable agents alone with rabbits because individual reactions to the drugs may be unusual, especially with barbiturates, where there is a very narrow range of safety [4, 5]. Pupil size and corneal or pedal reflexes are too unreliable to be used to monitor the depth of anesthesia, so again jaw tension and respiration should be closely observed [6, 7, 13]. Ketamine hydrochloride, commonly used, can provide a range from sedation to anesthesia at doses from 20 to 50 mg/kg intramuscularly (IM), but with poor muscle relaxation and analgesia [47, 10, 11]. There are many drug combinations available today to sedate rabbits and rodents. The author favors a “cocktail” of ketamine 10 mg/kg, alfaxalone 2 mg/kg, dexmedetomidine 10 mg/kg, and midazolam 0.2 mg/kg, all mixed into one syringe and given IM or SQ. The advantage of the mixture is that each drug is used at a low dose, minimizing the chances of harmful side effects. The other advantage of this combination is that dexmedetomidine can be reversed with an equal volume of atipamazole and midazolam can be reversed with flumazenil given at 0.1 mg/kg. The reversal allows the patient to recover quickly and safely. If the patient is somewhat debilitated (which is often the case), a combination of butorphanol 0.2 mg/kg, alfaxalone 2 mg/kg, and midazolam 0.2 mg/kg, all in one syringe, can be given IM or SQ. If needed, one can always top up with more alfaxalone. Monitor the patient carefully as alfaxalone may cause apnea. It is always best to have the patient intubated.

As with most work in the oral cavity, intubation is ideal, but with untrained personnel this can be difficult and even traumatic with the small oral opening and lateral skin folds. Use of topical anesthetics to control laryngospasms should be restricted [13]. Three techniques are available. (i) Blind intubation, where the patient is in the sternal position with its head extended upward. The small uncuffed tube is inserted slowly with the ear of the operator close to the end of the tube. Air movement can be heard; on inspiration the tube is dropped into the entrance of the trachea. This technique is sometimes much easier to describe than to perform. (ii) Stylet guided intubation, where a stiff bendable internal tube guide is inserted into the trachea and the endotracheal tube is slid over the guide (Figure 21.3a,b). This is certainly a versatile technique that does not require lots of extra equipment. (iii) Endoscope guided intubation, where a small (1.9–2.7 mm diameter) rigid endoscope is used to push the tongue ventrally, to visualize the tracheal opening, and to serve as a guide for the endotracheal tube. It is the nicest technique but requires an endoscope. A fairly recent paper describes how to intubate GP on their back, using an otoscope cone cut in half lengthwise [14]. For very short procedures where intubation can be avoided, IV induction can be followed by masking with isoflurane or sevoflurane. The induction drugs are then reversed to allow a quick recovery. Respiratory assistance can be provided with a straw or eye dropper placed into the back of the mouth and blowing into the trachea. External cardiac massage with rapid, light digital pressure application to the ribs behind the forelegs can provide some cardiopulmonary resuscitation (CPR) effects.

2 Photos of a rabbit in supine position, intubated by hands using a stylet introduced into the trachea using a large cone otoscope (left), and then endotracheal tube slid over the stylet down the trachea (right).

Figure 21.3 (a) Intubation of a rabbit using a stylet guide. The stylet is introduced into the trachea using a large cone otoscope. (b) The endotracheal tube is then slid over the stylet down the trachea and tied into place.

21.4 Anatomy

Even with excellent techniques of examination and visualization, whether sedated or not, the practitioner must be familiar with the normal anatomy and physiology of the patient being treated. The dentition is a primary reason rodents and lagomorphs were first placed into a similar classification, and later separated [15].

