Contagious Cancer Lurking In US Lakes

Oil spill on a sandy beach with waves in the background — Photo: Danny E Hooks / Shutterstock

A cancer that spreads not through genetics or bad habits, but through the water itself, has now been confirmed in fish living in an American lake, adding a strange new chapter to a growing scientific story about contagious tumors in the wild.

Quick Take

Scientists confirmed the first known transmissible cancer in a fish species, found in brown bullhead catfish in Vermont’s Lake Memphremagog.
The disease is a form of skin cancer that appears to spread directly from catfish to catfish, not from any outside toxin or pollutant.
Marine shellfish like clams, mussels, and cockles have carried similar contagious cancers for years, spread by free-floating cancer cells in seawater.
Researchers say human activity, including shipping and coastal development, may be helping move these cancer cell lines into new waters and new species.

A New Kind of Contagion Discovered in a Vermont Lake

Researchers at the University of Vermont, working with the U.S. Geological Survey and state wildlife officials, found something odd in brown bullhead catfish pulled from Lake Memphremagog. Dark, blemish-like tumors kept turning up on different fish, and the growths looked almost identical under the microscope. That similarity turned out to be the clue that cracked the case wide open.

How Genetic Testing Proved the Cancer Spreads Between Fish

Genetic analysis showed something a normal cancer should never show. Tumors from separate fish were more closely related to each other than each fish was related to its own body. That pattern rules out a cancer growing naturally inside each catfish. Instead, it points to a single cancer lineage jumping from one fish to another, published this year in the journal Nature.

Scientists are calling it the first confirmed transmissible cancer found in any fish, and the first ever documented in fresh water. Before this discovery, contagious cancer had only been recorded in three other groups of animals anywhere on Earth, none of them fish and none of them living in lakes or rivers.

Shellfish Have Carried This Threat for Years

Long before the Vermont catfish made headlines, marine biologists already knew contagious cancer existed in ocean waters. Clams, mussels, and cockles along multiple coastlines carry a disease called bivalve transmissible neoplasia, a leukemia-like cancer that spreads between animals of the same species and sometimes even jumps to different species entirely.

The mechanism behind that spread is what makes this story unsettling. Researchers studying the disease in soft-shell clams found that cancer cells can survive on their own in seawater and get released directly from sick animals into the surrounding water. That means the tumor itself becomes the infectious agent, floating free until it finds its next victim.

A Small But Growing List of Known Cases

Scientists have now cataloged fourteen separate transmissible cancer lineages across the animal kingdom, and eleven of them live in marine bivalves alone. Dogs and Tasmanian devils round out the rest of that short but disturbing list. The Vermont catfish discovery marks only the fourth documented type overall, and the very first one ever found outside the ocean.

That rarity matters. These are not common cancers sweeping through every species in the water. They are specific, clonal cell lines tied to particular animals in particular places, closely tracked by researchers rather than spreading unchecked across entire ecosystems. Wildlife officials in Vermont have stated plainly that they have not seen the disease jump to other fish species sharing the same lake.

Human Activity May Be Helping It Travel

One theory researchers keep coming back to involves people, not nature alone. Studies on shellfish populations suggest that human activity, including shipping traffic and coastal development, may be inadvertently carrying these cancer cell lines into new locations and even into new species that never encountered them before.

None of this means Americans should fear their local lake or the seafood counter. These cancers infect fish and shellfish tissue, not humans, and there is no evidence of any risk to people who swim, fish, or eat properly cooked seafood. What it does mean is that biologists have found one more example of nature finding strange new ways for disease to move, and it is worth watching closely.