Researchers have identified a rare transmissible skin cancer among brown bullhead catfish in Lake Memphremagog and other New England and Canadian waters. Affecting roughly 30 percent of the population, the contagion behaves like a parasite rather than a conventional tumor, making it the fourth known contagious cancer in nature and the first in a freshwater fish.
For more than a decade, anglers pulling brown bullhead catfish from Lake Memphremagog noticed something unsettling. The fish, famously eaten across the region for generations, carried raised black skin patches and inky-tar lesions. Biologists first suspected a virus, pollution from industrial runoff, or environmental toxins washing into the cross-border lake shared by Vermont and Quebec. Instead, genetic sequencing revealed an unexpected biological reality: a naturally occurring skin melanoma that actively passes from fish to fish.
Genetic Surprises in Lake Memphremagog’s Bullhead Population
When tissue samples arrived at University of Vermont laboratories, researchers expected to find standard genetic mutations triggered by environmental stressors or inbreeding. Population surveys conducted between 2014 and 2017 showed that between one-quarter and one-third of all brown bullhead in Lake Memphremagog carried these growths. Using whole-genome sequencing, molecular geneticists compared the DNA of the tumors against the host fish.
That genetic signature turned conventional oncology upside down. The tumor cells were more closely related to each other than to the fish carrying them, demonstrating that living cancer cells act as infectious agents that outlive their original hosts. The condition represents only the fourth known transmissible cancer in the animal kingdom, joining the ranks of facial tumors in Tasmanian devils, venereal tumors in dogs, and leukemia-like diseases in marine bivalves.
Investigating Environmental Suspects and Landfill Concerns
The discovery prompted immediate scrutiny regarding potential industrial pollution.
Researchers also explored whether sun exposure or heavy metal accumulation following extreme weather events played a role. While historical sightings exist, the modern concentration in Vermont and Canadian waters pushed scientists to evaluate how bottom-dwelling habits might facilitate the spread.
Spawning Behavior and Parasite-Like Transmission Dynamics
Because the disease bypasses typical host genetics, biologists are studying how living cells move between animals without triggering a fatal immune rejection. Unlike standard tumors, these cells survive in the water column or transfer through direct physical contact.
“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 at the University of Vermont
During seasonal spawning, brown bullhead cluster tightly in shallow waters, creating opportunities for physical friction that can rub tumor cells into open wounds. As scaleless bottom-dwellers, they interact continuously with lake sediment that may harbor free-floating cells. State biologists sampling regional water bodies have since identified the same melanoma in a few other ponds and lakes across New Hampshire, Maine, and New Brunswick.
Public Health Guidance and Long-Term Ecological Uncertainty
The cancer cells are biologically incapable of infecting humans or other fish species residing in the lake.
Even so, caution is advised for anglers. Long-term monitoring continues as scientists evaluate whether the transmissible cancer will permanently alter bullhead population structures or stabilize alongside its host.
