The idea that infectious cancer cells are drifting in the ocean, spreading a devastating leukemia-like disease may sound like a dystopian fantasy. But scientists say that is exactly what is happening — in clams.
It is no surprise that clams suffer from the disease. For at least 40 years, outbreaks of the clam equivalent of leukemia have been hammering populations of soft-shell clams (Mya arenaria), also called steamers and little necks, along the East Coast from Maine to the Chesapeake, causing declines in harvest and loss of jobs.
But the cause of the disease and how it spread were unknown. Investigations had pointed to environmental factors — at least affecting the susceptibility of the clams — and researchers had recently come to suspect the disease was transmitted by a virus.
When American and Canadian researchers looked into that possibility, they ended up sequencing the genes of the cancer cells, much as a forensic expert would sequence DNA at a crime scene. And they found that the cancer cells in clams from waters from different locations — Prince Edward Island, Maine, and New York — pointed to the same culprit.
“We realized that maybe this was a clone of cells that had spread,” said Stephen P. Goff at Columbia University. One original cancer in one clam had escaped its host, and spread from one clam population to another.
Cancers usually arise in an animal when its own cells start growing out of control. That is why cancer cells have the same genes as the person or animal suffering from cancer. But, Dr. Goff said, he and his colleagues found that none of the cancer cells had DNA matching the clams they were found in. Instead, the cancer cells were like one another; except for minor differences, they all had the same DNA. That meant they all came from one original case of cancer in one clam.
As Dr. Goff and his colleagues, Michael J. Metzger at Columbia and Carol Reinish and James Sherry at Environment Canada, reported in the journal Cell, it was not a virus hopping from clam to clam but the cancer cells themselves. They may last only hours in seawater, but that is long enough to reach other clams and infect them.
This is the third such cancer known in nature. A devastating facial tumor in Tasmanian devils spreads by biting, and a tumor in dogs spreads by sexual contact.
Elizabeth Murchison, of the University of Cambridge, who studies these transmissible cancers, said in an email that she was not that surprised that a third transmissible cancer had been discovered. But, she added, “I would not have guessed that it would be clams!”
Dr. Goff said that he and Dr. Metzger were approached by Dr. Reinish because of their expertise in mouse leukemia viruses. They started out looking for evidence of viruses but quickly came to suspect that the cancer cells themselves were spreading, and further tests confirmed that conclusion.
It may be, say the authors, that contagious cancers may be more common in nature than had been thought. Cockles, mussels and oysters have similar diseases, and Dr. Goff and his colleagues will investigate them next.
There are so few cancers that spread in this way, Dr. Murchison said, because to do so, the cancer cells must escape the host, survive long enough to reach another host, and overcome the reaction of the immune system to reject foreign tissue.
Dr. Goff said the way cancers make such a transition may be connected to how cancers spread within a body from one organ to another, called metastasis, a step that makes cancers turn deadly.
But there is nothing to suggest anyone should avoid swimming or worry about eating clams because of cancer fears. Cancers are adapted to individual species, and the human immune system would block such foreign tissue.
Barbara Jacoby is an award winning blogger that has contributed her writings to multiple online publications that have touched readers worldwide.