Good news for lovers of the world’s cutest thing-named-after-something-evil*!
I'm serious, whose idea was it to call this thing a devil?
Recent studies published in Science and Proceedings of the Royal Society B: Biological Sciences have shown promising results in the fight against a very unusual disease: Devil Facial Tumour Disease (DFTD). This is a pandemic occurring in Tasmanian devils, the world’s largest carnivorous marsupial, and until very recently looked to be almost certain to drive them to extinction.
This disease was first observed by humans in the mid-90s, and in the mid-2000s it was shown that DFTD was actually a form of transmissible cancer. This terrifying idea: a lethal cancer which is highly contagious, is thankfully rare: there are known cases of humans transmitting cancer to each other but the circumstances leading to this happening are extremely specific and even then actual transmission is highly uncommon. It’s most commonly observed in nature in dogs (as a nonfatal venereal disease), and as DFTD, though other species do exhibit transmissible cancers. The circumstances leading to transmission of DFTD on the other hand, are extremely common in the Tasmanian devil community: biting. Humans famously don’t bite each other very much, but in devils this behaviour happens all the time, most commonly during mating and contesting for food. But really, what does contagious cancer really mean?
Contagious cancer: the bad news
It’s important to understand a generalised definition of cancer here. The google definition of cancer is ‘a disease caused by uncontrolled division of abnormal cells in a part of the body’, along with the blunt example sentence of “he’s got cancer”, and that seems to me just fine as a description. Of course, there are many kinds of cancer caused by all kinds of different things, but this is what they tend to have in common: they’re caused by cells within the host’s body multiplying at an abnormal rate. Typically, cell division is a very controlled process, and most cells have programmed deaths in order to not cause too much cell build-up in an area. When DNA mutations cause this cell division to become uncontrolled, growths of abnormal cells begin.
Transmissible cancer is similar in theory, but to me is best understood as a virus: all DFTD cells are believed to have come from one originating tumour which began as a cancer in an original host devil some years ago. The carrier for this tumour passed it to another devil, which in turn kept passing it along. We know this because the cells found in all DFTD tumors are more similar in terms of DNA to each other than any of them are to the DNA of their host organism. So, what makes this kind of cancer contagious exactly? Well, it’s a little to do with the cancer, and a little to do with the devil. Tasmanian devils have been noted to be extremely non-diverse with regard to genes known as MHC genes, which play a part in immune responses. These genes, common amongst all vertebrates, are the most diverse genes within species with regards to actual genetic sequence. This is helpful for immune responses, as difference in MHC genes allows immune systems to detect foreign cells by differences in MHC genes, and so can easily reject the cells. However, as Tasmanian devils are so non-diverse for these genes, the cancer genes aren’t recognised as foreign, and are accepted as a part of the individual’s system. Yikes.
The good news
Three factors appear to be ‘flattening the curve’ of Tasmanian devil deaths from DFTD. The first of these is related to population density: as this gets lower, devils are increasingly less likely to come into contact with one another. The second is behavioural: infected individuals withdraw from contact as they become sicker, making them less likely to infect others. Finally, a genetic factor appears to be at play: there are a small number of genes in Tasmanian devils which have a heavy influence over their susceptibility to DFTD, and these genes appear to be making the surviving population more and more robust against the sickness. The devils looked set for an untimely extinction as little as 10 years ago, but this now appears unlikely. With any luck, their numbers will continue to stabilise and we can continue gawking at these native marsupials for many years to come.
*This is a completely made-up category and awkward turn of phrase and I didn’t stop to think other things named after evil things might be cuter. Though now I am trying to think of some and I’m coming up blank. Let me know of any contenders you feel were robbed!
Read more:
https://science.sciencemag.org/content/370/6522/eabb9772 - Paper suggesting genetic changes influencing survival
https://royalsocietypublishing.org/doi/10.1098/rspb.2020.2454 - Paper showing behavioural changes influencing survival
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1999395/ - MHC diversity and its influence on transmissibility of cancers
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3228048/ - A little on contagious cancers
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4698198/ - A little more on contagious cancers
This is a bit of a special case, as it's a foreign body entering the devil that the animal can build a tolerance to. It'd be like human populations developing genetic resistances to fatal diseases because the people who didn't have the resistance genes died before they could have children. In that sense it's quite unlike a cancer in humans, which comes from mutations in genes we already have.
Important also to note that 'building a tolerance' to something on a genetic level happens to a population, not an individual. In terms of what we can (and are legally able) to do with genetic medicine these days, an individual can't become more genetically resistant to something, but over the course…
When you say making more robust against the sickness, is that the same as building a tolerance to it? Can your genes build a tolerance to cancer?