Researchers Discover First Transmissible Cancer in Freshwater Fish Species

The investigation began in 2012 when anglers and fisheries biologists started pulling brown bullhead catfish out of Lake Memphremagog with unsettling physical abnormalities. Many of the fish featured dark, raised patches across their bodies. Biologists noted the growths resembled black tar on the skin, appearing on their fins, gills, lips, and occasionally covering their eyes entirely.

Surveys conducted between 2012 and 2013 revealed that between 23 percent and 37 percent of the catfish sampled carried the lesions. By 2014, roughly one in three fish in the cross-border watershed showed signs of the disease, according to findings detailed by the University of Vermont.

Genetic Sequencing Confirms a Clonal, Transmissible Cancer

When these unusual tumors first surfaced, researchers suspected an environmental culprit. Some questioned whether massive regional flooding caused by Tropical Storm Irene in 2011 had washed pollutants and carcinogens into the lake, degrading water quality and sparking the outbreaks.

Investigators examined the fish for toxins and even looked into whether a virus was driving the growths. But extensive testing failed to turn up viral pathogens or elevated toxin levels linked to nearby sites.

Instead, a team led by researchers at the University of Vermont turned to whole-genome sequencing to compare DNA from the tumors against healthy tissue from the host fish. The results upended conventional assumptions about how tumors form.

“These tumors were very different from the host fish that they came from, which is not what we expected or what anyone would expect.”

Julie Dragon, University of Vermont

Instead of originating independently within each animal, the cancer cells shared hundreds of thousands of genetic variants that were entirely absent from the healthy tissues of the fish carrying them. Julie Dragon, an associate professor of microbiology and molecular genetics, explained that the family trees placed all the tumors in one cluster and all the host fish in another.

Contagious Cancer: First Transmissible Type Discovered in Quebec Fish

The discovery places the Memphremagog catfish condition into an extremely exclusive category of diseases. Prior to this finding, naturally occurring contagious cancers had been documented in only three other groups: Tasmanian devils, dogs, and certain shellfish species like clams and mussels.

Spawning Behavior and Ecological Spread Across Watersheds

How the cancer moves between animals in an aquatic environment remains an active question, though researchers have developed working hypotheses. Brown bullheads are bottom-dwelling fish that lack scales, meaning their skin interacts closely with lake sediment where free-floating cancer cells might persist.

Surveys in other regional waters indicate the disease is not confined to a single basin.

Public Health Safety and Broader Implications for Cancer Research

Because Lake Memphremagog serves as a drinking water source for more than 175,000 residents, researchers have addressed public health concerns directly. Experts emphasize that the catfish melanoma poses no threat to humans or other mammalian species.

The American Cancer Society maintains that cancers cannot be transmitted between humans through casual contact, and researchers stress that the fish cells are incapable of surviving or infecting people or other animals outside their specific host species.

Beyond local fisheries management, researchers believe studying how these cancer cells survive, evade immune systems, and establish themselves in new hosts will provide valuable comparative models for understanding tumor mechanics in general biology.

“By studying how cancers survive and spread outside their original host, we can learn a great deal about what keeps cancers contained and what happens when those boundaries break down.”

Julie Dragon, University of Vermont