Anglers have long been puzzled about what’s causing the splotchy black tumors commonly found on brown bullhead catfish, or mud pout, living in Lake Memphremagog. Now, University of Vermont researchers say they have the answer: a transmissible cancer that was not previously known to science.
The findings, published in the prominent journal “Nature,” indicate that the raised black skin lesions on the fish are actually melanoma. Researchers emphasized in an announcement that the disease the fish carry “poses no known threats to humans.”
“The cancer cells behave more like parasites than conventional tumors, moving from fish to fish,” Julie Dragon, the UVM scientist who co-led the study, said in a statement announcing the finding.
Fishermen began noticing dark lesions in brown bullhead catfish in 2012. By 2014, almost a third of that species of fish found in Lake Memphremagog showed signs of disease.
“We wanted to know how a bottom-dwelling fish was getting a cancer we associate with exposure to too much sunlight,” Dragon said in the announcement. Scientists found that the DNA in the cancer cells from different fish were more closely related to each other than they were to the fish themselves, meaning the cancer was transmissible.
Seven Days reported on the contamination concerns in Lake Memphremagog in 2021. The 32-mile-long lake straddles the Vermont-Québec border. Environmental officials in Québec detected trace amounts of PFAS, aka forever chemicals, in the lake, which provides drinking water for 175,000 Canadians. A 2022 update from the Vermont Agency of Natural Resources found that certain PFAS were detected in the water and tissues of fish living in the lake.
Environmentalists have pointed to the Coventry Landfill, which towers over the lake’s southern end, as a potential source of pollutants. The landfill has operated there for decades. Dragon’s team found no evidence of a link between the landfill and the fish lesions.
“The cancer is found evenly throughout the 32-mile-long lake, in reproductively isolated populations of the fish. It’s not clustered in the lake’s South Bay, near the landfill, as would be expected if the landfill were the culprit,” the announcement reads.
The team expanded its search area across New England and Canada and detected the cancer elsewhere in lakes that lacked nearby sources of pollutants.
Researchers are now investigating the history of this transmissible cancer, the first to ever be detected in freshwater fish. Transmissible cancer has been documented in dogs, Tasmanian devils and mollusks.
In 1852, the author Henry David Thoreau described what could be a form of this cancer while fishing on the Concord River in Massachusetts. In his journals, he wrote about a “pout” that he said was “covered with an inky-black kind of leprosy, like a crustaceous lichen,” the researchers noted.
Importantly, scientists believe that further research into this form of cancer could turn up broader discoveries about how cancer survives and spreads, including in humans. These fish, Dragon said, provide an opportunity to examine the “evolution of cancer.”
“In the case of transmissible cancers, it may be that we don’t see many simply because we aren’t looking for them,” Dragon said.
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