The Latin verb “rodere” means “to gnaw,” a reference primarily to the prominent incisors and the fact that most are herbivores, although some rat species are omnivorous [16]. The Rodentia order is the largest order of mammals, with a wide variety of species from mice, rats, Guinea pigs, gophers, beavers, chinchillas, nutria, and many more. The Lagomorpha order, on the other hand, is quite small, consisting principally of the Leporidae family with rabbits, hares, cottontails, and pikas. Of all rodents and lagomorphs, the patients most commonly seen by veterinarians are domestic rabbits, Guinea pigs, and chinchillas, but the occasional “wild” cousin may be of need of dental assistance [17]. Both rodents and lagomorphs have a heterodont dentition, with the varying tooth shapes of incisors, premolars, and molars [17]. Rabbits and hares possess very temporary deciduous teeth (probably non‐functional), which place them in a diphyodont classification [18]. The rabbit’s deciduous teeth start exfoliating, possibly even before birth, with the tips of the maxillary incisors present at that time, and total replacement by three to five weeks of age [19]. In rodents there has also been a claim that Guinea pigs have deciduous teeth [20], but these may be fetal in nature and non‐functional [19]. The question of functionality of these teeth and whether they are fetal or true deciduous teeth appears to be pivotal in the decision of their classification by anatomists. Other variations in both rodent and lagomorph dentition indicate the general lack of in‐depth research into the area.

The dental formula of rodents can vary from 2× (I 1/1; C 0/0; P 0/0; M 3/3) = 16 in the Cricetid rats to 2× (I 1/1; C 0/0; P 1‐2/1; M 3/3) = 20 or 22 in Sciuridae squirrels. The presence of four incisors, no cuspids, few to no premolars, 8–12 molars and a large diastema between the incisors and cheek teeth are the common traits (Table 21.2).

Table 21.2 Dental formulas for rodents [8, 10, 19, 20].

Hamster (Mesocricetidae) 2× (I 1/1, C 0/0, P 0/0, M 2–3/2–3) = 12–16
Old World rats and mice (Muridae) 2× (I 1/1, C 0/0, P 0/0, M 2–3/2–3) = 12–16
Rats and mice (Cricetidae) 2× (I 1/1, C 0/0, P 0/0, M 3/3) = 16
Gerbil (Merionidae) 2× (I 1/1, C 0/0, P 0/0, M 3/3) = 16
Guinea pig (Cavidae) 2× (I 1/1, C 0/0, P 1/1, M 3/3) = 20
Chinchillas (Chinchillidae) 2× (I 1/1, C 0/0, P 1/1, M 3/3) = 20
Capybara (Hydrochoeridae) 2× (I 1/1, C 0/0, P 1/1, M 3/3) = 20
Nutria (Capromyidae) 2× (I 1/1, C 0/0, P 1/1, M 3/3) = 20
Old World porcupines (Hystricidae) 2× (I 1/1, C 0/0, P 1/1, M 3/3) = 20
New World porcupines (Erethizontidae) 2× (I 1/1, C 0/0, P 1/1, M 3/3) = 20
Beavers (Castoridae) 2× (I 1/1, C 0/0, P 1/1, M 3/3) = 20
Squirrels (Sciuridae): 2× (I 1/1, C 0/0, P 1–2/1, M 3/3) = 20–22

The primary difference of the lagomorph dentition is the presence of a total of four maxillary incisors as compared to two in rodents, as well as additional premolars [6, 13]. The most widely accepted dental formula for rabbits is 2× (I 2/1; C 0/0; P 3/2; M 3/3) = 28 (Table 21.3) [5 2123]. At least one author notes a possible variation in the number of maxillary molars (either two or three are possible, for a total count of 26–28) [8], which may be due to the last set of molars being exceptionally small and potentially difficult to visualize.

Table 21.3 Dental formula for lagomorphs [7, 8, 13, 18, 22].

