Subcutaneous Cervical Mass in the Northern Tree Shrew (Tupaia belangeri)

History

An 8-year-old, 260-g, singly housed, experimentally naive, female northern tree shrew (Tupaia belangeri) housed at the University of Pennsylvania presented to the veterinary team with a 5-day history of a fast-growing cervical mass. The patient had a body condition of 4.5 on a scale of 1 (emaciated) to 5 (obese), with normal appetite, water intake, fecal output, and activity level. The mass had reportedly increased in size by 25% within 1 week of its initial observation. The tree shrew originally arrived at the University of Pennsylvania in 2017 from the Max Planck Florida Institute for Neuroscience (Jupiter, FL) and had regularly received routine cage-side health assessments. Due to her high body condition score, she had been maintained on a low glycemic diet for weight control since 2022. Twelve other tree shrews living in separate enclosures within the same vivarium and were reported to be healthy.

The tree shrew was hand-caught for an awake examination. Physical examination revealed a 1.5 × 1.5 × 1.5-cm, tan to pink, round, soft, freely moveable subcutaneous mass with 2 pinpoint black foci located at the caudoventral aspect of the right mandible. Examination otherwise revealed normal vital signs and no other abnormalities.

Differential Diagnoses

Given the patient’s advanced age (estimated lifespan up to 11 years in captivity) and the accelerated growth of the mass, the primary differential diagnosis was neoplasia.¹ However, since this presented as a focal subcutaneous mass, a cyst, hematoma, abscess, and granuloma were initially considered, as well. Given the localization to the ventral neck (Figure 1), we considered the tissue of origin to be one of the following: lymph node, salivary gland, thyroid, skin, or surrounding soft tissue structures. This allowed us to narrow our differential diagnoses to lymphoma, salivary adenoma/carcinoma, thyroid adenoma/carcinoma, melanoma, carcinoma, and soft tissue sarcoma. Further diagnostics were required to determine the nature of this mass.

Figure 1.

Intraoperative Images of the Cervical Mass. The mass is focal, well circumscribed, tan to pink, soft, and subcutaneous around the base of the right mandible.

Treatment and Management

The tree shrew was placed under anesthesia for further diagnostics and surgical excision of the mass due to the high risk of malignancy. No additional masses were palpated on sedated physical examination, and intraoperative thoracic and abdominal ultrasonography revealed no significant abnormalities. However, intraoperative blood glucose measured at 498 mg/dL (reference interval 111-188 mg/dL), raising suspicions for a diagnosis of diabetes mellitus, which would represent a notable comorbidity in this case. Fine-needle aspiration of the cervical mass and impression smears were collected once the mass was successfully excised (Figure 2). Cytologic evaluation of the aspirates and impression smears revealed numerous sebocytes, lymphocytes, and large round cells suggestive of either lymphocytic or basal cell origin (Figure 3). Histopathologic evaluation was then recommended for a definitive diagnosis, which revealed a well-demarcated, partially encapsulated, densely cellular, multilobulated, malignant epithelial neoplasm with abundant intracytoplasmic lipid vacuoles and mitotic figures within sebocytes and basaloid reserve cells, consistent with a sebaceous carcinoma (Figures 4 and 5). Surgical margins were complete, and due to the typically localized behavior of sebaceous carcinomas, excisional biopsy was likely curative. Her low glycemic index diet was continued and her recheck blood glucose level was within normal limits (144 mg/dL). The facility quantitates water levels for all tree shrews, and this animal had no history of polyuria or polydipsia. Hence, the high intraoperative blood glucose measurement was attributed to stress and α-2 agonist delivery during surgery rather than to diabetes mellitus. Five months after the initial presentation, the tree shrew was reported to be healthy with no evidence of recurrence and no clinical signs of diabetes mellitus.

Figure 2.

Excised and Bisected Cervical Mass. The mass is mottled red and solid on the cut surface, indicative of hemorrhage and a neoplasm, respectively.

Figure 3.

