The current state of knowledge regarding the origins of cancer is that it arises in people and animals due to irreversible DNA damage and uncontrolled cell replication.
These rogue cells disregard the signals that provide essential checks and balances to cell growth, replication and death.
Ultimately, cancer cells accumulate mutations on their road to immortality, invade adjacent tissues, spread to distant organs and grow rampantly. Without medical intervention, many types of cancer are a death sentence.
But there is another, lesser-known type of cancer that has achieved immortality and not just in a single host. These cancers are contagious — they readily spread between unrelated animals through direct physical contact and are just as deadly.
Unlike run-of-the-mill cancers, transmissible cancers outlast the original host by decades, if not centuries, by spreading from host to host like a fleshy parasite.
So-called normal cancers cannot survive if passed between individuals due to the immune system. The only exceptions include rare accidents like cancerous cells hiding in a transplanted organ.
One example of the rare contagious cancers is canine transmissible venereal tumour (TVT). As the name suggests, cancerous cells are transmitted through sexual contact between individual dogs and tumours grow on the genitals.
It can also affect wild members of the canine family including wolves, coyotes and jackals.
No matter where in the world you look, affected dogs have nearly identical cancer cells. This suggests a long-term co-existence between the cancer and its canine hosts.
Indeed, scientists estimate this disease originated 11,000 years ago, making these cancer cells the oldest known cell line.
As domestic dogs spread around the world with their humans, so did the tumours. Many of you have probably not heard about this condition because it is extremely rare in North America due to the widespread accessibility of veterinary care as well as the chemotherapy treatment.
Dogs in developing countries are less lucky, but the tumours often spontaneously regress and only rarely cause death.
Another puzzling transmissible tumour occurs in endangered Tasmanian devils.
The tumour cells are spread between these pint-sized marsupials through biting. Living up to their name, aggressive interactions between devils are frequent and exuberant during mating and eating activities.
The tumour cells are deposited into skin wounds and start their deadly cycles of replication and growth. The facial tumors affecting devils can grow to such an extent that they impair eyesight, breathing and eating, ultimately killing the animals.
Unlike the canine TVT, devil facial tumour emerged recently and quickly kills affected individuals, pushing this creature toward extinction.
Another example is a newly discovered transmissible leukemia in several species of shellfish.
This is the first example of a transmissible cancer in a marine ecosystem and also the first to jump between species.
Environmental issues including pollution and habitat loss may have caused this deadly white blood cell cancer to emerge.
These are the only three naturally occurring examples of transmissible cancers ever discovered in nature.
Of these, canine TVT and devil facial tumour occur in animals that have a relatively narrow genetic pool. Even dogs that look as different as great danes compared to chihuahuas share remarkably similar DNA codes.
The Tasmania devil also has an extremely limited genetic pool. Scientists believe the genetic similarity between individual animals allows the transmissible cancer cells to invade and start growing long before the host’s immune system realizes there is a problem.
Learning more about these biological outliers may provide insight into how cancer originates and how it interacts with its host’s immune system.
Perhaps there are other transmissible cancers lurking in nature that await discovery.