Ameloblastic odontoma in the mandible of a llama

Abstract

A 4-year-old llama had an aggressive, multiloculated, expansile bone lesion involving the rostral mandible. The mandibular lesion was imaged using radiography and computed tomography. Antemortem diagnosis of an ossifying fibroma was made histologically. Postmortem findings showed the lesion to be limited to the mandible. Final diagnosis was ameloblastic odontoma.

A 4-year-old, intact male llama was presented to the Boren Veterinary Medical Teaching Hospital (BVMTH) at Oklahoma State University with a chief complaint of swelling of the right lower lip of approximately 1 week's duration. This llama had been healthy since birth and was fed grass hay. All the llamas in the herd received annual vaccinations with a 7-way Clostridium vaccine (Vision7 with SPUR; Bayer, Agriculture Division, Animal Health, Shawnee Mission, Kansas, USA), a rabies vaccine (Imrab3; Rhone Merieux, Athens, Georgia, USA), and tetanus toxoid (Tetanus Toxoid; Fort Dodge Laboratories, Fort Dodge, Iowa, USA); they were also dewormed with injectable ivermectin (Ivomec Injection for Cattle and Swine 1%; Merck AgVet Division, Merck, Rahway, New Jersey, USA) every 4 mo. There was no history of trauma. The owner considered his other llamas to be normal.

The llama weighed 62 kg, was slightly hypothermic (rectal temperature, 37°C; reference range, 37.5°C to 38.9°C) and tachycardic (102 beats/min; reference range, 60 to 90 beats/min). The llama's lower right lip protruded, secondary to a palpable, firm nodular mass (4 × 3 × 3 cm). The mass contained a central soft area located on the right mandible, extending from the rostral quarter of the mandible to approximately 0.5 cm from the mandibular symphysis and from the gum-line to the ventral aspect of the mandible. The llama did not resist palpation of the mass. There was no identifiable mass on the left mandible, and no other abnormalities were found on physical examination. Differential diagnoses included abscess, osteomyelitis, granuloma, cyst, and neoplasia. The diagnostic plan included doing a complete blood cell (CBC) count with fibrinogen and protein concentration measurement, taking radiographs (and possibly computed tomography) of the right mandible, and aspirating the central area of the mass for cytologic examination and bacterial (aerobic and anaerobic) culture with sensitivity testing.

The red blood cell count (7.29 × 10¹²/L; reference range, 11.3 to 16.9 × 10¹²/L) (BVMTH values), hemoglobin (92.5 g/L; reference range, 126 to 178 g/L) (BVMTH values), hematocrit (0.209 L/L; reference range, 0.28 to 0.39 L/L) (BVMTH values), and total plasma protein by refractometry (48 g/L; reference range, 57 to 70 g/L) (BVMTH values) were all decreased. Fibrinogen levels and differential cell counts were within normal limits. The CBC count results were interpreted as those of a normocytic normochromic anemia.

The llama was sedated with xylazine (Rompun Injectable; Bayer), 0.44 mg/kg BW, IM, to facilitate recumbent positioning of the animal for routine radiographs. A large (6.5 × 3.5 cm), multiloculated, expansile, moderately aggressive, mixed pattern bone lesion of the rostral extent of the right hemimandible was identified radiographically (Figure 1A). The soft tissues ventral to the rostral aspect of the mandible were swollen. The lesion extended from the right mandibular incisor teeth to the level of the right mandibular canine tooth. Slight caudodorsal displacement of the right mandibular incisor teeth was present.

Figure 1A. Oblique lateral projection of the mandibles from a 4-year-old llama. An aggressive, multiloculated, expansile bony mass (white arrows) is present on the right rostroventral aspect of the mandible with caudodorsal displacement of the 2nd (i₂ and 3rd (i₃) incisor teeth (black arrows).

The rostral half of the skull was imaged (kVp = 120, mAs = 351, FOV = 14 cm) by computed tomography (CT) with 3-mm slices made at 2.5-mm intervals (General Electric Sytec 4000i; Milwaukee, Wisconsin, USA). A multiloculated expansile bone lesion was noted on the rostral part of the mandible, primarily on the right side (Figure 1B). The ventral and right lateral aspects of the mass were irregularly marginated. As a result of this expansile mass, the right 2nd and 3rd mandibular incisor teeth were displaced dorsally, while the right 1st mandibular incisor tooth was displaced to the left.

