Along the quiet border waters of Lake Memphremagog, where Quebec meets Vermont, something has been moving through the brown bullhead catfish that defies one of biology's oldest assumptions: that cancer is a solitary fate, born and buried within a single body. Scientists have now confirmed, in a study published in Nature, that melanoma tumours are transmitting directly between fish — only the handful of such contagious cancers ever documented in any animal. The discovery does not merely mark a biological anomaly; it opens a deeper question about what conditions in the living world allow disease
Scientists identify first transmissible cancer in fish species
The cancer cells were more similar to each other than to the fish they inhabited
Mark
How did anyone even notice this was happening? Fish in a lake—it's not like they're getting regular checkups.
Mimi
Anglers started seeing it. People who fish notice when something looks wrong with the catch. Dark lesions on the skin, tumours visible to the naked eye. That's how it surfaced in 2012. Once it was on the radar, researchers could start asking why.
Mark
And they ruled out the obvious things—pollution, viruses, inbreeding. But then what? How do you figure out a cancer is contagious?
Mimi
You look at the cancer cells themselves. Dragon's team compared the genetic makeup of the tumours to the genetic makeup of the fish carrying them. The cancer cells were more similar to each other than to their hosts. That's the smoking gun. It means they're all descended from the same original cancer, being passed around.
Mark
So it's like a clone spreading through the population.
Mimi
Exactly. One cancer, replicating itself, jumping from fish to fish. It's not that each fish is developing melanoma independently. It's that the same cancer is establishing itself in new bodies.
Mark
But how does it jump? How does a cancer in one fish get into another?
Mimi
That's the question nobody can answer yet. The mechanism of transmission—whether it's through water, through contact, through some biological vector—that's still unknown. They've ruled out what it's not. Now they have to figure out what it is.
Mark
And a third of the catfish in that lake have it now?
Mimi
Yes. Which means whatever is enabling this transmission, it's efficient enough to spread through a significant portion of the population. That's what makes it so unusual and so important to understand.
El Pulso
- Since 2012, anglers on Lake Memphremagog noticed dark tumours spreading across catfish skin — what seemed like a local curiosity has grown into a finding that reshapes cancer biology.
- Roughly one-third of the lake's brown bullhead catfish population now carries the melanoma, signalling that whatever is happening, it is happening efficiently and at scale.
- Researchers methodically eliminated pollution, viral infection, and inbreeding as causes, only to arrive at a more unsettling conclusion: the cancer itself is the contagion.
- Genetic fingerprinting revealed the tumours are clones of a single cancer lineage — more related to each other than to the fish hosting them — confirming direct transmission between animals.
- How the cancer moves between fish, and what biological or environmental conditions permit it, remains unanswered, leaving scientists with a living laboratory and a set of questions still taking shape.
Along the quiet border waters of Lake Memphremagog, where Quebec meets Vermont, something has been moving through the brown bullhead catfish that defies one of biology's oldest assumptions: that cancer is a solitary fate, born and buried within a single body. Scientists have now confirmed, in a study published in Nature, that melanoma tumours are transmitting directly between fish — only the handful of such contagious cancers ever documented in any animal. The discovery does not merely mark a biological anomaly; it opens a deeper question about what conditions in the living world allow disease itself to evolve into something that travels.
In 2012, fishermen pulling brown bullhead catfish from Lake Memphremagog — straddling the Quebec-Vermont border — began noticing dark melanoma tumours on the fish's skin. What read as a local oddity has since become a scientific landmark: the first confirmed case of cancer spreading directly between members of a fish species, published in Nature.
Transmissible cancers are among the rarest phenomena in biology. Most cancers arise within a single organism and perish with it. Yet in these catfish, melanomas are moving from body to body with enough consistency that researchers now estimate roughly one-third of the lake's population carries the disease.
Investigators first pursued familiar explanations — pollution, viral infection, inbreeding-weakened immunity. Each was ruled out through genetic testing, which also revealed strong diversity in the fish population. The decisive breakthrough came when researcher Julie Dragon and her team at the University of Vermont compared the cancer cells to the healthy tissue of their hosts. The tumours were genetically closer to one another than to the fish carrying them — proof that a single cancer lineage was replicating across multiple animals, functioning less like a disease of the body and more like a pathogen.
"Something has to be happening to allow this to happen," Dragon told CBC News, acknowledging how little is yet understood about the mechanism of transmission. The catfish of Lake Memphremagog and surrounding northeastern waters now carry questions that science is only beginning to formulate — about ecosystem vulnerability, about which other species might be susceptible, and about what this strange, travelling cancer can teach us about the nature of the disease itself.
In 2012, anglers fishing Lake Memphremagog—a body of water straddling the Quebec-Vermont border—began noticing something wrong with the brown bullhead catfish they were catching. Dark tumours, melanomas, were appearing on the fish's skin. What started as a local oddity has become a scientific landmark: researchers have now confirmed this is the first known case of cancer spreading directly between members of a fish species, a discovery published in Nature that expands our understanding of how disease moves through the natural world.
The finding is striking because transmissible cancers are extraordinarily rare. Only a handful have ever been documented in any animal. Most cancers arise from mutations within an individual organism and die with that organism. But in these catfish, something different is happening. The melanomas are jumping from fish to fish, establishing themselves in new hosts, and spreading through the population with enough efficiency that researchers estimate roughly one-third of Lake Memphremagog's brown bullhead catfish now carry the disease.
When scientists began investigating the outbreak, they pursued the usual suspects. Could pollution be triggering the tumours? Was a virus responsible? Could inbreeding have weakened the population's immune defences? Genetic testing provided clear answers: no viruses were present, and the fish showed strong genetic diversity, ruling out inbreeding as a cause. The pollution hypothesis, too, fell away under scrutiny.
The breakthrough came when Julie Dragon, a researcher at the University of Vermont's Larner College of Medicine and co-leader of the study, and her team compared the cancer cells themselves with the healthy tissue of the fish carrying them. What they found was counterintuitive and decisive: the cancer cells were more genetically similar to each other than they were to the fish in which they were growing. This meant the tumours were not arising independently in each fish. They were clones of a single cancer, being transmitted from one animal to another, establishing themselves in new bodies, and replicating. The cancer had become a pathogen.
Dragon acknowledged the rarity of what they were observing. "These conditions are incredibly rare in nature," she told CBC News. "So something has to be happening to allow this to happen." That something remains unclear. The research team has ruled out the obvious culprits, but the mechanism of transmission—how the cancer moves between fish, what environmental or biological conditions enable it—is still an open question. The catfish in Lake Memphremagog and other northeastern lakes are now living laboratories for understanding a phenomenon that challenges our basic assumptions about how cancer works.
The implications ripple outward. If a transmissible cancer can establish itself in a wild fish population, what does that mean for aquatic ecosystems? What other species might be vulnerable? And what can this discovery teach us about cancer biology itself? For now, the brown bullhead catfish of Lake Memphremagog carry answers to questions scientists are only beginning to ask.
Citas Notables
These conditions are incredibly rare in nature. So something has to be happening to allow this to happen.— Julie Dragon, University of Vermont's Larner College of Medicine
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