Fourth Transmissible Cancer Found in Catfish at Lake Memphremagog, July 27

Researchers at the University of Vermont reported in Nature this week that brown bullhead catfish in Lake Memphremagog, which straddles Vermont and Quebec, carry a contagious melanoma that spreads from fish to fish. The cancer cells themselves transmit between animals during spawning season, making this the first transmissible cancer ever found in a fish and only the fourth known to science. About 30 percent of the bullhead population in the lake now carries the disease.

How the cancer spreads like a parasite

A transmissible cancer originates in a single founder animal, then cancer cells metastasize to infect new hosts. The cancer cells themselves are the agent of transmission, not a virus or bacteria. Genetic testing revealed tumours from different catfish shared hundreds of thousands of genetic variants with each other, forming a single genetic lineage passed between animals. Scientists believe the cancer spreads during spawning season when bullhead cluster tightly together in shallow water, giving tumour cells physical contact to jump from one fish to another.

Discovery began with anglers noticing black lesions in 2012

Anglers and biologists began observing brown bullhead with raised black lesions in Lake Memphremagog in 2012. Between 2014 and 2017, researchers found melanomas in 23 to 37 percent of brown bullhead sampled from the lake. Researchers initially hunted for common causes like chemical carcinogens or infections. They tested the watershed for polycyclic aromatic hydrocarbons after Tropical Storm Irene flooded the region in 2011, but found no virus, microbe, or chemical explanation. Years of sequencing turned up no pathogen, leading researchers to look for shared mutations instead.

Three other transmissible cancers exist in dogs, Tasmanian devils, and molluscs

The catfish melanoma joins only three other transmissible cancers known in nature. Dogs carry canine transmissible venereal tumour, a genital cancer passed during mating that descends from a single dog that lived in northeastern Asia between 6,000 and 11,000 years ago. Tasmanian devils suffer from a facial tumour disease that devastates populations. Clams and mussels carry a leukemia-like disease. The catfish cancer is the first in a fish and the first in fresh water.

No known risk to humans, but monitoring matters for water quality

Researchers say there is no known risk to humans and the fish are safe to handle, though they recommend not eating catfish with visible tumours. Lake Memphremagog is the source of drinking water for more than 175,000 people. Brown bullhead are considered an indicator species of water quality, and the high incidence of cancer could signal a local cancer-causing contaminant or contagion. The disease has since turned up in other lakes across New England and Canada, prompting ongoing monitoring by biologists and public health officials.

Final Thoughts

The discovery of transmissible melanoma in Lake Memphremagog catfish marks a rare natural event that challenges how scientists understand cancer transmission. While no human risk exists, the finding underscores the importance of monitoring water quality in shared freshwater ecosystems.

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