A catfish cancer begins as superficial spots, but can invade internal organs. Image Credit: VERMONT FISH & WILDLIFE DEPARTMENT
Contagious Fish Cancer Is Spreading Through a New England Lake—and Scientists Are Fascinated
Imagine catching a fish only to discover it's covered in black, cancerous-looking spots. That's exactly what's happening in parts of New England, where researchers have identified one of the world's rarest diseases: a contagious form of cancer that spreads from fish to fish.
Scientists say the discovery could change what we know about cancer—and raise new questions about how diseases evolve in the wild.
A Mysterious Disease in Lake Memphremagog
Lake Memphremagog stretches about 31 miles (50 kilometers) from northern Vermont into southern Quebec and is known for excellent fishing. Anglers flock there for lake trout, salmon, rainbow trout, and perch.
But another common resident—the brown bullhead catfish—is drawing attention for a much more unsettling reason.
Researchers found that nearly one-third of the lake's brown bullhead catfish are covered in dark, black tumors caused by melanoma, a type of skin cancer.
This Cancer Doesn't Behave Like Normal Cancer
Most cancers develop from a person's—or animal's—own cells.
This one appears to be different.
According to a new study published in Nature, the melanoma may actually be a single cancer cell line that has been spreading from one fish to another, almost like an infectious disease.
If confirmed, it would become the first known contagious cancer ever discovered in a fish.
One of the World's Rarest Diseases
Transmissible cancers are incredibly uncommon.
Only a handful have ever been documented, including:
- The facial tumor disease that spreads among Tasmanian devils through biting.
- A contagious venereal tumor that spreads between dogs during mating.
- Leukemia-like cancers that infect certain shellfish, including clams and mussels.
The brown bullhead melanoma would be the newest—and perhaps one of the most mysterious—members of this unusual group.
The Clues Were Hidden for More Than a Century
Oddly enough, this disease may not be new.
In 1858, author Henry David Thoreau described a catfish with an "inky-black" growth covering its head.
Decades later, in 1925, scientists reported widespread black tumors in catfish on Cape Cod.
Those reports were largely forgotten until Vermont anglers began reporting alarming numbers of diseased fish in 2012.
Researchers quickly realized the condition was surprisingly common.
DNA Reveals a Surprising Answer
Scientists compared DNA from healthy fish and cancerous tumors throughout New England.
Normally, a tumor is genetically closest to the animal it grows in.
Instead, researchers found something remarkable: all of the tumors were genetically more closely related to each other than to the fish carrying them.
That strongly suggests the cancer itself is acting like a living clone that has been passed from fish to fish over time.
Several cancer experts who reviewed the findings say the genetic evidence is compelling.
Not Everyone Is Convinced
Some scientists believe environmental pollution could still be playing a major role.
One researcher found unusually high levels of arsenic in affected fish and believes contaminants may either trigger the cancer or weaken the fish enough for it to spread more easily.
Local residents have also questioned whether pollution in Lake Memphremagog may be contributing to the outbreak.
Future laboratory studies will attempt to determine exactly how the disease spreads.
How Does It Spread?
Researchers still don't know.
One theory involves mating behavior.
Brown bullhead catfish are mostly solitary, but they gather together once they reach sexual maturity. During spawning, the fish may scratch one another with sharp pectoral fin spines, potentially transferring cancer cells.
Another possibility is even stranger—the cancer cells could survive in the water and infect nearby fish much like a pathogen.
Scientists hope controlled laboratory experiments will finally provide the answer.
Should Anglers Be Concerned?
Despite the widespread disease, brown bullhead populations remain healthy in Lake Memphremagog.
The cancer spreads throughout the fish's body but doesn't appear to kill them quickly, allowing infected fish to survive long enough to pass the disease to others.
There is currently no evidence that this cancer poses any risk to humans.
A Discovery That Could Change Cancer Research
Researchers believe this unusual melanoma may be just the beginning.
If one contagious cancer can spread through fish populations, others may exist unnoticed in wildlife around the world.
For scientists studying how cancer evolves, survives, and even behaves like an infectious organism, the discovery opens an entirely new chapter in understanding one of nature's most complex diseases.
