These Catfish Have Been Catching Cancer From Each Other

Cancer is contagious in Lake Memphremagog. Since at least 2012, brown bullhead catfish here have been passing tumors between themselves. The cells carry their own genetic identity, unrelated to any host they infect, all descended from one cancer in one original catfish.

About 30 percent of the lake's brown bullhead population now carries the condition: black, crusty melanomas on the skin, gills and eyes, with some fish showing tumors stacked on old tumors, the cancer reaching the brain and liver in severe cases. There is no clear mortality signal. The fish appear to live with it.

Biologists noticed the lesions in 2013 and suspected a nearby landfill, then a viral infection. Both hypotheses failed. Whole-genome sequencing showed the tumors were genetically closer to each other than to any of the fish they grew on, sharing hundreds of thousands of variants across different individuals and tracing to a single founding lineage.

When the lineage began is unknown. Henry David Thoreau described what was probably this same condition in Massachusetts catfish in the 19th century, recording their dark, encrusted lesions without knowing what he was looking at.

Only three other animals are known to carry transmissible cancers: dogs, Tasmanian devils and several shellfish species. This is the first documented case in fish, and only the fourth in any animal. How it passes between hosts is not yet confirmed; researchers suspect direct contact during spawning, when the fish press together in close clusters and sharp pectoral fin spines break skin.

Only four animals are confirmed to carry transmissible cancers. Researchers suspect that number is low.

Read the full story at NPR, July 22, 2026

Read the research at Nature, July 22, 2026

Hot Take: Thoreau described this in the 19th century and had no framework for what he was seeing. Science caught up 150 years later. The cancer was not waiting.