A cancer that spreads through water like a virus has jumped into fish for the first time ever recorded, and scientists just confirmed it in a Vermont lake.
Story Snapshot
- Scientists confirmed the first transmissible cancer ever found in a fish species, in catfish from a Vermont lake.
- The cancer spreads between catfish through living cancer cells released into the water, not through normal growth inside each fish.
- Similar contagious cancers already spread through seawater in clams, mussels, and cockles along U.S. coasts.
- Researchers say human activity near waterways may help these rare cancers reach new places and species.
First Contagious Cancer Found In A Fish
Researchers at the University of Vermont, the U.S. Geological Survey, and the Vermont Fish and Wildlife Department confirmed that brown bullhead catfish in Lake Memphremagog carry a transmissible cancer. Their study appeared July 22 in the journal Nature. The tumors show up as inky black spots of melanoma on the fish’s skin. Scientists say the disease passes directly from catfish to catfish, making it the first known transmissible cancer ever found in a fish.
Genetic testing revealed something unusual. Tumors taken from different fish matched each other more closely than they matched their own hosts’ bodies. That pattern means the cancer itself moves between animals as a living cell, rather than growing fresh inside each fish on its own. Researchers combined mitochondrial and nuclear DNA evidence across several years of samples to confirm one shared, contagious cancer lineage.
How Cancer Cells Travel Through Water
Scientists have already tracked how these unusual cancers move outside a host’s body. A 2022 study on bivalve transmissible neoplasia, a disease found in clams and other shellfish, showed the cancer cells “survive well in seawater” and “are released into the water by diseased animals”. That finding backs the idea that the cancer cells themselves, not a virus or bacteria, act as the infectious agent as they drift through open water.
The catfish case adds a new twist because it happened in fresh water, not the ocean. Until this year’s discovery, transmissible cancer had only been documented in dogs, Tasmanian devils, and marine shellfish. Scientists counted 14 separate transmissible cancer lineages across those groups. No fish had ever joined that short list before the Lake Memphremagog findings.
A Pattern Already Seen In Shellfish
Contagious cancer is not new to American waters overall. For years, researchers have tracked bivalve transmissible neoplasia moving through clams, mussels, and cockles along U.S. and European coastlines. The disease behaves like a slow-moving infection, jumping between individual shellfish and sometimes even between different species living in the same waters, according to genetic studies of the cancer’s spread.
So far, the fish outbreak appears contained to one lake. Researchers studying the Vermont site say they have not found the cancer in other catfish populations or bottom-dwelling fish nearby, and they do not believe it poses any risk to humans who touch or eat the fish. Lake Memphremagog remains the only confirmed location of fish-to-fish cancer transmission found anywhere in the world.
Human Activity May Be Fueling The Spread
A 2023 study
