Although cancer can on occasion be caused by infectious agents such as specific bacteria, parasites, and viruses, it is not generally considered a transmissible disease. In rare circumstances, however, direct communication from one host to another has been documented. Direct transmissions of cancers are not entirely restricted to animals. There are approximately 3,500 women per year in the United States who develop a malignancy during pregnancy, and in rare cases, mother-to-fetus transmission of melanoma, lymphoma, leukemias, and carcinomas have been reported as well as fetus-to-fetus transmission in twins. Acute leukaemia cells have also been transferred between fetuses (in mothers with a multiple pregnancy) followed by the development of disease in both fetuses. There are also several reports of transmissible tumors (reticulum cell carcinoma) in laboratory golden hamsters. The cancer can be transferred simply by housing infected and uninfected animals together.
Canine transmissible venereal tumor, CTVT, also known as canine venereal sarcoma and Sticker's sarcoma, is a very old contagious cancer that emerged thousands of years ago in wolves or one of the ancient breeds of dog, making it the oldest continuously passaged cell line in the world. The cancer is typically transmitted during mating when the malignant tumor cells from one dog are directly transferred to another dog via coitus, licking, biting, and sniffing tumor-affected areas (the genitals, nose, or mouth). Although capable of metastasizing, CTVT often does not require treatment, as spontaneous regression is the general rule. Most cases of CTVT are eventually rejected by the host dog, who then is conferred lifelong immunity. Although customarily CTVT is ultimately rejected, it has some fascinating properties that have allowed it to persist and be transmitted for so many generations.
Canine transmissible venereal tumor is most commonly seen in the Southern United States. Free-roaming, sexually intact dogs are at greater risk. This cancer is most coomon in dogs of 2 to 5 years of age. No heritable breed-related predisposition has been found. Tumor growth originally appears on the external genitalia, lips and mouth within 2 to 6 months of mating and can either grow slowly and unpredictably for years or grow invasively and eventually metastasize. Metastasis usually occurs in the skin, eyes, liver, brain and bone marrow. Tumors initially appear as small papules that progress by fusing together into a cauliflower-like growths with an ulcerated inflamed surface. The tumor often oozes a clear or blood-filled fluid. A high Leishmania parasitisation of canine venereal tumors has been observed. In dogs infected with Leishmania parasites the cancer is always aggressive and does not regress. CTVT responds to many therapies, however, chemotherapy is the most effective. The overall prognosis for this canine cancer is good to excellent.
Source: Ernesto del Aguila III, NHGRI
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- Contagious Cancer
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- Issues in Veterinary Practices and Specialties: 2013 Edition