How we found a new transmissible cancer in bullhead catfish

For more than a decade, anglers on Lake Memphremagog — which straddles the Québec-Vermont border — have been catching brown bullhead catfish marked with distinctive black lesions.

In 2019, scientists from the United States Geological Survey (USGS) and the Vermont Fish and Wildlife Department confirmed the lesions to be malignant melanoma .

Yet, until recently, there was no confirmed explanation for how the disease emerged.

When my colleagues and I at the University of Vermont turned to whole-genome sequencing, we expected to find a genetic explanation for why some brown bullheads had melanoma and others didn’t.

Instead, we discovered evidence of something seen only in a handful of cases in the animal kingdom: transmissible cancer .

That’s not where anyone expected the investigation to lead.

Various theories When scientists first documented cases in 2012, the most likely explanation seemed to be environmental factors.

The previous year, Hurricane Irene had caused devastating flooding across Vermont.

Could the bullhead’s habitat have been tainted by pollutants stirred up during the hurricane? This seemed to be a more likely culprit than ultraviolet exposure, which is the leading cause of human melanoma.

The brown bullhead is a benthic (bottom-dwelling) fish that feeds at night.

Cancer-causing microbes in the waterways were another theory.

But a decade of research yielded no evidence to support these hypotheses.

After years of dead ends, DNA offered a way to ask a fundamentally different question.

USGS and Vermont Fish and Wildlife scientists contacted the University of Vermont, where I am a genomics and data scientist and a member of the UVM Cancer Center.