NEWPORT, VERMONT, July 25 (Nationwide Times) — Researchers say they have identified the first known freshwater transmissible cancer in fish, a melanoma outbreak in brown bullhead catfish from Lake Memphremagog, a border lake shared by Vermont and Quebec. The study was published July 22 in Nature, and the authors say the work is complete enough to show a real pattern, but not enough to explain exactly how the disease spreads.
What did the study find?
The researchers sampled brown bullhead catfish from 2014 through 2017 and found visible lesions or growths in roughly 23% to 37% of the fish. In plain terms, that means about one-quarter to one-third of the sampled fish showed signs of the disease.
Genetic analysis showed that the tumors were melanomas, a kind of skin cancer, and that the tumor cells were more closely related to one another than to the fish carrying them. The study team interpreted that as evidence that this was not just many fish developing similar cancers on their own, but one cancer line spreading between fish.
Why is this unusual?
Most cancers do not spread from one animal to another. In this case, the authors say the cancer cells themselves appear to be the infectious agent. That is why they describe it as transmissible, or contagious.
The study authors said they found no evidence that a virus or other microorganism caused the outbreak. They also said the cancer appears to affect brown bullhead catfish, and that they have not seen it in other catfish or other bottom-dwelling fish in Lake Memphremagog.
How long has this been happening?
Anglers and biologists began reporting more raised black skin lesions on brown bullhead in about 2012. The study also looked at whether water-quality changes after Tropical Storm Irene in 2011 may have played a role, but the authors did not establish a cause.
That matters because Lake Memphremagog is more than a fishing spot. The lake is a drinking-water source for about 175,000 people, so any unusual disease pattern there draws attention from both wildlife managers and public health officials.
Do scientists know how it spreads?
Not yet. The researchers do not know the exact transmission route.
One leading idea is that close physical contact during spawning may help spread the cancer. Brown bullhead gather in groups to reproduce, and the authors said larger fish of spawning age seem to be the ones most often affected. But that is still a theory, not a proven explanation.
The study team also said there is no evidence yet that the disease passes to other species.
Is it a risk to people?
The authors said the fish pose no health threat to humans and are safe to handle and study. They still advise against eating infected fish.
That distinction matters. A disease can be serious for wildlife without being dangerous to people, and the study does not show that the catfish cancer threatens the lake’s drinking water.
What about arsenic?
The researchers reported elevated arsenic levels in the cancer cells and noted an overlap between the disease’s location and areas with naturally occurring arsenic. But they did not establish that arsenic caused the cancer.
What happens next?
Researchers from the University of Vermont, the Vermont Department of Fish and Wildlife, and the U.S. Geological Survey have secured funding for follow-up work. One planned test will place infected bullhead with healthy fish in a tank to see whether the cancer can spread under controlled conditions.
For now, the main finding is simple but striking: scientists say a transmissible cancer is moving through a freshwater fish population in Vermont for the first time ever identified, and they still have to figure out how and why it is happening.
