Publicatiedatum: Exoticscon, september 2018

Acquired Dental Disease in Guinea Pigs

Authors: Eva Stoffels, DVM, PhD, and Frank Van De Goot, PhD.
Affiliation: From the Marumoto Veterinary Practice, Veldenstraat 18, 2470 Retie, Belgium; Northwest Clinic Alkmaar, Wilhelminalaan 12, 1815 JD Alkmaar, The Netherlands.

Abstract: Acquired dental disease (ADD) is a progressive debilitating condition that affects rodents and rabbits. Symptoms include pain, anorexia and weight loss. In this study 28 cases of ADD in guinea pigs were investigated by means of medical imaging and histopathology. In more than 50% of cases, clinical findings support the inflammatory (post)-infectious etiology of ADD. Medical imaging reveals degenerative changes in the jaws, such molar hypertrophy and alveolar bone lesions; the histopathologic appearance is consistent with periodontitis and attendant osteolysis. Treatment options based on multimodal pain management, antimicrobial and anti-inflammatory therapy, and suppression of osteoclastic activity are evaluated. Prognosis depends on the general condition of the patient as well as on the location and severity of lesions. Radiography provides a reliable diagnostic tool with a high prognostic value.

Introduction
Dental problems are common in guinea pigs, other rodents and in rabbits. Guinea pigs appear to be exceptionally prone: the prevalence can become as high as 20% in animals older than 3 years. (1)
Acquired dental disease (ADD) is a degenerative condition associated with anorexia, pain, weight loss, often elongation of clinical crowns, and typical radiographic findings suggestive of molar degeneration with alveolar bone involvement. (1) Pathogenesis has not yet been fully resolved. Various etiologic factors have been considered such as paucity of abrasive feed, renal1 and nutritional (2) hyperparathyroidism and (sub) clinical scurvy. Recently, infectious causes have been proposed. (3) The current work is a retrospective study based on selected clinical cases of ADD in guinea pigs. Inflammatory and in particular infectious causes are investigated in detail. Medical imaging techniques including digital radiography, computed tomography (CT) and magnetic resonance imaging (MRI) are involved. Furthermore, lesions are visualized by histopathology. Treatment options based on multimodal anti-inflammatory therapy, inhibition of osteolysis and combating the infection are discussed.

Materials and Methods
Clinical examination and medical imaging
Twenty eight guinea pigs (17 males and 11 females) presenting with anorexia, pain and weight loss were examined. The average age was 3.9 years. Fourteen animals had a history of (untreated) jaw infections such as periapical abscesses. Five animals had a record of recent severe gastrointestinal disorders (enterocolitis). Bacteriologic examinations revealed Streptococcus spp.,Staphylococcus spp., Enterococcus spp. and Escherichia coli as the causative agents, respectively. The remaining nine animals had a record of chronic kidney disease.

Following the routine inspection of the oral cavity with a speculum (Heine), radiography of the skull was per- formed using a dental X-ray source (Trophy CCX) with AGFA CR15-X plate reader. Per case, left and right oblique (15-25o) and rostro-caudal projections were recorded. When indicated, ultrasound examination was performed (Mindray M7 12 MHz with L14-6s linear transducer). Two animals were examined post mortem using CT (Philips Mx8000 Dual CT Scanner) or MRI (Siemens 15T).

