NEWPORT, VERMONT, July 25 (Nationwide Times) — A study published in Nature says brown bullhead catfish in Lake Memphremagog carry what appears to be the first known freshwater transmissible cancer in fish.
The finding matters because Lake Memphremagog is more than a fishing spot. It is a drinking-water source for about 175,000 people. But the researchers say the fish do not pose a health threat to people, and the cancer appears to be limited to the catfish themselves.
What did the study find?
Researchers sampled brown bullhead catfish in Lake Memphremagog between 2014 and 2017 and found raised black skin lesions, or melanomas, in roughly 23% to 37% of the fish they studied.
The team says the tumors are not just similar cancers appearing separately in many fish. Genetic analysis showed the tumor cells were more closely related to one another than to the fish carrying them. The authors interpret that pattern as evidence that the cancer is clonal — meaning the cancer cells themselves are spreading from fish to fish.
That is what scientists mean by a transmissible cancer: the cancer acts like an infectious agent.
Julie Dragon, a University of Vermont researcher and co-author, told NHPR, “It is the infectious agent itself.”
Why is this unusual?
Cancer usually starts inside one animal and stays there. A transmissible cancer is rare. The best-known examples are in some dogs, Tasmanian devils, and a few marine animals.
The Nature study says this is the first known transmissible cancer found in freshwater fish.
That makes the Lake Memphremagog case important for scientists studying how cancer can move through animals and how disease can spread in a lake ecosystem.
Which fish are affected?
The study focuses on brown bullhead catfish.
Dragon told WBUR that researchers have not seen the cancer in other catfish or other bottom-dwelling fish in Lake Memphremagog, and that they do not believe it spreads to other species.
Mark Henderson, a fish biologist and study co-leader at the University of Vermont, told The Weather Network that the tumors seem to show up in larger fish of spawning age.
How might it spread?
The exact transmission route is still unknown.
Researchers suspect close contact during spawning may help pass the cancer along, because bullhead catfish gather in groups during reproduction. Dragon told WBUR that the team thinks the fish may be exposed when they spawn and come together in large groups.
But the authors have not proved that this is how transmission works.
The study also says the researchers found no evidence that a virus or other microorganism caused the outbreak.
Is it a risk to people?
The authors say the fish are safe to handle and study, and that the cancer does not appear to infect other species.
Even so, they advise against eating infected fish. Vermont Fish and Wildlife has also warned against consuming bullhead with skin lesions.
Why does the lake setting matter?
Lake Memphremagog sits on the Vermont-Quebec border and serves as a drinking-water source for a large population. That made the outbreak stand out to researchers and state wildlife officials.
The Vermont Department of Fish and Wildlife has not documented dramatic population declines in the lake since the melanomas were first noticed, according to WBUR’s report.
What do researchers still not know?
The biggest open question is why the cancer developed in the first place.
The study reports elevated arsenic levels in the cancer cells and an overlap between the disease’s location and areas where arsenic occurs naturally, but it does not establish a causal link.
Researchers have also discussed whether runoff and pollution after Tropical Storm Irene in 2011 may have affected water quality in the lake. That idea remains a hypothesis, not a proven cause.
The team is planning follow-up work, including tank studies with infected and healthy fish, to learn more about how the cancer spreads.
For now, the study establishes a new and unusual disease in a freshwater species, but it does not answer every question about where it came from or how exactly it moves through the lake.
