Table 1.

Transmissible cancers in animal species

		Transmission route	Immuno-avoidance	References
1.	CTVT (one clone)	Sexuala	Down-regulation of MHC	[82, 83]
2.	Tasmanian DFTD (two clones with sub-clonal divergence)	Bitinga	Down-regulation of MHC + inbred host	[84–86]
3.	Leukaemia in marine bivalves: soft shell clams mussels cockles (two clones) golden carpet shell clams (clone derived from another clam species)	Unknown, but suggested to be via filter feeding	?	[87, 88]
4.	Transmissible Syrian hamster reticulum cell sarcoma (multiple clones)b	?	?	[89, 90]

? Uncertain.

^aUnclear if transmission of cancer cells is via direct blood contact or via transfer of other fluids (saliva, seminal fluid).

^bThese early reports have not been followed up and so the status of this example remains uncertain.