First Transmissible Cancer Found in Brown Bullhead Catfish

Scientists have discovered the first known transmissible cancer in fish, a melanoma spreading among brown bullhead catfish in lakes across Quebec and the northeastern United States. The findings, published on July 22, 2026, in the journal Nature, confirm the disease as one of only a handful of contagious cancers ever identified in the animal kingdom.

The unusual tumors were first reported by anglers in Lake Memphremagog, a body of water straddling the Vermont-Quebec border, in 2012. Surveys later indicated that by 2014, roughly one-third of the lake's brown bullhead catfish were afflicted with dark, raised skin lesions. Initial investigations explored whether pollution, viruses, or inbreeding were responsible. However, genetic testing found no viral cause and showed the fish population had strong genetic diversity.

The breakthrough came when researchers compared the tumors with healthy fish tissue. They found the cancer cells were genetically far more similar to each other than to the fish they were growing on, a hallmark of a clonally transmissible cancer where the tumor cells themselves act as infectious agents. The study reports hundreds of thousands of shared genetic...