Tuesday, October 11, 2011

The Tasmanian Devil's Dilemma

Because I couldn't resist an excuse to take a break from schoolwork to go look at lots of pictures of Tasmanian devils...

The Tasmanian devil went from being a populous species to being an endangered one within the course of a little over a decade (1996 to the present), being declared endangered in May of 2009. Unlike many other cases, the cause of this is not human activity; it is disease.

Tasmanian devils (simply called ‘devils’ in the literature—who says scientists don’t have a sense of humor?) are the largest extant carnivorous marsupials since the thylacine went extinct in the twentieth century. They are, as their name suggests, native to Tasmania. They are also adorable. Before 1996, devils were widespread and quite common, (despite extensive human effort) with an estimated population size of around 150,000.

In 1996, a devil with facial tumors was photographed around Mt. Williams, Tasmania. During the next five years, numerous devils were observed to have similar tumors, and the population declined dramatically. Studies revealed that the cause of these tumors was an infectious cancer, quickly dubbed Devil Facial Tumor Disease (or DFTD). Infectious cancers are odd but do exist; the two other documented cancers of this sort affect dog genitals and inbred hamsters, respectively. Only the cancer affecting dog genitalia is known in the wild.

DFTD is invariably fatal. The cancer is transmitted when an infected devil bites another, more or less injecting cancer cells from its own mouth into the face of the other devil. Tumors soon develop. Though the cancer metastasizes rapidly, this is not the cause of fatalities; the facial tumors themselves are so large and painful that the animal soon is unable to eat and starves to death. There is no immunity within the devil population.

Since Tasmanian devils' courtship involves rather a lot of biting, the spread of DFTD has been rapid. Estimates of the species’ decline range up to 70% (O’Neill 2010). Indeed, the decline has been so dramatic that it has begun to affect the age at which females are breeding; the usual age before the advent of the disease was about 2 years, (with an expected lifespan of 6 years), but now, females have been observed breeding as young as 1 year of age. The rapid progress of the disease means that few females survive more than one breeding season.

DFTD is not the only problem the devils have to deal with. Aside from human activity, Tasmania has a problem with invasive red foxes, which prey upon smaller marsupials and compete with the devils. They are also thought to predate on juvenile Tasmanian devils. It has been proposed that the large devil population prior to 1996 helped prevent fox invasion, and the rapid decline facilitated the current fox population explosion.

In an effort to control this, the Tasmanian government started an eradication program, putting out poisoned bait for the foxes. The only problem was that the Tasmanian devils seem to find the bait just as interesting as the foxes do. The current suggestion to remedy this problem is to bury the bait at a depth of 15 centimeters, but this only dissuades the devils; it is not a reliable solution.

Multiple efforts are ongoing to try to save the species. Isolating uninfected devils, captive breeding and even measures so drastic as culling infected individuals, and storing sperm from deceased individuals have all been suggested or are underway.

((For those of you who are curious as to what the effects of the disease look like, here’s Wikipedia’s page image. Warning: Not for the faint of heart. Or those thinking about eating in the near future. http://en.wikipedia.org/wiki/File:Tasmanian_Devil_Facial_Tumour_Disease.png))


Hughes Channing; Gaffney Robbie; Dickman Christopher R. A Preliminary Study Assessing Risk to Tasmanian Devils From Poisoning for Red Foxes. Journal of Wildlife Management, volume 75, issue 2, pages 285-392, February 2011

Keely, T; McGeevy, P.D.; J. K. O’Brien. Characterization and Short-Term Sperm Storage of Tasmanian Devil Sperm Collected Post-Mortem. Theriogenology, volume 76, issue 4, pages 705-714. September 2011.

Lachish Shelly; McCallum Hamish; Mann Dydee; et al. Evaluation of Selective Culling of Infected Individuals to Control the Tasmanian Facial Tumor Disease. Conservation biology, volume 24, issue 3. Pages 841-851, June 2010

McCallum, Hamish, Tasmanian devil facial tumour disease: lessons for conservation biology, Trends in Ecology & Evolution, volume 23, issue 11 pages: 631-637, November 2008

O’Neill, Iain D. Tasmanian Devil Facial Tumor Disease: Insights Into Reduced Tumor Surveillance From an Unusual Malignancy, International Journal of Cancer, volume 127, issue 7. Pages 1637–1642, October 2010

Lachish, Shelly; McCallum, Hamish; Jones, Menna. Demography, Disease and the Devil: Life-History Changes in a Disease-Affected Population of Tasmanian Devils (Sarcophilus harrisii), Journal of Animal Ecology, volume 78, issue 2 pages 427–436, March 2009


http://en.wikipedia.org/wiki/Tasmanian_devil ((I also nicked the image from here. Yay Wikipedia!))


Cassidy Anton said...

One of the most remarkable parts of this story to me is that it is a transferable cancer. I do not entirely understand what this means. Is this a cancer cell which is passed to another animal and then multiplies in the animal without being destroyed by antibodies upon arrival? Is it the same as a virus except it comes with its own set of functioning organelles? Finally, does this mean that human cancers if passed between individuals could become contagious?

John Latto said...

Yes. No and Not likely.

Yes - it is actually the cancer cells that are passed from one individual to another and then these cancerous cells grow in the new individual! This is only possible because of low diversity in MHC genes in this species (essentially the genes that identify 'self' to the immune system). Presumably a liver or heart transplant from one devil would be easily accepted by another (compare this to humans).

No, it's not really like a virus (although some viruses are capable of inducing cancer). A virus infects host cells in each host individual. Here the host is infected by cells from another devil.

And finally, no, this seems unlikely for human cancers because a) we don't have such aggressive interactions with other individuals (when was the last time you bit somebody's head? Mike Tyson excepted..) and b) human's have much more variation in the MHC gene so a cancer cell from another individual would almost certainly be recognized as 'foreign' and attacked by you immune system in a way that your own cancerous cells, sadly, sometimes are not.

If you think this transmissible cancer is weird you are correct. It is. Very weird.

Anonymous said...

a new paper proposes that the tumor may represent cancer stem cells - could be very interesting
See http://www.ncbi.nlm.nih.gov/pubmed/21956952
though this one isn't free like his other paper :o(