An unexpected transmissible cancer has been detected in North American waterways, prompting new concerns about how the disease developed.
Beginning with discoveries in Lake Memphremagog, a glacial lake spanning both Quebec and Vermont, researchers have found strange black cancerous lesions on fish in the waterways of New England and Canada, revealing an environmental mystery.
University of Vermont researchers announced the startling cancer discovery in a recent paper published in Nature, describing how the cells act similarly to a parasite, transmitting between hosts.
This discovery marks the first time a freshwater species has been identified as afflicted with a transmissible cancer, whose long-term population effects are a major concern.
Fish Cancer
Many questions swirl around this discovery, which go far beyond the specific locations and even species affected—such as how cancer operates and how this transmissible form could have originated.
The first indications that something was amiss came from anglers and biologists who both noticed more brown bullhead catfish with raised black patches coming out of Lake Memphremagog. From their first identifications in 2012 to 2014, the prevalence of the disease increased to one in three of these fish.
These particular catfish are often used as a barometer for environmental health, leading the University of Vermont team to consider a pollution-related pathogen after they had ruled out a viral one. Yet under close examination, they discerned the lesions were something even less expected: a type of skin cancer called melanoma, typically caused by sunlight, and therefore, exceedingly rare in bottom-dwelling fish.
Genetic Research
The melanoma finding set the team on a research course looking to identify whether a genetic mutation had left this catfish population particularly susceptible to skin cancer. What they found was surprising: these cancer cells were more closely related to one another than to their host fish’s healthy cells, suggesting transmission.
These transmissible cancers are extremely unusual, with only three types having been discovered previously. None have been found in freshwater or fish species before this; they have been relegated to Tasmanian devils, dogs, and mollusks.
So far, researchers have only identified the malady in larger, spawning-age fish, leading the team to suspect that transmission occurs during spawning behavior. These events do appear to be a likely candidate, as the fish cluster tightly together in shallow water, providing ample opportunity for physical contact among many fish. Furthermore, due to their lack of scales and bottom-dwelling nature, they are more susceptible to the sediment, which could harbor free-floating cancer cells.
In other species, these transmissible cancers may or may not have devastating population-scale effects. Currently, how the Lake Memphremagog catfish population will fare is unknown, although some fish appear to survive years after the infection is first noted.
The University of Vermont team says that the cancer cannot affect humans, and the lake’s drinking water is safe, but they would caution against eating the cancerous fish.
The Root of Cancer
While the Coventry Landfill is in close proximity to the lake, the cancer is spread evenly along the 32-mile-long lake, instead of concentrating near the landfill. Additionally, water tests show that contamination levels are no higher than those of similar North American waterbodies such as Lake Champlain.
Instead, more concerning than a single source of pollution, the team says they have now discovered this cancer in fish located in other ponds and lakes in this region, none of which were close to pollution sources and had pristine water quality.
The one correlation identified so far is that the cancer cells have high arsenic levels, an element naturally found in the region’s soil in high concentrations. Along with this possible link to a natural cause, the researchers are beginning to suspect that the cancer has existed for quite some time, but has simply gone unrecognized. A potential line of evidence for this hypothesis comes from the journals of Henry David Thoreau, who noted a catfish with a similar condition back in the summer of 1852. He even noted that he had come across the fish disease previously, and again wrote of it in 1858.
“By studying how cancers survive and spread outside their original host, we can learn a great deal about what keeps cancers contained—and what happens when those boundaries break down,” co-author Julie Dragon concluded. “These fish provide an opportunity to look at the evolution of cancer.”
The paper, “Brown Bullhead Catfish Melanoma Represents a Novel Transmissible Cancer,” appeared in Nature on July 22, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.