An intact 5-year-old female mixed breed dog was presented in an emaciated condition to a veterinary teaching hospital. Unfortunately, the stray dog died before undergoing further examinations, including cytology and imaging tests. Multiple masses were seen in the inguinal and abdominal pairs of mammary glands and subcutaneous tissue. Necropsy of the case revealed no masses in the external genitalia and other tissues and organs. Tissue samples were taken from the masses for histopathology and immunohistochemistry. Histopathological examination of the mammary masses showed rows and cords of round to ovoid neoplastic cells with large round to polyhedral nuclei containing a single nucleolus. Three to five mitotic figures per high-power field were observed. Immunohistochemical staining of the mass for vimentin revealed strong cytoplasmic positivity. Intense and moderate immunoreactivities for Ki-67 and c-Myc were detected in the cells. The neoplastic cells were negative for cytokeratin, S-100 and CD20 markers, but were weakly positive for CD3, p53 and CD68. The findings achieved from histopathology and immunohistochemistry established a diagnosis of disseminated transmissible venereal tumour (TVT) in the mammary glands of a mixed-breed dog. This case highlighted that TVT should be considered as a differential diagnosis for extragenital masses or mammary gland tumours in dogs, particularly in regions with a high prevalence of stray dogs.

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