Researchers confirm first known transmissible cancer in fish species

Scientists have confirmed that mysterious black skin lesions observed on brown bullhead catfish (Ameiurus nebulosus) in Lake Memphremagog—a body of water spanning Vermont in the United States and Quebec in Canada—and other nearby lakes and ponds represent a novel form of transmissible cancer. The findings, published on July 22, 2026, in the journal Nature, establish this melanoma as the first known contagious cancer in any fish species and the first identified in a freshwater ecosystem (Nature).

Since around 2012, anglers and biologists noted a sharp rise in raised black patches on the fish. By 2014, surveys indicated that roughly one-quarter to one-third of brown bullheads in the lake carried the lesions, which histopathological analysis confirmed as malignant melanomas. Initial hypotheses focused on environmental contaminants, such as runoff or a nearby landfill, or a viral pathogen, given that brown bullheads are bottom-dwelling and nocturnal and thus unlikely to receive high ultraviolet exposure typically linked to melanoma. Water quality tests and microbiome analyses did not support those explanations (University of Vermont).

Whole-genome sequencing of tumor tissue and matched healthy host tissue from affected fish, along with samples from unaffected reference populations, revealed the key evidence. Tumor mitochondrial and nuclear genomes clustered more closely with one another across different hosts than with the genomes of the individual fish carrying them. Hundreds of thousands of genetic variants were shared among the tumors but absent from host tissues—far exceeding patterns seen in conventional cancers that arise independently in each individual. This clonal signature indicates the cancer originated in a single founder animal and has since spread by transmission of living cancer cells themselves, which behave more like parasites than typical tumors (Nature; Gizmodo).

The discovery brings the total of naturally occurring transmissible cancers in animals to four types: canine transmissible venereal tumor in dogs, devil facial tumor disease in Tasmanian devils, disseminated neoplasia in multiple bivalve mollusk species, and now this melanoma in brown bullheads. Researchers, including co-lead authors Julie Dragon of the University of Vermont and colleagues from Vermont Fish and Wildlife and other institutions, note that the cancer has also been detected in additional ponds and lakes across New England and into New Brunswick, including some with pristine water quality (University of Vermont; NPR).

Transmission mechanisms remain under investigation. The lesions appear primarily in larger, spawning-age fish. During spawning, bullheads form dense clusters in shallow water, creating opportunities for physical contact, biting, or scratching via their spines that could allow cancer cells to transfer. It is also possible that cells persist briefly in sediment. Elevated arsenic levels in the tumor cells and a geographic correlation with natural arsenic in surrounding soils have been observed, though the precise role is unclear. Heavily affected fish can survive for years, and long-term population impacts are not yet known; some other transmissible cancers are highly lethal while others are more benign (New York Times; University of Vermont).

Historical accounts, including journal entries by Henry David Thoreau in the 1850s describing similar black spots on catfish in Massachusetts waters, and early 20th-century reports of black tumors, suggest the lineage may be older and more widespread. Researchers emphasize there is no known risk to humans: the cancer cells do not infect other species, and the fish remain safe to handle. The lake supplies drinking water to more than 175,000 people. The work opens avenues for studying cancer evolution, immune evasion, and metastasis outside traditional models (Nature; New York Times).