Mast cell tumor and pheochromocytoma in a rescued Malayan sun bear

Abstract

In 2019, a wild-caught Malayan sun bear (Helarctos malayanus), rescued from the illegal pet trade and relocated to the Edinburgh Zoo (UK), was diagnosed with mast cell tumor (MCT) affecting a hindlimb. Surgical excision was performed at the initial site in the tarsal region, with a repeat excision 2 y later following a relapse, without evidence of metastatic spread. In 2023, MCT metastases were detected in the popliteal lymph node and prostate, prompting a novel treatment approach combining prednisolone and toceranib to alleviate the clinical signs. Four months after initiating treatment, the sun bear was euthanized on welfare grounds. The autopsy and histopathology confirmed the presence of a MCT involving the prostate and the popliteal and iliac lymph nodes. An incidental pheochromocytoma was identified in an adrenal gland. MCT and pheochromocytoma in the Ursidae family, with detailed description of these neoplasms, their microscopic and immunohistochemical features, have not been reported previously, to our knowledge.