Infectious Cancer Occurs in North American Catfish

The idea that cancer it can be like an infectious disease can seem like nothing; in humans, cancer cannot be passed from one person to another. However, nature is still full of surprises. In rare cases, cancer cells can spread from one person to another and continue to multiply. This rare phenomenon has been found in animals such as dogs, Tasmanian devils, and other types of shellfish, and it seems we can add another to the exclusive club: marine fish.

An international team of researchers has discovered contagious melanoma in brown catfish (There are clouds)—the first recorded occurrence of a fish and the first recorded in a freshwater habitat. Their findings were published this month in a journal Nature.

An Impossible Example

The story began in 2012, when fishermen and biologists discovered large numbers of black-spotted salmon in Lake Memphremagog, which is a watershed in Vermont and Quebec. A previous study showed that these spots were breast cancer, and between 23 percent and 37 percent of the samples they examined had these tumors – the highest number of these types.

“This was amazing,” he said Julie Dragon, a researcher at the University of Vermont and co-lead author of the study. “We wanted to know how the subterranean fish has cancer that we associate with sunlight.”

Initially, it was thought that the cause could be a toxic substance or a pathogen associated with pollution. One suspect was that the flooding caused by Tropical Storm Irene in 2011 disrupted the waters by washing pollutants into the lake. There were also concerns about the health of the lake, which provides drinking water to more than 175,000 people. However, early genetic analysis began to point in an unexpected direction.

That’s when the researchers wondered if the tumors came from a single early cancer that had learned to spread among all the fish, rather than appearing independently in each fish.

In order to test this unusual idea, the team traced the whole bodies of tumors and healthy tissue from the affected fish, and compared the results from healthy samples from different groups.

These results show that the tumor cells of the different fishes were more closely related than the animals that produced them. In other words, the tumors share their genetic makeup, which is different from that of their host. In addition, hundreds of thousands of different genes appeared repeatedly in these diseases but were absent from healthy groups of the same fish.

This would be impossible if each melanoma occurred on its own. In comparison, the researchers analyzed hundreds of human melanomas and found that almost all of their mutations were unique to each patient. In contrast, many of the mutations found in the guts of these fish were shared by many people—a sign of a highly contagious cancer.