Treatment
After trimming the clinical crowns, pharmacotherapy (multimodal pain management, antimicrobial therapy and prokinetics) was started. The medications included either meloxicam (0.5 mg/kg PO q12h; Meloxivet, Kela) or metamizole (80 mg/kg SC q12h; Vetalgin, MSD Animal Health, Belgium). Acetaminophen (50-100 mg/kg PO q12h; Eurogenerics, Belgium) and tramadol (5 mg/kg PO q12h; Eurogenerics, Belgium) were given in combina- tion with assisted feeding, and when necessary metoclopramide (1 mg/kg PO q12h; Primperan, Sanofi, Belgium). Doxycycline (5-10 mg/kg PO q12h; Doxyral, Ecuphar, Belgium) was administered. This basic treatment was sustained for 4-6 weeks. Animals with persisting jaw abscesses received additionally azithromycin (25 mg/kg PO q24h; Sandoz, Novartis, Belgium) for 10 days. NSAIDs doses were reduced in animals with renal dysfunction.
Fifteen animals underwent extended treatment. The medications included either calcitonin (1 IU/kg SC q24h; Miacalcic, Novartis, Belgium), or disodium tiludronate (1 mg/kg SC q24h; Tildren, Ceva, Belgium) for 10 days. Pentoxifylline (20 mg/kg PO q12h; Trental, Takeda, Czech Republic) was used as an additional anti-inflammatory agent. Calcium gluconate (200 mg/kg PO q12h; Gluconal CAM-P-OR, Purac, The Netherlands) was given. In 3 cases, apicoectomy or extraction of the affected molar(s) was performed.

Histopathology
Two animals with end-stage ADD (post-infectious) were euthanized with ketamine (50 mg/kg SC; Nimatek, Eurovet) and xylazine (20 mg/kg SC; Sedaxyl, Kela, Belgium), followed by intracardiac injection of sodium pentobarbital (100 mg/kg, IC; Kela, Belgium); their jaws were dissected, fixed using 4% buffered formaldehyde and decalci- fied by soaking in 8% HCl for 2 days. Paraffin-embedded sections were stained with hematoxylin-eosin (HE).

Results
General presentation
Guinea pigs in early stages of disease appeared alert and interested in feeding, but displayed signs of pain while masticating. In advanced cases the animals had a greatly reduced lateral excursion of the mandible and sali- vated profusely. Secondary contact dermatitis of the chin was commonly seen. Atrophy of mastication muscles (especially m. masseter) resulted in a characteristic hollow appearance of the cheeks. In the end stage the animals were usually cachectic and presented with gastrointestinal complications and hepatic lipidosis (confirmed by ultrasonography and by necropsy dependent on the owner’s consent).
Upon inspection of the head and the oral cavity, abnormalities were noted such as malocclusion and asymmetric wear-off of the incisors, moderate mandibular prognathism, bony protrusions in the ventral aspect of the man- dible (Fig 1), and occasionally visible or palpable abscesses, dyspnea and purulent nasal discharge or unilateral exophthalmos. Interestingly, elongation of clinical crowns of molars leading to entrapment of the tongue was not frequently observed. More commonly, enlargement, brittle structure and discoloration of molars, as well as formation of diastemata was seen. The majority of defects (22 out of 28 animals, 79%) were located in the mandibular arcade.

Figure 1. A: Enlarged and ventrally distended apices (arrow) of the right mandibular arcade in advanced acquired dental disease (necropsy). B: Incisor assymetry indicative of an inflammatory process involving molars.

Medical imaging
The most prominent radiographic sign was elongation of the reserve crowns, accompanied by ventral, lateral of even caudal displacement of the mandibular molar apices (Figs 2-3). In guinea pigs with previous records of jaw infections (n = 14), periapical radiolucency was a common finding (Fig 2B). The normal architecture of molars and the adjacent bone was disrupted; hypertrophic molars (“giant teeth”) were formed, as could be clearly visualized by computed tomography (Fig 3). When the maxillary arcade was involved, molar apices extended dorsally penetrating nasal conchae or the infraorbital space (Fig 4). Additional diagnostics such as biopsy and MRI (Fig 4) allowed to detect persisting abscesses in 7 animals.

In guinea pigs with a history of gastrointestinal problems (n = 5), radiographic signs were less pronounced (Fig 2C). Bilaterally symmetric moderate extension of the mandibular apices (especially P1) but no periapical radiolucency was seen. In animals with renal dysfunction (n = 9), radiographic appearance was variable. Severe deformities including giant teeth were detected in end stage kidney disease (Fig 2D).

No abnormalities were observed in the temporomandibular joint (TMJ), even in case of severe molar deformi- ties (giant teeth, Fig 5). Ultrasound examination revealed intact soft tissues and ruled out (sub) luxation of TMJ.

