Cancer can apparently spread by sexual contact, at least in dogs. The extremely rare type of cancer called canine transmissible venereal tumor, or CTVT, affects only dogs and is the world's oldest and most widely spread form of cancer.

According to a new study published in the journal Science, Jan. 23, the canine cancer has been around for as long as 11,000 years and has been passed from one dog to another through sexual contact.

"It's intriguing how a tumour has survived for more than 10,000 years travelling round and infecting a very large number of dogs," said Kerstin Lindblad-Toh of the Broad Institute in Cambridge, Massachusetts. "If more examples [of transmissible cancers] were found, it would be interesting to see what genetic changes characterise this type of long-lived tumour."

Today, the CTVT still retains the genetic signatures of its first host which scientists believe is an Alaskan malamute or husky-type canine. The researchers determined the genetic makeup of the cancer's first host by sequencing the genomes of CTVTs from two living dogs located in Australia and Brazil.

"These tumours are in dogs all around the world, yet they all originate from one single dog that lived all those years ago," said Elizabeth Murchison of the Wellcome Trust Sanger Institute in Hinxton, UK. "We do not know why this particular individual gave rise to a transmissible cancer but it is fascinating to look back in time and reconstruct the identity of this ancient dog, whose genome is still alive today in the cells of the cancer that it spawned."

Researchers noted the importance of studying CTVT even though they occur only in dogs. Mike Stratton, senior author of the study and director of the Wellcome Trust Sanger Institute, pointed out that studying transmissible cancers allows scientists to better understand the evolution of cancer which could be crucial if they start to occur in humans. "The genome of the transmissible dog cancer will help us to understand the processes that allow cancers to become transmissible," he said. "Although transmissible cancers are very rare, we should be prepared in case such a disease emerged in humans or other animals."