A Sixth Modality of Infectious Disease: Contagious Cancer from Devils to Clams and Beyond.

Infectious agents come in many forms, but they have been grouped into five distinct classes of agents: viruses, bacteria, fungi, parasites, and prions. Cancer is not normally on this list. Infectious agents like Human papilloma virus (HPV) or Human T-lymphotropic virus type 1 (HTLV-1) can cause cancers in infected hosts, but these cancers are generated within each new individual from oncogenic changes within the hosts’ own cells, and they stay within that individual. If cancer cells did travel from one individual to another, a normal immune system would be able to recognize them as foreign and reject them. Cancer is thus usually a self-limiting disease—it either regresses or it kills its host, and the death of the host marks the death of the cancer lineage.But this is not always the case. Transmissible cancers have been identified as spreading within two vertebrates, dogs (Canis lupus familiaris) and Tasmanian devils (Sarcophilus harrisii), and more recently, multiple independent lineages of transmissible cancer have been found in four species of bivalves (Fig 1). This is an infectious modality that has significant effects on animals in both the terrestrial and marine environments, as well as in both vertebrates and invertebrates. While large-scale transmission of cancer has not been observed in humans, transmission between humans has been observed on a small scale in a number of circumstances, often in the context of immune suppression. With more research, more cases are likely to be found in humans as well as other animals.Fig 1Comparison of the known lineages of infectious cancers in natural populations.