Infectious cancer hasn’t done much over the last 4,000 years

Enlarge / Does this dog look concerned because he realizes he can catch an infectious cancer? (credit: Martin Astley)

Cancer is a horrific disease, with its damage only limited by the fact that it only harms the individual in which it arises-except when it doesn't. In a few extremely rare cases, cancerous cells have evolved the ability to move from host to host, essentially becoming an immortalized parasite. The best known instance of this is in dogs, where a cancer has essentially become a sexually transmitted disease.

While the cancer's been known about for some time, there's been a bit of confusion about its origins. Now, a huge team of researchers has looked at parts of the genome gathered from hundreds of dogs from around the globe, and they've reconstructed the cancer's history and evolution. In the process, the team found that it's not actually doing much evolving anymore.

Dogs can catch cancer while having sex

It's hard to know what to call this thing. Cancer? Parasite? Disease? To an extent, its formal name covers things nicely: canine transmissible venereal tumor, or CTV. As the "venereal" implies, dogs transmit CTV during sex. It results in tumor growth, often on the external genitalia. But, unlike the cancer spreading among Tasmanian devils, the immune system quickly suppresses CTV, and the tumors quickly regress. But it does last long enough to spread throughout dog populations. With fewer working dogs and strays, CTV is rare in Europe and North America, but it remains prevalent elsewhere.

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