Startling New Evidence Cancer Is Contagious

Scientists studying tumors in Tasmanian devils and dogs have found evidence some cancer cells are being passed between animals through physical contact such as sex or biting. Science reporter and author David Quammen puts the findings in context.

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According to the latest data from the Center for Disease Control and the American Cancer Society, more than ten million Americans are living with some form of cancer. And while there are new drugs and treatments constantly being tested, some with good rates of success, cancer is still largely an enigma. Who gets it, and why? Those of us who are cancer-free take quiet solace, perhaps, in the assumption that cancer is not contagious. But is that really true?

Scientists studying cancer in two very different animal species, Tasmanian devils and dogs say they found evidence that some cancer cells are being passed between animals through physical contact, like sex or biting. If they're right, this means the cancer is literally jumping from one animal to the other, much like a parasite. Now, important note, we're talking about animals here, not humans, but it does raise some concerns about how cancer is evolving.

We wanted to go to Tasmania to check this out, but when we found out science reporter and author David Quammen had already been there, we decided to rip it from the headlines. David Quammen wrote the article "Contagious Cancer: The Evolution of a Killer" for the April issue of Harper's Magazine. He joins us now to share his reporting. Hi, David.

Mr. DAVID QUAMMEN (Science Reporter, Harper's Magazine; Author, "The Reluctant Mr. Darwin"): Good morning.

MARTIN: Thanks for being here. Let's start with the Tasmanian devils. This disease has been killing these animals for a decade. What made you decide to go out and investigate this phenomenon now?

Mr. QUAMMEN: Well, I had heard about the disease and since 1996, it's been sweeping across the island, killing what seemed to be thousands of Tasmanian devils. And I decided that it seemed to be possibly connected with some other research I had heard about in cancer more recently in Philadelphia, done by a fellow, Carlo Maley, on the evolution of cancer in humans. He studies a phenomenon called Barrett's Esophagus.

It's a pre-cancerous condition and has written some papers about the evolution of tumors into more and more aggressive forms. Aware of these two things, I thought, I wonder if they're connected. I wonder if the evolution of cancer toward more aggressive forms is somehow involved in what's going on with the Tasmanian devil.

MARTIN: OK, before we get to that link and what you found out, how exactly is the disease being passed from one Tasmanian devil to another? And what it is about this species that makes them more likely to contract cancer?

Mr. QUAMMEN: It's a couple of things. The Tasmanian devil is a little marsupial carnivore and they have a tendency to bite one another in the face. And they do it when they're feeding over carcasses. They do it when they're fighting. They do it during competition for mates. Males and females even doing it during the mating period, and they end up with these little facial injuries and that seems to be the way that the tumor cells are transferred from crumbly friable tumors on the face of one devil into the facial wounds on another devil.

MARTIN: Now what - beyond concerns about extinction of this particular species, why should people - why should humans be concerned about this?

Mr. QUAMMEN: Well, as far as I'm concerned, the first reason is that it helps us understand cancer better. Cancer is a very lonely disease and one of the reasons why it feels lonely, seems lonely, is this axiomatic belief that it's not transmissible. It's never a infectious disease. In fact, it's almost never an infectious disease, but the Tasmanian devil shows it can be an infectious disease.

MARTIN: But does that mean, David, that if we went around biting each other, could people with cancer biting people who didn't have cancer in the face that those people are doing to contract tumors on their face?

Mr. QUAMMEN: No, absolutely not. There's some special conditions with the Tasmanian devil. One is that it has a very small gene pool. It is unpopular with ranchers in Tasmania because it occasionally eats lambs. It has been nearly exterminated a couple of times, or at least the population driven low. So it's been through genetic bottleneck. The population has recovered, but the genetic diversity has not recovered so that all the Tasmanian devils that exist are so similar to one another genetically that their immune systems don't seem to be able to tell one from another. And that's an important condition in this particular cancer.

MARTIN: Now you have, though, in your article in Harper's, you do reference some rare cases of cancer and tumors being transmitted among humans, people who came into contact with the cancer cells, mainly through syringes. That's disconcerting.

Mr. QUAMMEN: Or surgical accidents. That's right. There are a few cases, very rare, very special, in the medical literature in which the cancer from one person took root and grew in another person. For instance, there was a 53-year-old surgeon who cut his hand while he was doing surgery on, if I recall correctly, on abdominal cancer in a woman, and he ended up with a tumor of his hand that came from the woman's own cancer.

He was cured with surgery, and there was another case where a medical technician stuck herself with a syringe, got a tumor in her hand, and then there was also a case back in the 1920s of a medical student named Henry Vadaneaux (ph) who stuck himself with a syringe. He got cancer of the hand from another person, and he was not cured. He died of the cancer.

MARTIN: But maybe we should define what it means to be contagious. I mean, when I hear those examples, I think, well, it kind of makes sense. I mean, if you're injecting the cancer cells directly into your bloodstream that doesn't sound like it's a definition of contagious. I guess I'm thinking more airborne, is that - what do you mean when you say contagious? How are you using it here?

Mr. QUAMMEN: Contagious could be any situation in which the cancer cells from one person are passed to another. Now, those are very artificial situations, you're absolutely right. And there is no known case, at least not known to me, of cancer being transferred from one human to another during normal physical contact. Although it is true in the Tasmanian devil and also in dogs, there is this cancer called Canine Transmissible Venereal Tumor, CTVT, that is essentially a sexually transmitted disease. It's passed from one dog to another during normal sexual contact.

MARTIN: So what did you learn through this trip and through your research?

Mr. QUAMMEN: Well the first thing I learned is that cancer is an evolving disease. This comes from the work of Carlo Maley and his colleagues working on Barrett's Esophagus. And they have shown that tumors actually have the capacity to evolve towards more aggressive forms within a given person. How can they do that? The different cells of a tumor actually have subtle variations in their genetic form because of mutations as the cells reproduce.

Darwinian evolution requires genetic diversity plus competition that yields the survival of the fittest. This actually can take place on the cellular level within a given cancer and can evolve toward being, for instance, resistant to chemotherapy, more likely to metastasize, and ultimately, at least potentially, towards the ability to transmit from one individual to another.

MARTIN: Wow, well, we're far from saying that cancer is contagious for humans. It is some disconcerting evidence.

Mr. QUAMMEN: We're very far from that, I think the real point is understanding cancer better, understanding the biology of cancer, and that will, I hope, ultimately, make it a less lonely disease and a more treatable one.

MARTIN: David Quammen, science journalist and author of the recent book, "The Reluctant Mr. Darwin." Thanks very much for sharing your report. We appreciate it.

Mr. QUAMMEN: You're welcome.

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