How Dog Cancer Became a New Species (CTVT)

You probably think of cancer as being not infectious, right? One of your cells made a pro gamer move and started dividing uncontrollably, but it’s not like that could become someone else’s problem. I mean, I can’t even get a kidney transplant, gaining superhuman urine production with the power of three kidneys, without needing drugs to stop my body from rejecting it. How is some dumb cancer cell supposed to do that?

Well, turns out Transmissible cancer is possible, though very rare. Canine Transmissible Venereal Tumor (CTVT) is a strain of infectious cancer that spreads through dogs and several other canine species through sex.

• What is Cancer?
• How does Infectious Cancer Evolve?
• CTVT Remains Infectious Across the Species Gap
• Are there More?
• Is it a Species?
• The SCANDAL Hypothesis
• Conclusion

What is Cancer, Anyway?

Cancer, fundamentally, can be thought of as what happens when your cells stop working for the greater good and turn back into independent organisms.

But, of course, the vast vast vast majority of the time this results in the cancer dying. A very unsuccessful reversion to single cellularity. Of course cancer isn’t going to be able to live on its own. It’s completely unadapted to any environment other than inside the human body. But it can still evolve to be better at living inside a body parasitically.

As I discussed in the article I previously mentioned, cancer often evolves into “cancer stem cells” by taking on stem cell characteristics which helps them survive. They also tend to mutate and evolve mechanisms of suppressing the immune system. Simply because cancers that don’t do these things die before you even notice them.

But just because a cancer cell is good at living inside its host doesn’t mean it’s going to be good at jumping into someone else. Sure, cancer does tend to suppress the immune system a bit. But that probably won’t be enough to prevent someone else’s immune system from recognizing it as foreign tissue. Convincing a natural killer cell¹the immune cell that checks cells for cancer and viruses that you aren’t a mutant is a lot easier than convincing it that you aren’t from an entirely different, genetically distinct person.

Plus, most cancers wouldn’t even be able to reliably get from one body to another. They can’t live outside the body very long after all.

What’s worse is that there’s no way for the cancer to know that it even needs to do those things. Evolution is blind. Sure, the cancer needs to jump ship before it burns through its current host and dies with them. But natural selection only selects for cells that are reproducing the most right now. The cancer will never actually evolve what’s good for it in the long term unless that happens to also be good in the short term and it’s extremely lucky. So it makes sense why transmissible tumors are so incredibly rare. But they’re not impossible.

How does Infectious Cancer Evolve?

Let’s say that you’re a very lucky cancer cell. You came from a cancerous histiocyte cell, which is a type of amoeba-like immune cell that lives in the skin and eats dead cells. You have also mutated into a cancer stem cell, and so you are pretty good at breaking away from your current tumor and building new tumors elsewhere in the body. You have also evolved to be pretty good at suppressing the immune system, as the host’s immune cells have been fighting you for a long time. 

It is 11,000 years ago. You live on a dog’s dick.

What luck, the host dog is now having sex with another dog which it is closely related to. You get broken off from your tumor and find yourself on the other dog’s body. Since this other dog is closely related to your original host (in fact, this entire pack of feral dogs happen to all be closely related to each other due to inbreeding) you don’t have too much trouble stopping her immune system from killing you. You are able to build a tumor and are now the common ancestor of an entire massive lineage of transmissible cancer.

Sure, while this pack of dogs are closely related, your descendants still need to evolve some better immune system suppression abilities to thrive. Which they do. Eventually they spread to more distantly related dogs and evolve even better immune suppression. Eventually they even spread to wolves, coyotes, etc by evolving extremely well evolved immune suppression.

One thing that helps is getting rid of unnecessary DNA. One of the major ways the immune system detects foreign tissue is by finding proteins made from genes not found in your own genome. Even relatively closely related people will still have some proteins that are slightly different from each other. 

For example, women are less likely to accept an organ transplant from a man because that organ has a Y chromosome which the woman does not. But the reverse is not true, men can more easily accept female tissue because the immune system doesn’t care if it’s missing DNA, only if there’s different DNA.

So Canine Transmissible Venereal Tumor (CTVT) simply got rid of most of its DNA, keeping only the chromosomes that it needs to survive. After all, a tumor doesn’t really need to know how to make fur or how to be a good boy. So why keep all that dog DNA when all it’s doing is making you more sus?

So that’s basically how Canine Transmissible Venereal Tumor happened.

CTVT Remains Infectious Across the Species Gap

Though CTVT originated among dogs, it has since managed to spread to several other canine species. Namely coyotes and foxes. But how did that happen?

