Tasmanian devils are threatened by one of the craziest pathogens I’ve ever heard of: an infectious cancer. You might be wondering, “How the heck can a cancer be infectious? Shouldn’t the host’s immune system recognize the parasitic cells as foreign and kill them?” Well, yeah, it should. But Tasmanian devils have very low genetic diversity in their Major Histocompatibility Complex (MHC) genes, a group of immune system genes associated with recognizing foreign/non-self materials. As a result, Tasmanian devil immune systems can’t tell that the parasitic cell line is different from the animals’ own cells. Major bummer!
So, how is this infectious cancer transmitted among Tasmanian devils? Well, you’ve probably heard that devils are very aggressive: males fight over females during the breeding season and females will fight to defend their dens. There is also fighting over carcasses. So, there’s lots of biting happening among devils, and many of these bites occur on the devils’ heads. And guess what? The infectious tumors usually occur on the faces of devils, which is why the disease is called Tasmanian devil facial tumor disease. So, people suspected for a long time that biting and transmission were linked.
Ok, now for the cool part! Hamede et al. (2013) hypothesized that the devils with the most bites on their heads would be the most likely to become infected by Tasmanian devil facial tumor disease. However, they found the opposite trend! The devils with the most bites on their heads were less likely to become infected. Hamede et al. (2013) suggested that instead of an infected biter transmitting the infectious cells to an uninfected bite recipient (more bites on devil = higher infection probability), it may be that uninfected biters become infected after biting the tumors of infected bite recipients (more bites given to other devils = higher infection probability). Devils often have open mouth wounds and the tumors often start growing inside the oral cavity, and these observations support the idea of infection resulting from biting tumors.
Dominant individuals are probably more likely to do lots of biting (as opposed to receiving many bites), so it may be that dominant individuals have higher risk of infection. This is a really cool possibility, because it suggests that some hosts (in this case the dominant individuals) in a population are “super receivers” of infection. A lot of attention has been given to disease super spreaders, like Typhoid Mary, and the super receiver concept is a neat addition to our understanding of heterogeneity in pathogen transmission rates.
Finally, I just want to point out that not all infectious cancers are transmitted by biting. For instance, there’s an infectious cancer of canines that is sexually transmitted. Crazy!
Hamede, R.K., H. McCallum, and M. Jones. 2013. Biting injuries and transmission of Tasmanian devil facial tumour disease. Journal of Animal Ecology 82: 182-190.