Cancer can be very personal disease: our own cells turn against us and multiply out of control. But some cancers are contagious. These transmissible cancers can be thought of, not as an outgrowth of the afflicted animal, but as an independent organism.
For example, down in Tasmania, a horrible form of transmissible cancer, known as devil facial tumor disease, is bringing down the native devils. Those tumors, which sprout from devils' faces, are not the devil's own cells. Rather, genetic analysis indicates that the tumor cells line up with those of the first Tasmanian devil to ever contract the disease.
In new research, says the BBC, researchers have analyzed the genetics of the world's “oldest-known living cancer,” a strain of cancer cells that has been passing from dog to dog, sustaining itself, over the past 11,000 years.
Known as canine transmissible venereal tumour, says Nature, this disease spreads between dogs during sexual contact. Sampling and genetically analyzing tumor cells from dogs separated by half a world, researchers were able to track back the history of the cancer to its likely origin. Science magazine:
The genes of this original dog, they concluded, suggest that it had short, dark fur and a body that resembled today’s Alaskan malamutes. Genetics couldn’t determine whether the dog was a male or female, or where it lived, but its genome did show signs of inbreeding, a potential clue as to how the tumor got started.
From this first dog the cancer spread, first in small pockets, and then, eventually, around the world.
“As sinister as CTVT may seem,” says Carl Zimmer for his blog, The Loom, “it could be a lot more dangerous. You need only compare it to the only other known example of contagious cancer in the wild–a facial tumor that is spreading among Tasmanian devils.”
Like CTVT, the devil’s tumor spreads by taking advantage of the contact Tasmanian devils make with each other–instead of mating, they spread when the devils bite each other in the face during fights. But they’re drastically different in how they affect their host. CTVT typically disappears spontaneously from dogs. The devil’s tumor can balloon so fast that it often kills a Tasmanian devil in a matter of months.
Fortunately for us, these two contagious cancers are the only known transmissible cancers, so far.