Clinical Exposures: Canine transmissible venereal tumor: The cytologic clues


Clinical Exposures: Canine transmissible venereal tumor: The cytologic clues

Jun 01, 2008

A 3-year-old castrated male Labrador retriever with a history of blood dripping from its penis was referred to the Foster Hospital for Small Animals at the Cummings School of Veterinary Medicine at Tufts University. The dog had been adopted from a breed rescue group about six weeks before presentation. Before being acquired by the rescue group, the dog had lived in Texas and Tennessee. Bloody penile discharge had been noted since the adoption, although the new owners reported that it had initially seemed to resolve without any intervention.


Figure 1
On physical examination, a large mass was noted on the shaft of the penis (Figure 1). The remainder of the physical examination findings were unremarkable.

Figure 2
Cytologic examination of impression smears made from tissue excised from the mass revealed a markedly cellular sample with a predominant population of homogeneous, discrete round cells with mild anisocytosis and anisokaryosis (Figure 2). These cells had a moderate amount of pale-blue cytoplasm, often containing a few small punctate vacuoles. The nuclei were round with coarse chromatin and a single prominent nucleolus. Moderate mitotic activity was observed (0 to 2/50X field) (Figure 3). Occasional neutrophils, lymphocytes, plasma cells, and squamous cells were noted in the background, as were variable numbers of erythrocytes. The cytologic diagnosis was transmissible venereal tumor.


Figure 3
The dog was treated weekly with intravenous vincristine. A dramatic reduction in tumor size occurred after the first dose, with complete tumor remission after seven doses. The dog was tumor-free at its last recheck one month after the last vincristine injection.


Canine transmissible venereal tumor, also called Sticker's sarcoma, is a naturally occurring, horizontally transmitted round cell tumor found in domestic dogs and potentially other canids such as gray wolves and coyotes.1,2 Although there have been reports of this tumor in most parts of the world, a high incidence seems to exist in temperate climates. The tumor is seen most commonly in young, sexually active, intact dogs allowed to roam freely (including stray dogs).1-3 These tumors are usually spread during coitus or other social behaviors such as sniffing and licking.1,2 Thus, the typical locations of these tumors are the external genitalia and the nasal and oral cavities.1-3 Other less common locations include the anal mucosa and the skin and subcutaneous areas.3,4 A recent report of a multicentric, extragenital, cutaneous canine transmissible venereal tumor in a sexually immature 11-month-old virgin female mixed-breed dog suggests that transmissible venereal tumor cells can be inoculated into puppy skin lesions by the mother during social interactions such as grooming and other mothering behavior.4 Transmissible venereal tumor metastasis has been seen involving the lymph nodes, skin, eyes, liver, musculature, abdominal viscera, lungs, and brain.3-5