Transmissible Cancer Found in Catfish in Vermont

The Vermont Fish & Wildlife Department runs an annual fish census in Lake Memphremagog, which lies across the border from Quebec. In 2013, researchers noticed something off about the brown bullhead catfish populating the lake. Many of them had alarming dark streaks on their skin.

“It looks like black tar on the skin — not very pretty,” remarks Peter Emerson, a fish biologist involved in the census since 2009. “You can tell it doesn’t help the fish; they were appearing in the mouth, on the fins, even on the lips or covering the eyes.”

Interestingly, a bit over a quarter of the catfish displayed these black marks. Although anglers had reported similar observations the previous year, there hadn’t been any signs of this issue before that. “They arrived very suddenly,” says Emerson.

In a recent study published in Nature, Emerson and his colleagues revealed that these dark blemishes represent a type of transmissible skin cancer that seems to transfer from one catfish to another.

Julie Dragon, a data scientist from the University of Vermont and a co-author of the research, explains, “It is the infectious agent itself. Those cells can somehow escape the host, possibly linger in the environment, and then infect a new host.”

Dragon believes that studying these catfish could provide insights into how tumors spread within our own bodies. “We still don’t fully grasp the mechanism by which a cell exits the tumor, travels through the bloodstream, and enters a new organ,” she says. “In the catfish, it might involve some of the same processes that allow these cells to invade a new host and bypass the immune system.”

A Mysterious Affliction

After the unusual lesions were spotted, the Vermont Fish & Wildlife Department collaborated with researchers to identify them as melanomas. This discovery was puzzling because catfish typically spend little time at the surface where UV exposure could lead to such cancers. Emerson suspected chemical pollutants might be the culprit, especially considering the nearby Coventry Landfill.

However, tests didn’t reveal significant toxic substances in the fish. That’s when Dragon was brought in. Initially, she explored whether a virus might be responsible, but found no evidence of one.

“Over time, genome sequencing got cheaper and easier to do,” she notes, suggesting that whole genome sequencing of both fish and tumors would be a good next step.

Normally, tumors are similar to the host’s genetic makeup, but in this case, the tumors differed significantly from the catfish they came from. “These tumors were very different from the host fish that they came from,” explains Dragon. “This wasn’t what we expected.”

The genetic close resemblance among the tumors indicated that they likely originated from a single fish some time ago and have been spreading since.

A Cancer with a Life of Its Own

Transmissible cancer has been recorded in a handful of animals, including Tasmanian devils, dogs, and some mollusks; now it seems catfish have joined that list. While it’s unclear how the catfish transfer the cancer, Dragon suspects it might occur during spawning. Notably, young fish don’t show signs of melanoma.

“They bunch together and flop around on each other,” she explains, “and they have sharp spines behind their pectoral fins. We think that they might accidentally scratch one another, providing a potential pathway for the cancer cells to invade a new host.”

Dragon plans to investigate further, considering whether environmental factors might be weakening the catfish immune systems. Approximately 30% of catfish across both the U.S. and Canadian sides of the lake currently have melanoma, but according to Dragon, “they do seem able to live with these tumors for a while.”

“Typically, cancers lead to dead ends for their hosts,” observes Vincent Lynch, an evolutionary biologist at the University at Buffalo. “When the host dies, so does the cancer.” Therefore, it would be advantageous for cancer cells to find ways to survive by infecting new hosts. “The cancer wants to survive; now it has a life of its own.”

Lynch also notes that these catfish could be valuable in laboratory settings to study cancer metastasis, potentially revealing ways to prevent it. However, that goal seems to be a long-term aspiration.

For now, researchers intend to study melanomas in catfish found in other lakes. Interestingly, Henry David Thoreau might have documented a similar melanoma in a Massachusetts catfish, describing it as having “an inky-black kind of leprosy, like a crustaceous lichen.”

The study’s authors suggest that transmissible cancers may be more widespread than once thought.