🌅 The Fourth Transmissible Cancer Known to Science

SOURCE

Curd et al. (2026)

WHAT TO KNOW

• A new study by researchers at the University of Vermont documents what appears to be the first transmissible cancer ever observed in fish, identifying a type of contagious skin cancer among nearly a third of brown bullhead catfish in Lake Memphremagog, a 30-mile-long waterway straddling the border of Vermont and the Canadian province of Quebec. The cancer, a type of melanoma, manifests as ink-black tumors on the fins, gills, lips, and eyes of the fish. Genetic analysis revealed the cancer cells are near-identical clones of one another, suggesting they descended from an ancestral cancer cell that emerged in a single, long-dead “patient zero” years ago. It also appears to be the first transmissible cancer observed in fresh water, which surprised scientists since cells typically require salty fluid to survive.

WHY IT MATTERS

• Cancers are almost always caused by mutations in the host’s own cells. However, transmissible cancers represent a rare exception in which living tumor cells can escape one host organism and take root in another. Prior to the present study, only three groups of species were known to have transmissible cancers: dogs, Tasmanian devils, and mollusks. The authors of the present study aren’t certain how the catfish are spreading the cancer to one another, but they suspect it happens during spawning season, when adult bullheads crowd tightly in shallow waters.

CONNECT THE DOTS

• Some research suggests that transmissible cancers are a new species in their own right, while other work has gone so far as to argue that entire animal species might have begun life as cancer cells millions of years ago. While the skin cancer affecting catfish is just now being extensively studied, it may actually be more than a century old: In 1858, famed naturalist Henry David Thoreau wrote about seeing “inky-black” lesions on brown bullheads while visiting the Assabet River in Massachusetts.