Canine Transmissible Venereal Tumor

Canine transmissible venereal tumors (TVTs) are cauliflower-like, pedunculated, nodular, papillary, or multilobulated in appearance. They range in size from a small nodule (5 mm) to a large mass (>10 cm) that is firm, though friable. The surface is often ulcerated and inflamed and bleeds easily. TVTs may be solitary or multiple and are almost always located on the genitalia. The tumor is transplanted from site to site and from dog to dog by direct contact with the mass. They may be transplanted to adjacent skin and oral, nasal, or conjunctival mucosae.

The tumor may arise deep within the preputial, vaginal, or nasal cavity and be difficult to see during cursory examination. This may lead to misdiagnosis if bleeding is incorrectly assumed to be hematuria or epistaxis from other causes. Initially, TVTs grow rapidly, and more rapidly in neonatal and immunosuppressed dogs. Metastasis is uncommon (5%) and can occur without a primary genital tumor present. When metastasis occurs, it is usually to the regional lymph nodes, but kidney, spleen, eye, brain, pituitary, skin and subcutis, mesenteric lymph nodes, and peritoneum may also be sites.

Because of their homogenous populations of large, round cells with distinctive centrally located nucleoli, TVTs are usually easily diagnosed by cytologic examination of fine-needle aspirates or impression smears or by histopathologic evaluation of biopsies. TVTs may be difficult to distinguish from other round cell tumors, particularly lymphosarcomas, when they occur in extragenital locations. TVT cells contain specific, long, interspersed nuclear elements (LINEs) inserted upstream of the myc gene, which can be differentiated from other neoplasms using a PCR-based detection assay. Compared with cytologic or histologic diagnosis, PCR yields improved diagnostic accuracy and can facilitate the decision of when to discontinue chemotherapy.

TVT has a worldwide distribution, with reports of clinical cases from all continents except Antarctica. In most of north and central Europe as well as North America, TVT is uncommonly reported. However, TVT is enzootic in the rest of the world, with increased prevalence in more tropical and subtropical urban environments (eg, central and South America, the Far East and Middle East, and parts of Africa). TVT is the most common canine tumor in the Bahamas, Japan, and India.

Although spontaneous regression can occur, TVTs are usually progressive and treated accordingly. Complete surgical excision, radiation therapy, and chemotherapy are effective treatments; however, chemotherapy is considered the treatment of choice. Vincristine sulfate (0.5–0.7 mg/m², IV, once weekly for 3–6 weeks) is reported to be effective. The rate of tumor regression is negatively correlated with tumor size, older age, and season. Usually, total remission can be expected by the sixth treatment. Adriamycin (30 mg/m² for dogs weighing >10 kg; 1 mg/kg for dogs weighing ≤10 kg; IV, once every 3 weeks) and radiation therapy have been effective for those animals not responding to vincristine.

The prognosis for total remission with chemotherapy or radiation therapy is good, unless there is metastatic involvement of organs other than skin. Complete surgical excision often cannot be achieved because of the anatomic location of many of these tumors. Recurrence is likely in such cases unless adjunct radiation or chemotherapy is used.