Elizabeth Murchison, a professor at the University of Cambridge, and Peter Emerson of the Vermont Department of Fish and Wildlife collect catfish for testing in Newport, VT. Emerson was one of the researchers who published a new article in the journal. Nature describing the first known transmissible cancer in fish. Joshua Brown/University of Vermont hide
Elizabeth Murchison, a professor at the University of Cambridge, and Peter Emerson of the Vermont Department of Fish and Wildlife collect catfish for testing in Newport, VT. Emerson was one of the researchers who published a new article in the journal. Nature describing the first known transmissible cancer in fish.
Joshua Brown/University of Vermont
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Joshua Brown/University of Vermont
Each year, the Vermont Department of Fish and Wildlife conducts a fish census in Lake Memphremagog, which straddles northern Vermont and the Canadian province of Quebec. In 2013, the team was surprised to see some of the brown bullhead catfish they were finding in the lake. The fish were marred by disturbing dark stripes.
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“It looks like black tar on the skin, not very pretty,” says Peter Emerson, a fish biologist with the department who has participated in the census since 2009. “You can tell it doesn’t help the fish because they’re inside the mouth, they’re on the fins.” Sometimes the dark markings are found on the lips or gills or even cover the fish’s eyes.
Just over a quarter of the catfish had black spots. The fishermen had noticed them the year before, but before that there was nothing. “They came very suddenly,” Emerson says.
In an article published Wednesday in NatureEmerson and his colleagues report that these dark spots are a type of transmissible skin cancer, which appears to spread from one catfish to another.
“It’s the infectious agent itself,” says Julie Dragon, a data scientist at the University of Vermont and co-author of the study. “Those cells can somehow escape the host, maybe live in the environment (the aquatic environment) and then infect a new host.”
Beyond this lake, Dragon believes these catfish may hold clues to how tumors spread through our bodies. “We still don’t really understand this event where a cell leaves the tumor, moves through the bloodstream and reaches a new organ,” he says. In catfish, “there could be some of the same processes that allow these cells to invade a new host, avoid the immune system, and then establish themselves.”
A fishy mystery
Shortly after the ominous lesions first appeared, the Vermont Department of Fish and Wildlife worked with researchers to determine they were melanomas.
They found this to be strange, since catfish don’t typically spend much time at the surface, where they would be exposed to the sun’s potentially harmful ultraviolet rays. Emerson and his colleagues thought that perhaps the melanoma was being caused by a chemical or pollutant that leached into the lake. (Coventry landfill is nearby, after all.) So they looked for toxins in the fish’s bodies. fish bodies.
“But it turns out, as we did the studies, they didn’t have much of that,” Emerson says.
That’s when the team approached Dragon. He began looking to see if there could be a virus responsible for the cancers, but found no evidence.
“Over time, genome sequencing became cheaper and easier to perform,” he says, “so we recommend whole-genome sequencing of fish and tumors.”
A tumor usually appears because something goes wrong in the cells of the host organism. This means that a tumor tends to genetically resemble its host.
However, this was not the case with these catfish. “These tumors were very different from the host fish they came from,” Dragon says, “which is not what we expected or what anyone would expect.”
Instead, the genetics of the tumors closely resembled each other, which suggested to Dragon that these cancers were clones that arose in a fish some time ago and have been spreading from catfish to catfish ever since.
A cancer with a “life of its own”
So far, transmissible cancer has been identified in only a handful of animals, including Tasmanian devils, dogs, certain mollusks and now, apparently, fish.
Dragon and his colleagues aren’t sure how catfish infect each other, but they suspect it may occur during spawning. First, they don’t see melanoma in young fish.
Second, “they clump together in large groups, stand on top of each other, and have spines behind their pectoral fins that are quite sharp,” Dragon says. “And we think they actually accidentally scratch each other, and that’s a potential pathway for cancer cells to invade a new host.”
This is partly what Dragon will consider next, as well as whether there is something environmental or viral that depresses the catfish’s immune system. Today, an average of 30% have melanoma across the lake on both the U.S. and Canadian sides, although Dragon says “it looks like they can live with these tumors for some time.”
“Normally, cancers are kind of a dead end,” says Vincent Lynch, an evolutionary biologist at the University at Buffalo who was not involved in the research. “When the host dies, when the person with cancer dies, so does the cancer.”
This means that it would be advantageous for the cancer to be able to escape and invade another host to stay alive. “Cancer wants to survive,” says Lynch. “Now it has a life of its own.”
He adds that these catfish can be useful in the laboratory to study how cancers metastasize and migrate within our own bodies. “It could tell us something about how metastases evolve and originate within an individual,” he says. “If you could prevent that from happening, then you could prevent people from dying of cancer.”
However, this is likely a long way off.
Meanwhile, researchers will study catfish melanomas that have appeared in lakes and ponds elsewhere. For example, this particular melanoma may have been first documented by Henry David Thoreau, who observed a catfish in a tributary of the Concord River in Massachusetts “with a kind of inky-black leprosy, like a crustacean lichen.”
The authors of this study argue that it is worth considering whether transmissible cancers are perhaps more common than we thought.
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