Researchers have been able to track the global spread of a rare transmissible cancer found in dogs, and transmitted between them sexually. The disease, known as canine transmissible venereal tumor (CTVT), forms tumors in the genitals of dogs, which manifest themselves in a “cauliflower-like” appearance. The new study has looked into how the cancer has spread to six continents, matching its movement around the world with the global migration of dogs.

It is the tumor cells themselves that are infectious, and are spread from dog to dog through mating. This means that the tumor cells growing and forming the cancer are not actually genetically related to the host dog that they are infecting. CTVT is in fact one of only four cancers known to be transmissible, with another being the facial tumors currently decimating the Tasmanian devil populations in Australia, which are also derived from cells of the very first Tasmanian devil to develop the disease.

From this, researchers have been able to figure out that the very first dog to have contracted the cancer lived an astonishing 11,000 years ago, making CTVT the oldest cancer lineage known to exist. But the authors of the new study, published in eLife, have found that the genetics of the cancer are in no way set. In fact, they have discovered that at least five times in the past 11 millennia, the tumors cells have taken mitochondrial DNA from the host dogs.

^{CTVT cells, stained and viewed under a microscope. Joel Mills/Wikimedia Commons} 

These five events have therefore created five “families” of CTVT tumor, which have allowed the researchers to build a family tree and track how the cancer has changed through time, as well as giving them the ability to trace how it has spread around the world. One of the mitochondrial transfer events, which occurred around 500 years ago, amazingly matches up with the maritime trade routes during that period of history. They were able to date the time since the transfer by counting the number of mutations, which occur at a constant rate.

“We found it along the coast of West Africa, in the Cape Verde islands, Brazil, South Africa, India, and some parts of southern Europe,” explained Dr. Elizabeth Murchinson, the paper’s senior author, to the BBC. “You can kind of imagine those dogs on boats, which must have taken that tumour around with them.” Wherever the boats landed, and the dogs disembarked, they would have mated with the local canines and spread the cancer.

After studying DNA from 449 tumors, taken from dogs living in 39 countries, the researchers also found something else of interest and never seen before. In one sample taken from a dog in Nicaragua, they found that the mitochondrial DNA derived from the host dog had been spliced with mitochondrial DNA already present in the tumor. While researchers had suspected that mitochondrial DNA may get mixed up, up until now there had been little evidence of it, and none previously from a cancerous cell.