Radiographic findings correlated well with the duration of illness and severity of symptoms. Giant teeth were typically found 2-3 months following the onset of symptoms, and could be considered end stage disease. The affected animals were usually lethargic. The same held for guinea pigs with lesions in the maxillary arcade.

Figure 2. Right oblique radiographic projections visualizing the right ventral aspect of the mandible. A: Normal dentition, with a well-demarcated alveolar bone. B: A periapical abscess involving M1, note a giant M1 (red arrow) and periapical radiolucency in P1 (white arrow). C: Enlargement and ventral distension of the P1 apex (white arrow), in a patient with enteritis-induced acquired dental disease. D: End stage acquired dental disease (renal) with severe deformation and caudal displacement of mandibular molars (white arrows).

Figure 3. A giant right mandibular M3 in right lateral and rostral views (arrows). Note malocclusion and asymmetry of the incisors. Computed tomography (courtesy of the Veterinary Clinic Causus, Oudenburg, Belgium).

Figure 4. Acquired dental disease with a defect in the maxillary arcade. A: A right oblique radiographic projection visualizing the apices of the left maxillary arcade, note distention and projection visualizing the apices of the left maxillary arcade, note distention and periapical radiolucency periapical radiolucency of P1 (white arrow). B: An MRI image displaying an enhanced signal around of the P1 apex (yellow arrow). Post mortem biopsy revealed an abscess. arrow). Post mortem biopsy revealed an abscess. 

Figure 5. A rostrocaudal projection showing an intact TMJ and bilateral giant P1. 

Histopathology
Axial sections of molars revealed pathologic changes that extended over the whole length of the reserve crown (Fig 6). Most striking was increased cellularity and expansion of the periodontal ligament, infiltration of inflammatory cells into the enamel space, and presence of giant cells. Enamel itself displayed an irregular surface and numerous cavities. The adjacent alveolar bone consisted mainly of woven bone with broadened and fringed cement lines. Expansion, increased cellularity and moderate fibrosis of the intertrabecular space, as well as numerous resorption lacunae with osteoclasts could be seen. This picture is suggestive of a chronic inflammatory process (periodontitis) accompanied by enhanced osteoclastic activity and pathologic bone resorption.

Figure 6. Histopathology of the mandible. A: Normal periodontal ligament (pl) and enamel (e) in a control animal. B: Advanced acquired dental disease with an expanded periodontal ligament, infiltration of inflammatory cells and irregular enamel surface. C: Enamel in acquired dental disease, inflammatory cells and irregular enamel surface. C: Enamel in acquired dental disease, note numerous cysts, red arrow: dark granules of tetracycline deposited in newly mineralized tissue. cysts, red arrow: dark granules of tetracycline deposited in newly mineralized tissue. D: Alveolar bone in acquired dental disease featuring lacunae with osteoclasts (white arrows) and pronounced cement lines (yellow arrow). Hematoxylin & eosin stain.

Treatment outcome
Recovery was defined as symptomatic improvement (ie, normal jaw excursion during mastication, cessation of pain symptoms and maintaining a stable body weight without assisted feeding for at least 6 months following termination of therapy). The recovery statistics are summarized in Table 1. The outcome strongly depended on the history of the disease, age and condition of the animal, as well as on the extent of molar and bone lesions. Aged (> 4 year) guinea pigs that had suffered longer than 2 months, became cachectic and developed giant teeth, usually did not respond to treatment. Only one case with pronounced bilateral giant teeth survived and regained good condition; however, molar deformities did not regress, as confirmed by radiography. Furthermore, therapy failed in 5 out of 6 animals with maxillary lesions. The outcome of apicoectomy (3 cases) was unsatisfactory, too. The best results were obtained in the group with recent gastrointestinal infections. Extended treatment appeared superior to basic treatment; no apparent difference in efficacy between bisphosphonates and calcitonin was noted.

Table 1. Summary of treatment outcome.

