Transmissible melanoma discovered in brown bullhead catfish of Lake Memphremagog

Researchers from the University of Vermont and collaborators published a study in *Nature* describing the first known transmissible cancer in a freshwater fish. The disease is a melanoma that appears as raised black skin lesions on brown bullhead (Ameiurus nebulosus) catfish in Lake Memphremagog, a 32‑mile lake that straddles northern Vermont and the Canadian province of Quebec.

Anglers first reported the dark lesions in 2012, and by 2014 roughly a third of the sampled catfish showed the condition. Whole‑genome sequencing revealed that tumor cells from different fish were more closely related to each other than to their host fish, demonstrating that the cancer cells themselves spread from one fish to another—behaving “more like parasites than conventional tumors,” said co‑author Julie Dragon.

The scientists investigated possible environmental causes, including PFAS contamination and a nearby landfill, but found no evidence linking pollutants to the disease. The cancer does not appear to pose any risk to humans; the fish are safe to handle but are not recommended for consumption. This finding constitutes the fourth naturally occurring transmissible cancer documented in animals and the first in a fish or freshwater species, offering new insights into how tumor cells can evade immune systems and spread between hosts.