Transmissible melanoma identified in catfish from Lake Memphremagog

Researchers with the Vermont Fish & Wildlife Department first noticed dark, tar-like markings on brown bullhead catfish in Lake Memphremagog in 2013. About 25 % of the fish examined that year showed the lesions, which appeared on fins, lips, gills and sometimes eyes. The abnormalities were later identified as a transmissible skin cancer.

A study published in Nature reports that the melanoma cells are genetically distinct from their host fish and closely resemble each other, indicating they are clonal and spread from one catfish to another. Whole-genome sequencing showed the tumors did not match the genetic profile of the individual fish, supporting the conclusion that the disease is infectious.

"It is the infectious agent itself," said Julie Dragon, a data scientist at the University of Vermont and co-author of the paper. "Those cells can somehow escape the host, maybe live in the aquatic environment, and then infect a new host."

The researchers propose that transmission may occur during spawning, when fish gather in dense clusters and could inadvertently scratch each other with spines near their pectoral fins. About 30 % of catfish across both the U.S. and Canadian sides of the lake now carry the melanoma, though many appear to survive with the tumors for extended periods.

Evolutionary biologist Vincent Lynch, who was not involved in the study, noted that most cancers die with their host, making a transmissible form advantageous for the cancer cells. He suggested that studying this fish melanoma could help illuminate mechanisms of metastasis in humans, though practical applications remain distant.

The findings add to the short list of known transmissible cancers, which includes diseases in Tasmanian devils, dogs and certain mollusks, and raise the possibility that such cancers may be more widespread in wildlife than previously recognized.