Oral Squamous Cell Carcinoma in a Pigtailed Macaque (Macaca nemestrina)

An adult, gravid, female pigtailed macaque (Macaca nemestrina) presented for facial swelling centered on the left mandible that was approximately 5 cm wide. Differential diagnoses included infectious, inflammatory, and neoplastic origins. Definitive antemortem diagnosis was not possible, and the macaque''s condition worsened despite supportive care. Necropsy findings included a mandibular mass that was locally invasive and expansile, encompassing approximately 80% of the left mandibular bone. The mass replaced portions of the soft palate, hard palate, sinuses, ear canal, and the caudal–rostral calvarium and masseter muscle. Histologically, the mass was a neoplasm that was poorly circumscribed, unencapsulated, and infiltrative invading regional bone and soft tissue. The mass consisted of polygonal squamous epithelial cells with intercellular bridging that breached the epithelial basement membrane and formed invasive nests, cords, and trabeculae. The mitotic rate averaged 3 per 400× field of view, with occasional bizarre mitotic figures. Epithelial cells often exhibited dyskeratosis, and the nests often contained compact lamellated keratin (keratin pearls). The neoplasm was positive via immunohistochemistry for pancytokeratin, variably positive for S100, and negative for vimentin, smooth muscle actin, and desmin. The gross, histologic, and immunohistochemical findings were consistent with an aggressive oral squamous cell carcinoma. The neoplasm was negative via PCR for papilloma virus. In general, neoplasia in macaques is rare. Although squamous cell carcinomas are one of the most common oral neoplasia in many species, to our knowledge this case represents the first reported oral squamous cell carcinoma in a pigtailed macaque.Abbreviation: SCC, squamous cell carcinomaSquamous cell carcinomas (SCC) are one of the most commonly reported oral tumors. They are characterized as firm, nodular to irregular, soft-tissue masses that are often ulcerated.⁶ These tumors are frequently highly invasive to local bone and muscle and occasionally metastasize to local and regional lymph nodes.⁶ Histologically, SCC are characterized by keratin pearls, intercellular bridges, and positive cytokeratin staining on immunohistochemistry.^6,18 SCC have been associated with carcinogen exposure (such as bracken fern toxicosis in cattle), actinic radiation, and rarely with papillomatosis.⁸In general, neoplastic diseases are rare in nonhuman primates, and SCC and lymphoma are the 2 most commonly reported oral neoplasms in these species.³ SCC have most commonly been reported in rhesus macaques (Macaca mullata) and baboons (Papio spp.) among nonhuman primate species.⁹ In rhesus macaques, SCC has occurred in the oral cavity,⁹ integument,^9,22 esophagus,⁹ stomach,²¹ lung,^9,13 prepuce–penis,¹⁰ cervix,⁹ uterus,⁹ and eye.⁹ These neoplasms have also been reported to occur in cynomolgus macaques,^14,15,17,19 marmosets, squirrel monkeys, tree shrews, capuchins, tamarins, black spider monkeys, sooty mangabies, a spectacled langur, and an orangutan.⁹ No report describing SCC in a pig-tailed macaque has been published previously. The oral cavity is the most common site of SCC in nonhuman primates, and metastasis occurs in approximately 23% of cases.⁹ The average age at diagnosis of oral SCC in rhesus macaques is 17.6 y.²² In baboons, SCC is the third most common neoplasm, after intestinal adenocarcinoma and lymphosarcoma.⁴ The following case report describes an oral SCC in a pregnant pig-tailed macaque.