The word “contagious” is not usually used to qualify cancer. While undoubtedly influenced by external factors, conventional knowledge suggests, in all but a few rare cases, that cancer arises and remains in an individual. However, recent research on disseminated neoplasia, a leukemia-like disease in bivalves, suggests that cancerous cells could be transmissible (Nature 534, 705–709; 2016). The study sampled three bivalves—mussels (Mytilus trossulus), cockles (Cerastoderma edule), and golden carpet shell clams (Polititapes aureus). They sequenced and compared host tissue and hemocytes in normal and diseased specimens to identify whether the DNA in neoplastic hemocytes matched the individual, as would be expected, or if it exhibited a distinct genetic origin.
In all three species, the DNA from the neoplastic hemocytes did not match the bivalve tested, suggesting a contagious origin in the wild populations. The cancerous cells in mussels came from a single external source, while neoplasia in cockles may have had two sources. Most curiously, the neoplastic hemocytes sampled from clams originated in an entirely separate (albeit closely related) species. These results add new evidence to cancer transmission and propose that intra- and inter-species infection may occur, at least in marine environments, more commonly than previously thought. EPN
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