Adam’s Journal
I read a story about a contagious form of cancer in catfish. Is it only a matter of time before we all start catching cancer from one another?
Dr. Scofield Prescribes
Last month, scientists published a study detailing a form of melanoma they’d discovered in catfish living in a lake that borders Vermont and Quebec.
Anglers who fished the lake first noticed the growths – raised black spots – more than a decade ago. Wildlife biologists first ruled out the most common cause of the skin cancer, ultraviolet light, as well as the possibility that pollutants triggered the disease.
Ultimately, they determined that every cancer they found on the fish likely started with a single catfish. That catfish developed a tumor, which shed cancer cells into the water. Those cells then spread to other catfish.
Wash, rinse, repeat.
Catfish aren’t the only species in which scientists have found contagious tumors. Researchers have also documented the phenomenon in dogs, Tasmanian devils, and some kinds of clams and other shellfish.
In dogs, the transmissible cancer – known as canine transmissible venereal tumor, or CTVT – spreads through sexual contact. Studies of the cancer, which is rarely lethal, suggest they all likely descend from a single dog that lived thousands of years ago.
Among Tasmanian devils, a contagious cancer spreads through exposure to blood, which usually happens when the animals fight with one another. Unlike in dogs, this spreadable cancer is lethal, and it has brought the species to the verge of extinction.
In shellfish – and, perhaps, the catfish – cancers seemed to spread through the water when the aquatic creatures are in close proximity to one another.
We’ve seen no evidence of similar contagious cancers in people. Although as far back as the Renaissance scholars have speculated that cancers could be contagious, human cancers almost always arise from mutations in our own cells’ DNA.
We do know that several viruses can cause many different forms of cancer. One is human papillomavirus (HPV). That’s why experts recommend that children get immunized for the virus, which is usually spread through sexual contact.
However, not all people who get HPV develop cancer. And even if they do, they don’t contract that cancer from another person; it’s the exposure to the virus that, in some, causes their cells to mutate.
Epstein-Barr virus is similar. Almost everyone is infected with this virus, which can cause mononucleosis, and a very small fraction of those infected will one day develop cancer. Still, a person cannot catch that cancer from another.
Is it possible that we’ll one day discover a contagious cancer in humans? Yes. But at least as of now, the number of species in which we’ve never observed such cancers dwarfs those where we have. So, there’s good reason to hope we’ll remain in that vast majority.
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Dr. Hal Scofield is a physician-scientist at the Oklahoma Medical Research Foundation, and he also serves as Associate Chief of Staff for Research at the Oklahoma City VA Medical Center. Adam Cohen is OMRF’s senior vice president and general counsel. Send your health questions to contact@omrf.org.
