Researchers identify first known transmissible cancer in a freshwater animal: melanoma in brown bullhead catfish

Scientists have identified a clonally transmissible melanoma affecting brown bullhead catfish (Ameiurus nebulosus) as the first known contagious cancer in a freshwater animal and the first in any fish species. The discovery, detailed in a study published July 22 in Nature, stems from research on mysterious raised black skin lesions that anglers and biologists began noticing in high numbers around 2012 in Lake Memphremagog, a water body spanning Vermont, USA, and Quebec, Canada (Nature).

By 2014, nearly one in three sampled brown bullhead in the lake showed the dark lesions, which histopathological analysis confirmed as malignant melanomas. Prevalence remained elevated in subsequent years. Brown bullheads are bottom-dwelling, scaleless fish widely distributed in eastern North America and often used as indicators of environmental quality. Researchers initially suspected a virus, contaminant, or pollution-related cause linked to events such as flooding from Tropical Storm Irene in 2011 (EurekAlert/University of Vermont; UVM News).

Whole-genome sequencing of paired tumor and healthy tissues from affected fish, along with samples from unaffected reference fish, revealed a different explanation. Tumor mitochondrial and nuclear genomes were more closely related to one another across different host fish than to the genomes of their individual hosts. Hundreds of thousands of genetic variants were shared among the tumor samples but absent from host tissues—far exceeding patterns seen in conventional cancers that arise independently in each individual. This is the genomic signature of a clonally transmissible cancer, in which living cancer cells themselves act as the infectious agent, spreading between animals like a parasite (Nature).

The finding establishes this melanoma as only the fourth documented type of naturally occurring transmissible cancer in animals. Previously known examples are canine transmissible venereal tumor in dogs, facial tumor disease in Tasmanian devils, and bivalve transmissible neoplasia (leukemia-like diseases) in multiple species of clams, mussels, and related mollusks (Nature; EurekAlert/University of Vermont).

The cancer has been confirmed in brown bullhead from Lake Memphremagog and a number of other ponds and lakes in Vermont, New Hampshire, Maine, and New Brunswick. It appears concentrated in larger, spawning-age fish. Researchers hypothesize transmission may occur through physical contact when fish cluster tightly in shallow water during spawning, or via free-floating cancer cells in sediment with which the bottom-dwellers interact. Naturally occurring arsenic, present at elevated levels in some regional soils and detected in the cancer cells, may play a contributing role by affecting immunity or carcinogenesis, though the cancer occurs in waters of varying quality and is not clustered near potential pollution sources such as a nearby landfill (UVM News; EurekAlert/University of Vermont).

Ecological impacts remain unclear. Some fish with extensive tumors survive for years, but long-term effects on populations are unknown. The cancer poses no known risk to humans: the cells cannot infect other species, and the fish are safe to handle. Lake Memphremagog supplies drinking water for more than 175,000 people. Scientists note historical observations of similar black spots on catfish in Massachusetts journals of Henry David Thoreau from the 1850s and are investigating possible earlier origins and wider distribution (UVM News).

Lead researchers include Julie Dragon of the University of Vermont Cancer Center, Mark Henderson of UVM’s Rubenstein School of Environment and Natural Resources, and Peter Emerson of the Vermont Fish and Wildlife Department. The work opens avenues for studying cancer evolution, transmission mechanisms, and the potential influence of environmental stressors on wildlife disease.