Scientists studying fish populations in Lake Memphremagog, which borders Vermont and Quebec, have documented an unusual disease outbreak among brown bullhead catfish. Beginning in 2013, roughly one-quarter of the catfish population displayed dark, tar-like lesions on their skin, fins, and around their eyes and mouths. The blemishes emerged suddenly, with no prior documented cases in the lake, though anglers had observed them shortly before the research team’s discovery.
Investigators initially suspected environmental factors, including potential contamination from a nearby landfill, or damage from sun exposure. However, testing revealed no significant toxin accumulation in affected fish. Genetic analysis ultimately revealed that the lesions were melanomas spreading between individual catfish rather than arising independently in each host. The tumor cells were genetically similar to one another but distinctly different from the fish carrying them, suggesting they originated from a single individual and have propagated through the population since.
Researchers believe transmission likely occurs during spawning season when catfish cluster together and may accidentally wound each other with sharp spines, creating an entry point for cancer cells. Currently, an average of 30 percent of the catfish population throughout the lake—on both sides of the United States-Canada border—carry the melanoma. Scientists report the fish can survive with the tumors for extended periods.
This represents only the second known instance of transmissible cancer among vertebrates and the first documented in fish. The finding has captured scientific attention because the disease mechanism may provide insights into how cancers metastasize in human patients, particularly how tumor cells escape their original site and establish themselves in new locations while evading immune responses. Researchers plan to investigate whether similar transmissible melanomas exist in other freshwater systems and whether historical records, possibly including observations by naturalist Henry David Thoreau, document earlier occurrences of the disease.
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