# Scientists Find a Rare Contagious Cancer Spreading Through Catfish

By The Current Tribune · Published Wed, 22 Jul 2026 16:02:55 GMT · Updated Wed, 22 Jul 2026 22:02:55 GMT
Source: The Current Tribune — https://currenttribune.com/article/contagious-cancer-brown-bullhead-catfish

The discovery of a rare contagious cancer in brown bullhead catfish isn’t just a weird fish story. It’s a jolt to how scientists think about cancer, evolution and the health of freshwater ecosystems — and it’s happening in a lake that supplies drinking water to more than 175,000 people.

Researchers have confirmed that the inky black skin tumours disfiguring brown bullhead catfish in Lake Memphremagog, on the border between Vermont and Quebec, are not ordinary cancers. They’re a transmissible melanoma — a clonal lineage of cancer cells that jumps from fish to fish — making this only the fourth known example of naturally contagious cancer in the animal kingdom.

## A strange cancer cluster in a border lake

Biologists and anglers first started flagging the problem in 2012, when they noticed an unusually high number of brown bullheads with raised, black skin lesions in Lake Memphremagog, a long, narrow lake stretching from Newport, Vermont up to Magog in Canada. Follow-up sampling between 2014 and 2017 found that roughly a quarter to more than a third of the brown bullheads carried these melanomas, with estimates hovering around 23–37% of the fish examined.

The lesions are hard to miss. Some are flat, dark patches on the body or near the eyes. Others bulge from the skin as crusty, jet-black growths, with some fish showing what researchers described as “tumours on top of tumours”. In severe cases, the cancer doesn’t stay put: tumour cells spread into the brain, liver and other organs.

Yet the fish population didn’t show an obvious die-off. Cancers appeared in both younger and older mature fish, and scientists haven’t seen a sharp age threshold where afflicted fish suddenly vanish from the population. That murky clinical picture made the cause even more confusing. Pollution was high on the list of suspects, as was a viral infection, especially given concerns about runoff and water quality in a lake used as a municipal water source.

## From pollution suspicion to genetic smoking gun

The breakthrough came when researchers turned to the genome. Instead of just cataloguing lesions and local conditions, they sequenced DNA from tumours and matched healthy tissue from the same fish, hunting for shared mutations that could point to a common environmental trigger.

What they found was stranger — and far more telling. Tumour cells taken from different fish shared hundreds of thousands of genetic variants with each other, forming a single, coherent lineage that didn’t match the genetic profile of the host fish they were growing in. Within a given animal, the cancer’s DNA looked more like the tumours in other fish than like the healthy cells in its own body.

That pattern is the calling card of a transmissible cancer: a disease where the tumour itself becomes the infectious agent. At some point in the past, a normal brown bullhead developed a conventional melanoma. Instead of dying with its host, that cancer lineage found a way to survive by engrafting onto new fish, moving through the population like a clonal parasite.

This kind of transmissible cancer is vanishingly rare. Until now, scientists had only documented three naturally occurring examples: a sexually transmitted tumour in dogs, two aggressive facial tumour lineages decimating Tasmanian devils, and multiple leukaemia-like cancers in bivalves such as clams, cockles and mussels. The Lake Memphremagog melanoma adds a freshwater fish to that short, unsettling list.

## Why a contagious cancer in catfish is such a big deal

Transmissible cancers rewrite the normal rules of oncology. Instead of each cancer starting from scratch in one body, they act like independent organisms, evolving over years to spread and survive.

In dogs, a single transmissible venereal tumour lineage has been circulating for thousands of years. Most infected animals survive, so the cancer persists without wiping out its hosts. Tasmanian devils drawn into brutal biting fights aren’t so lucky: their facial tumours, which arose at least twice independently, are typically lethal, and have helped push the species towards extinction.

Marine bivalves show yet another pattern. Their transmissible neoplasias have emerged multiple times across at least ten species, sometimes even jumping into new host species and carrying bits of mitochondrial DNA with them. The common thread is that once a cancer becomes transmissible, evolution takes over — favouring variants that spread efficiently but don’t always kill so quickly that the lineage burns itself out.

The brown bullhead melanoma now slots into this small but growing catalog. Its relatively high prevalence in Lake Memphremagog, combined with the lack of clear evidence that it is rapidly killing off hosts, suggests a lineage that has already found some kind of grim equilibrium with its fish population.

