Overview
- An international team reported in Nature that tumor cells from brown bullhead (brown catfish) are genetically closer to one another than to their hosts, a pattern that indicates a single clonal cancer moving between fish.
- The study, published in Nature on July 22, 2026, followed years of reports from Lake Memphremagog where 23 to 37 percent of sampled fish had confirmed melanomas and researchers expanded sampling to nearby lakes and ponds.
- Researchers found no consistent virus or bacteria in tumor samples and instead conclude the transmissible agent is likely the cancer cells themselves, though an infectious role in origin has not been ruled out.
- Scientists are testing how the cancer spreads with leading hypotheses that include close contact during spawning, short-term survival of tumor cells in water or sediment, and changes in reproductive immunity, and they note larger, spawning-age fish are most affected.
- Tumor tissues show elevated arsenic that correlates with regional soil levels but does not prove causation, there is no evidence the cancer can infect people, and authorities say local drinking water remains safe while ecological impacts are still being mapped.