Contagious Cancer Found in Lake Catfish, Scientists Report

Scientists have discovered a contagious cancer spreading among brown bullhead catfish in a freshwater lake straddling the Vermont-Quebec border — the first documented case of a transmissible cancer in fish and only the fourth known example in the animal kingdom. The finding, published in the journal Nature on July 22, reveals that between 35% and 45% of mature brown bullhead catfish (Ameiurus nebulosus) in Lake Memphremagog are affected by melanoma, a rate of cancer never before documented in any wild fish population.

A Decades-Old Mystery

Anglers and wildlife biologists first noticed unusual black skin lesions on catfish in Lake Memphremagog in 2012. By 2014, surveys indicated that roughly one-third of the lake’s brown bullhead catfish carried these inky black, sometimes crusty tumors. Some fish displayed “tumors on top of tumors,” with the cancer spreading to the brain, liver and other organs, according to Julie Dragon, a data scientist and associate professor at the University of Vermont who led the research.

Initial suspicions focused on environmental pollution or a viral infection. The lake, which provides drinking water for over 175,000 people in Quebec, has faced previous contamination concerns including PFAS from a nearby landfill. However, as Nature News reported, genetic testing ruled out a widespread viral cause and pointed to something far more unusual.

Genetic Evidence of a Parasitic Cancer

Whole-genome sequencing of tumor cells from different fish revealed hundreds of thousands of shared gene variants that formed a single genetic lineage — one completely distinct from the healthy cells of their individual hosts. This is the hallmark of a transmissible cancer, where living cancer cells physically transfer from one individual to another, essentially becoming a parasitic cell line that can outlive its original host.

“We had no inkling that this was going to be the case,” Dragon told Gizmodo. “It’s still mind-blowing.” She explained that in conventional cancers, comparing more tumors reveals fewer shared variants. “In the case of transmissible cancers, it is the opposite. The more you sample, the more shared variants you find.”

To put this in perspective, the researchers compared their findings to human melanoma data from The Cancer Genome Atlas. Among 463 human melanomas, 94% of genetic variants were unique to a single cancer. In the catfish, the majority of tumor-specific variants were shared across nearly all sampled fish.

Only the Fourth Known Example

Transmissible cancers have previously been documented in only three other animal groups: canine transmissible venereal tumor in dogs (spread through sexual contact), devil facial tumor disease in Tasmanian devils (spread through biting), and bivalve transmissible neoplasia in at least 10 species of clams, cockles and mussels.

Michael Metzger, a biologist at the Pacific Northwest Research Institute, told Nature he “wasn’t expecting it in a freshwater species.” His research has shown that marine bivalve cancer cells can survive for days in saltwater but die instantly in freshwater, raising questions about how the catfish cancer transmits.

Open Questions and Implications

How the cancer spreads remains unknown. It has only been found in sexually mature catfish, suggesting transmission may occur during spawning season when fish huddle closely together. Alternatively, cancer cells might survive in lake-bottom sediments. The tumors also contain high levels of arsenic, a known carcinogen, which may make fish more vulnerable even if not the direct cause.

A critical question is how the cancer evades the fish’s immune system. “We will be exploring that for sure because it can tell us something about how cancers avoid the immune systems in humans as well,” Dragon said.

The cancer has also been found in catfish populations in other ponds and lakes across New England, including locations without major human-made pollutants. Historical accounts suggest it may have existed for centuries — naturalist Henry David Thoreau wrote about similar-looking fish in Massachusetts in the mid-1800s.

No Human Risk

The cancer does not pose any danger to humans. However, researchers emphasize that the discovery challenges fundamental assumptions about cancer biology. As Dragon noted, “What I hope folks take away is that cancer goes beyond humans, and genomics is a powerful tool for researching cancer and other diseases.”

The researchers argue that transmissible cancers may be “more widespread, but we are just not noticing them.” The finding opens new avenues for understanding how cancers evolve into parasitic cell lines and how they evade immune systems — insights that could ultimately benefit human medicine.

What’s Next

The research team plans to continue studying affected catfish populations across New England to determine the cancer’s age, origin, and long-term impact on fish populations. Unlike Tasmanian devil facial tumor disease, which has a 90% fatality rate, the health effects on catfish remain unclear. Determining the transmission mechanism and population-level consequences will be critical for conservation efforts and for understanding whether similar phenomena exist in other fish species.