This week, a healthy population of endangered Tasmanian devils was found in a remote part of their homeland. Why is this important news, and why are they in trouble?
What is a Tasmanian devil, and what has happened to them?
The Tasmanian devil (Sarcophilus harrisii) is a marsupial carnivore endemic to the Australian island of Tasmania. Historically widespread across the mainland, competition from the introduced Dingo has isolated them to Tasmania for the last 400 years. Although once bounty hunted for supposedly hunting livestock, they have been protected since 1941 to avoid repeating the mistakes that lead to the extinction of the Thylacine, or Tasmanian tiger. Once common, Devil populations have fallen by 80% in 20 years due to a highly unusual disease first observed in 1996, involving large, horrifying tumours in and around the mouth and on the head and neck. This is what is known as ‘Devil Facial Tumour Disease’ (DFTD). Tasmanian devils are now endangered, and this bizarre and poorly understood disease has driven them to the brink of extinction.
How does DFTD work?
Like many cancers, DFTD tumours grow from a single cancer cell which divides uncontrollably, which can then spread to other parts of the body and form new tumours, impairing the functioning of affected organs. This is known as metastasis. The exponential division occurs due to abnormalities in cancer cell cycles. It has been debated how cancer cells do this; they may divide independently from the growth factors normal cells require, or may simply be able to produce their own growth factors. Alternatively, inhibitors which stop cell cycles may themselves be inhibited. The unpredictable nature of the spread of cancer makes treatment difficult.
‘Devil Facial Tumour Disease’ has a highly aggressive prognosis. Dramatic oral swelling prevents individuals from eating until they succumb to starvation. Tumours also spread to the lymph nodes and lung epithelial cells, causing respiratory problems. Death usually comes within 6 to 12 months, and the mortality rate is 100% once symptoms are visible. Since 1996, devil numbers have dropped by 70% across Tasmania, and up to 90% in the regions where the disease originated.
To make matters worse, DFTD is one of very few known contagious cancers. Other examples include a sexually transmitted cancer in domestic dogs and a leukaemia-like disease in shellfish. Each type has different transfer mechanisms, although, as a general rule, they are spread by the physical transfer of cancer cells from the infected to new hosts. The cancer then begins independent runaway evolution as a virus-like, free-living asexual ‘parasite’. How and why this happens is not yet understood.
In Tasmanian devils, the disease is spread via transmission of living tumour cells from host to host through biting, which unfortunately plays an important social role during mating and fighting. DFTD is contagious as soon as tumours are exposed. Strangely, all tumour cells found in the Devils are genetically identical to that of the first individual that died from the disease, a relic from the first mutation that created it. This classifies DFTD as a ‘mammalian parasitic disease’. Complicating things further, a second form (DFTD2) was discovered in 2016, suggesting that Tasmanian devils are uniquely unlucky in being particularly susceptible to contagious cancers, as this second form has an independent origin to the first.
This may be because, like many animals that exhibit contagious cancer, Tasmanian devils have low genetic diversity from a prehistoric bottleneck, worsened by persecution from European settlers. This generally hinders their adaptability, and their lack of diversity in almost all immunological genes makes them highly sensitive to all kinds of diseases. Seemingly, all populations share similar MHC genes, which are involved in differentiating between ‘self’ and ‘non self’ (antigens) which allow the immune system to identify and attack infections and not their own cells. There is normally no immune response against the cancer cells, as they are Tasmanian devil tissue. DFTD has also evolved to down-regulate the presence of surface MHC complexes, making it even harder for the devil’s already ineffective immune system to combat the disease.
How is it being combated?
Despite the harrowing nature of the disease and its seemingly unstoppable spread, there are signs of hope.
In the wild, certain populations show signs of evolving resistance in response to the cancer’s strong selection pressure, as some populations may be resistant due to differences in MHC complex, or another gene. One hypothesis is that a less aggressive strain of the disease could become dominant, in the same way a similar infection in dogs eventually coevolve to become non-fatal.
Although developing a successful vaccine for cancer has yet to be achieved for any organism, it is theoretically possible to develop one by artificially recreating MHC complexes on tumour cells so that the immune system can identify them, though this would still be incredibly difficult and risky to implement in an endangered, wild population. Immunisation trials using dead tumour cells showed great promise, and 39 vaccinated devils released into the wild developed immune responses.
As for treating the infected, a preliminary experiment on 5 of Devils found that administering grafts of DFTD with MHC complexes induced immune responses, including the disease not taking hold in one individual, and immunotherapy caused the tumours to reduce. Research published this month that mapped the origins of DFTD suggested targeted therapy for tumours is a possibility.
Several solutions attempted by the Tasmanian government and NGOs have prevented the 2011 prediction that the disease could spread across the entire population within 5 years from becoming a reality. Selective culling proved ineffective, only taking the place of disease-related deaths. If efforts to save the devils in the wild fail, zoos and reserves like Healesville Sanctuary run a sustainable captive breeding program of healthy individuals for later reintroduction, possibly not until after the disease has run its course. Establishing populations on Tasmania’s neighbouring islands have been proposed, although introducing new species has proven throughout history to be harmful to local ecosystems, so may not be desirable.
The discovery of 14 healthy individuals in the remote Southwest of Tasmania this week is a highly promising sign – not only are they cancer free, but they appear to have lived unaffected for several years over a large range. Keep an eye out for the implications and details of this discovery in the coming months.
Despite a seemingly unstoppable pandemic, Tasmanian devils were more secure than they had been in a decade in 2015. Nevertheless, the species is in a precarious position. The plausibility of a treatment remains uncertain, and without extensive support to conservation and scientific research, the cancer could still wipe out the world’s last great marsupial carnivore.