- Since 2012, brown bullhead catfish in Lake Memphremagog (Vermont/Quebec) have shown a high rate of melanomas, suggesting a causal contaminant or contagion.
- Whole-genome sequencing revealed that tumour mitochondrial and nuclear genomes are more closely related to each other than to their hosts, indicating a clonal transmissible cancer.
- Hundreds of thousands of genetic variants are shared among tumour samples but absent from host fish, vastly exceeding the levels seen in conventional cancers.
- This melanoma represents the fourth documented type of naturally occurring transmissible cancer in animals, after dogs, Tasmanian devils, and several bivalve species.
- Phylogenetic analyses of mitochondrial and nuclear DNA, structural variants, and copy-number variation all support a single clonal tumour lineage in the lake.
- No viruses or microorganisms were uniquely associated with the tumours, further supporting that cancer cells themselves are the infectious agent.
- The cancer likely originated outside Lake Memphremagog and may spread through reproductive behaviors or the aquatic environment.
- The long-term impact on brown bullhead populations is unclear, but the cancer's rapid spread (23–37% incidence) raises conservation concerns.
- This is the first documented transmissible cancer in a fish species and in a freshwater ecosystem.