Today I Learned: Some animal cancers are contagious!
But luckily for us, this does not normally occur in humans, just other animals.
Earlier today, I was reading about the difference between antibiotic resistance, which I study in my PhD research, and cancer resistance, a topic I might get to research soon. I came across the phrase “transmissible cancer” and thought….woah?!?!?! What??
This is not going to be a very long or very technical article, I just want to share some of the broad information I learned because this was mind-blowing to me. I’ll what I think are some frequently asked questions about the topic, and then I’ll link some more resources in case you want to learn more!
Which animals can get a transmissible cancer?
The natural transmissible cancers which have been discovered have been discovered in three main groups of animals:
- Dogs, through canine transmissible venereal tumor (CTVT)
- Tasmanian devils, with two separate lineages, devil facial tumor diseases 1 and 2 (DFT1 & DFT2)
- Marine bivalves (such as soft-shell clams, cockles, mussels, etc), through bivalve transmissible neoplasia (BTN), which encompasses many different lineages
Humans are luckily not on this list. Transmission between two distinct humans has occurred, but not from typical “infections.” Cancer can be transmitted from mother to fetus and from organ transplantation, but both are exceptional medical situations, not ordinary contagious spread, and they are very different from the naturally circulating transmissible cancers seen in dogs, Tasmanian devils, and bivalves. In other words, you won’t “catch cancer” by being near someone with cancer like you would be being near someone with a cold.
How do those animals transmit their cancer?
- Dogs: Direct transmission of living tumor cells, usually during mating.
- Tasmanian devils: transmission of tumor cells into facial wounds through biting.
- Marine bivalves: transmission by shedding cancer cells into the seawater, which can then seed disease in other animals.
Importantly, and what distinguishes these cancers from something like human papillomavirus (HPV), which is a virus that can cause cancer, is that the cancer cells are directly transmitted as the pathogen itself.
How did the transmissible cancers develop in the first place?
Each of the above lineages began as an ordinary cancer in an individual animal, and the cancer acquired the ability to survive transfer to a new host, similar to typical pathogens such as bacteria, viruses, and parasites.
Get Leah Childers’s stories in your inbox
Join Medium for free to get updates from this writer.
These traits developed as a result of accumulated genetic mutations in cancer cells, similar to how bacterial cells under antibiotic treatment can mutate many times over their short replication time, selecting for the ability to replicate in the presence of antibiotics.
For the dog case, all known CTVT tumors descended from one original dog’s cancer from a dog estimated to have lived ~11,000 years ago. For the Tasmanian devil cases, each lineage developed separately and did not branch off the same initial lineage. Marine bivalves also have multiple independent lineages.
Could this happen to a human cancer?
The processes by which transmissible cancers arise center around extremely rare and unlikely events. No naturally circulating human transmissible cancer has ever been observed, but that does not mean it could never happen.
So could it happen? In principle, perhaps; in practice, it appears extraordinarily unlikely.
The human immune system is very efficient at distinguishing foreign cells, and the mutations necessary to allow cancer to be transmitted like a pathogen are very rare.
Additionally, the risk of a dog or a Tasmanian devil spreading their transmissible cancer to a human is very low due to species barriers and immune rejection. A DFT-infected Tasmanian devil’s tumor is made of living Tasmanian devil cells. If the Tasmanian devil bit a human, the cells would have to survive the bite, implant in human tissue while avoiding the human immune system, and keep multiplying. The bitten human would be more likely to catch a normal bacterial infection from the wound; receiving the transmissible cancer is almost impossible and has never been observed.
So to conclude: transmissible cancers are real and sound scary, but have never been observed in humans and are very unlikely to develop.
References + other resources:
“Transmissible Cancers in an Evolutionary Perspective” https://pmc.ncbi.nlm.nih.gov/articles/PMC7327844/
“Immunology of naturally transmissible tumours” https://pmc.ncbi.nlm.nih.gov/articles/PMC4264906/
