Researchers Find the First Transmissible Cancer in a Freshwater Fish, and It Appeared Overnight

In a Vermont Lake, Cancer Learned to Jump Between Fish

Transmissible cancer is one of biology's rarest phenomena. Only three types were known to exist in the entire animal kingdom: a facial tumor that has devastated Tasmanian devils, a venereal tumor in dogs, and a leukemia-like disease in marine bivalves. Now there is a fourth, and it turned up in a catfish on the Vermont-Quebec border.

A study published July 22 in the journal Nature identifies the first transmissible cancer ever found in a freshwater fish species. The tumors are melanoma, and they are spreading through brown bullhead catfish in Lake Memphremagog. Nearly one in three fish in the lake carry the distinctive black skin lesions. Genetic analysis confirms that every tumor traces back to a single fish: the cancer cells are clones, more closely related to each other than to the animals they inhabit 1.

In other words, these cancer cells are not dying with their hosts. They are moving between fish like parasites.

The discovery began with a mystery that has been building for over a decade. Since 2012, anglers and biologists on Lake Memphremagog had been pulling up brown bullhead covered in raised black patches. By 2014, the affliction had reached epidemic proportions in the population 1. Researchers initially suspected a virus. They suspected pollutants in a lake that supplies drinking water to more than 175,000 people.

Whole-genome sequencing told a different story. When the team compared DNA from tumors and healthy tissue across affected fish, they found that the cancer cells shared hundreds of thousands of genetic variants with each other and almost none with their hosts. That pattern is the molecular fingerprint of a clonally transmissible cancer: a tumor that originated in one individual and then spread its own cells to new hosts 1.

"It seems to only happen in larger fish that are of spawning age," said Mark Henderson, a fish biologist and study co-leader at UVM's Rubenstein School of Environment and Natural Resources. During spawning, bullhead cluster tightly in shallow water, and as scaleless bottom-dwellers they maintain constant physical contact with each other and with sediment that could harbor free-floating cancer cells 1.

What makes the discovery unsettling is not the disease itself but the question of what triggered it. Transmissible cancers are vanishingly rare because they require tumor cells to accomplish something extraordinary: survive outside their original host, evade a new host's immune system, and colonize fresh tissue. For that to happen spontaneously in a freshwater lake, something in the environment may have shifted.

The strongest lead is arsenic. The cancer cells carry elevated levels of the element, a known carcinogen, and the disease's geographic footprint tracks with naturally occurring arsenic in surrounding soils across New England. The team has since found the cancer in lakes and ponds across Vermont, New Hampshire, Maine, and New Brunswick, many with pristine water and no proximity to any industrial source 1.

The nearby Coventry Landfill, which some environmentalists suspected as a source, does not fit the evidence. The tumors appear evenly throughout the 32-mile-long lake rather than clustered near the dump, and water quality tests show pollutant levels comparable to other clean lakes in the region 1.

The ecological stakes are uncertain. Some transmissible cancers are catastrophic. The Tasmanian devil facial tumor has wiped out as much as 90% of some populations. Others, like the dog venereal tumor, have coexisted with their hosts for thousands of years. In Lake Memphremagog, heavily diseased bullhead can survive for years, and long-term population effects remain unknown 1.

The cancer poses no known risk to humans. The tumor cells cannot infect or survive in another species, and the fish are safe to handle and study 1. But the broader question the study raises is what environmental pressures, from chemical contamination to climate-driven habitat stress, might be creating conditions where a solitary disease evolves into a contagious one.

Julie Dragon, a scientist at the UVM Cancer Center and co-leader of the study, frames the implication with a shift in perspective. Maybe transmissible cancers are not as rare as the three previously known cases suggest. Maybe scientists have not been looking. "In the case of transmissible cancers, it may be that we don't see many simply because we aren't looking for them" 1.

If she is right, the catfish of Lake Memphremagog are not an anomaly.