For more than a decade, anglers on Lake Memphremagog have been catching brown bullhead catfish that are riddled with tumors.
The cause has long eluded scientists, but now, a team of local researchers says the animals have a rare form of cancer that is contagious and spreads like a parasite.
"We have not seen this cancer in other catfish or other bottom-dwelling fish in Lake Memphremagog, so we don't believe that they can pass it to any other species," said Julie Dragon, a molecular geneticist at the University of Vermont who helped lead the research. "We obviously haven't tested it, but it doesn't appear that it can. So I don't think there's a threat to the public."
In a paper published today in the prominent scientific journal Nature, researchers report Memphremagog’s brown bullhead — also called “horned pout” locally — is one of just four animals on earth to succumb to this rare type of transmissible cancer. Similar conditions have been documented in dogs, Tasmanian devils and clams.
It’s a breakthrough finding for scientists from the University of Vermont, federal government and state who have been scouring the lake for years looking for a cause.
The cancer manifests as raised black lesions on the fish’s skin, along with lumpy tumors. Tom Jones, a fish biologist with Vermont Department of Fish and Wildlife, recalls hearing startling early accounts of it from anglers who told him their families have caught and eaten these famously delicious fish for generations.
“One fellow that I can recall, he described it as, ‘Hey, this black tar-like substance came off in my hands, you know, when I handled the fish,’” Jones said.
A population survey from 2014-2017 found that between one-quarter and one-third of all brown bullhead in Lake Memphremagog have these growths.
Genetic analysis at the University of Vermont confirmed the tumors were melanomas, which have been documented in fish on other lakes, but never at such high rates.
Scientists with the Vermont Fish and Wildlife Department and the U.S. Geological Survey collected tissue samples from afflicted animals caught on Lake Memphremagog and brought them to Julie Dragon’s genetics lab at UVM.
Dragon compared the genes of the tumors with those of the surrounding fish tissue, and found — to her surprise — that the genes in the tumors were more closely related to each other than to the tissue of the host fish.
“Whether it was caused by UV or some other pollutant, cancers usually look more like their host than they do like another random person,” she said.
This pattern of tumor tissue being more related to itself than to its host, found on multiple fish living within one body of water, led Dragon and her colleagues to conclude the bullhead have a “clonal cancer,” where cancer cells spread between host organisms, similar to how a parasite behaves.
Researchers hypothesized previously that poisoning from lead or other chemicals on the floor of Newport’s formerly industrial South Bay could be to blame. The scientists even explored the possibility that the bottom-dwelling fish had skin cancer from sun exposure, or that the lesions were caused by excessive pollution running into the lake in the aftermath of Tropical Storm Irene.
Transmissible cancers have never been formally documented in fish, but the New England naturalist Henry David Thoreau described brown bullhead with similar spots in some ponds in Massachusetts in the 1850s.
Right now, Dragon believes the cancer was somehow introduced to the population in Lake Memphremagog in 2012 — either as a spontaneous mutation, possibly triggered by exposure to pollution from excessive runoff after Tropical Storm Irene, or through a fish that was introduced from another lake or pond where the cancer was already circulating.
Researchers still don’t know how the tumors spread between bullhead, nor do they know why they suddenly infected roughly a third of the catfish in Lake Memphremagog in recent years.
“We think that it is something that they're exposed to maybe at maturation when they spawn and they gather together in these big aggregate groups,” Dragon said.
Scientists don’t know what the contagious cancer means for the bullhead population in the lake, or for fish broadly.
Despite their appearance, brown bullhead in Lake Memphremagog appear to be doing all right — Vermont Fish and Wildlife has not documented dramatic population declines in the lake since they started finding fish with melanomas.
For now, the Vermont Department of Fish and Wildlife is recommending people do not eat bullhead from Lake Memphremagog with skin lesions.
Researchers with the Vermont Department of Fish and Wildlife and the U.S. Geological Survey have secured funding to study how the fish transmit the cancer to each other next spring, by putting infected bullhead from Lake Memphremagog in a tank with fish that are healthy.
