Scientists Discover First Known Transmissible Cancer in Freshwater Fish

Scientists investigating mysterious black skin patches on catfish across cross-border lakes in New England and Canada have uncovered an unusual biological phenomenon. According to researchers at the University of Vermont, the dark lesions ruining the skin of brown bullhead catfish are not caused by a virus or environmental pollution as initially suspected, but by a living, transmissible melanoma.

The discovery, detailed in the journal Nature, represents only the fourth type of transmissible cancer ever identified in the animal kingdom. Unlike conventional tumors that arise from a host’s own mutated cells, these cancer cells act as infectious agents, spreading directly from animal to animal.

Genetic Sequencing Reveals Clonal Transmission in Catfish

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Anglers and biologists first noticed a striking increase in brown bullhead catfish displaying raised black skin patches in Lake Memphremagog starting in 2012. By 2014, field observations indicated that nearly one in three fish in the shared Vermont and Quebec waterway showed the dark lesions. Because brown bullhead are bottom-dwelling fish often used as indicators of environmental quality, researchers initially suspected a pathogen or contaminant tied to pollution.

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Julie Dragon, a scientist at the University of Vermont and co-leader of the new study, explained that early hypotheses centered on a viral infection. When those tests failed to pan out, Dragon and her colleagues at the UVM Cancer Center in the Larner College of Medicine used whole-genome sequencing to examine the genetics of the tumor cells, expecting to find an underlying genetic mutation in the host fish.

Instead, DNA comparisons between tumors and healthy tissues revealed that the cancer cells shared hundreds of thousands of genetic variants that were entirely absent from the host fish. This genetic fingerprint proved that the tumor cells were closely related to each other rather than to the fish carrying them, confirming the presence of a clonally transmissible cancer.

Spawning Behavior and Environmental Factors Under Investigation

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Prior to this finding, transmissible cancers were known in only three other wildlife categories: the Tasmanian devil’s facial tumor disease, a transmissible venereal tumor in dogs, and leukemia-like diseases affecting clams, mussels, and other bivalve mollusks. This discovery marks the first time the phenomenon has been documented in any fish or freshwater species.

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Mark Henderson, a fish biologist and study co-leader in the Rubenstein School of Environment and Natural Resources, noted that the transmission mechanism appears selective. It seems to only happen in larger fish that are of spawning age, Henderson said, suggesting that close physical contact during tight spawning clusters in shallow water could facilitate the spread of free-floating tumor cells. As scaleless, bottom-dwelling animals, the bullhead also interact heavily with lake sediment, which might harbor infectious cells.

Researchers also noted that weakened immune systems caused by pollutants, hormonal shifts, or naturally occurring arsenic could make the lake’s fish more vulnerable to the infection.

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Environmental concerns surrounding the health of Lake Memphremagog have prompted questions regarding whether the nearby Coventry Landfill might be leaching toxins into the water and driving the cancer rates. However, the research team found no evidence supporting this theory. According to the study, the cancer is distributed evenly throughout the 32-mile-long lake among reproductively isolated fish populations rather than clustering near the landfill in the lake’s South Bay.

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Because Lake Memphremagog serves as a drinking water source for more than 175,000 people, scientists emphasized that the disease poses no known risk to humans. The specialized cancer cells cannot survive or infect other species, and the fish remain safe to handle for research purposes. Even so, caution is advised for anyone considering consuming the catch. And, personally, I wouldn’t eat the ones with tumors, Henderson said.

While heavily diseased bullhead can survive for years in the lake, the long-term ecological consequences for fish populations remain entirely unknown as researchers continue to study how habitat stress and climate change influence disease evolution in the wild.