Overview
- A study published on July 22, 2026 reports that melanoma tumors on brown bullhead catfish from Lake Memphremagog share hundreds of thousands of genetic variants and form a single lineage distinct from their hosts.
- Researchers interpret those shared genomes as strong evidence the tumors are a transmissible cancer lineage, making this the first confirmed example in any fish and the first in a freshwater species.
- Laboratory transmission between fish has not yet been demonstrated, so scientists stress the finding is genomic evidence that needs follow-up experiments to prove how the disease spreads.
- Earlier work found higher concentrations of several metals, including arsenic, in affected fish and PFAS testing is ongoing, leaving open the possibility that environmental contaminants influence susceptibility or spread.
- The tumors are widespread locally and seen in other New England and Quebec waters, prompting public-health precautions such as Quebec's recommendation not to eat visibly affected fish and coordinated research to test transmission, immune evasion, and ecological impact.