As I mentioned, my research focuses on emerging infectious diseases--that is, diseases that have been newly discovered, or are expanding in geographic range, or perhaps moving into new populations. Typically when we see organisms in that first group--novel to man--we think of bacterial or viral pathogens, such as SARS or E. coli O157:H7. However, sometimes things get even more weird, such as the strange case of the contagious Tasmanian devil facial cancer.
It's long been known that viruses can cause cancer. The first "cancer virus," which caused sarcoma in chickens, was discovered in 1911 by Peyton Rous, who later won the Nobel Prize for his work. Since this time, we've seen that viruses such as Hepatitis B and the human papilloma virus (HPV) can also cause liver and cervical cancer, respectively---so the idea of cancer caused by an infectious agent isn't new.
However, things are much different regarding the cancers in Tasmanian devils. First, the infectious organism isn't a bacterium or virus at all--it's a cancer cell that originated in a Tasmanian devil, and then took on a life of its own as a distinct pathogen, transmitted between animals as they fight and bite each other around the face and mouth. These wounds provide an opportunity for direct inoculation of the cancerous cells into the bloodstream of the next animal, allowing these transmissible tumor cells to find a home and proliferate, seeding the new animal with the disease. First seen in 1995, this highly lethal pathogen has since spread widely, and is almost 100% fatal. Death comes either from metastasis of the cancer throughout the body, or more commonly, because of starvation as the mouth tumors make it impossible for the animals to eat.
A new report in the Proceedings of the National Academy of Sciences notes that one reason these tumor pathogens have been able to spread so widely in the Tasmanian devil population is because these animals have little genetic diversity. The animal where the cancer originated was genetically very similar to all other animals in the current population--and so none of them are recognizing the tumor cells as "foreign" when they're infected. Typically, when one is infected with a pathogen, the body's immune system reacts in a number of ways to eliminate the foreign substance. However, in this case, because the devils are all so genetically alike, the immune system thinks the tumor cells are simply a normal part of the body--so they can proliferate unchecked.
This is potentially very bad news for the Tasmanian devils. Because these cells are so like their own, something like a vaccine won't work to stem the spread of disease. Rather, a better way to protect uninfected devils is to remove a cancerous animal from the population as soon as they're seen to be diseased. And while the issue of this particular cancer is unique to Tasmanian devils, other species of animals are also at risk because of their low genetic diversity:
"What also worries me is that many other wildlife populations are going through similar bottlenecks - koalas on Kangaroo Island, platypuses on King Island," she said.
"Loss of genetic diversity in these genes just opens the door for emergence and rapid spread of new and old disease."
For the devils, this spreading infection puts the entire species in peril:
Once the cancer becomes visible and spreads internally through the body, the animal usually dies within a few months from starvation and the breakdown of body functions.
Scientists predicted at a devil forum in Hobart earlier this year that the species faced extinction within 10 to 20 years at the current rate of decline.
Reference
Siddle et al. 2007. Transmission of a fatal clonal tumor by biting occurs due to depleted MHC diversity in a threatened carnivorous marsupial. PNAS. Link.
