Cancer that spreads like an infection? A third of catfish in one lake share the same tumor — here's why

Cancer does not spread from one body to another — that has long been accepted as common knowledge, much as it cannot be caught the way a cold can. Now a fourth exception to that rule has been identified: a skin cancer found in catfish living in a lake straddling the US-Canada border.

The cancer did not originate within the fish themselves. Malignant cells had transferred wholesale from other fish, effectively acting as the infectious agent.

A joint research team led by professor Julie Dragon and Dr. Emily Curd of the University of Vermont published their findings in the journal Nature in 2026.

The team caught 103 catfish from Lake Memphremagog, which spans Vermont and Quebec, Canada, and sequenced their entire genomes.

Starting in 2012, anglers and biologists began reporting unusual fish in the lake — animals covered in dark, raised growths.

Tissue analysis confirmed the growths were malignant melanoma. Between 2014 and 2017, between 23 and 37 percent of catfish caught in the lake were affected.

Melanoma had been reported in the same species elsewhere, but never at such high rates. The lake also serves as a drinking water source for 175,000 people.

When the research team analyzed the cancer cells, the results defied expectations.

For cancer to develop, a cell's own DNA must mutate. That would mean the genetic makeup of a tumor should closely resemble the genome of the fish it grew in.

The opposite was true. The cancer cells' genes were far more similar to the cancer cells of other fish than to the genome of their own host.

When the team constructed a phylogenetic tree, all the cancer cells clustered together in a single group. Tumors from fish caught in different years — 2015, 2019 and 2023 — all fell on the same branch, indicating they descended from a single ancestral cell.

Analysis of 686,296 nuclear genome variants produced the same result.

The numbers make the distinction even clearer.

The research team identified 245,189 mutations present only in the cancer cells and absent from normal cells.

Of the mutations found exclusively in cancer cells, 59 percent were shared by at least 14 of the 16 fish sampled. By contrast, 83.4 percent of the mutations in normal cells appeared in only a single fish.

For comparison, the team analyzed human melanoma data from an international cancer genome database. Among 463 human melanoma cases on record, 94 percent of mutations appeared in only one patient. Out of 498,648 cases, just 40 — or 0.008 percent — involved a mutation shared by 10 or more patients.

Even when the team randomly drew 16 cases from that database and repeated the process 100 times, not a single run produced a mutation shared across all 16. That confirms that independently arising cancers do not share genetic variants.

The team also investigated whether a virus might be responsible. They screened all tissue samples for viruses and microorganisms but found no common pathogen attached exclusively to the cancer cells.

Until now, only three types of transmissible cancer had been documented in nature: cancers found in dogs, Tasmanian devils and bivalves.

The canine cancer arose thousands of years ago and typically resolves on its own. The Tasmanian devil cancer emerged only decades ago but is fatal in nearly every case, having reduced the species' population by 90 percent.

This is the first time a transmissible cancer has been confirmed in fish, and the first such cancer found in freshwater.

The transmission route has not been identified. Dogs spread the cancer through mating; Tasmanian devils through territorial fighting; bivalves exchange cancer cells through seawater.

The research team looked to the catfish's own behavior for clues. The cancer has never been found in fish that have not yet reached reproductive age.

Catfish congregate in tight groups during spawning season, pressing their bodies together. Because they lack scales and live close to the bottom, the team also raised the possibility that cancer cells mixed into the sediment could attach to passing fish.

However, the team said it is not yet known what effect the cancer has on the catfish. It also remains unclear whether the cancer has already spread to catfish populations elsewhere in North America.

DOI: 10.1038/s41586-026-10828-6

Curd, E.E., Hart, S.F.M., Lubkowitz, J. et al. Brown bullhead catfish melanoma represents a novel transmissible cancer. Nature (2026).