Pretty much anything a dog has ever stuck its dick into can theoretically get exposed to CTVT, which is a long list of things. After that, it’s just a question of whether or not that CTVT can actually survive in that host. If that CTVT tumor happens to have a cell with a mutation that makes it resistant to fox immune systems and/or that fox happens to have a weak immune system then a new tumor will form. Cells in that tumor may then evolve to be even better at countering fox immunity and spread through the fox population.  

Don’t worry though. There are no known cases of CTVT infecting humans. Turns out it’s pretty easy for our immune system to tell the difference between your own cells and mutant dog cancer, so it gets wiped out before you even notice it. 

But we do have “exposures” concerningly often, so maybe we will get a strain of our own eventually. There are parasitic worms that can trick our immune system into not noticing it, and I’m pretty sure there’s a lot more difference between a human and a worm than a human and a dog. So it’s not impossible that CTVT could mutate and jump the gap.

Plus, there are many instances where cancer can infect someone who is immunocompromised. And those are just normal cancers that weren’t adapted to be infectious. CTVT is. So people who are taking anti-rejection drugs because of a transplant or have a weak immune system for some other reason should really avoid having sex with dogs. Y’know, keep any unnecessary exposures to a minimum.

If we keep rolling those dice eventually they’ll come up snake eyes. Just imagine seeing your doctor about your weird genital warts and them having to explain that you have Canine Transmissible Venereal Tumor. You’re patient zero. That you need to inform everyone you’ve slept recently that they should get themselves tested for dog dick cancer. Imagine how awkward that would be?

Are there More?

CTVT is not the only known transmissible cancer. Tasmanian devil facial tumor disease (DFTD), is similar, though it affects Tasmanian devils²Yes, that is a real species not just a loony toons character. and spreads through bites.

Syrian hamsters also used to have one, known as contagious reticulum cell sarcoma. It was discovered in 1945, but has since died out.

At least four species of bivalves (clams, oysters, muscles, etc) are each known to have their own independent strain of transmissible leukemia. And unlike all the other examples of transmissible cancers, this cancer can spread without need of direct contact. Makes sense, as clams aren’t exactly known for hugging each other or anything.

But these are almost definitely not the only examples of transmissible tumors. For one, CTVT is only 11,000 years old while DFTD likely emerged in the 90s, which an instant ago evolutionarily speaking. If such a thing is so rare as to have only happened seven times, what are the chances that all of them would’ve happened so recently? Or that over half of them would have come from clams?

I think it’s likely that parasitic cancers exist in many more species. We only discovered CTVT because it affects a companion species to which a massive portion of our veterinary and biological research focuses on. 

Similarly, we only noticed DFTD because it’s single handedly threatening to drive a beloved loony toon species into extinction. It’s hard to get grants to study communicable cancers in something like lampreys or some obscure tree frog, believe me.

Not to mention how easy it would be to misdiagnose infectious cancers as cancer causing viruses or noninfectious cancers. These were difficult to discover even in animals we study closely.

Is it a Species?

I would argue that at least CTVT, if not also the other transmissible cancers, are now their own species, not just a cancer. Remember that multicellular organisms can be described as being made of trillions of single cellular animals with a very high degree of cooperation. The same way that you can describe an ant colony as a superorganism. Cancer is just what happens when that cooperation fails and some cells start acting as independent creatures again.

But CTVT isn’t just an uncooperating cell. It’s found a sustainable ecological niche as an obligate parasite of dogs and several other canids. Again, it’s been around for 11,000 years and there is no reason to expect it will die out any time soon. It reproduces and evolves on its own. Etc.

This perspective would also explain the unusual pathological stability of transmissible cancer cells. Normal cancer tends to be highly unstable and mutate rapidly, with cancer cells all looking a bit different from each other. CTVT and DFTD cells, on the other hand, all look very uniform. The tumors are also have a very consistent appearance, described as cauliflower-like. ³If you want to see an image feel free to look that up. But be warned, you will see doggie dick. A highly stable genome and consistent morphology is not a sign of defective cells, it’s a sign of an established species.

There only reason why it seems so strange, even heretical, to suggest that it’s a new species is because then the Canis genus would have to include dogs, wolves, coyotes, jackals… and an asexually reproducing single cellular parasite. Not only that, but it would be yet another⁴http://ravingsofamadscientist.com/the-truth-about-the-orders-of-mammals-school-lied-about/ example of a species under Mammalia which does not have fur, milk, or give live birth. (Yes, there are other mammals that don’t, your grade school science teacher lied to you.)