Permanent teeth
2× (I 2/1, C 0/0, P 3/2, M 2/3) = 26
Rabbits (Oryctolagidae)
Deciduous teeth
2× (I 2/1, C 0/0, P 3/2, M 0/0) = 16
Permanent teeth
2× (I 2/1, C 0/0, P 3/2, M 2–3/3) = 26–28
Hares (Lepidae)
Deciduous teeth
2× (I 1/0, C 0/0, P 3/2, M 0/0) = 12
Permanent teeth
2× (I 2/1, C 0/0, P 3/2, M 3/3) = 28

The standard single row of maxillary incisors in rodents is known as simplicidentata, or simple dentition [24]. In lagomorphs, the location of the two smaller rudimentary maxillary incisors (peg teeth) [21] directly caudal/palatal to the two large grooved incisors is a double row dentition or duplicidentata (Figure 21.4). The large incisors of both groups are continuously growing and considered to be aradicular hypsodonts, which means long‐crowned without a true root structure [25]. The exposed or clinical crown is the supragingival portion, while the reserve crown is subgingival, and combined, they form the anatomical crown. The submerged segment is sometimes called the clinical root, but it is not a true root structure, though using the term open‐rooted is acceptable for these teeth. This can be contrasted with equine teeth, which are radicular hypsodonts or rooted, long‐crowned teeth, with distinct roots that eventually mature into a closed‐root structure [18, 26]. These also have a subgingival reserve crown portion that continues to erupt (not grow) in a coronal direction as the exposed crown is worn away.

Image described by caption and surrounding text.

Figure 21.4 Example of duplicidentata (note the peg teeth present in a rabbit maxilla).

All lagomorph cheek teeth and caviomorph rodent (chinchilla, Guinea pig) molars are also aradicular hypsodonts. Most of the other rodents, such as rats, mice, hamsters, and gerbils, have premolars and molars that are brachyodont (short‐crowned, closed roots) that do not continuously grow or erupt. With the aradicular hypsodont incisors, these latter individuals have a mixed dental crown classification, as compared to a mixed dentition (both deciduous and permanent counterparts present at the same time).

The continuously growing teeth have some interesting characteristics that fit well with their form and function. To begin with, in the periodontal ligament in rodent teeth (and in developing human teeth), there is an intermediate group of collagen fibers that attach either to the alveolar bone or cementum, not both, with splicing in between. This intermediate plexus differs from the traditional view of ligament fibers running the entire distance from bone to tooth, and may provide a more suitable mechanism by which continually growing or erupting teeth can have extensive tooth movement using this middle zone [27, 28].

Enamel on the incisors of both rodents and lagomorphs is thickest on the facial/labial surfaces, thinning as it extends on to the distal and mesial surfaces, and is nearly non‐existent on the lingual aspect, which is covered with softer dentin and some cementum [29]. This configuration of dental hard tissue promotes a wearing pattern that results in a sharp, chisel‐like tooth [23]. In the rabbit, the large maxillary incisors grow at a rate of 2.0 mm per week, while the mandibular incisors grow 2.4 mm per week rostromesially. Apparently, there is a faster attrition rate, typically due to dietary influences alone, of the lower incisors to compensate for the difference [21, 22, 30, 31]. Incisor teeth of the chinchilla may grow as much as 6–8 cm per year [32] and are yellow‐orange in color, as are most mature rodent incisors.

The position of the most apical portion of incisors may vary, depending on the animal. Rat maxillary incisors extend for two‐thirds of the diastema, while the mandibular incisors reach distal to the last molar. The maxillary incisors of hamsters reach to one‐half to two‐thirds of the diastema (mice up to three‐quarters), and are level to or distal to the last molar for the mandibular incisors. Guinea pigs have maxillary incisors that end near the mesial aspect of the first cheek tooth, with their mandibular counterparts traveling lingually and to the level of the second cheek tooth. Chinchilla maxillary incisors reach to one‐half of the diastema, and the mandibular incisors to the first or second cheek teeth (Figure 21.5). Lagomorph maxillary incisors extend for one‐third of the diastema and the mandibular incisors reach the mesial surface of the first cheek tooth (Figure 21.6).

Image described by caption and surrounding text.

Figure 21.5 Lateral X‐ray of a chinchilla skull showing the maxillary incisors reaching one‐half of the diastema and the mandibular incisors extending to the third cheek tooth.

Image described by caption and surrounding text.