Cytologic Evaluation of the Impression Smears. There are numerous sebocytes (*) and lymphocytes (arrowhead). Diff-Quik stain. 40x.

Figure 4.

Histologic Evaluation of the Subcutaneous Mass. Expanding the subcutis and raising the epidermis is a completely excised, well-demarcated, partially encapsulated, densely cellular, multilobulated, malignant epithelial neoplasm with multifocal regions of hemorrhage. Hematoxylin and eosin stain. 0.75x.

Figure 5.

Histologic Evaluation of the Subcutaneous Mass at Higher Magnification. Higher magnification of the neoplastic lobules highlights multiple mitotic figures and hemorrhage with surrounding sebocytes and reserve cells. Hematoxylin and eosin stain. 40x.

Discussion

Neoplastic disease in tree shrews is relatively uncommon, accounting for 5.6% (63/1,132) of cases in one study; among these, cutaneous neoplasms represented 7.9% (5/63) of all tumors identified.² In the same study, >82% of tree shrews >8 years old, as in the case described here, developed tumors.² The most common types of cutaneous neoplasia in tree shrews appear to be adenocarcinomas and sarcomas.² Adenocarcinoma is the most common tree shrew cutaneous neoplasm, primarily affecting the jugulo-sternal glands (3/63 [4.8%]).² Sarcomas are the second most common tree shrew cutaneous neoplasm and most often occur within the nasal subcutis or the mammary skin (2/63 [3.2%]).² To our knowledge, this is the first report of a spontaneously occurring sebaceous carcinoma in a female northern tree shrew.

Sebaceous carcinomas are malignant epithelial neoplasms with neoplastic cells showing sebaceous differentiation. The only reported cases in tree shrews, to our knowledge, is in reference to neoplasms arising from the sebaceous regions of the jugulo-sternal glands of male tree shrews.² We had not initially considered this organ when we first identified the mass; however, for those currently or considering working with tree shrews, it is important to be cognizant of this gland. In cats and dogs, these tumors arise primarily on the head and are typically locally invasive.^3–6 Due to the presence of moderate to marked hemorrhage and neoplastic sebocytes with intracytoplasmic lipid vacuoles, large pleomorphic nuclei, prominent nucleoli, and mitotic figures within both neoplastic sebocytes and reserve cells, this neoplasm is consistent with a sebaceous carcinoma versus a sebaceous adenoma or sebaceous epithelioma. Although neoplastic cells frequently contained intracytoplasmic lipid vacuoles, the multilobulated appearance of this neoplasm and epithelial cell origin allow it to be differentiated from a liposarcoma.

Several potential etiologies have been proposed for the development of sebaceous carcinoma in humans, including UV radiation exposure, immunosuppression, and aberrant hormonal activity.^3,7 None of these factors appear applicable to the present case, so the underlying cause remains undetermined. Clinically, sebaceous carcinomas in most species manifest as localized swellings associated with sebaceous tissue, most often involving the head, eyelids, thorax, or abdomen.^4,6,8,9 The treatment of choice in cats, dogs, and humans is wide surgical excision.^8,9 When surgery is not feasible, chemotherapy or radiation therapy may be considered, although these modalities are generally reserved for cases with confirmed metastasis in humans.⁹

In conclusion, a sebaceous carcinoma arising from the ventral cervical region was identified in a geriatric laboratory tree shrew and confirmed using histology. To our knowledge, this is the first reported case in a female northern tree shrew and should be included as a differential diagnosis for a focal, fast-growing subcutaneous mass in that species. In general, there is a dearth in scientific literature pertaining to tree shrew pathology, and we aim for this report to be useful for the comparative medicine community.

Conflict of Interest

The authors have no conflicts of interest to declare.

Funding

The University Laboratory Animal Resources at the University of Pennsylvania provided internal funding of this case report.

Ethics statement

All procedures were performed within an AAALAC-accredited facility, with the approval of the University of Pennsylvania IACUC, and in accordance with the Guide for the Care and Use of Laboratory Animals and the Animal Welfare Act.