Transmissible tumours in Tasmanian devils

These mostly solitary furry creatures are passing deadly diseases between themselves...
11 April 2022

Interview with 

Hannah Siddle, University of Southampton & Healesville Sanctuary at Zoos Victoria

TASMANIAN DEVIL

tasmanian devil, Sarcophilus harrisii

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One animal that's experiencing a rare cancer outbreak are Australia's Tasmanian devils. This disease, which only appeared for the first time in recent decades, has pushed the species to the brink of extinction. Threatened species keeper Ben from the Healesville sanctuary at Zoos Victoria in Australia explains how they're helping to save the Tasmanian devil to Julia Ravey, and Hannah Siddle from the University of Southampton talks through what has led to the devils decline in numbers…

Chris - Cancers are made from a person's own cells going rogue. If a cell acquires the right combinations of mutations or changes in its genetic code, it can divide uncontrollably and lead to the formation of a tumour. As these cells contain a unique individual's DNA, if they end up in another individual's body, it should sound the immune alarm, just like an Incompatible organ transplant. They should be recognised as foreign and destroyed. Now some viruses, which are of course also infectious, like the human papilloma virus or HPV, can also cause cancer. But this is because they can trigger changes in a cells genetic code that leads to tumour formation. And in these sorts of cases, the virus is infectious rather than the cancer itself. In the very rare instances of contagious cancers, though, it's the cancer cells themselves that are passing from one individual to another. And that's exactly what we're gonna be exploring this week. To emphasise, these cancers haven't been observed naturally yet in humans, but they have been spotted in other animals and one animal that's experiencing a rare cancer outbreak are Australia's Tasmanian devils. This disease, which only appeared for the first time in recent decades, has pushed the species to the brink of extinction. Threatened species keeper Ben from the Healesville sanctuary at Zoos Victoria in Australia explains how they're helping to save the Tasmanian devil to Julia Ravey during, well, feeding time at the zoo...

Ben - So today we're going to feed some of our Tazzy devils. All our food is labeled with little tags so we know whose food is whos because everyone has different weights. So again, we're doing a scatter feed here -we throw foods throughout their habitat. We spread it around so that they have to go forage for it. Some of the devils over here are pretty shy. Tasmanian devils don't really, truly follow their name and they're actually very timid by nature. Tazzy devils are threatened species. So with some of our breeding females that we're hoping to breed this season, we've got camera footage monitoring them feeding. So we just make sure that's turned on every night. And we check that every day just to see if the females have eaten overnight. The main reason we need to check this is because if they're going through oestrus, they don't eat their food as often, or as frequently as normal. And that way we can tell that they are going through their oestrus, and we can start to prepare for breeding.

Julia - Sanctuaries like the Healesville sanctuary supported by Zoos Victoria in Australia are playing a vital role in helping the Tasmanian devil avoid extinction through their breeding programs. These animals are also being released into the wild on islands and mainland Australia to up their numbers. This is because their numbered sightings have dropped by 80% in the last two decades and as Ben said, they are officially an endangered species. Hannah Siddle, an assistant professor in molecular immunology from the University of Southampton, explains what has caused this drastic decline...

Hannah - The majority of the decline has been caused by what we call DFT1 which is devil facial tumour disease. And that is the tumor that emerged sometime just before 1995. We do now have a second contagious cancer in the Tasmanian devil population termed DFT2. And it is now responsible for some declines, but in a very limited geographic area where it's still located.

Julia - And you say this is a contagious cancer. So how does it spread from one animal to another?

Hannah - It's a little bit like for us a virus or bacteria, because it can pass when the devils contact each other. Rather than being a virally induced tumour, this is actually the cancer cells themselves that are able to pass between individuals. And we think in the case of the Tasmanian devils, that this is primarily occurring when they bite each other, which unfortunately they do tend to do when they're fighting and in the mating season as well. Our best evidence at the moment is that these cells are actually grafting into the next devil when this biting behaviour occurs. And once these tumour cells then seed into the next individual, they grow and establish and make a new tumour, which can then be passed on to the next individual.

Julia - That's rare, isn't it? Because cancer is made up of a unique individual cells that have gone rogue in a way. So what is it you think that makes this cancer able to transmit in a population?

Hannah - We think that these tumours are taking advantage of the fact that although genetic diversity is not so low that these animals are clonal or exactly the same, they do have a lower level of genetic diversity than in populations that have more healthy numbers. We do think that these tumours may be taking advantage of this fact, and that may be allowing them to initially start to spread between a few individuals. After that though, the idea that there is not enough genetic diversity in the population and so this is allowing the tumour to take hold, that idea breaks down a little bit because we do have some genetic diversity, and enough genetic diversity in the population to stop the tumour spreading. But tumours are very good at acquiring new mutations and they adapt to their environment. And so they will acquire adaptations that are then helping them to then passage between individuals. And we have found that one of the adaptations that they have made, these tumours, is to lose some of the proteins or what we call antigens on the cell surface of the tumours, that are usually a flag for the immune system and would stop the tumour from transferring between individuals. This adaptation then allows the tumors to essentially become silent, or invisible in the population.

Julia - Are the cells in this tumour the same in every individual infected?

Hannah - Essentially, yes. So when we say they're the same, they all have derived from a single ancestral tumour cell, which is a cell in a Tasmanian devil that became malignant a long time ago. The tumours that are currently circulating in the Tasmanian devil will be slightly different to what they were when that tumour first arose and started transferring because they've acquired new genetic mutations, new adaptations to their environment.

Julia - You mentioned that there are two of these contagious cancers in Tasmanian devils. So what's happened there? Is this the same disease, but it's just a really mutated version, or is it something new entirely?

Hannah - It is something new entirely. It arose in a completely independent manner from a different individual, this second tumour. Really, really unusual to have that occur. And actually this is the only mammalian species where we know that that's occurred. We think that the second tumour arose sometime around 2014. And interestingly, we think we've been tracing this from nearly the beginning of its genesis. There is a lot of effort going into trapping Tasmanian devils in the geographic location of DFT2, which is in the Southeast of Tasmania and it's quite confined at the moment. Although recent efforts have shown that it is moving northwards, and so it is starting to spread, unfortunately, and become more prevalent.

Julia - It seems like an unusual phenomena for this to happen twice in one species. Is this a case of lightning striking twice in the same place or are these animals, do you think just susceptible to these types of conditions?

Hannah - I think it probably is lightning striking twice, as you say, but I think we need to probably be more aware in wildlife generally. Possibly these tumours are arising at different times, but that they don't have the success that DFT1and DFT2 have had in the population. So I think it's probably likely that these tumours do arise sometimes.

Julia - With communicable diseases in humans, we aim to treat them, we aim to develop vaccinations or other medical treatments. It's anything in the line like that for Tasmanian devils? Will there be a vaccine for these tumours, do you think?

Hannah - Transmissible tumours should be, not easy to vaccinate against, but they should present some sort of routes for vaccination. And certainly we've been able to show that if we can restore some of these antigens that I was talking about before that usually coat the tumour cells, then if you put those into a Tasmanian devil or in a co-culture situation, which is what we call this, then we can actually see an immune response against these antigens on the tumour cells. So that tells us that the Tasmanian devil immune system is capable of responding. But it does seem as if this type of vaccine with these antigens on the surface of the tumour cells, it does work in some animals, but it doesn't seem to work in 100% of animals and we're not quite sure what the efficacy of that is and why it doesn't work in some instances. So I think we need to do just a bit more work to understand what it is that the immune system responds to and whether then we could make a more targeted vaccine based on that information.

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