Researchers have discovered the first known transmissible cancer in a freshwater fish species, identifying clonal skin tumors spreading among brown bullhead catfish in Lake Memphremagog, according to a study published in the journal Nature.
The mystery began back in 2013 across Lake Memphremagog, a large freshwater lake spanning the border of Vermont and Quebec. Anglers and biologists hauling in brown bullhead catfish noticed raised black spots on their bodies. Later sampling confirmed that between 23% and 37% of the fish carried melanoma lesions, a type of skin cancer. While melanomas had been reported previously in brown bullhead catfish elsewhere, such high rates had never been documented in the species before.
Investigating Environmental Theories and Ruling Out Pollution
Initially, experts suspected an environmental cause behind the outbreak. Tropical Storm Irene had unleashed massive regional flooding in 2011, leading researchers to question whether pollution and cancer-causing contaminants had triggered the unusual illness. Environmentalists also wondered if the nearby Coventry Landfill might be leaching toxins into the lake. However, the research team does not see any evidence this is the cause, noting that the cancer is found evenly throughout the 32-mile-long lake rather than clustering near the South Bay closest to the landfill.
Instead of an external contaminant, genetic analysis revealed a much stranger reality. Researchers from the University of Vermont and other institutions took a deep dive into the genetics of the lesions, expecting to find a genetic mutation that made the fish susceptible to developing their own tumors. Instead, whole-genome sequencing demonstrated that the cancer cells were far more closely related to each other than to their host fish.
Whole-Genome Sequencing Reveals Clonal Transmission
When the team compared DNA from tumors and healthy tissues in affected fish, they uncovered the definitive signature of a clonally transmissible cancer, where tumor cells themselves act as infectious agents moving from animal to animal. Hundreds of thousands of genetic variants are shared among tumor samples but absent from host fish, vastly exceeding levels seen in conventional cancers
, the study reports.
This discovery marks only the fourth type of transmissible cancer identified in the animal kingdom, and the first known in any fish or freshwater species. Previously documented transmissible cancers include the Tasmanian devil’s facial tumor disease, a transmissible venereal tumor in dogs, and transmissible leukemia-like diseases found in bivalve mollusks such as clams and mussels.
How the Parasite-Like Cancer Spreads Among Catfish
Scientists are still working to determine the exact transmission vector in the wild, though researchers have noted that the behavior of the fish likely plays a critical role.
“It seems to only happen in larger fish that are of spawning age,” said Mark Henderson, a fish biologist and study co-leader in UVM’s Rubenstein School of Environment and Natural Resources. “Maybe some part of spawning behavior leads to the spreading of the cancer between animals.”
Mark Henderson, fish biologist and study co-leader in UVM’s Rubenstein School of Environment and Natural Resources
During spawning season, brown bullheads cluster tightly in shallow water, creating opportunities for physical contact that could transfer tumor cells between hosts. As scaleless, bottom-dwelling fish, they also interact closely with sediment that might harbor free-floating cancer cells. Researchers also note that pollutants, naturally occurring arsenic, or hormonal changes could potentially weaken immune systems, making the lake’s fish more vulnerable to infection.
Geographic Spread and Public Health Implications
The phenomenon is not limited to a single body of water. According to researchers, the condition is widespread.
Because Lake Memphremagog serves as a drinking water source for more than 175,000 people, scientists have explicitly addressed public health concerns. Researchers emphasize that the disease poses no known risk of transferring to humans, noting that the cancer cells cannot infect or survive in other species. While the ecological impact on long-term fish populations remains uncertain, heavily diseased bullhead can survive with the condition for years, though experts advise caution regarding consumption.