Discussion
Pathogenesis
Medical imaging and histopathology data indicate that acquired dental disease is associated with periodontitis, alveolar bone resorption, deformation and hypertrophy of molars. ADD appears to be a common sequel to bacterial infections of the jaw, or to chronic kidney disease with hyperparathyroidism. (1) Interestingly, current results suggest that gastrointestinal infections can also trigger this disease. In order to elucidate the pathogenesis, one has to review physiological processes that govern the growth of hypsodont rodent teeth.

Formation of dental tissue is closely linked to, yet not subject to the same humoral regulation as bone remodeling. In particular, mineralization of the cementum and enamel precursor matrix is a passive process that utilizes calcium and phosphate released locally from the alveolar bone. (4) In a physiologic situation there is a delicate equilibrium between humorally regulated osteoclastic bone resorption and passive matrix mineralization. This equilibrium is easily disrupted by factors that enhance osteoclast recruitment and maturation, more specifically, by the effectors of the RANK-RANKL pathway. The prime effectors are parathyroid hormone (PTH) and calcitriol; the link between dental disease and hyperparathyroidism has been investigated in the past; (1,2) however, inflammatory mediators such as prostaglandins (e.g. PGE2), cytokines (tumor necrosis factor-alpha (TNF-alpha), macrophage colony stimulating factor (M-CSF), interleukin-1 (IL-1)) are also known to upregulate RANKL expression. (5) Furthermore, TNF-alpha upregulates several pro-osteolytic factors independent of the RANK/RANKL pathway, such as matrix metalloproteinases and inducible nitric oxide synthase (iNOS). Hence, TNF-alpha is considered to play a pivotal role in the pathogenesis of periodontitis. (5)

In light of the above, it is plausible to assume that inflammatory cytokines produced in a local inflammatory process (eg, a periapical abscess, osteomyelitis) are capable of triggering periodontitis and alveolar bone loss. Alternatively, high circulating levels of cytokines in response to a (systemic) infection may elicit similar effects. High serum TNF-alpha levels are well documented in rodent models of enterocolitis.(6) This mechanism can be described as “reactive ADD” and is often observed in guinea pigs with previous digestive tract infections.

Regressing alveolar bone releases minerals and creates space in the jaw, which allows molars to expand uncontrollably and form giant teeth. As the disease progresses, the demineralized bone fails to provide sufficient mechanical support to the molars. This explains bone deformities and displacement of molar apices. Moreover, tissue compression exerted by expanding molars exacerbates inflammation and leads in turn to further bone loss. This vicious circle may be responsible for the progressive degenerative character of the disease.

Treatment, prognosis and prevention
To the authors’ knowledge, untreated ADD results in nearly 100% mortality within 2-4 months following the onset of symptoms. The present approach concentrates on pain management, antimicrobial therapy, multimodal anti-inflammatory therapy and targeting osteoclasts. Tetracyclines are chosen due to their dual action: as broad spectrum antimicrobials and as potent matrix metalloproteinase inhibitors. (7) Pentoxifylline, a xanthine derivative, has been demonstrated to reduce circulating TNF-alpha levels. (8) Bisphosphonates exert a proapoptotic effect on differentiated osteoclasts and are widely used to combat pathologic bone resorption. (9) Calcitonin, the functional PTH antagonist, inhibits osteoclasts and promotes mineralization of bone defects. (10) Since both bisphosphonate and calcitonin lower serum calcium levels, simultaneous calcium supplementation is necessary in order to mitigate the parathyroid response. Prognosis is strongly dependent on the history, general condition of the patient and the grade of lesions visualized by radiography. Medical imaging has a high prognostic relevance in ADD.

Conclusions
Acquired dental disease has been identified as chronic periodontitis with consequential bone resorption and secondary malformations. There is often an infectious (systemic) underlying cause. Anti-inflammatory, antimi- crobial and anti-osteolytic treatments provide a recovery rate higher than 50%. These results are promising, but require further refinement. More research is needed to identify, quantify and target the inflammatory mediators as well as potential disease modulating factors.

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