Figure 1B. Transverse computed tomographic image of the rostral aspect of the skull from a 4-year-old llama with an aggressive, multiloculated, expansile (arrows) bony mass present on the right rostroventral aspect of the mandible. The image was made at the level of the incisor teeth by using a 3-mm slice and bone settings (WW = 2500, WL = 500). The aggressive mass is causing cortical bone lysis and is displacing the 2nd and 3rd right mandibular incisor teeth (i₂ and i₃) dorsally and the right first incisor tooth (i₁) to the left.

While the llama was sedated, the surface of the mass was aseptically prepared for an aspirate. An 18-gauge needle was inserted into the central soft area and approximately 3 mL of clear straw-colored fluid was removed for cytological analysis and bacterial (aerobic and anaerobic) culture with antibiotic sensitivity testing. There was a depressed area in the mass, both visually and by palpation, after the fluid was withdrawn. The firm peripheral swelling of the mass remained. The fluid contained 500 × 10⁶ white blood cells/L, but no red blood cells; had a protein content of 32 g/L; and consisted primarily of a few scattered mononuclear cells and a few nondegenerative neutrophils. A large amount of proteinacous material was in the sample. The interpretation was cystic fluid. After the aspirated sample had been collected, sedation was reversed with tolazoline (Tolazine Injection; Lloyd Laboratories, Shenandoah, Iowa, USA) (1.0 mg/kg BW, IV). Recovery was uneventful.

The llama had acclimated to the hospital environment by the 2nd day of hospitalization but still had a poor appetite, eating only a small amount of grass hay. Findings on physical examination, including temperature, pulse, and respiration, were all within normal limits by the day following admission. There was no change in the mass. Ceftiofur sodium (Naxcel; Pharmacia & Upjohn Company, Kalamazoo, Michigan, USA), 2.2 mg/kg BW, IM, q12 h, and procaine penicillin G (Pen-G; Phoenix Pharmaceutical, St. Joseph, Missouri, USA), 22 000 units/kg BW, IM, q12 h, were administered while awaiting the results from the bacterial (aerobic and anaerobic) culture and antibiotic sensitivity testing. Subsequently, no bacteria were isolated from the sample.

It was decided that even though bacterial culture results were still pending, a biopsy of the mass was warranted because the radiographic evidence was of an aggressive expansile lesion and the fluid analysis was unremarkable. Prior to the biopsy, the llama was administered a tetanus toxoid (Tetanus Toxoid; Fort Dodge Laboratories) booster. The ceftiofur sodium (Naxcel; Pharmacia & Upjohn) was continued at the same dosage. The llama was sedated with xylazine (Rompun Injectable; Bayer), 0.44 mg/kg BW, IM, and the area was aseptically prepared. A bone marrow biopsy needle (Jamshidi Bone Marrow Needle; Baxter Healthcare, McGaw Park, Illinois, USA) was inserted into the rostroventral base of the mass and directed caudad along the base of the mass until a sample was obtained. The procedure was repeated at an approximately 15- to 20-degree angle dorsolaterally from the 1st sample site. The samples were placed in 10% formalin and submitted for histopathological evaluation. Tolazoline (Tolazine Injection; Lloyd Laboratories) reversal agent was not administered, as the llama was recovering uneventfully.

The antibiotic treatment was continued pending receipt of the histopathological results. During hospitalization, the mass appeared to enlarge and the llama became resentful to palpation of the mass. The llama did not resent daily physical examinations. His appetite decreased and he was observed to lie down often. The decrease in appetite corresponded to a decrease in gastric motility. Temperature, pulse, respiration, and other physical examination findings were within normal limits. Ten days after admission the llama had lost 4.5 kg, or over 7% of his BW.

The biopsy material was interpreted to be that of an ossifying fibroma, based upon the histological appearance of irregular lobules of well-differentiated bone embedded within a dense arrangement of mature fibrous connective tissue. The biopsies contained no tissues that were consistent with stellate reticulum, ameloblasts, or odontoblasts. These tissues were similar to an ossifying fibroma, recently reported in a llama, that involved the medial canthus of the eye and extended to the soft palate (1). Information obtained from the biopsy ruled out an abscess, osteomyelitis, and granuloma, which, potentially, could have been treated medically. Treatment of this aggressive, expansile, neoplastic lesion would have required surgical removal. Rostral mandibulectomy has been performed in a heifer with ameloblastic fibro- odontoma (2). The report indicated at the 1-year follow-up, the heifer had gained weight and rejoined the herd. The heifer's tongue did protrude but the cosmetic appearance was acceptable to the owner (2). Cattle use their tongues to prehend food. Small ruminants and llamas use their incisors to prehend food. Therefore, it is unknown if surgical intervention would have been successful in this case. Due to the progression of the disease, decrease in appetite, and poor prognosis with or without any treatment, the owner elected to euthanize the llama.

