‘Canine transmissible venereal tumour’ is a cancer that spreads between dogs through the transfer of living cancer cells, primarily during mating. The disease usually manifests as genital tumours in both male and female domestic dogs. It first arose in an individual dog, but survived beyond the death of the original dog by spreading to new dogs. The cancer is now found in dog populations worldwide, and is the oldest and most prolific cancer lineage known in nature.

Artist’s impression of the 'founder dog' that first gave rise to CTVT between 4000 and 8500 years ago. (Credit: Emma Werner) Artist’s impression of the 'founder dog' that first gave rise to CTVT between 4000 and 8500 years ago. (Credit: Emma Werner)

One of the most remarkable aspects of these tumours is that their cells are those of the original dog in which the cancer arose, and not the carrier dog. The only differences between cells in the modern dogs’ tumours and cells in the original tumour are those that have arisen over time either through spontaneous changes in the cells’ DNA or through changes caused by carcinogens.

CTVT infection involves the direct transfer of CTVT cancer cells (shown) between dogs by physical contact. (Credit: Mayra Martinez-Lopez & Julio Ortiz) CTVT infection involves the direct transfer of CTVT cancer cells (shown) between dogs by physical contact. (Credit: Mayra Martinez-Lopez & Julio Ortiz)

An international team of researchers, led by scientists at the Transmissible Cancer Group at the University of Cambridge, has compared differences in tumours taken from 546 dogs worldwide to try to understand how the disease arose and how it managed to spread around the world. Their results are published today in Science.

“This tumour has spread to almost every continent, evolving as it spreads,” says Adrian Baez-Ortega, a PhD student in the Transmissible Cancer Group, part of Cambridge’s Department of Veterinary Medicine. “Changes to its DNA tell a story of where it has been and when, almost like a historical travel journal.”

Smiley belongs to a Buddhist monk living high in the Himalayas. She has recently recovered from CTVT. (Credit: Ruth Pye) Smiley belongs to a Buddhist monk living high in the Himalayas. She has recently recovered from CTVT. (Credit: Ruth Pye)

Using the data, they created a phylogenetic tree – a type of family tree of the different mutations in the tumours. This allowed them to estimate that the cancer first arose between 4,000 and 8,500 years ago, most likely in Asia or Europe. All of the modern tumours can be traced back to a common ancestor around 1,900 years ago.

Map showing original spread of canine cancer (BP = Before Present) Map showing original spread of canine cancer (BP = Before Present)

The researchers say that the cancer first spread from Europe to the Americas around 500 years ago, when European settlers first arrived at the continent by sea. Almost all the tumours found today in North, Central and South America descend from this single introduction event.

Canine cancer spread to the Americas around 500 years ago and then subsequently through European colonies. Canine cancer spread to the Americas around 500 years ago and then subsequently through European colonies.

From the Americas, the disease spread further, to Africa and back into the Indian subcontinent – almost all places that were, at the time, European colonies. For example, the cancer is seen in Reunion, but this was where European travellers would stop off on the way to India. All of this evidence suggests that the tumour was spread by sea-faring dogs, transported through maritime activities.