Contagious Skin Cancer Is Spreading Among Catfish in Quebec and U.S. Lakes

The world’s first known transmissible cancer in a fish has been discovered in catfish living in lakes across Quebec and the U.S.

Scientists have confirmed the world’s first known transmissible cancer in a fish. The rare skin cancer is spreading among brown bullhead catfish in Lake Memphremagog, where living cancer cells pass directly from one fish to another, much like a parasite ().
The landmark genetic study, co-led by Dr. Julie Dragon at the University of Vermont, found that the transmissible melanoma is circulating among brown bullhead catfish in Lake Memphremagog, which spans the Canada-U.S. border. Published in the journal Nature, the findings mark only the fourth known case of a naturally occurring transmissible cancer in animals.

Most cancers are not contagious. However, scientists have identified a handful of rare cases in the animal kingdom where living cancer cells spread directly from one individual to another. These include:

• Tasmanian devils, which transmit facial tumors through biting.
• Dogs, which spread a transmissible cancer during mating.
• Marine shellfish, where cancer cells spread through seawater.

The discovery in brown bullhead catfish adds a new chapter to cancer biology. It is the fourth naturally occurring transmissible cancer ever identified and the first known case in a freshwater fish, reshaping our understanding of how cancer can evolve and spread ().

Inside the Discovery of the World’s First Contagious Cancer

The quest begins when the fishermen started reporting bizarre black spots and lumps on the brown bullhead catfish in the region of Quebec and Vermont back in 2012. The earliest reports come from 1990s ().

Locals were worried as the incidences of the catfish with the inky-black spots tremendously increased after the huge flood caused by Post – tropical storm Irene. It was believed that the lesions appeared due to carcinogens being washed into the flooded lakes.

Earlier research by Vicki on brown bullhead catfish in the region suggested “skin with melanistic lesions had significantly higher concentrations of seven metals, including arsenic, a known carcinogen and zinc”.

Vermont’s Fish & Wildlife Department asked the researchers to check for viruses, but the results were inconclusive.

Then the researchers thought of doing genetic testing of healthy and infected fish with tumors to check if the inbreeding has led to any mutation in the fish. The fishes show marked genetic diversity which came as a great surprise to the scientists showing anunusual cancer endemic.

Unusual Cancer Spread

Exact transmissions remain unproven, but the researchers believe the Catfish are bottom dwellers that bite, scratch, and crowd each other during breeding. This physical friction rubs the living cancer cells into open wounds.

The mating rituals and the crowding increased the spread of the cancer, affecting almost one-third of the brown bullhead catfish in Memphremagog lake. This cancer behaves like an infectious agent, a parasite where it transmits the disease from one animal to another.

The Immortal Catfish Cancer

Usually, tumor mirrors the genetics of the affected fish, carrying this unique defect and might die with it. Researchers revealed that in the case of brown bullhead catfish there is a genetic shocker.

The tumors are more related to each other than to fish carrying them. Suggesting, these tumor cells have been alive, mutating, and travelling for years from one fish to another, outliving the original fish where the disease first started.

Can Humans Catch the Contagious Fish Cancer?

Quebec’s Ministry of the Environment has issued an official warning for the local fisherman to immediately discard and avoid eating any fish showing any visible black marks or lumps.

Quebec’s Ministry has also asked the fishermen to report any catfish tumor found in lakes other than Lake Memphremagog, Lake Magog and some other lakes in Maine and New Hampshire.

References:

1. Brown bullhead catfish melanoma represents a novel transmissible cancer – (https://www.nature.com/articles/s41586-026-10828-6)
2. Rare contagious cancer found in fish from Quebec and U.S. lakes – (https://www.cbc.ca/news/science/fish-transmissible-cancer-9.7279045)
3. Melanoma and Other Melanistic Lesions in Brown Bullhead Ameiurus nebulosus From Waterbodies in the Northeastern United States and Canada: Identification of Risk Factors
   – (https://onlinelibrary.wiley.com/doi/10.1111/jfd.70207)

Source-Medindia