I doubt that many people would expect to be reading about cancer in an infectious disease blog, and yet here you are. Could it really be that I’m about to tell you that cancer, one of the most feared diseases in the developed world, can be contagious?
The answer is yes. That said, you don’t need to panic about a Hollywood-style infectious cancer outbreak just yet. Although there are examples of human cancers accidentally being transplanted into a new individual (1), these are rare and barely deserve being described as ‘infectious’. It could of course be argued that there are many viruses which can cause cancer (in fact, infectious agents are thought to cause 16.1% of cancers!), for example human papilloma virus which causes over 90% of cases of cervical cancers (2). However, in this case I am talking about cancers which are infectious in themselves.
The unfortunate animals affected are the cute-yet-ferocious tasmanian devils. Now only found on the island of Tasmania (an Australian state to the south of the mainland), the population of these carnivorous mammals has become endangered with thanks to a rampant infectious facial tumour. The cancer is transmitted between individuals when they fight over food. The tumour grows and eventually prevents them from eating, starving them. For a long time it was not undertood how the cancer was able to spread – normally the immune system would recognise the cells that were from another individual and reject them (much like you often hear happening with transplants). What is different this time?
All normal cells express a class of molecule (known as MHCs) which lets the immune system know when a pathogen is in the cell. They are also the molecules which allow the immune system to spot foreign tissue, and are thereby the cause of transplant rejection. However, the cancerous cells don’t express these molecules, and so the immune system doesn’t respond to them (3). This means that when cancerous cells rub off onto another devil’s face, they are able to enter scratches and cuts and grow without being attacked by the immune system. Thankfully human cells are killed if they lack MHCs, so this should never evolve in human cancers.
- Gartner et al., (N Engl J Med, 1996) Genetic Analysis of a Sarcoma Accidentally Transplanted from a Patient to a Surgeon
- Center for Disease Control and Prevention
- Siddle et al., (PNAS , 2013) Reversible epigenetic down-regulation of MHC molecules by devil facial tumour disease illustrates immune escape by a contagious cancer