Catfish Are Spreading Cancer to Each Other Like a Parasite, Scientists Find

A Plague in the Water

In 2012, an angler pulled a brown bullhead out of Lake Memphremagog with what looked like tar smeared across its skin. He brought it to Peter Emerson, a fish biologist with the Vermont Fish and Wildlife Department, who had surveyed the lake since 2009. Emerson had never seen anything like it. Within a few years, what started as one unusual catch had become something far more alarming.

Since 2012, anglers and biologists reported a striking increase in brown bullhead catfish with raised black skin patches in the cross-border lake shared by Vermont and Quebec. By 2014, nearly one in three fish showed the dark lesions. Emerson described it plainly: "It looks like a plague." The growths turn up anywhere on the outside of the body, often inside the mouth and on the barbels — the whiskers a bottom-feeding catfish uses to find food.

Cancer That Jumps From Fish to Fish

Although researchers expected a virus or pollution-linked contaminant was the cause, they soon learned the fish were suffering from melanoma, an aggressive form of skin cancer. "We actually looked for viruses, and that was the first hypothesis," said researcher Julie Dragon. "We did genomic sequencing and could not find the virus that associated with the diseased fish."

Scientists were also baffled by how a bottom-dwelling fish was getting a cancer typically associated with too much sun exposure. Whole-genome sequencing ultimately revealed the answer: the tumor cells were more closely related to one another than to the fish carrying them — the genetic signature of a transmissible cancer, in which cancer cells themselves spread from animal to animal. The cancer cells behave more like parasites than conventional tumors, moving from fish to fish.

Researchers at the University of Vermont published their findings this week in the journal *Nature*, confirming the melanoma is contagious, spreading from one fish to another as living cancer cells. It is the fourth type of transmissible cancer known to science, the first in a fish, and the first in fresh water.

Only the Fourth of Its Kind — Ever

Previous examples of transmissible cancer include facial tumor disease in Tasmanian devils, a venereal tumor in dogs, and leukemia-like transmissible diseases in clams, mussels, and other bivalve mollusks. Until about ten years ago, transmissible cancer was believed to be an extremely rare phenomenon affecting only two mammalian species: dogs and Tasmanian devils. Over the last decade, however, it was discovered that this type of cancer is quite common among marine bivalves, to the extent that it may be a completely ordinary occurrence in those species.

Researchers believe the cancer may spread during spawning. "It seems to only happen in larger fish that are of spawning age," said Mark Henderson, a fish biologist and study co-leader at the University of Vermont. "Maybe some part of spawning behaviour leads to the spreading of the cancer between animals." Researchers believe spawning behavior may provide the close physical contact needed for cancer cells to transfer between fish.

Spreading Beyond One Lake

The diseased fish were first found in Lake Memphremagog on the Vermont/Quebec border, but the team has since found fish with this cancer in other lakes in New England and Canada. Water quality tests in Lake Memphremagog show the same low levels of pollutants as many other nearby waterbodies, including Lake Champlain, which isn't known to have any transmissible melanoma. The team has expanded its sampling to Vermont, New Hampshire, Maine, and New Brunswick — and found the cancer in "a few other ponds and lakes in this region." The researchers emphasize that the disease poses no known risk to humans.

The discovery raises a question that may reshape how scientists think about cancer in the wild. If transmissible cancers were once considered freakish anomalies limited to two land mammals, and are now turning up in shellfish and freshwater fish alike, the real question isn't how rare these diseases are — it's how many have simply gone undetected. The catfish of Lake Memphremagog may have just opened a much larger door.