Researchers have identified a rare form of melanoma that appears to spread directly between freshwater catfish, behaving more like a parasite than a conventional tumor. The discovery opens a surprising new chapter in cancer biology.
For more than a decade, biologists studying brown bullhead catfish in Lake Memphremagog, which straddles the Vermont-Quebec border, puzzled over an alarming sight: increasing numbers of fish bearing raised black lesions across their bodies. Now, researchers say those lesions are something extraordinarily rare—a transmissible melanoma capable of moving from one animal to another.
The discovery, published in Nature, marks the first known transmissible cancer in a freshwater species and only the fourth naturally occurring transmissible cancer ever documented.
Unlike conventional cancers, which arise independently within each individual, transmissible cancers spread when living cancer cells themselves pass between hosts, functioning almost like infectious parasites. Until now, naturally occurring transmissible cancers had been confirmed only in dogs, Tasmanian devils, and several shellfish species.
Researchers analyzed DNA from tumors and healthy tissue collected from affected fish. Instead of matching the genetic profile of each individual fish, tumor cells from different animals were found to be nearly genetically identical to one another, indicating that the cancer originated from a single ancestral tumor lineage rather than developing independently in each fish.
The disease has been observed since at least 2012, when anglers and wildlife officials first reported unusual black skin patches on brown bullhead catfish. Surveys later found that roughly one-third of the lake’s population showed signs of the disease. Researchers have since identified affected fish in additional lakes across New England and eastern Canada, suggesting the cancer may be more widespread than initially believed.
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Scientists do not yet know precisely how the cancer spreads, although they suspect transmission may occur when fish gather closely during spawning or through skin injuries that allow cancer cells to enter new hosts. Experimental studies to directly observe transmission are still underway.
The finding has implications well beyond fisheries biology. Understanding how these rare cancers evade immune defenses and survive outside their original host may provide new insights into the evolutionary limits of cancer and the biological mechanisms that normally prevent tumors from becoming contagious.
For now, researchers emphasize that the disease affects fish—not humans—and there is no evidence that this cancer poses any risk to people. But its discovery highlights how much remains unknown about cancer’s ability to evolve in the natural world, reminding scientists that even one of medicine’s oldest adversaries may still have unexpected forms waiting to be discovered.
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