But, where else exactly would it go on the evolutionary tree if not as a sister species to dogs from which it evolved?

The SCANDAL Hypothesis

I am not the only one to suggest that cancer can become species. In fact, there have been some to not only suggest that, but that entire clades of animals may be descended from cancer.

The SCANDAL(Speciated by CANcer Development AnimaLs) Hypothesis is a very contrived acronym. Not least because it’s creators have specified that in it’s plural form SCANDALS stands for “Speciated by CANcer Development AnimaLS”. It should also tell you just how controversial the position is.

Myxosporea is a clade of very simple Cnidarians (Cnidaria include jellyfish and many other much smaller stinging jelly organisms). Myxosporea is extremely simplistic even by the standards of Cnidarians. It includes some of the smallest animal species.

It also has a very unusual life cycle that makes it difficult to place in the tree of life. Notably, it’s life cycle includes a single cellular stage, a multicellular plasmodial stages, a stage where all of it’s cells are located inside of one of it’s other cells, and two completely morphologically different adult forms depending on what it happened to be parasitizing at the time⁵just look at this diagram of it’s life cycle, it’s such bullshit, wtf. https://www.researchgate.net/figure/Diagram-of-the-life-cycle-of-myxosporean-alternating-fish-and-annelid-hosts_fig3_224830030.

As far as SCANDALS are concerned, Myxosporea completely lacks any kind of oncogenes, which are genes that stop cancer from happening. Other Cnidarians have these, so what gives? The only reason that not having oncogenes would make any kind of evolutionary sense is if you used to be a cancer. After all, having a gene specifically designed to kill you in particular isn’t really great for natural selection.

Maybe, it is a highly evolved cancer of Polypodium, which is a slightly more complex Cnidarian parasite. Notably, Polypodium is one of the few animals that can live inside the cells of larger animals, which might explain how Myxosporea came to living inside itself.

But how exactly you get from literal cancer to a multicellular myxosporean is unclear. We would expect species evolved from transmissible cancer to be single cellular and infect species related to the one it came from. Myxosporeans are neither of those things ⁶though it is not unheard of for the cancer of a parasite to jump over into the host of that parasite. there was one known case where a human was infected by their tapeworm’s cancer. Though he was already immunocompromised due to HIV, it’s not impossible that a cancer could retain the immunosuppressive abilities of the parasite species. https://www.nejm.org/doi/10.1056/NEJMoa1505892.

It is known that cancer is often more complex than we give it credit for. Cancer stem cells are stem cell like cancer cells found at the center of metastatic tumors and make other types of cancer cells. (For more info on those, read this other article of mine http://ravingsofamadscientist.com/the-real-reason-why-we-cant-cure-cancer-cancer-stem-cells/.)

Conclusion

Does some mad scientist on a tiny irrelevant blog saying that CTVT is a species make it a species? No. In fact, I’m sure that most biologists who read this will strongly disagree. The subject of what is and isn’t a species is extremely vague and controversial. 

Are dogs their own species, a subspecies of wolves, or just normal wolves bred to have horrific birth defects? Are wolves and coyotes the same species since they can (and do) reproduce with each other without issue? What about ring species? Do viruses count as life? What the fuck did tardigrades even evolve from? Everyone you ask will give you a different answer, which is basically the same as saying that no one knows.  

But sometimes science is more art than science. Maybe instead of arguing we should just base our taxonomy on whether I happen to find it philosophically interesting? 

Do you agree? Do you violently disagree? Do you wish for me to suffer a slow painful death at the hands of CTVT? Discuss in the comments below.

Citations

https://vcahospitals.com/know-your-pet/transmissible-venereal-tumor

https://stemcellsjournals.onlinelibrary.wiley.com/doi/full/10.1002/stem.751#:~:text=Tasmanian%20devil%20facial%20tumor%20disease%20%28DFTD%29%20and%20canine,received%20little%20attention%20and%20not%20been%20formally%20examined.

Gartner HV, Seidl Ch, Luckenbach C, et al. Genetic analysis of a sarcoma accidentally transplanted from patient to a surgeon. N Engl J Med 1996;335:1494–1496

Panchin AY, Aleoshin VV, Panchin YV. From tumors to species: a SCANDAL hypothesis. Biol Direct. 2019 Jan 23;14(1):3. doi: 10.1186/s13062-019-0233-1. PMID: 30674330; PMCID: PMC6343361.