Scientists have discovered a new transmissible cancer spreading among brown bullhead catfish in Lake Memphremagog on the Quebec-Vermont border. Researchers used whole-genome sequencing to reveal that the fish’s inky skin lesions are clonal tumours passed directly between hosts, making it the fourth known transmissible cancer in the animal kingdom.
For more than a decade, anglers fishing the waters of Lake Memphremagog — which straddles the Canadian province of Quebec and northern Vermont — noticed something unsettling about the local brown bullhead catfish population. The fish carried dark, inky streaks and blotches across their bodies. It looks like black tar on the skin — not very pretty,
says Peter Emerson, a fish biologist with the Vermont Fish & Wildlife Department who has participated in the annual fish census since 2009. You can tell it doesn’t help the fish cause they’re inside the mouth, they’re on the fins.
Sometimes the dark marks cover the eyes, lips, or gills.
When the state wildlife agency first documented the condition during censuses, just over a quarter of the catfish showed these blemishes. Anglers had reported seeing them the year prior, but before then, the markings were absent. They arrived very suddenly,
Emerson explains.
Searching for Toxins and Viruses in Lake Memphremagog
When state biologists first identified the blemishes as malignant melanoma, the diagnosis baffled researchers. Catfish are benthic, bottom-dwelling fish that feed at night, meaning they have little exposure to the ultraviolet solar radiation that typically drives human melanoma. Suspecting an environmental pollutant, investigators looked toward potential toxins in the water, noting that the Coventry Landfill sits nearby, and considered whether historic events like the flooding caused by Hurricane Irene in 2011 had stirred up chemical carcinogens. Yet, extensive testing revealed the fish did not have significant levels of those suspected contaminants.
With environmental theories coming up short, scientists turned to infectious agents. Julie Dragon, a data scientist at the University of Vermont and a member of the UVM Cancer Center, investigated whether a virus might be responsible for triggering the cancers. She found no viral evidence. The breakthrough came only when falling costs made high-resolution genetic analysis feasible. Working alongside the Vermont Fish and Wildlife Department and USGS scientists, University of Vermont researchers sequenced the DNA of more than 80 brown bullheads—both diseased and healthy specimens caught in 2023, alongside archival samples from 2019—analysing both healthy tissue and tumour samples from the fish.
DNA Sequencing Reveals Clonal Tumours and a Rare Phenomenon
The original genetic hypothesis assumed that each tumour would trace back to the individual fish carrying it, forming an evolutionary branch linked by shared host-variant genes. Instead, research published in the journal Nature revealed an unexpected biological reality. The tumours taken from different fish were more genetically similar to one another than they were to the hosts carrying them. In some instances, a tumour matched healthy fish from entirely different bodies of water more closely than it matched the catfish on which it grew. Dragon explains that the growths were actually clones spreading from catfish to catfish.
This discovery establishes brown bullhead transmissible melanoma, or BBTM, as the fourth known transmissible cancer in the natural world. Prior to this finding, contagious cancers had been documented in dogs, Tasmanian devils, and bivalves. The revelation also sheds light on historical anomalies. In 1852, the essayist Henry David Thoreau wrote about observing bullhead catfish bearing black velvety patches in a Massachusetts river, and sixty years later zoologist Raymond Osburn investigated similar reports, successfully transferring bullhead tumour cells into a healthy fish in a laboratory setting. While researchers cannot confirm whether those historical observations were identical to BBTM, the accounts suggest the disease has deep historical roots in New England waterways.
“It is the infectious agent itself. Those cells can somehow escape the host, maybe live in the environment — the aquatic environment — and then infect a new host.”
Julie Dragon, Data scientist at the University of Vermont
Wider Implications for Cancer Biology
Beyond ichthyology, cancer biologists view the catfish discovery as a natural model to help answer broader questions about oncology. Understanding how these contagious cancer cells survive outside a primary host, bypass immune system defences, and establish themselves in a new organism could provide vital insight into how human cancers metastasize through the bloodstream to distant organs.
