Contagious Cancer Found in Brown Bullhead Catfish of Lake Memphremagog

An angler pulling a brown bullhead out of Lake Memphremagog noticed what looked like tar smeared across its skin. The angler brought the fish to Peter Emerson, a fish biologist with the Vermont Fish and Wildlife Department who had surveyed the lake since 2009. Within three years, similar black growths appeared on a quarter to 40 percent of the lake’s bullhead population.

Genetic Sequencing Reveals a Clonal Killer in Lake Memphremagog

Tissue samples made their way to the bioinformatics core at the University of Vermont starting around 2014. For years, genomic sequencing turned up no virus, bacteria, or microbe of any kind. Julie Dragon, associate professor of microbiology and molecular genetics who directs the core, then investigated whether a shared mutation drove the disease.

In standard cancer cases, every tumor is a close genetic relative of the host animal, placing tumors next to their respective hosts on a family tree. For the Memphremagog samples, the lineage trees inverted: all tumors formed one distinct cluster, while all fish formed another. The analysis revealed hundreds of thousands of mutations shared across cancers in different fish while remaining entirely absent from the host catfish themselves.

“It’s a clonal cancer. It’s not driven by a bacteria or a virus. It’s the infectious agent itself, and it’s moving from fish to fish.”

Contagious Cancer Identified in Brown Bullhead Catfish of Lake Memphremagog

Julie Dragon, associate professor of microbiology and molecular genetics at the University of Vermont

Joining a Rare Class of Contagious Malignancies Across Species

Contagious cancers bypass normal biological barriers by shedding living cells that transfer to a new host and continue dividing. Prior to this discovery, science recognized only three natural categories of transmissible cancer: facial tumor disease in Tasmanian devils, canine transmissible venereal tumors in dogs, and leukemia-like cancers affecting bivalve mollusks such as clams and mussels.

While Tasmanian devils transmit tumor cells through facial biting during mating and fighting, and dogs pass venereal tumors through direct physical contact, the exact transmission vector for catfish remains under investigation. The scientists behind the study have suggested it could be linked to mating behaviour, as in dogs and Tasmanian devils, or to catfish picking up cancer cells from the water, as happens in shellfish.

Geographic Distribution Across New England and Canada

Following initial discoveries along the Vermont and Quebec border, investigators expanded their surveys. Subsequent genotyping of bullhead from other water bodies confirmed a wider geographic footprint.

“It looks like it’s widespread, including New Brunswick, Nova Scotia, Maine, New Hampshire, Massachusetts, Pennsylvania… And some even in Wisconsin, New York … they’re everywhere.”

Peter Emerson, fish biologist with the Vermont Fish and Wildlife Department

Despite this broad distribution, local abundance varies.