Canine transmissible venereal tumour (CTVT)
A mysterious contagious cancer which plagues dogs throughout the world may be the first truly transmittable cancer known, a new study suggests.
The cancer cells themselves move directly from dog to dog, acting "parasitically" on each infected animal, the researchers say.
Canine transmissible venereal tumour (CTVT) spreads between dogs through sex or other forms of contact, such as licking and biting, they believe.
The same cancer appears to infect dogs throughout the world and probably originated from a cancer in a single wolf, or a dog closely related to a wolf, which lived between 250 and 1000 years ago, the researchers say.
Previously, viruses were suspected of spreading CTVT in the same way that the human papilloma virus - found in genital warts - spreads cervical cancer to women through sex.
But a new genetic analysis shows that the dog cancer cells are direct descendents of tumour cells from the long-dead animal in which the disease originated.
"The 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," says lead researcher Robin Weiss at University College London in the UK.
"The idea that this is caused by transfer of the cancer cells themselves, not a cancer-causing virus, has been around for 30 years," says Weiss. "Now we've proved it through forensic DNA analysis."
Weiss said that the discovery makes the cancer, otherwise known as Sticker's sarcoma, "the oldest cancer known to science", and possibly the world's longest-lived colony of cloned mammalian cells.
Weiss and his colleagues made the discovery after analysing samples of blood from 16 unrelated dogs from five different continents, all of which had the cancer.
Their study showed that the cancer cells all originated from a common ancestor cell, not from the dogs themselves. Comparisons of the cancer DNA with dog DNA from reference samples held by the US Kennel Club and Crufts showed that it most likely originated at least 250 years ago from a wolf or an "old" Asian dog breed such as a husky or Shih Tzu, and spread like wildfire thereafter.
It has survived since as a stowaway partly because it does not usually kill infected dogs. "But if mangy old strays get it, they usually die," says Weiss.
In healthy dogs, any tumours that form usually disappear after a few months, once the dog's immune system gets it under control. But some cells survive and pass the cancer to uninfected dogs.
Similar cancers are thought to be endangering the Tasmanian devil, a carnivorous marsupial whose numbers have plummeted by at least a third since the mid-1990s due to the spread of unsightly and often fatal facial cancers. They transmit the facial cancer to each other in bites (see Tasmanian devils felled by rare cancer).
Weiss has no proof, but says it is possible that some cancer cells might get transferred in the same way in humans, perhaps if a man with prostate cancer has sex with a male partner whose own immune system is impaired, perhaps by HIV, for example.
No such infectious cancers have yet been discovered in people, but cancers hidden in donated human organs have been known to grow and kill their unfortunate recipients.
Next Weiss would like to find out how the chromosomes in the cells have become so stable. The prevailing wisdom is that as they multiply, cancer cells become increasingly unstable with repeated division as chromosomes fragment and become ever more jumbled during reassembly.
Although the dog cancer cells have heavily disrupted chromosomes, they seem to have ceased further fragmentation and become "stable", says Weiss, contradicting the dogma that cancer cells always become more aggressive.
Journal reference: Cell (vol 126, p477)