But the Tasmanian devil isn’t being killed any of the deadly six.
Its possible extinction is at the hands of its own cells gone wrong.
It is suffering from an infectious cancer. – The Devil Facial Tumour Disease (DFTD).
Now some pathogens are known to cause cancers. Most notably, the Human Papilloma Virus (HPV) seems to be a trigger the vast majority of (if not all) cervical cancers (and other types too). Wonderfully, there is now a vaccine against the main forms of HPV which cause these cancers. And we can expect to see these cancers decrease significantly over the next few
decades (in most cases HPV doesn’t lead to cancer and when it does there is often a lag of decades before it does). Another score for medical science! (Yay Medical Science!)
And especially in countries like Australia where the HPV vaccination rates have been very strong since it started in 2007.
(Yay Australia!)
Devils. Tassy ones.
What causes their tumours?
Well tumour cells. - Well duh, of course.
But no. The tumour cells are the pathogens.
Genetic studies show that the tumours on the faces of these marsupial carnivores are not genetically identical to the rest of their faces, they show Tas Devil genes, but not necessarily the ones of the face they are growing on (ick).
In fact, and bizarrely, the tumours on all the devils are genetically identical to each other. They are clones of each other. What seems to have happened is that part of the tumour on the face of one devil has jumped ship and started to grow
on another’s.
But wait a minute. Won’t the immune system see these cells as "other"? Won’t they be killed off like a failed organ transplant?
You would think so wouldn’t you?
But no.
The surface markers that define a your own cells as “ you” are called the MHC markers and they are found on pretty much all your own cells. Other peoples organs get rejected because they are seen as “Not You” and attacked by the immune system.
We have a huge range of MHC types, meaning that almost all other people's organs will be seen as "other" and rejected. Few people match your MHC type and hospitals spend a lot of effort trying to ensure correct organ matches.
Tassy Devils have very few MHC types and it was assumed this was the reason why the tumours evade detection - almost all tassy's cells from different individuals look like "self". In fact about 80% of Devils have very similar MHC genotypes to the evil Tumour Cell Clones. But recently scientists have discovered that even Devils with quite different MHC groups seem to get infected. So WHAT IS GOING ON?
Rather nicely, the same researchers who proposed the MHC similarity explanation are the ones who also this year showed that it isn't the reason (or not the main reason) for immune evasion! Indeed they found that while the tumour cells have the genes for the MHC surface markers, they don't actually see the light of day (or the immune system), A couple of mutations in transport genes mean the MHC protein never gets to the cell surface and so they can never be checked by the immune system as self or non self. They are immune "invisible!"
So there you have it, an immunologically hidden pathogen of a very strange nature.
So what does this mean for Tasmania's Devils?
In some areas DFTD has reduced their numbers by 90%. Could this disease bring them to extinction in the wild?
Well the good news is that if it does, then the tumours die with them and maybe we could reintroduce captive bred Devils to replace them? Alternatively, maybe this new discovery as to why the cells are not seen by the immune system could be the springboard needed for discovering a vaccine which helps the immune system recognise maybe very low levels of the MHC molecule that might still trickle to the surface. Time and research by awesome scientists will tell! (Yay awesome scientists!)
Disclaimer: The editors of Fun Facts and Friends have included no pictures of the facial tumours as they are quite disturbing. You are all quite capable of using google images, but you have been warned!
We would also like to point out that gldfish have nothing to do with this Pathogenic factoid. Nor do they even connect to the analogy used. But we liked them.