Venereal cancer is transmitted by contagious cancer cells which act like parasites, infecting dogs around the world, according to scientists following their study of the oldest known cancer cells.

Canine transmissible venereal tumor, or CTVT, is a cancer that spreads between domestic dogs through sexual contact and also through licking, biting or sniffing affected areas. Remarkably what was found was that unlike cervical cancer in humans that is caused by a virus, the dog cancer spreads by the tumor cells themselves rather than a virus.

Lead researcher Robin Weiss of University College London said, "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."

Genetic analysis has suggested that 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 result of the study is published in Friday's issue of the journal Cell.

Blood and tissue samples from 16 unrelated dogs on five continents were chosen for the study. It was found that the tumor had adapted to evade the immune response in dogs.

Tumors in donor organs have emerged in people with compromised immune systems on rare occasions raising the possibility of sexually transmitted prostate or cervical cancer tumors in humans. However as Weiss said there is no proof and it would be difficult to study.