In mussels, cancer is contagious

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Scientists have discovered that cancer-contaminated mussels on the Pacific coast of Canada have transmitted their disease to their sisters in Latin America and Europe.

"There is no natural explanation for how it happened without human help," reports Michael Metzger, a biologist at the Pacific Northwest Research Institute in Seattle and co-author of a study on the subject, published in eLife magazine.

Cancer usually develops when cells acquire new mutations and then multiply. Only the immune system or the drugs are able to stop the process. When it's impossible, the cancer host dies and takes the disease with him.

From 1990, however, this perception of the disease has evolved. The Tasmanian devils began to develop tumors on their faces, but the DNA of the tumors was different from that of the affected animals.

Only one possibility: the cancers had been transmitted by other Tasmanian devils. When they fight, these animals pass tumor cells that migrate to the face and then turn into a tumor.

It seems that cancer is also transmissible in aquatic animals, as Dr. Metzger discovered when he was working at Columbia University. The sick molluscs would release cancer cells, which would float in the currents until they reach other animals.

Boat trip

This discovery allowed Nicolas Bierne, from the Institute of Evolution Sciences of Montpellier, to solve a mystery concerning the common mussels.

The researcher was unable to determine why genetic markers of Mytilus trossulus (which do not exist in Europe but could be called "bay mussels") were found in French mussels, while both species live in different waters and are unable to mate.

Cancer seems to be a plausible explanation. Common mussels may have been infected with the same cancer that had hit the mussels on Canada's Pacific coast – confirmed by DNA from French mussel cancer cells, which was closer to the DNA of infected Canadian mussels than that of healthy mussels.

At the same time, South American scientists, including Nuria Vázquez, discovered mussel banks infected with disease on their shores. Molluscs had the same characteristics as sick mussels in Canada and French mussels.

It remains to be seen how cancer cells have managed to cross several oceans. According to Dr. Metzger, human beings are at the origin of the displacement of the disease. Mussels hang easily to the hull of boats and travel in this way; they then arrive in new waters and infect local species.

While this discovery in mussels may seem anecdotal, it speaks volumes about the transmissibility of cancer and opens new perspectives for research on this disease. However, be reassured, this cancer can not be transmitted to humans, says Antonio Villalba, researcher at the Marine Research Center of the Junta de Galicia.

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