Figure 21.6 Lateral X‐ray of a rabbit skull showing the maxillary incisors reaching one‐third of the diastema and the mandibular incisor extending to the mesial surface of the first cheek tooth.

In rabbits, mastication is typically performed in a lateral, scissor‐like fashion due to a horizontal oral mandibular fossa of the TMJ. The mandibular incisor cuts back and forth in between the peg teeth and the larger maxillary incisors [8]. The lateral grinding movement is facilitated in the caudal teeth with a flat occlusal surface and deep transverse enamel folds [31, 33].

Rabbits also differ from caviomorph rodents in that they possess more cheek teeth (six in the maxilla and five in the mandible). The uneven number of cheek teeth in the maxilla and mandible creates a specific occlusion where each cheek tooth from one arcade contacts two cheek teeth from the opposite arcade (Figure 21.7). These points become important when one considers extracting a cheek tooth. Other structures of interest in the oral cavity of rabbits and Guinea pigs is the significant medial folding of skin near the diastema, which separates the rostral mouth from the caudal mouth, thereby permitting separate functions of the incisors and cheek teeth and limits visualization of the oral cavity [4, 8]. Golden hamsters have internal cheek pouches, while some other rodents have external, fur‐lined cheek pouches found near the oral opening [34].

Image described by caption and surrounding text.

Figure 21.7 A close‐up view of a rabbit cheek teeth showing that one tooth touches 2 of the opposite arcades with 6 teeth on the maxilla and 5 on the mandible.

21.5 Malocclusion

With the continually growing nature of lagomorph and rodent incisors (and some molars), it should be readily evident that any disruption in the normal attrition sequence can lead to significant problems with overgrowth [6, 13, 21]. The most common dental problem in rodents [34] and lagomorphs [6, 13, 22] alike is malocclusion. It is also the most common genetic problem in rabbits [11, 22, 36]. These malocclusions are typically classified into atraumatic and traumatic.

21.5.1 Atraumatic

Atraumatic malocclusion results from genetic malpositioning of teeth or dietary causes of insufficient attrition [11, 17, 34]. Rabbits as young as three weeks of age may exhibit mandibular incisors that are level to or even extend labial to their maxillary counterparts [5, 11, 21]. It is typically considered that an autosomal recessive gene for a shortened maxillary diastema is the probable cause [4, 6, 8, 9, 13, 33]. The mechanism of abnormal growth of the dorsal and basal skull bones that give the appearance of a longer mandible [33] may be compared to brachiocephalic conditions in some animals or a Class III malocclusion in man [37]. Dwarf breeds seem to be most affected.

If the teeth continue to grow without proper occlusive wear, the maxillary incisors start to curl or twist in the mouth and at times have been known to penetrate into the skull if left untreated, whereas the mandibular incisors grow into the upper lip and the nostrils [5, 11] (Figure 21.8a,b). Once the incisors are overgrown, the animal cannot eat properly, may drop its food (quidding), traumatize its tongue, and salivate excessively (ptyalism or “slobbers”), which can lead to wet dewlap (moist dermatitis) in rabbits [6, 8, 13, 36]. Excessive overgrowth may cause the maxillary incisors to penetrate into the sinuses or ocular sockets. Malocclusion and overgrowth of the incisors with failure to properly close the mouth can further lead to molar overgrowth and malocclusion in rabbits and rodents with aradicular hypsodont cheek teeth [5, 8]. If untreated, maxillary molars may flare outward, lacerating the buccal mucosa, while the mandibular molars overgrow lingually, potentially trapping the tongue either ventrally or dorsally.

Image described by caption and surrounding text.

Figure 21.8 (a) The elongated maxillary incisors curl into the oral cavity and if left unchecked can penetrate the palate. (b) The elongated mandibular incisors are less curved and end up labial to the maxillary; unchecked they will grow into the soft tissues of the nose.

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Aug 15, 2020 | Posted by in GENERAL | Comments Off on Small Mammal Oral and Dental Diseases

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