A highly contagious cancer is spreading among fish in the United States.

A highly contagious cancer is spreading among fish in the United States. It is the first transmissible cancer ever identified in a freshwater species.The disease was first noticed in Lake Memphremagog, which straddles the Vermont-Quebec border. What scientists initially thought might be a local infection or a pollution-related mutation proved to be a highly contagious melanoma.Using whole-genome sequencing, researchers at the University of Vermont determined that the cancer cells from infected brown bullhead catfish are genetically identical clones. Living cancer cells leave one host and physically transplant themselves onto other fish, behaving essentially like parasites.Published in Nature, this discovery is only the fourth known transmissible cancer in the animal kingdom and the first recorded in any freshwater species.Although the cancer poses no threat to humans, its rapid spread has prompted urgent study of how it transmits and what ecological effects it may have. The disease primarily affects mature, spawning-age fish, leading researchers to suspect that close contact during spawning or exposure to bottom sediments facilitates transmission.The same cancer has now been found in lakes across Maine, New Hampshire, and New Brunswick, showing that it is not limited to polluted waters. Scientists are examining possible connections to naturally occurring arsenic in New England soils and investigating how these rogue cells survive outside their hosts. The work offers new insight into the ways cancer can transcend ordinary biological limits.[Curd, E. E., et al. (2026). Brown bullhead catfish melanoma represents a novel transmissible cancer. Nature. DOI: 10.1038/s41586-026-10828-6]Science and facts💡