## How is the tumour spreading in freshwater?

One of the big scientific puzzles is how this catfish transmissible cancer actually moves between animals. In marine environments, researchers studying bivalve cancers have shown that tumour cells can survive for days floating freely in salt water, which fits with the idea that filter-feeding animals might pick up loose cancer cells as they pump seawater through their bodies.

Freshwater is another story. The same team that worked on marine systems found that these free-floating tumour cells die almost instantly in freshwater, which is why discovering a transmissible cancer in brown bullheads caught experts off guard. Lake Memphremagog is not a tidal estuary; it’s a freshwater lake.

That has led scientists to suspect a different style of transmission in catfish, more like what’s seen in dogs and Tasmanian devils. In those land animals, tumour cells spread primarily through direct physical contact — mating in dogs, and intense biting around the face in devils. Brown bullheads are bottom-dwelling fish that often interact closely in murky, crowded habitats, so skin-to-skin contact, minor injuries or even aggressive nips could, in theory, give the cancer cells the opportunity they need to invade a new host.

For now, the exact route of spread remains an open question. But the genetics leave little doubt that the tumours in different fish are all branches of the same ancestral cancer.

![brown bullhead catfish showing contagious cancer melanoma lesions in Lake Memphremagog](/media/2026/07/contagious-cancer-brown-bullhead-catfish-inline.webp)
*Brown bullhead catfish in Lake Memphremagog are now known to carry a transmissible melanoma lineage.*

## A warning sign for lake health — but not a simple story

The appearance of such a high cancer rate in a freshwater indicator species has raised alarms about the broader health of Lake Memphremagog. Brown bullheads are widely distributed across eastern North America and are often used as sentinels for water quality, because they spend their lives rooting around in sediments where pollutants accumulate.

One early hypothesis was that severe flooding from tropical storm Irene in 2011, which hit the region hard, might have washed contaminants into the lake, degrading water quality and helping trigger the melanoma outbreak. That’s still a plausible part of the story, but the new genetic evidence argues that a single transmissible cancer lineage plays a central role, not just a general wave of tumours popping up independently in stressed fish.

Those two ideas — environmental stress and contagion — aren’t mutually exclusive. A compromised immune system or chronic exposure to pollutants could make fish more vulnerable to infection with tumour cells, or could have helped the original founder cancer evolve in the first place. For residents who rely on Lake Memphremagog for drinking water, the discovery underscores why ongoing monitoring of both water quality and wildlife health is more than just an academic exercise.

## How this fits into the bigger picture of cancer and evolution

Every new transmissible cancer discovered forces scientists to rethink where the boundaries of an individual organism really lie. Once a tumour starts moving between hosts, it becomes a kind of microscopic hitchhiker, evolving traits that let it spread, evade immunity and tolerate new environments.

The brown bullhead melanoma adds another data point to a pattern that’s slowly coming into focus. Transmissible cancers have now appeared in mammals, marine invertebrates and freshwater fish, in both saltwater and inland habitats. That diversity hints that these diseases might be more common in nature than the handful of known examples suggests — we’ve just been bad at recognising them.

It also raises questions closer to home. Human cancers are not known to be naturally transmissible in this way, and there is no evidence that this catfish melanoma poses any risk to people. But studying how transmissible cancers arise, spread and adapt could still shed light on metastasis, immune evasion and long-term tumour evolution in our own species.

There’s a broader ecological angle, too. When a cancer becomes contagious, it doesn’t just threaten individual animals; it can reshape entire populations and the food webs they sit in. Tasmanian devils are the most dramatic example, but a persistent cancer in a common freshwater fish could quietly alter predator–prey dynamics, nutrient cycling and the way we interpret wildlife health signals from lakes and rivers.

## What This Means

The discovery of a contagious cancer in brown bullhead catfish marks Lake Memphremagog as a natural experiment that no one planned. A single rogue melanoma lineage is now a long-term resident of the lake’s ecosystem, sharing space with tourists, anglers and the communities that drink its water.

For scientists, the catfish transmissible cancer is both a warning and a tool: a reminder that cancer can escape the body that spawned it, and a rare chance to watch that process unfold in real time. For local managers and residents, it’s another reason to keep a close eye on water quality and fish health — not because the lake suddenly became dangerous to people, but because a system that can nurture a contagious cancer is one that deserves careful, sustained attention.