The first thing you should know about canine transmissible venereal tumor is that you should never do a Google image search for “canine transmissible venereal tumor.”
The second thing you should know about canine transmissible venereal tumor, which we’ll shorten to CTVT, is that it’s actually the business end of a contagious cancer that seems to have plagued a small population of dogs for the past 11,000 years. This makes CTVT one of just three cancers we know of that can pass from animal to animal like a rock ’n’ roll-loving demon. The other cases include Tasmanian devil facial tumor disease and a sarcoma scientists infected Syrian hamsters with in the 1960s
But unlike the afflictions of devils and hamsters, CTVT is an STD—a sexually transmitted disease. The cancer spreads when tumor cells are shed by the host during a moment of intimacy and make contact with another canine, at which point they set up shop in the new dog’s private parts. Symptoms of infection range from bleeding genitals to what an early 19th century veterinary practitioner described as “an ulcerous state, accompanied with a fungous excrescence.”
However, cancer biologists like Elizabeth Murchison have learned to look past CTVT’s cauliflower-like exterior to appreciate the science within. A native Tasmanian, Murchison was lured into the wonderful world of transmissible cancers by studying her island’s devils. “From there, I learnt about the dog cancer, and found it completely fascinating,” she told me in an email.
Murchison and her colleagues recently sequenced the genomes of CTVT samples from dogs found across the world from each other—an Aboriginal camp dog from Australia and a cocker spaniel from Brazil. Despite the fact that the dogs were separated by more than 10,000 miles and an ocean, the team found the tumors’ genetic makeups to be strikingly similar.
Further analysis of the tumors’ mutation rates suggests that the cancer cells infecting the dog living in the Australian Outback and the one living in Brazil’s favelas shared a common ancestor 460 years ago. “We note that the estimated timing of this divergence coincides with the era of rapid human global exploration,” the researchers report in the journal Science.
But a lineage that dates back to Columbus’s time is still relatively recent compared to how long ago the cancer itself originated. The team estimates that CTVT blinked into existence in a dog that lived between 10,179 and 12,873 years ago—this dog’s mutated cells were the source of the endlessly contagious cancer. By comparing DNA from the modern tumor cells to the genotypes of 1,106 dogs, wolves, and coyotes, the researchers conclude that the first animal to suffer from CTVT was a black-ish, relatively inbred canine resembling a malamute. And from this whelp, a miracle was born.
The researchers, in their own peer-reviewed and suitable-for-publication way, can barely contain their wonder. “It is remarkable,” they write, “that a somatic genome whose DNA would normally have survived for no more than 15 years during the life of one dog has continued to exist for several millennia as a parasitic life form.”
Truly, CTVT is a survivor beyond comparison. Unlike most cancers, which are caused when the host’s own cells multiply uncontrolled, CCVT is an independent lineage that has parasitized dogs around the world. And every CTVT cell marches to the beat of the same war drum. It also isn’t picky and will just as happily colonize other canids, such as wolves, foxes, and coyotes.
Even now, the yelps you hear in the night may be an alley mutt carrying out the cancer’s ancient legacy—replicate, fornicate, disseminate.
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