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Some Dogs Carry 'Contagious' Cancer
by: Ed Edelson
Researchers are describing what seems to be a real-life medical nightmare: A cancer that spreads from animal to animal like an infection.
Luckily for humans, this malignancy occurs only in dogs, and there's no need for people to be worried about it, experts say.
"It's a scientific curiosity," said Robin Weiss, professor of viral oncology at University College London, and a member of the team reporting the discovery in the journal Cell. "There is no evidence of transfers of human cancers from one person to another, except in very special circumstances, so we should not say that a human cancer patient is dangerous to others."
The cancer, called canine transmissible venereal tumor (CTVT), was first isolated from 16 dogs in Italy, India and Kenya. In each case, a study of the tumors' genetic material showed that it differed from that of the dog in question -- suggesting that it had been passed from another dog.
Further study of cancers from 40 other dogs in five continents found that the tumors were almost genetically identical, meaning that they originally came from a single source and had somehow spread across the globe.
Working with geneticists and computer experts in Chicago, the researchers compared the genetic material of tumors to that of specific breeds of dogs. They concluded that the cancer most likely arose more than 250 years ago -- perhaps as long as 1,000 years ago -- in a wolf or Asian dog such as a Husky or Shih Tzu.
CTVT is transmitted primarily through sexual contact, but experts believe it can also be picked up as dogs lick, bite or sniff tumor-affected areas. It is seldom fatal and usually disappears in three to nine months, just long enough for the dog to pass it on.
"One aspect where this is related to human cancer is not in the mode of transmission, but what it tells us about the nature of cancer," Weiss said.
Generally, as cancers become more aggressive, they become less stable genetically, he said. But CTVT has had the same genetic makeup for centuries and is "the oldest tumor cell lineage known to science," which means that it has become genetically stable, Weiss said.
"This questions the theory of instability," he said. "I don't think that instability is inevitable as a tumor gets worse and worse."
The report also raises wildlife conservation issues, added Elaine Ostrander, chief of the cancer genetics branch at the U.S. National Human Genome Research Institute, who wrote an accompanying commentary.
Similar cancers are known to exist in two other species, the Tasmanian devil and the Syrian hamster, Ostrander said. For these types of endangered species, exposure to CTVT might endanger the population's survival, she wrote.
There appears to be no danger to humans from the sort of cancers seen in these animals, Ostrander said. While CTVT may occur in stray dogs, pedigreed dogs are usually not allowed casual sex, and the cancer "can't be transmitted to humans by handling dogs," she said.
"We always wonder when we see something in the animal kingdom if we will see the same thing in humans," Ostrander said. "We don't see any human evidence in this case."
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