Canine transmissible venereal tumor
Canine transmissible venereal tumor (CTVT), also called transmissible venereal tumor (TVT), Sticker tumor and infectious sarcoma is a tumor of the dog and other canids that mainly affects the external genitalia, and is transmitted from animal to animal during copulation.
Canine TVT was initially described by Russian veterinarian M.A. Novinsky (1841-1914) in 1876, when he demonstrated that the tumor could be transplanted from one dog to another by infecting them with tumor cells.
It has been proposed that the tumor cells responsible for canine transmissible venereal tumor be considered a parasitic cancer. It has been estimated that the tumour cell line originated 200 to 2,500 years ago in a wolf, coyote, or old Asian dog breed such as a Husky or Shih Tzu. The tumour cells are themselves the infectious agents. 
TVT tumor cells have fewer chromosomes than normal cells. Dog cells normally have 78 chromosomes; TVT tumor cells contain 57 - 64 chromosomes that are very different in appearance from normal dog chromosomes. All dog chromosomes except X and Y are acrocentric, having a centromere very near to the end of the chromosome, while many of the TVT chromosomes are metacentric or submetacentric, having a centromere nearer to the middle. There is no evidence that the tumor is caused by a virus or virus-like organism. All tumor cells of this type of cancer share extremely similar genetic code, often if not always unrelated to the DNA of their host. Specifically, the LINE-1 (Long interspersed nuclear element) element in the tumor cells is in a different location than in normal canine DNA. 
TVT is most commonly seen in sexually active dogs in tropical and subtropical climates. The disease is spread when dogs mate, and it can even be transmitted to other canine species, such as foxes and coyotes. Spontaneous regression of the tumor can occur, probably due to a response from the immune system. TVT undergoes a predictable cycle: the initial growth phase of four to six months (P phase), a stable phase, and a regression phase (R phase), although not all TVTs will regress. The tumor does not often metastasize (occurring in about 5 percent of cases), except in puppies and immunocompromised dogs. Metastasis is most commonly to regional lymph nodes, but can also be seen in the skin, brain, eye, liver, spleen, testicle, and muscle. Biopsy is necessary for diagnosis.
Signs and symptoms
In male dogs, the tumor affects the penis or prepuce. In females, it affects the vagina or labia. Rarely, the mouth or nose are affected. The tumor often has a cauliflower-like appearance. Signs of genital TVT include a discharge from the prepuce and in some cases urinary retention, from blockage of the urethra. Signs of nasal TVT include oronasal fistulae, nosebleeds and other nasal discharge, facial swelling, and enlargement of the submandibular lymph nodes.
Chemotherapy is very effective for TVT, but surgery alone often leads to recurrence. Surgery may be diificult due to the location of these tumors. The prognosis for complete remission with chemotherapy is excellent. The most common chemotherapy agents used for TVT are vincristine, vinblastine, and doxorubicin. Radiation therapy may be effective when chemotherapy does not work.
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- Devil facial tumour disease - a similarly transmissible "immortal cell" cancer in the Tasmanian Devil.
- HeLa cells - immortal cell line used in biomedical research