Clonally transmissible cancer
A parasitic cancer or transmissible cancer is a cancer cell or cluster of cancer cells that can be transmitted from animal to animal. They are quite rare in humans and other animals. These cancers are distinct from cancers caused by infectious agents such as oncoviruses and cancer bacteria, which are more common.
Examples in animals
Cancer is not normally a contagious disease, but there are four known exceptions in dogs, Tasmanian devils, Syrian hamsters, and soft-shell clams. These cancers have a relatively stable genome as they are transmitted. Because of their transmission, it was initially thought that these diseases were caused by the transfer of oncoviruses, in the manner of cervical cancer caused by HPV.
- Canine transmissible venereal tumor (CTVT) is sexually transmitted cancer in dogs. It was experimentally transplanted between dogs in 1876 by M. A. Novinsky (1841–1914). A single malignant clone of CTVT cells has colonized dogs worldwide, representing the oldest known malignant cell line in continuous propagation.
- Contagious reticulum cell sarcoma of the Syrian hamster can be transmitted from one Syrian hamster to another by means of the bite of the mosquito Aedes aegypti.
- Soft-shell clams, Mya arenaria, have been found to be vulnerable to a transmissible neoplasma of the hemolymphatic system — effectively, leukemia.
Instances of transmission of human cancer
Transmissible cancers are rare in humans. A malignant fibrous histiocytoma was contracted from a patient by a surgeon when he injured his hand during an operation. More recently, Barozzi and colleagues found that a significant fraction of Kaposi's sarcoma occurring after transplantation may be due to tumorous outgrowth of donor cells. Although Kaposi's sarcoma is caused by a virus (Kaposi's sarcoma-associated herpesvirus), in these cases, it appears likely that transmission of virus-infected tumor cells—rather than the free virus—caused tumors in the transplant recipients.
Animals to humans
Only one known instance of cross-species cancer transmission is known, that of a 41-year-old man in Colombia with a compromised immune system due to HIV. The man's tumor cells were shown to have originated from the dwarf tapeworm, Hymenolepis nana, making this a case of parasite-to-host cancer transmission. 
- Retrovirology A sexually transmitted parasitic cancer
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- "A Man In Colombia Got Cancer And It Came From A Tapeworm".