Cancer as an infectious disease

If we think about infectious diseases (as opposed to non-communicable diseases) we probably know that these can be caused by viruses, bacteria, fungi, parasites and prions.

Cancer is not on this list. While some viruses can cause cancers in infected hosts, these cancers do not infect others. The cancer dies with the patient or the cure.

But times are changing. Transmissible cancers have been identified as spreading within two vertebrates, dogs (Canis lupus familiaris) and Tasmanian devils (Sarcophilus harrisii), and multiple independent lineages of transmissible cancer have been found in four species of bivalves[1].
[1] Canine Transmissible Venereal Tumor
The first transmissible cancer to be identified was Canine Transmissible Venereal Tumor (CTVT), a solid tumor that spreads within populations of dogs through sexual contact. The etiology of the disease was initially uncertain, but it is now known that transmission occurs through transfer and replication of the cancer cells themselves, rather than through viral modification of cells in each new host. An unexpected feature of CTVT is that it has repeatedly acquired new mitochondrial genomes from its hosts throughout its evolution. Analysis of the CTVT genome found that the cells have been spreading as a transmissible cancer lineage for 10,000–12,000 years[2]. Thus, these cells have been evolving and spreading as an asexual parasitic organism that has outlived its original canine host by more than 500 generations.

[2] Tasmanian Devil Facial Tumor Disease
The first case of Devil Facial Tumor Disease (DFTD) in a Tasmanian devil was found in 1996. DFTD is a solid facial tumor that spreads from animal to animal through physical contact when devils bite each other. It was identified as a transmissible cancer after sequencing of the genomes of DFTD[3]. The fatal disease continues to spread through the devil population and threatens them with extinction.
A recent report identified, a second, apparently completely independent, lineage of DFTD (termed DFT2) in a small number of devils[4]. The original Devil Facial Tumor Disease is now termed DFT1.
[Healthy Tasmanian Devil pup]

[3] Bivalve Transmissible Neoplasias
The finding of transmissible cancer in soft-shell clams and some evidence from disseminated neoplasia in mussels suggested that it could also be transmissible[5]. Recently, disseminated neoplasia was found in mussels (Mytilus trossulus), cockles (Cerastoderma edule) and golden carpet-shell clams (Polititapes aureus)[6].
As multiple lineages of transmissible cancers are spreading through multiple bivalve species, scientists decided to term these diseases 'bivalve transmissible neoplasias' (BTN).

[1] Metzger, Goff: A Sixth Modality of Infectious Disease: Contagious Cancer from Devils to Clams and Beyond in PloS Pathogens – 2016
[2] Murchison et al: Transmissible dog cancer genome reveals the origin and history of an ancient cell lineage in Science – 2014
[3] Murchison et al: Genome sequencing and analysis of the Tasmanian devil and its transmissible cancer in Cell – 2012
[4] Pye et al: A second transmissible cancer in Tasmanian devils in Proceedings of the National Academy of Sciences – 2016
[5] Carballal et al: Neoplastic diseases of marine bivalves in Journal of Invertebrate Pathololy – 2015
[6] Metzger et al: Widespread transmission of independent cancer lineages within multiple bivalve species in Nature - 2016

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