Transmissible cancer discovered in fish in US and Canada
Vermont scientists stunned to find melanoma that moves from catfish to catfish like a parasite—first-ever transmissible cancer in a freshwater animal
University of Vermont
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Scientists in Vermont have been studying catfish with mysterious black lesions--like this one. They were surprised to learn that the lesions are melanoma skin cancer and more surprised to discover that the cancer is transmissible, moving from fish to fish like a parasite. This is the first time a transmissible cancer has been found in a freshwater animal and only the fourth time in any animal. The diseased fish were first found in Lake Memphremagog on the Vermont/Quebec border, but now the team has found fish with this cancer in other lakes in New England and Canada.
view moreCredit: Vermont Fish & Wildlife
A team of Vermont researchers has discovered that mysterious black skin lesions afflicting catfish in Lake Memphremagog and other New England and Canadian lakes are caused by a transmissible cancer—the first ever identified in a freshwater fish species.
The cancer cells behave more like parasites than conventional tumors, moving from fish to fish.
The discovery, published July 22 in the journal Nature, sheds light on how cancer can spread in the wild and raises important questions about where this cancer originated, how it will affect fish health and populations—and how cancer works in all animals including humans.
Much More Strange
Since 2012, anglers and biologists have reported a striking increase in brown bullhead catfish (Ameiurus nebulosus) with raised black skin patches in the cross-border lake shared by Vermont and Quebec. By 2014, nearly one in three fish showed the dark lesions. “At first, we thought this disease might be a virus, but that wasn’t panning out,” said Julie Dragon, a scientist at the University of Vermont and co-leader of the new study. Because brown bullhead are considered indicators of environmental quality, the researchers suspected a contaminant or pathogen linked to pollution.
Instead, the lesions turned out to be melanoma, a skin cancer. “This was surprising,” Dragon said, “and we wanted to know how a bottom-dwelling fish was getting a cancer we associate with exposure to too much sunlight.” So Dragon, a researcher at the UVM Cancer Center in the Larner College of Medicine, and her colleagues took a “deep dive,” she says, into the genetics of the cancer cells. They expected to find a genetic mutation that makes the fish susceptible to having their own cells become cancerous.
They found something much more strange.
Using whole-genome sequencing, the team compared DNA from tumors and healthy tissues in affected fish. They found that the cancer cells were much more closely related to each other than to their host fish—the signature of a clonally transmissible cancer, in which tumor cells themselves act as infectious agents.
“Hundreds of thousands of genetic variants are shared among tumor samples but absent from host fish, vastly exceeding levels seen in conventional cancers,” the study reports.
Chasing the Cause
This discovery marks only the fourth type of transmissible cancer in the animal kingdom—and the first in any fish or freshwater species. Previous ones include the Tasmanian devil’s facial tumor disease, a transmissible venereal tumor in dogs, and transmissible leukemia-like diseases in clams, mussels, and other bivalve mollusks.
The research team is now working to understand how the cancer cells spread between animals.
"It seems to only happen in larger fish that are of spawning age,” said Mark Henderson, a fish biologist and study co-leader in UVM’s Rubenstein School of Environment and Natural Resources. “Maybe some part of spawning behavior leads to the spreading of the cancer between animals."
During spawning, bullhead cluster tightly in shallow water, allowing physical contact that could transmit tumor cells. As scaleless, bottom-dwelling fish, they also interact closely with sediment, which might harbor free-floating cancer cells. The study notes that pollutants or hormonal changes could weaken immune systems, making the lake’s brown bullhead fish more vulnerable to infection.
Naturally occurring arsenic may also play a role.
The cancer’s ecological impact remains uncertain. While some transmissible cancers are devastating—the Tasmanian devil tumor has wiped out 90% of some populations—others, like the dog tumor, coexist with their hosts and have for thousands of years. In Lake Memphremagog, heavily diseased bullhead can survive for years, but long-term population effects are still unknown.
Because Lake Memphremagog provides drinking water for more than 175,000 people, the researchers emphasize that the disease poses no known risk to humans. The cancer cells cannot infect or survive in another species, and the fish are safe to handle and study. However, the findings raise broader questions about how pollution, habitat stress, and climate change may influence the evolution and spread of diseases in wildlife. “And, personally, I wouldn’t eat the ones with tumors,” says Henderson, who notes that about 30% of the lake’s brown bullhead appear to be afflicted.
Landfill?
Some environmentalists have wondered if the nearby Coventry Landfill might be leaching toxins into the lake, causing the cancer. The research team does not see any evidence this is the cause. The cancer is found evenly throughout the 32-mile-long lake, in reproductively isolated populations of the fish. It’s not clustered in the lake’s South Bay, near the landfill, as would be expected if the landfill were the culprit. Additionally, the team’s water quality tests in Lake Memphremagog show the same low levels of pollutants as many other nearby waterbodies and lakes, including Lake Champlain, which isn’t known to have any transmissible melanoma.
More importantly, the team has expanded its sampling to other waters in Vermont, New Hampshire, Maine, and New Brunswick—and found the cancer in “a few other ponds and lakes in this region,” said Peter Emerson, a biologist for the State of Vermont and co-leader on the new study. Many of these newly found sites, Emerson notes, have pristine water and no proximity to any landfills or toxic outflows.
However, the team did find that the fish cancer cells have elevated levels of arsenic—a notoriously poisonous element and carcinogen. The geographic distribution of the cancer appears to have correlation with levels of arsenic in the lakes’ surrounding soil.
“Arsenic is found naturally throughout New England, and there's really high concentrations of it in the Vermont’s Northeast Kingdom as well as up through Maine, which is where we've also seen evidence of this cancer,” says Henderson.
Thoreau’s find?
The team of scientists who conducted the new Nature study are beginning to explore how long this cancer may have been around—and how common transmissible cancers may be. "Our contention is that maybe it's not as rare as everyone thinks. We're just not seeing it,” says Dragon. "In the case of transmissible cancers, it may be that we don't see many simply because we aren't looking for them."
In fact, the first documented observation of the catfish cancer may be from Massachusetts, in the journals of Henry David Thoreau. In July of 1852, Thoreau was on the Concord River and wrote, “one of these large pouts [catfish] had a very large velvet-black spot, which included the right pectoral fin; a kind of disease I have often observed on them.” Six years later, Thoreau again wrote in his journal: “I see a pout this afternoon in the Assabet”—a tributary of the Concord River— “lying on the bottom, near the shore, evidently diseased. He permits the boat to come within two feet of him. Nearly half the head, from the snout backward diagonally, is covered with an inky-black kind of leprosy, like a crustaceous lichen.”
The scientists are currently exploring ponds and rivers in Massachusetts to learn more about the distribution of the cancer and to see if it may have originated there.
Julie Dragon emphasized that understanding this newly discovered transmissible cancer could offer new insights into cancer biology more broadly. “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,” she said. “These fish provide an opportunity to look at evolution of cancer.
Journal
Nature
Method of Research
Experimental study
Subject of Research
Animals
Article Title
Brown bullhead catfish melanoma represents a novel transmissible cancer
Article Publication Date
22-Jul-2026
