Scientists Discover a Contagious Cancer Spreading Among Freshwater Catfish

A Cancer That Jumps From Fish to Fish

A team of researchers from Vermont has identified what they say is the first-ever transmissible cancer in a freshwater fish species. The discovery, published on July 22, 2026 in the prestigious journal Nature, upends what scientists thought they knew about how cancer spreads — and raises urgent questions about the health of aquatic ecosystems across the northeastern United States and Canada.

Researchers emphasized that the disease the fish carry "poses no known threats to humans." But the implications for wildlife are striking. As lead study author Julie Dragon, a data scientist at the University of Vermont, put it: "The cancer cells behave more like parasites than conventional tumors, moving from fish to fish."

A Mystery That Started With Black Spots

Anglers in Lake Memphremagog, which spans the border between Quebec and Vermont, first reported catching brown bullhead catfish with strange black spots and lumps in 2012. They turned out to be melanoma skin cancer tumors. The diagnosis baffled scientists. "This was surprising," Dragon said. "We wanted to know how a bottom-dwelling fish was getting a cancer we associate with exposure to too much sunlight."

After a virus was ruled out, experts suspected an environmental contaminant, since brown bullhead are considered an indicator species for water and environmental quality. Instead, further testing revealed melanoma. Some environmentalists wondered if the nearby Coventry Landfill might be leaching toxins into the lake, but the research team found no evidence for this. The cancer is found evenly throughout the 32-mile-long lake, in reproductively isolated populations of fish — not clustered near the landfill, as would be expected if it were the culprit.

What the DNA Revealed

Researchers tested the hypothesis that the affliction represents a clonally transmissible cancer — a rare phenomenon in which cancer cells spread between individuals, behaving more like parasites than conventional tumors. Whole-genome sequencing of tumor and matched non-tumor host tissues revealed that tumor genomes are more closely related to each other than to their hosts. Hundreds of thousands of genetic variants are shared among tumor samples but absent from host fish, vastly exceeding levels seen in conventional cancers.

These findings indicate that melanoma in these brown bullheads represents the fourth documented type of naturally occurring transmissible cancer in animals, after dogs, Tasmanian devils, and several bivalve species. Now, about a third of the brown bullheads living in the lake have tumors. It's not entirely clear how sick the fish become. In some cases, the cancer spreads to other organs, such as the brain or liver. But many fish with lesions seem relatively healthy, and some older fish even have them, suggesting they can live with the disease for a time.

What Comes Next

Study co-leader Mark Henderson, a fish biologist at UVM's Rubenstein School of Environment and Natural Resources, noted that "it seems to only happen in larger fish that are of spawning age" — a detail that could be key to understanding how the cancer transmits between individuals, possibly during close contact at breeding time.

The study also found genetic evidence suggesting the cancer lineage may have originated outside Lake Memphremagog, though researchers say more work is needed to determine where it first emerged. The findings raise broader questions about how pollution, habitat stress, and climate change may influence the evolution and spread of diseases in wildlife. For now, scientists have cracked open a door into a strange and unsettling corner of nature — one where cancer itself is contagious, and a freshwater lake holds clues that could reshape our understanding of disease in the wild.