Scientists confirm rare 'parasite-like' cancer in bullhead from Lake Memphremagog

Researchers have identified a rare, transmissible form of cancer affecting brown bullhead catfish in Lake Memphremagog. The discovery, detailed in a paper published today in the journal Nature, provides an explanation for a decade-long mystery regarding why a significant portion of the lake’s population of the fish — locally known as “horned pout” — has been afflicted with persistent tumors.

The findings indicate that the cancer behaves like a parasite, spreading between individual fish. This makes the brown bullhead one of only four animals on earth to succumb to this rare type of transmissible cancer, a list that includes dogs, Tasmanian devils, and clams. 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.

A Shift in Scientific Understanding

The research team, comprised of scientists from the University of Vermont, the U.S. Geological Survey, and state agencies, conducted a years-long investigation into the cause of the growths. Initially, authorities explored various environmental factors, including the potential for chemical contamination from Newport’s formerly industrial South Bay. Investigators also considered whether the bottom-dwelling fish developed skin cancer due to sun exposure, or if the lesions were linked to significant pollution runoff following Tropical Storm Irene.

Genetic analysis, conducted at the University of Vermont, ultimately revealed a different mechanism. Julie Dragon, a molecular geneticist, compared the genes of the tumors with those of the surrounding host tissue. She discovered that the genes within 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."

Julie Dragon, molecular geneticist at the University of Vermont

This unique genetic clustering led researchers to conclude that the fish possess a “clonal cancer,” where cells move between organisms. Although genetic analysis confirmed the growths were melanomas, similar to those found in fish in other lakes, the high rate of occurrence in Lake Memphremagog proved anomalous.

Observations and Public Health

Anglers have reported startling encounters with the afflicted fish for years, noting the growth of black, tar-like lesions and lumpy tumors. Tom Jones, a fish biologist with the Vermont Department of Fish and Wildlife, recalled accounts from anglers who have fished the waters for generations. One individual described how a black, tar-like substance would come off the fish during handling.

Population surveys conducted between 2014 and 2017 found that between one-quarter and one-third of the brown bullhead in Lake Memphremagog carried these growths. Despite this high infection rate, the Vermont Fish and Wildlife Department has not observed a dramatic decline in the overall bullhead population.

"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,"

Julie Dragon, molecular geneticist at the University of Vermont

"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."

Julie Dragon, molecular geneticist at the University of Vermont

Historical Context and Future Study

Researchers suggest the cancer was 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. Current theories suggest that transmission likely occurs during spawning, when the fish gather in large groups.

To further investigate these findings, researchers have secured funding for a study beginning next spring. The project will involve placing healthy bullhead in tanks with infected individuals to observe the transmission process directly.

In the interim, the Vermont Department of Fish and Wildlife has issued a clear recommendation for the public: do not consume brown bullhead from Lake Memphremagog that exhibit skin lesions.