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Scientists say that now although it may be hard to believe infectious cancer cells are drifting in the ocean, spreading a devastating leukemia— in clams. 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. 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 typically 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. It was not a virus hopping from clam to calm 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. 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.

http://www.nytimes.com/2015/04/14/science/a-clam-cancer-outbreak-spread-by-one-set-of-cells.html?ref=topics

A Clam Cancer Outbreak, Spread by One Set of Cells

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