A strange transmissible cancer is spreading through the catfish in this lake
JUANA SUMMERS, HOST:
For more than a decade, there's been something fishy going on in a lake that straddles the state of Vermont and the Canadian province of Quebec. It does involve a kind of catfish, but also cancer and possibly clues about how tumors spread through the body. Here's NPR science reporter Ari Daniel.
ARI DANIEL, BYLINE: Every year, Vermont Fish & Wildlife does a fish census in Lake Memphremagog in the very north of the state. In 2013, the team was startled by some of the brown bullhead catfish they were finding. They were marred by these dark streaks that anglers had first noticed the year prior. Peter Emerson is a fish biologist with the department.
PETER EMERSON: Looks like black tar on the skin. Not very pretty. You can tell it doesn't help the fish because they're inside the mouth, they're on the fins.
DANIEL: Sometimes they're on the lips or the gills or even covering the fish's eyes. About a quarter of the catfish had the black blotches, which were, in fact...
EMERSON: Melanoma.
DANIEL: Skin cancer, which was odd since catfish don't usually hang out at the surface, where they'd be exposed to the UV rays of the sun. Emerson and his colleagues thought maybe there was some chemical or pollutant leeching into the lake that was causing the melanoma.
EMERSON: But it turns out, as we did the studies, they don't have much of that at all.
DANIEL: So the team reached out to Julie Dragon. She's a data scientist at the University of Vermont who ultimately conducted a whole genome sequence of the fish and their cancers. Now, a tumor usually erupts because something's gone awry in the cells of the organism. So you'd expect a tumor to resemble its host genetically, but that wasn't the case for these catfish.
JULIE DRAGON: These tumors were very different from the host fish that they came from.
DANIEL: Rather, the tumors resembled one another quite closely, suggesting to Dragon these cancers were clones that arose in a fish some time ago and have been spreading from catfish to catfish ever since.
DRAGON: It is the infectious agent itself. Those cells can somehow escape the host, maybe live in the aquatic environment, and then infect a new host.
DANIEL: Transmissible cancer has been identified in only a handful of animals so far, Tasmanian devils, dogs, certain mollusks and now fish, it seems. Dragon and her colleagues aren't sure how the catfish are infecting one another, but they suspect it may happen during spawning.
DRAGON: They aggregate into these big clusters. and they have these spines. And we think that they actually scratch, accidentally, each other, and that is one potential avenue for the cancer cells to invade a new host.
DANIEL: Dragon will look into this possibility next, alongside whether there's something environmental or viral depressing the catfish immune systems, since 30% on average now have the melanoma throughout the lake on both the U.S. and Canadian sides. The research is published in the journal Nature. Vincent Lynch is an evolutionary biologist at the University at Buffalo, who wasn't involved in the study.
VINCENT LYNCH: Normally, cancers are sort of a dead end. When the host dies, when the person with the cancer dies, so does the cancer.
DANIEL: So it'd be advantageous to the cancer to be able to escape and invade another host to stay alive.
LYNCH: The cancer wants to survive. Now it's got a life of its own.
DANIEL: And Lynch says these catfish may be useful in the lab to study how cancers migrate and metastasize within our own bodies.
LYNCH: It could tell us something about how metastases evolve and originate within an individual. If you could stop that from happening, then you can actually stop people from dying of cancer.
DANIEL: Though, that's likely to be a ways off, says Lynch. In the meantime, the researchers say it's worth considering whether transmissible cancers are perhaps more common than we once thought.
Ari Daniel, NPR News.
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