THE direct descendants of a cancer that first afflicted a single wolf between 250 and 1000 years ago are still spreading from dog to dog all around the world. The contagious cells represent the oldest surviving cancer known to science.
Canine transmissible venereal tumour (CTVT) is transmitted directly between dogs, probably through sex, licking and biting. It was thought that a virus was responsible for spreading the cancer from one dog to the next, in a similar way to human papilloma virus, which lives in genital warts and spreads cervical cancer to women through sex. Now a genetic analysis has shown that the cancer cells found in dogs today are descended from tumour cells from the long-dead animal in which the disease originated.
鈥淭he cancer escaped its original body and became a parasite transmitted from dog to bitch and bitch to dog until it had colonised all over the world,鈥� says lead researcher Robin Weiss of University College London.
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Weiss and colleagues analysed blood samples from 16 unrelated dogs on five continents that had the cancer. They found that the cells all originated from a common ancestor, and not from the dogs themselves. Comparisons of the cancer DNA with dog DNA from reference samples showed that it probably originated at least 250 years ago from a wolf or an 鈥渙ld鈥� Asian dog breed such as a husky or shih-tzu ().
The cancer has persisted partly because it doesn鈥檛 usually kill infected animals. Instead, a dog鈥檚 immune system tends to destroy most of the tumour cells within a matter of months. The few cells that do survive can then pass the cancer to uninfected dogs. These tactics have allowed CTVT to spread throughout the world. Quarantine laws have eradicated the cancer in some countries, including the UK, but it is still prevalent among strays.
Similar cancers are thought to be endangering the Tasmanian devil, a marsupial whose numbers have plummeted by at least a third since the mid-1990s.
