For more than a decade, a puzzling health issue has affected brown bullhead catfish in Lake Memphremagog, a body of water spanning the border between Vermont and Quebec. Beginning in 2012, anglers observed that some bullheads were covered with velvety black growths. Subsequent investigations revealed that these growths were melanomas—skin cancers—which are highly unusual in catfish. More than a third of the bullheads in the lake were found to be afflicted.

Researchers now believe the cancers are the result of a contagious cancer spreading through the population. According to a recent study, a single catfish initially developed a tumor that shed malignant cells into the water. These cells then invaded other catfish, effectively creating a transmissible cancer outbreak. Such contagious cancers have previously been identified only in a limited number of species, including dogs, Tasmanian devils, and certain shellfish.

Julie Dragon, a geneticist at the University of Vermont and co-author of the study, said the discovery raises questions about the prevalence of transmissible cancers in nature. “Is this really as rare as we think it is, or are we just not looking?” she asked.

The investigation began in 2014, when Vermont Fish and Wildlife Department biologists sought Dr. Dragon’s expertise to investigate the unusual prevalence of melanomas in the lake’s catfish. Initial hypotheses considered ultraviolet light exposure, given its known link to melanoma in other animals, but the catfish’s nocturnal, bottom-dwelling habits made this unlikely. Researchers also examined pollution and microbial infection without finding a clear cause.

By analyzing the DNA of affected fish and their tumors, Dr. Dragon’s team made a striking discovery: rather than each tumor being genetically related to its host, all tumor cells were closely related to each other but genetically distinct from the individual fish carrying them. After repeated verification, the evidence pointed to a transmissible cancer rather than individual tumors arising independently.

Transmissible cancers have historical precedent, albeit rare. Canine transmissible venereal tumor (C.T.V.T.), spread during mating, is thought to descend from a single dog thousands of years ago. Tasmanian devils have experienced a similar but more deadly transmissible cancer transmitted through biting during fights, which has seriously endangered the species. Invertebrates such as clams have also been documented to carry transmissible leukemias, wherein cancer cells spread through seawater to infect others.

The new discovery marks the first confirmed case of a contagious cancer spreading in freshwater. Experts suggest the bullheads’ spawning behavior—where large numbers of fish gather tightly in shallow water, frequently biting or scratching each other—likely facilitates transmission. Michael Metzger, a researcher in transmissible cancers in shellfish, noted that the ability of cancer cells to survive and spread in freshwater overturns previous assumptions about environmental limitations on transmissible cancers.

Additional evidence suggests similar cancers may exist in catfish elsewhere in the northeastern United States. Historical accounts by naturalist Henry David Thoreau describe spotted catfish in Massachusetts rivers in the 19th century, which some scientists suspect may have been early observations of this disease.

Dr. Dragon and her team are encouraging anglers and researchers to report instances of catfish with black, raised tumors. They request photographs, location details, and, if possible, frozen specimens for further analysis.

While there is currently no indication that such transmissible cancers pose risks to humans, researchers urge caution. Peter Emerson, fisheries biologist with the Vermont Fish and Wildlife Department, advised anglers to avoid consuming bullheads exhibiting tumors, citing a precautionary approach despite no evidence of harm.

Elizabeth Murchison, a specialist in transmissible cancers at Cambridge University, noted that although transmissible cancers remain rare, their existence in diverse species underscores the possibility they could emerge in humans in the future.