Contagious cancer cells originated from 1 wolf or dog

Contagious cancer cells act like parasites, infecting dogs around the world, say scientists studying the oldest known cancer cells.

Canine transmissible venereal tumour,or CTVT, apparently spreads between domestic dogs through sexual contact and may also spread through licking, biting or sniffing affected areas.

Unlike cervical cancer in humans, which is caused by a virus, the dog cancer spreads by the tumour cells themselves rather than a virus.

"The cancer escaped its original body and became a parasite transmitted from dog to bitch and bitch to dog until it had colonized all over the world," said lead researcher Robin Weiss of University College London.

The cancer probably originated from a cancer in a single wolf or closely related Asian breed such as a husky or Shih Tzu between 200 and 2,500 years ago, the genetic analysis suggests. It appears in Friday's issue of the journal Cell.

To reach the conclusion, the team examined blood and tissue samples from 16 unrelated dogs on five continents.

Cancer challenge

They found evidence the tumour had adapted to evade the immune response in dogs.

On rare occasions in humans, tumours in donor organs have emerged in people with compromised immune systems. The findings raise the possibility of sexually transmitted prostate or cervical cancer tumours in humans, but there is no proof and it would be difficult to study, Weiss said.

As cancer develops, chromosomes in the tumour cells tend to become more unstablewith repeated divisions, but the researchers found no evidence of that in CTVT.

The findings challenge the idea that there is "an inevitable progression of cancer towards more instability," Weiss said.

It's also possible that small populations of endangered species like the Tasmanian devil might be prone to other transmissible cancers.

Australia's population of the carnivorous marsupial is threatened by a disfiguring and often fatal cancer that is transmitted through bites.