Doggy Style

For some time (since 1876) veterinarians have been describing a type of tumor in dogs that occurs in the genital region of dogs. it is known by various monickers including CTVT (Canine Transmissible Venereal Tumor), Sticker Tumor, and Infectious Sarcoma.

It has long been thought of as being a sexually transmissible disease- and it is- but with a twist.

Ok- so there is nothing new about sexually transmitted infections, and really nothing new about sexually transmitted viruses that contain ongogenes that cause cancer. Well known examples are genital wart viruses (of certain genotypes) that lead to squammous cell cancers of the cervix, and Hepatitis B virus that can lead to cirrhosis and hepatocellular (liver) cancer. (Three cheers for the Australian scientists who discovered a vaccine to prevent HPV induced cervical cancer).

The amazing thing about this dog venereal tumor is that THERE IS NO VIRUS (or anything else) that causes it. Here comes the amazing part: it is the tumor ITSELF that is infectious!

This is quite bizarre. We all know that "cancer " isn't supposed to be "catching" or contagious.

This condition is more like a parasitic skin graft (allograft) than any previously known infectious disease. Genetic studies have shown that it probably arose a few thousand years ago in a single dog- and has been transmitted through biting and dog sex ever since. It probably began life as a skin tumor that then mutated to somehow avoid being detected by he dog's immune system (perhaps by downregulating expression of MHC surface molecules). In the process, it's ditched a few unwanted chromosomes completely.

So... in summary... this organism is a thousand-year-old bit of dog tumor. Should it be classified as a sub-species of canine? .. or should it be classified next to "Dog" on the evolutionary tree as an infectious unicellular organism?

It turns out that this sort of thing also happens to a certain type of russian hamster- and also may be resposible for a mysterious "Facial Tumor Disease" currently decimating the population of Tasmanian Devils on the Island of Tasmania (off the south-eastern corner of Australia for the eographically challenged). The dog disease, however, rarely metastasizes and only kills puppies (so thats OK).

I wonder if any human diseases will/do/have evolve(d) in this manner. (Sounds like a plot to a B-grade shlock-horror film).

CELL article pdf from last year
Wikipedia here