Ed Yong in Not Exactly Rocket Science:
CTVT, or canine transmissible veneral tumour, is a cancer that has evolved into an independent global parasite. Most cancers (including those that affect humans) aren’t contagious. Although some infectious diseases can lead to cancer, you cannot actually catch a tumour from someone who has one. But CTVT is an exception – the cancer cells themselves can spread from dog to dog, through sex or close contact.
A Russian veterinarian called Mistislav Novinski first discovered the disease in the 1870s, but it took 130 years for others to discover its true nature. In 2006, Robin Weiss and Claudio Murgia from University College London compared CTVT samples from 40 dogs across the world. All of them carried distinctive genetic markers that set them apart from the cells of their host dogs. They all had a common ancestor – an ancient tumour that escaped from its original host and took the world by storm.
CTVT is one of two types of contagious cancer. The other plagues Tasmanian devils and might drive them to extinction. While this second type is confined to Tasmania, CTVT has become a global success story. Hopping across continents on the bodies of dogs, this cancer cell has become an immortal parasite. The ones that Novinski studied in the 1870s were probably largely identical to the ones that Weiss and Murgia looked at 130 years later.
But this immortality comes at a price. The contagious cancer sometimes gets a glitch in its power supply, and it has to swap to a new set of batteries. Claire Rebbeck from Imperial College London has found that as the cells spread, they can pick up small structures called mitochondria. These are the batteries that provide our cells with energy, and the tumours can replace their set by raiding their hosts.