Disseminated Metastatic Tubulopapillary Mammary Carcinoma in a Captive Fennec Fox ( <i>Vulpes zerda</i> )

BACKGROUND: Mammary gland tumours (MGTs) are common in women and domestic carnivores but appear to be rare in wild canids. CASE PRESENTATION: A 14-year-old female fennec fox (Vulpes zerda) was hospitalised due to dyspnea, received supportive treatment, but died 3 days later and was submitted for necropsy. Gross examination revealed an emaciated fox with multiple firm white nodules involving the mammary gland region, left pectoral muscle, lungs and pleural surfaces. Histologically, these nodules consisted of an infiltrative mammary epithelial neoplasm forming tubulopapillary structures with marked nuclear pleomorphism, characterised by hyperchromatic, clumped nuclei. Immunofluorescence labelling revealed strong estrogen receptor alpha (ERα) expression confined to the cytoplasm of tumour cells across all examined sites, an unusual pattern, compared with the typically nuclear ERα staining reported in MGTs. CONCLUSIONS: Based on these findings, the MGT in this fennec fox was diagnosed as a Grade II tubulopapillary carcinoma. This case represents the first description of a mammary gland carcinoma with widespread systemic metastasis in a fennec fox.