A modern form of infectious cancer has been passed from dog to dog for thousands of years, according to a new analysis. A study published in Science on Thursday, January 23, 2014, describes the cancer, canine transmissible venereal tumor (CTVT), as emerging in an ancient dog approximately 11,368 years ago.
Researchers sequenced samples of CTVT from infected dogs living in Australia and Brazil to determine the genetic makeup of the original host dog. Analysis showed that the tumor first occurred in a medium to large size dog with a solid black coat. The animal carried a mixture of doglike and wolflike traits, resembling a malamute or husky.
Geneticist Elizabeth Murchison of the University of Cambridge, lead author of the paper published in Science, discovered that the cancer contains over two million mutations, more than any known human cancer. Inbreeding provided a clue to transmission, since most cancer cells would be recognized as foreign and die in a potential host.
“If this dog was inbred, maybe it was in a very inbred overall population,” Murchison says about the original host dog. “If so, that may have been an environment which was especially conducive for the emergence of a transmissible cancer.”
CTVT spreads through sexual contact or licking of tumor cells. “It’s a cancer that’s really a parasite,” Murchison says. Cancer cells transfer from one dog to the next and form a new tumor, often on the genitals. According to the National Center for Biotechnology Information, most cases of CTVT are not fatal and eventually rejected, giving the host dog lifelong immunity.
The study reveals that the two modern afflicted dogs likely shared a common ancestor “about 460 years ago,” with the disease spreading worldwide by companion dogs during human global exploration. CTVT cannot be transmitted to humans, but may prove to be an important tool in the study of cancer cell adaptation, as researchers describe the survival of the tumor as “remarkable.”