Korean Journal of
Veterinary Service

The odontogenic fibroma is a benign neoplasm of odontogenic ectomesenchymal origin, characterized by well-developed collagenous fibrous tissue intermingled with varying amounts of odontogenic epithelium (Baiju and Rohatgi, 2011). This tumor, when located peripherally and involving gingival inflammation, is termed peripheral odontogenic fibroma. It’s a relatively prevalent tumor-like lesion in the oral cavity, often seen in dogs and cats where it accounts for 37∼67% of all canine odontogenic tumors (Fonseca et al, 2014; Brierley et al, 2019). Factors that lead to its development are local irritations such as poor-quality dental restorations, dental plaque, and calculus (Fonseca et al, 2014), and most lesions occur on the maxillary anterior interdental papilla (Shulman et al, 2004; Fonseca et al, 2014; Brierley et al, 2019). Clinically, this tumor presents as an asymptomatic, outward-growing, smooth or sometimes ulcerated mass of mucosal color, displaying various rates of growth (Fonseca et al, 2014). Typically, the lesions measure between 10 mm to 20 mm in diameter at the time of discovery; larger or rapidly growing lesions are often incorrectly identified as cancerous (Fonseca et al, 2014).

Peripheral odontogenic fibroma is not only common in dogs, cats, and sheep but also occurs in humans, though it’s rare in other farm animals and wildlife (Barker et al, 1997; Gelberg, 2001). Occasionally referred to as ‘gingival enlargement’, this condition denotes abnormal, excessive growth of periodontal tissues (Khera et al, 2005). While it’s seldom seen and studied in non-human primates, extensive research has been done in humans (Lewis and Reiter, 2005). It has noted that in dogs, gingival hyperplasia is most common at around 7 to 8 years of age with an incidence of 2.08% (Barker et al, 1997). An uptick in reported cases is attributed to environmental factors, longer lifespans of animals, and more frequent post-mortem examinations (Hitt et al, 2003). Reports of gingival enlargement from non-human primates are scarce, including in species like rhesus macaques (Macaca mulatta) (Ulland et al, 1970), mustache guenon monkeys (Cercopithecus cephus) (Sheldon, 1967), stump-tailed macaques (Macaca arctoides) (Staple et al, 1977), baboons (Papio ssp.) (Hodosh et al, 1971), gorillas (Gorilla gorilla) (Fagan and Oosterhuis, 1979), other than the Formosan rock macaque, also known as the Taiwanese monkey (Macaca cyclopis).

Distinguishing peripheral odontogenic fibroma from fibrous gingival hyperplasia is often difficult. Peripheral odontogenic fibroma typically presents as a solitary mass, while gingival hyperplasia usually shows multiple foci or extends linearly along the gingiva. Histologically, gingival hyperplasia features fewer fibrocytes in its collagen, which is coarser, denser, more mature, and bundled, compared to the fine, fibrillar collagen found in the cell-rich peripheral odontogenic fibroma (Munday et al, 2017).

This study focuses on a recurring case of peripheral odontogenic fibroma in a Formosan rock macaque, aiming to enhance the clinical understanding and management of such cases.

This case study focuses on a 10-year-old male Formosan rock macaque (Macaca cyclopis) residing in a zoo, which has repeatedly developed a cabbage-like growth on the gum near its right incisor and molar teeth on right maxilla. over the last three years, with these masses re-emerging annually. Surgical removal has been the consistent treatment each year. Upon oral examination, the area surrounding the masses showed signs of lesions and tissue degradation. The masses themselves, appearing in tandem with these symptoms, were a firm, pinkish lump, about 0.5×0.4×0.4 cm in size, respectively, taking on a cabbage-like shape around the incisors and molars. When cut open, the masses revealed pinkish hue and were filled with fibrous tissue (Fig. 1). These masses underwent surgical excision and was then processed for histopathological analysis, involving fixation in 10% neutral buffered formalin, embedded in paraffin, and sectioned into 4 μm thick slices. These sections were then stained with Hematoxylin and Eosin (H&E) for examination under a light microscope.

Figure 1. Gross appearance of peripheral odontogenic fibroma in a 10-year-old male Formosan rock macaque. The masses were firm, pinkish nodule, measuring 0.5×0.4×0.4 (A) and, on cut surface, showed white aspect with a fibrous consistence (B).

