Abstract

Each individual cancer is a clonal cell lineage that emerges through an evolutionary process when a single cell of the body acquires somatic mutations that drive proliferation and survival. As it develops, cancer can spread through the body to invade distant tissues. However, it does not normally spread or survive outside of the body. Clonally transmissible cancers are clonal cell lineages that survive beyond the deaths of their hosts by acquiring adaptations for transmission between hosts. Rare examples of cancer transmission between humans have been reported due to transfer of cancer cells in utero, by surgical injury, by experimental inoculation and by inadvertent transplantation of cancer cells with donated organs. However, there are only two known naturally occurring clonally transmissible cancers that have spread between multiple hosts and these are the transmissible venereal tumour of dogs and the facial tumour of Tasmanian devils. These two cancers are specialised parasitic clonal cell lineages that spread between individuals through physical contact and have survived long after the deaths of the animals from which they originally emerged.