Scientists Discover First Transmissible Fish Cancer in Vermont Catfish

For more than a decade, anglers pulling brown bullhead catfish out of Lake Memphremagog noticed a troubling disfigurement. The fish were frequently covered in velvety black patches and lumpy growths that felt like tar. What started as localized observations by fishermen in 2012 eventually grew into a widespread ecological puzzle. By 2014, health surveys revealed that roughly one in three brown bullhead catfish in the binational lake carried the dark lesions.

Researchers initially suspected an environmental toxin, a virus, or industrial pollution. Brown bullheads are widely used as sentinel species because they develop liver tumors in water contaminated with polycyclic aromatic hydrocarbons, making them reliable indicators of watershed health. Scientists explored whether heavy runoff from Tropical Storm Irene in 2011 had stirred up a point source. However, genetic and spatial testing quickly ruled out those local contamination theories.

Genetic Sequencing Reveals a Clonal Cancer Outbreak

To pinpoint the cause, a team of researchers from the University of Vermont, state wildlife agencies, and federal institutions took deep tissue samples. Led by molecular geneticists and biologists, the team performed whole-genome sequencing on both healthy tissues and tumor samples. They anticipated finding standard genetic mutations caused by environmental carcinogens or sun exposure. Instead, the sequencing data revealed a profound genetic shocker.

Researchers Identify Transmissible Cancer in Brown Bullhead Catfish

Ordinarily, a tumor’s DNA mirrors the genetic profile of the host animal it grew inside. In Lake Memphremagog’s catfish, however, the tumor genomes were far more closely related to each other than to the individual fish carrying them. Hundreds of thousands of genetic variants appeared consistently across tumors while remaining entirely absent from the host fish’s healthy cells. This definitive signature exposed a clonally transmissible cancer, where living cancer cells act as infectious agents that jump directly from one animal to another.

Dr. Dragon further explained the mechanism, noting that it’s not driven by a bacteria or a virus. It’s the infectious agent itself, and it’s moving from fish to fish. Operating much like a parasite, the cancer cells survive outside a single host and establish new infections in subsequent animals, defying the traditional biological rules where a host immune system instantly rejects foreign tissue.

Contagious Cancer Found in Brown Bullhead Catfish of Lake Memphremagog

A Rare Biological Phenomenon Across Animal Species

Transmissible cancers remain exceedingly rare across the animal kingdom.

• Tasmanian devils, which spread fatal facial tumors through biting during fights and mating.
• Dogs, which transmit canine transmissible venereal tumors during mating.
• Marine shellfish and bivalves, which pass cancerous blood cells through open seawater as they filter feed.

The addition of brown bullhead catfish expands this select biological club into freshwater ecosystems for the first time. While bivalves transmit cancers through open water currents, researchers are still investigating the precise vector for catfish. Field biologists suspect that spawning behavior plays a central role. During spawning seasons, brown bullhead cluster tightly together in shallow waters, creating prime physical contact that can transmit tumor cells.

Contagious Cancer Identified in Brown Bullhead Catfish of Lake Memphremagog

Geographic Reach and Ecological Stakes in Lake Memphremagog

The geographic distribution of the disease presents another mystery. Field surveys conducted across the binational watershed demonstrate that the melanoma has saturated Lake Memphremagog, yet remains absent from many surrounding bodies of water. Out of dozens of lakes in the region, only specific waters harbor the transmissible lineage.

The visual impact on the fish is striking.

Experts stress that the disease poses no known risk to humans, as the cancer cells cannot infect or survive in other species.