Outbreak: Cancer of the clam

A soft-shelled clam.
A soft-shelled clam.

Cancer is not supposed to be a cold; it’s not supposed contagious. It may metastasize and spread from your face to your lungs, but I can’t catch it from you. For humans cancer isn’t contagious; for clams it is.

The clam, Mya arenaria, is the famous ‘fried clam’, a staple of clam shacks all along New England coastal roads. It also challenges our notions of cancer biology. Recently, a type of bivalve leukemia has been discovered, one that is transmissible between clams. Just as I can catch a cold from you, a clam can catch a cancer from its neighbor . Contagious cancer breaks our original epidemiological dogma of cancer that it stays within an individual and does not jump to another. Science is aware of only two other transmissible cancers – one in Tasmanian devils and one in dogs spread via venereal diseases.

What makes the clam cancer remarkable is its method of transmission. In devils and dogs (I wrote ‘gods’ at first, whoops!), cancer is transmitted via direct routes of fluid exchange (biting, sexual exchange). Clams are not known to bite or fornicate physically. The cancer leaves and lives outside the host floating on ocean currents long enough to infect another clam. It is unknown how long a cancer can survive outside the clam. Sperm cells in corals can last up to 4 months in the water. If cancer cells have similar survival then that means a movement of 100’s or 1000’s of kilometers on currents of cancer. Regardless if clam cancer cells move millimeters or miles, that they can survive in seawater outside the host is remarkable. Remember that when your wading the waters of Cape Cod this summer.

Fret not steamer slurpers and fried clam connoisseurs! A cancerous clam infects only other clams. We know of no cancers that jump from one species to another. Yet.

For the nitty gritty science: Metzger et al. 2015. Horizontal transmission of clonal cancer cells causes leukemia in soft-shell clams 

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