Histopathological analysis showed significant growth of the gum’s surface epithelium covering the masses, with areas displaying keratinization and multiple keratin layers. The tumor comprised spindle and stellate cells, forming a mass encircled by collagen-rich matrix and well-developed blood vessels (Fig. 2A). The cells exhibited eosinophilic cytoplasm typical of primitive fibroblasts and nuclei that varied in shape from round to elongated.

Figure 2. Peripheral odontogenic fibroma. A 10-year-old male Formosan rock macaque. Delicate fibrillar collagen and strands of denser collagen occasionally intersect the mass, which was separated from irregular hyperplasia of surface epithelium (A). Cellular connective tissue comprises delicate fibrillar collagen and evenly spaced stellate cells (B). H&E. Scale bar=50 μm.

The tumor cells on the periphery of the tumor tissue had scant cytoplasm and eosinophilic nuclei (Fig. 2B). The diagnosis for this case, based on these observations, was identified as peripheral odontogenic fibroma in the gums of the Formosan rock macaque.

This case report describes peripheral odontogenic fibroma in the Formosan rock macaque also known as the Taiwanese monkey. There is a longstanding controversy whether or not neoplastic fibromatous lesions of the gingiva in domestic animals are equivalent to human odontogenic fibromas, reclassification of fibromatous epulis to peripheral odontogenic fibroma has the advantages of more closely aligning the classification systems of odontogenic tumors in humans and domestic animals and avoiding the awkward use of a clinical descriptive term (epulis) as part of a morphological diagnosis (Gardner, 1996). The histological description and presentations of human odontogenic fibroma has many similarities to lesions that in domestic animals have historically been referred to as fibromatous epulis of the periodontal ligament. Also, the peripheral odontogenic fibroma should be differentiated from a gingival hyperplasia (Gardner, 1996). The histopathological examination revealed that the lesion comprised mature fibrous and collagenous tissue with a low density of cells, showing epithelial reticulation. Additionally, there was evidence of localized hypertrophy due to inflammation, aligning with the known histopathological features of peripheral odontogenic fibroma. These findings facilitated the differentiation of this condition from other types of gingival hyperplasia and oral tumors. Commonly known as fibrous proliferation, peripheral odontogenic fibroma is prevalent in both humans and animals, notably in brachycephalic and boxer dog breeds, where it occurs in approximately 30% of dogs aged over five years (Barker et al, 1997). In animals, the cause of peripheral odontogenic fibroma is known to be the initial development of gingivitis in the gums. As this inflammation of the gums becomes chronic, inflammatory cells such as lymphocytes and plasma cells infiltrate the area, and there is proliferation of fibrous tissue and mucosal surface squamous epithelium. The lesion grows in size in a nodular form, leading to the development of peripheral odontogenic fibroma (Head et al, 2002).

Peripheral odontogenic fibroma is known for its tendency to reappear, Woodward (2002) documented an instance where a peripheral ossifying fibroma, previously removed from the area between a dog’s left upper premolars, did not recur four months following the initial excision. In this instance of peripheral odontogenic fibroma, it seems that the chronic inflammation surrounding the tooth’s crown is the primary cause. Diagnosing this condition requires a surgical biopsy of the oral lesion. In cases where squamous cell carcinoma might be a concern, assessing the nearby lymph nodes is vital because of the high likelihood of local metastasis. Symptoms can include reduced appetite, excessive salivation, bleeding, difficulty swallowing, and bad breath. Treatment typically involves surgical removal, and although recurrence post-surgery is possible, the overall prognosis is generally positive. The rate of recurrence varies, with some studies suggesting a range from minimal to around 39% (Michaelides, 1992).

Conclusion

This report details a case of a 10-year-old male Formosan rock macaque (Macaca cyclopis) from a zoo, which presented with a recurring, cabbage-shaped masses near its incisors and molars. Visual oral examination revealed lesions and tissue loss around the masses. The co-existing masses, firm and pinkish, measured around 0.5×0.4×0.4 cm and formed a cabbage-like arrangement around teeth, respectively. Histopathological assessment showed a pedunculated tumor with spindle and stellate cells, rich in collagen and well-vascularized, leading to a diagnosis of peripheral odontogenic fibroma. This case is notable as it represents the first recorded instance of peripheral odontogenic fibroma in a Formosan rock macaque in a South Korean zoo.

This work was supported by “Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry and Fisheries (IPET) through Agriculture, Food and Rural Affairs Convergence Technologies Program for Educating Creative Global Leader, funded by Ministry of Agriculture, Food and Rural Affairs (MAFRA) (320005-04).

No potential conflict of interest relevant to this article was reported.