Cancer poses one of the greatest human health threats of our time. Fortunately, aside from a few rare cases of cancer transmission in immune-suppressed organ transplant recipients or a small number of transmission events from mother to fetus, cancers are not spread from human to human. However, transmissible cancers have been detected in vertebrate and invertebrate animals, sometimes with devastating effects. Four examples of transmissible cancers are now known: 1) canine transmissible venereal tumor (CTVT) in dogs, 2) a tumor in a laboratory population of Syrian hamsters that is no longer cultured, 3) infectious neoplasias in at least four species of bivalve mollusks, and 4) two independently derived transmissible cancers (devil facial tumor disease [DFTD]) in Tasmanian devils (Fig 1A and 1B). The etiologic agents of CTVT, the bivalve cancers, and DFTDare the transplants (allografts) of the neoplastic cells themselves, but the etiologic agent is unknown for the hamster tumor.

The effects of these transmissible cancers on their respective host populations vary. CTVT is spread in dogs through sexual contact and is at least 11,000 years old, placing the timing of its origin close to that of the domestication of dogs. Although genomic analyses of the tumor suggest evasion of multiple components of the dog immune system, dogs most commonly survive and often show evidence of spontaneous tumor regression within a year of initial diagnosis. For the infectious bivalve neoplasias, which have existed for at least 40 years, population effects vary from enzootic infections with no noticeable effects on population sizes to evidence of a catastrophic population decline. In Tasmanian devils (Fig 1A), the first infectious tumor discovered (DFT1; Fig 1B) has spread across approximately 95% of the geographic range of Tasmanian devils since 1996 (Fig 1C). DFTD is almost always fatal (Fig 1B), with >90% declines in infected localities and an overall species-wide decline exceeding 80%. Transmission dynamics appear consistent with frequency dependence, with DFTD spread by biting during social interactions, resulting in predictions of extinction from standard epidemiological models. Despite these predictions, long-infected devil populations persist at reduced densities, suggesting that individual-level variability in fecundity and tumor growth rate in infected individuals are key for understanding epidemiological dynamics. Additionally, the origin of the second, independent lineage of DFTD (i.e., DFT2) within 20 years of the discovery of DFT1 suggests that transmissible cancers may be a recurring part of the Tasmanian devils' evolutionary history, without causing extinction.