Scientists have documented an outbreak of transmissible melanoma among brown bullhead catfish in Lake Memphremagog, which straddles Vermont and Quebec. The disease first appeared in 2013, with roughly 26% of the catfish population initially displaying dark lesions on their skin, fins, lips, gills, and eyes. The outbreak was unexpected, as anglers had not reported such markings the previous year, and the condition appeared suddenly across the lake.
Researchers initially pursued several hypotheses to explain the melanomas. Since catfish typically inhabit deeper waters with minimal sun exposure, UV radiation seemed unlikely as a cause. The team investigated whether chemical pollutants or toxins from a nearby landfill might be responsible, but testing revealed minimal contamination. A viral origin was also explored but ultimately ruled out through genome sequencing studies.
Genetic analysis revealed the critical finding: tumor cells from different fish were nearly identical to one another but genetically distinct from their host fish. This pattern indicated the cancers were clones spreading between individual catfish rather than independent malignancies arising within separate hosts. Researchers hypothesize transmission occurs during spawning season, when catfish congregate in dense clusters and may inadvertently scratch one another with sharp spines, potentially creating entry points for cancer cells.
Transmissible cancers remain exceedingly rare in nature, previously documented only in Tasmanian devils, certain dog populations, and some molluscs. Current infection rates in Lake Memphremagog average approximately 30% across the lake’s catfish population on both the United States and Canadian sides, though affected fish appear capable of surviving with the tumors for extended periods. Researchers intend to investigate whether environmental factors or immune suppression contribute to the outbreak’s progression, while also examining similar cases in other freshwater systems to determine whether transmissible cancers may be more prevalent than previously recognized.
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