Diagnosis and treatment of Merkel cell carcinoma in a puppy: A comparative oncology approach.

A 5-month-old male golden retriever dog was presented with a solitary, rapidly growing cutaneous mass on the left thoracic wall. The medical history was unremarkable and no other clinical abnormalities were identified. After wide surgical resection, histopathological evaluation revealed a malignant tumor of small, round, atypic basophilic neoplastic cells positive for neuron-specific enolase and pancytokeratin, providing a diagnosis of Merkel cell carcinoma. With clean histological margins and absence of local or distant metastasis, no recurrence was reported during a minimum follow-up period of 20 mo. The dog developed an oral nodule 12 mo after surgery. Histopathological evaluation revealed a papilloma, and PCR was positive for Papillomavirus sp. Although DNA extraction and PCR for papillomavirus detection were carried out on the Merkel cell carcinoma tumor, no papillomavirus was detected. Merkel cell carcinomas can easily be confused with other cutaneous neoplasms in young dogs. Despite being malignant, they are frequently associated with a good prognosis after complete surgical excision. Key clinical message: i) Merkel cell carcinoma should be included in the differential diagnosis of a cutaneous mass in a puppy. ii) Wide excision provided an excellent long-term outcome in this clinical case.