The Tasmanian Devil Bites Back

Tasmanian devils are rapidly evolving resistance to deadly facial tumours, changing the way we understand cancer.

Devil Facial Tumour Disease

The picture that Looney Tunes painted of Tasmanian devils as ferocious, aggressive creatures is not far from the truth. Unfortunately, it is the same extreme aggression that made Tasmanian devils famous in the western world that has led to their rapid demise. Once widespread across Australia, the species has declined by around 80% and is now confined to several small populations around the island of Tasmania (1). This decline has been attributed to a contagious cancer known as Devil Facial Tumour Disease (DFTD). DFTD causes horrendous facial and throat tumours, leading to asphyxiation, starvation and in most cases death. The devils’ fierce nature has facilitated the rapid transmission of DTFD as their mating and fighting involves violent biting, allowing the transfer of living cancerous cells (2).

taz-devil

Figure 1. Image source: public domain

DFTD is one of only three contagious cancers known to science. The other two occur in dogs and clams; however the virulence and severity of these cases is far less than in Tasmanian devils. In fact, in 2008 Tasmanian devil populations were described as being endangered with extinction. What is unique about DFTD is the speed at which it has spread, reducing populations by up to 90% in some areas in just 20 years (3).

Why should we care?

As the world’s largest marsupial carnivore, the Tasmanian devil is an iconic species that shapes the ecosystems it inhabits.  Without it, carcasses would be left to rot and potentially spread disease to humans and wildlife. A 2013 study reports an increase in feral cats and decrease in native eastern quolls following declines in Tasmanian devil populations (4). DFTD puts not only the Tasmanian devil, but also the entire Tasmanian ecosystem at risk.

mapFigure 2. Map showing the widespread distribution of DFTD cases in Tasmanian devils. Data obtained from Save the Tasmanian Devil Program

Scientific study

Scientists have recently been able to study Tasmanian devils’ genetic information in attempt to understand DFTD further. They discovered in 2014 that two types of cancer exist, both causing facial tumours. These originated from separate outbreaks in two different Tasmanian devils (5).

Further study revealed that three separate populations of Tasmanian devils have built-in defenses against DFTD (1). Upon analysing samples, scientists found that devils tested after the emergence of the disease have more associations with cancer defense and immunity than those sampled before DFTD became widespread. This suggests that in those three areas, Tasmanian devils protected against DFTD will reproduce and pass this defense on to offspring. This will lead to an overall increase in disease-resistant Tasmanian devils.

The speed of DFTD resistance evolving in only 4-6 generations has never been documented in a wild population, and thus impacts upon the way we view cancer outbreaks (1).

angry-devil

Figure 3. The Tasmanian devil’s aggressive nature has facilitated the spread of DFTD. Image source: H.K Colin, public domain

New implications for cancer biology

The unanticipated discovery of a second type of transmissible cancer in Tasmanian devils (5) suggests either:

  1. Transmissible cancer is more common than we previously thought

In humans, cancer has been known in rare cases to transfer during pregnancy or organ transplants. This study suggests the alarming potential for more types of transmissible cancer to emerge, potentially in our own species.

  1. Tasmanian devils are particularly at risk of developing these kinds of disease

As Tasmanian devil populations reduce, they go through what is known as a bottleneck event, where individuals become almost ‘clones’ due to inbreeding. This, combined with their frequent aggressive interactions may have enabled the cancer to reach such unusually high proportions, and drive the devils’ unexpected resistance.

sad-devil

Figure 4. A Tasmanian devil with a relatively mild case of DFTD. Image source: Rodrigue Hamende, public domain

 A glimmer of hope… or not?

The realisation that some Tasmanian devils are becoming resistant to cancer presents some hope for these fierce creatures. An effective vaccination against DFTD may prove successful, particularly for captive populations which could be bred and be re-released into the wild.

 However, although promising, DFTD resistance may not be enough to ensure the entire species’ success in avoiding extinction. Many wild populations are still succumbing to DFTD and the disease continues to be widespread in all corners of the island of Tasmania. It is likely that a new type of DFTD could rapidly evolve and threaten resistant Tasmanian devils, as previously witnessed.

The future – what is next for the Tasmanian devil?

The future may not be so bleak for the Tasmanian devil. The government-funded Save the Tasmanian Devil Program set up in 2003 has established an insurance population to secure the species against extinction while monitoring wild populations. Many smaller complimentary projects are in place for Tasmanian devil conservation. These include reducing the number of deaths by road accidents and controlling the reproduction of diseased females through contraception.

DFTD is a unique disease sparking innovative research and fascination among scientists and the media. A better understanding of DFTD and the loss of Tasmanian devils on the island ecosystem is required.  This information can then be applied to other worldwide emerging infectious diseases.

References

  1. Epstein, B., Jones, M., Hamede, R., Hendricks, S., McCallum, H., Murchison, E. P., Storfer, A. (2016). Rapid evolutionary response to a transmissible cancer in Tasmanian devils. Nature Communications, 7, 12684.
  2. Murchison, E. P. (2008). Clonally transmissible cancers in dogs and Tasmanian devils. Oncogene, 27 Suppl 2(S2), S19–S30.
  3. McCallum, H., Tompkins, D. M., Jones, M., Lachish, S., Marvanek, S., Lazenby, B., Hawkins, C. E. (2007). Distribution and impacts of Tasmanian devil facial tumor disease. EcoHealth.
  4. Hollings, T., Jones, M., Mooney, N., & Mccallum, H. (2014). Trophic Cascades Following the Disease-Induced Decline of an Apex Predator, the Tasmanian Devil. Conservation Biology, 28(1), 63–75.
  5. Pye, R. J., Pemberton, D., Tovar, C., Tubio, J. M. C., Dun, K. A., Fox, S., Woods, G. M. (2015). A second transmissible cancer in Tasmanian devils.
Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s