At necropsy, significant gross lesions were limited to the rostral fourth of the mandible. The rostral mandible was focally expanded and effaced by a relatively well-demarcated, hard, egg-shaped mass that was 5 cm in length and 3 cm in diameter. The mass displaced the incisors and had caused mild, multifocal ulceration of the overlying oral mucosa. On cross-section, the mass was multicavitated and consisted of an extensive network of thin seams of hard, white-grey tissue that surrounded either white or dark red soft tissue.

Several sections from different regions of the tumor were examined microscopically and each was similar. The mass was nonencapsulated and well demarcated from the mandibular cortical bone. Most of the mass was comprised of numerous irregular and coalescing islands of eosinophilic, homogeneous material that was regionally either acellular or sparsely populated by cells within lacunae (resembling bone). The islands were separated by a loosely arranged stellate reticulum. The stellate reticulum contained multifocal, solid, and occasionally cystic islands and cords of epithelial cells. The epithelial cell islands had a distinct arrangement consisting of a peripheral arrangement of tightly packed, tall columnar cells (interpreted to be ameloblasts). The central regions consisted of larger, often keratinizing, squamous cells that were abruptly distinct from the peripheral columnar epithelium. In central regions, intercellular bridges were conspicuous, and structures resembling enamel inclusions were seen between the cells. In a few foci, the peripheral columnar cells interfaced with seams of amorphous eosinophilic material that was either acellular or sparsely populated by cells within lacunae (resembling dentin) deposited by a receding line of odontoblasts. Rarely, the dentin material was organized into rudimentary tooth-like structures with a central pulp cavity.

The definitive diagnosis was complicated by the appearance of several differentiating tissue types within the solitary mass. Histologically, the arrangement of numerous islands and cords of epithelial cells was most consistent with a neoplasm arising from the dental lamina epithelium and previously described as ameloblastoma or keratinizing ameloblastoma (3,4). The proliferating ameloblasts were accompanied by several apparent inductive changes in the surrounding stellate reticulum. The most prominent was the deposition of bone-like tissue that contributed to the bulk of the mass. Attempts were made to characterize this material as either osteoid or dentin; however, examination of sections specially stained by the Roush-Breur-Wilson method for dental structures was equivocal. There were several foci of dentin deposition associated with the proliferating ameloblasts, and, rarely, a combination of ameloblasts, odontoblasts, dentin, and surrounding stroma was organized into structures slightly resembling teeth. These criteria prompted the diagnosis of an ameloblastic odontoma. Lastly, according to Walsh et al (5), the connective tissue stroma of ameloblastic odontomas consists of extensive masses of osteodentin, which is somewhat compatible with the large deposition of bone-like material that comprised the bulk of this mass.

This llama had a normocytic, normochromic anemia. Chronic inflammatory processes, chronic infections, severe trauma, or neoplasia are frequently associated with anemia. This type of anemia is often slowly progressive and of little clinical significance. This llama's anemia was attributed to the neoplasm. Since the mass was a neoplasm instead of an infectious process, it is not surprising that bacteria were not isolated from the sample.

Retrospectively, the proliferating epithelial component present in the neoplasm was absent in the biopsy samples. This emphasizes that multiple biopsies from different regions of this type of mass are often necessary to achieve a correct diagnosis. In this case, however, the clinical management and prognosis of ossifying fibroma versus ameloblastic odontoma would not have been significantly different.

The differences in the detection of bony changes between routine radiography and CT are several fold. Due to the better physical density discrimination of CT compared with radiography, areas of bone lysis and bone production are more apparent (6,7). As CT generates 2-dimensional transverse images in which anatomy is displayed in a relatively less complex format compared with that of radiographs, the superimposition of anatomic structures is eliminated. This display allows for the direct visualization of the region in question. In addition, gray scale manipulation allows enhancement of different physical densities within a CT image that may help delineate bony changes.

Dental tumors are considered rare in domestic animals. These tumors can be locally invasive, but rarely metastasize (3,8). Ameloblastic odontomas have been reported in other species including dogs (9), horses (10,11), nonhuman primates (12), sheep (13), and cattle (5,14). An anaplastic sarcoma of the mandible in a llama has been diagnosed at necropsy (15). To our knowledge, this is the first report of an ameloblastic odontoma in the mandible of a llama. CVJ