It’s rare that cancerous tumours can spread from one individual to another. But a genetic study suggests that’s what’s happening with melanoma tumours among catfish in Quebec and the northeastern U.S.
The discovery represents the first known transmissible cancer in fish and one of very few transmissible cancers ever found, reported the study published in the journal Nature on Wednesday.
Julie Dragon, co-leader of the new study, noted that only three other transmissible cancers have ever been identified so far — in Tasmanian devils, dogs and shellfish.
“These conditions are incredibly rare in nature,” she said. “So something has to be happening to allow this to happen.”
Anglers in Lake Memphremagog, which spans the border between Quebec and Vermont, first reported catching brown bullhead catfish with strange black spots and lumps in 2012.
They turned out to be melanoma skin cancer tumours, which humans can also get.
Now, about a third of the brown bullheads living in the lake have them.
It’s not clear how sick the fish become. In some cases, the cancer spreads to other organs, such as the brain or liver. But many fish with lesions seem relatively healthy and some older fish even have them, suggesting they can live with the disease for a time.
Because of the sudden appearance of the black lesions in Lake Memphremagog the year after a huge flood caused by post-tropical storm Irene, locals worried they were caused by carcinogens washed into the lake by floodwaters.
Tests for carcinogens haven’t been conclusive, although a study published in May found higher concentrations of seven metals, including zinc and the carcinogen arsenic, in the skin of fish with the tumours.
So in 2014, Dragon, an associate professor of microbiology and molecular genetics at the University of Vermont’s Larner College of Medicine, was asked by Vermont’s Fish & Wildlife Department to look into the possibility that a virus was to blame.
Genetic testing didn’t find any viruses.
Dragon considered another possibility — that the lake’s catfish were inbred and had a mutation that made them more susceptible to cancer. She decided to test the genetics of both healthy and infected fish, along with the tumours.
The tests showed the fish weren’t inbred — they had quite a lot of genetic diversity.
But the biggest surprise? While most cancer cells mutate from the host’s cells and resemble their host, “these cancer cells were more similar to each other than they were to the fish [they were found on],” Dragon said.
Dragon and her team were puzzled. They started asking other scientists what they thought. Sam Hart, a University of Washington researcher and co-author of the new study, had previously worked on the transmissible shellfish cancer. He suggested something similar was infecting the catfish.
Dragon said the tumours seem to shed cells easily: “They do sort of fall apart when you touch them.”
Because they affect only adult fish, one theory is that the disease is transmitted during spawning, when they form big clusters and could come into contact with each other’s spines, behind their pectoral fins.
“They… swim all over each other and we think it’s possible that they poke each other and cells can get into other fish that way,” Dragon said.
So far, the researchers haven’t actually shown transmission of the tumours from one fish to another, even in the lab, although they hope to later this year.
Because of that, Vicki Blazer, lead author of the May study that found higher levels of toxins in the sick fish, isn’t convinced this is a transmissible cancer.
“I guess my first concern is to call something a transmissible anything when you haven’t transmitted it,” said Blazer, a retired scientist with the U.S. Geological Survey.
She is now testing infected fish for PFAS, another possible contaminant that could make them more susceptible to cancer.
Stephen Goff, a Columbia University researcher who was part of the shellfish cancer study but not this one, said he’s convinced the brown bullheads are infected with a transmissible cancer, as the genetic evidence is “totally compelling.”
The study was funded by U.S. and Vermont government agencies including the U.S. National Institutes of Health and the Great Lakes Fishery Commission. It was also supported by Quebec’s Ministry of the Environment, whose staff helped collect the brown bullheads for testing.
Quebec’s Ministry of the Environment recommends not eating the fish with tumours, even though there’s no evidence it’s harmful to human health. Dragon said Vermonters sometimes do eat them and she believes they’re safe to eat.
Catfish with tumours have also been found in Quebec’s Lake Magog and some other lakes in Maine and New Hampshire. Anglers are asked to email photos of any they catch in other water bodies in Quebec to the Ministry of the Environment for tracking purposes.
