Contagious Canine Cancer Spread by Parasites

Dogs have a form of sexually transmitted cancer that for 200 to 2,500 years has apparently spread via contagious tumor cells that escaped from their original body and now travel around the world as parasites.

These cells are the oldest cancers known to science thus far, and could shed light on how cancers survive and evade the immune system.

The researchers investigated canine transmissible venereal tumor, a cancer found in the domestic dog and potentially in relatives such as the gray wolf and coyote. It is spread through sex and licking, biting and sniffing cancerous areas. The tumors usually regress three to nine months after their appearance, leaving the dogs immune to reinfection, although providing enough time for dogs to pass the disease on.

Some human cancers, such as cervical cancer, are caused by viruses.

What is unique about this dog cancer is that, for 30 years, scientists have suggested it was caused by spreading the tumor cells themselves rather than a virus or other contagious agent. Prior research showed, for instance, the disease could not spread from tumor cell extracts or dead tumor cells, but only via living tumor cells. Still, virus-like particles seen in the tumor cells clouded the issue.

Cancer researcher Robin Weiss at University College London and his colleagues analyzed genetic markers in recently collected and archived tissue from dogs spanning five continents, from locales in Italy, India, Kenya, Brazil, the United States, Turkey and Spain. They found the tumor cells did not actually belong to the dogs they were in. Rather, the cells were all genetically nearly identical, apparently stemming from a wolf or a closely related ancient dog breed from China or Siberia.

The tumor cells themselves act as parasites, the new study concludes.

The researchers found the cancer secretes compounds that inhibit facets of the immune systems of their hosts, allowing them to avoid detection. At the same time, the immune inhibition they cause rarely results in death of the infected animal, to help guarantee the host passes the disease on.

Judging by the number of mutations the cancer's DNA accumulated, the researchers estimate it emerged 200 to 2,500 years ago. Instead of becoming progressively more genetically unstable over time, as scientists widely supposed happens to cancer, these cancer cells "do not go on getting more and more genetically unstable," Weiss told LiveScience.

The study is detailed in the Aug. 11 issue of the journal Cell.