Since 2012, fishermen and biologists at Lake Memphremagog, on the border between Vermont (USA) and Quebec (Canada), had been observing something disturbing: a growing number of brown catfish ( Ameiurus nebulosus ) were developing raised, black spots on their skin . By 2014, nearly one in three specimens showed these lesions. What seemed like a local pollution problem or a virus has turned out to be something much stranger: a cancer that is transmitted from one fish to another like a parasite.
The discovery, published in the journal Nature by researchers at the University of Vermont, represents the first documented case of transmissible cancer in a freshwater species and the fourth known case in the entire animal kingdom, following Tasmanian devil facial tumor, canine venereal tumor, and contagious leukemias in bivalve molluscs. The news has made headlines worldwide, but scientists want to emphasize that there is no danger to humans.
A tumor that does not belong to its bearer.
To understand what was happening, the team led by Julie Dragon and Mark Henderson sequenced the complete genome of the tumors and compared it to healthy tissue from the same fish. The result was conclusive: the cancer cells from different individuals shared hundreds of thousands of genetic variants that did not appear in the host fish. In a typical cancer, each tumor is genetically unique because it originates from the patient's own cells. Here, however, the tumors were virtually identical to one another, which can only be explained if the malignant cells had spread from one fish to another.
“We thought it might be a virus or a contaminant,” Dragon admits. “But when we saw that the tumor genomes were more closely related to each other than to those of their hosts, we knew we were dealing with a transmissible clonal cancer.” In other words, the tumor cells themselves act as infectious agents , colonizing new hosts and multiplying within them.
How is melanoma transmitted among fish?
The exact transmission mechanism is still being investigated, but the authors point to the breeding season as the key time . During spawning, catfish congregate in shallow waters and maintain very close physical contact. "It seems to only occur in large, reproductively mature fish," Henderson explains. "Perhaps some aspect of their reproductive behavior triggers the spread of the cancer."
Furthermore, as benthic fish that live on the seabed and lack scales, they constantly interact with the sediment , where shed tumor cells could accumulate. The study also points out that marine and aquatic pollution or hormonal changes could weaken the fish's immune system, making them more vulnerable to infection.
The fourth known transmissible cancer in animals
Until now, science had only documented three examples of transmissible cancer in nature. The oldest is canine transmissible venereal tumor (CTVT) , which is transmitted during mating and has been circulating globally for between 6.000 and 11.000 years. Then there is Tasmanian devil facial tumor (DFTD) , which has decimated this species since the 90s by being transmitted through bites. And the third group consists of several contagious leukemias in bivalve mollusks such as clams and mussels, which are spread through seawater.
Brown catfish melanoma is the first of its kind in a fish and in a freshwater ecosystem. "Until ten years ago, transmissible cancer was thought to be extremely rare," says Adrián Báez-Ortega, a researcher at the University of Cambridge. "But the aquatic environment could be more conducive to transmission. It is very likely that there are many other cases that have gone undetected."
Natural arsenic and its possible link to Thoreau
Researchers ruled out the Coventry landfill, located near the lake, as the cancer was caused by the tumors because they appeared uniformly along the lake's 35 miles (51 kilometers) and in other clear-water bodies of water. However, they did find a correlation with naturally occurring arsenic levels in the soil. "Arsenic occurs naturally throughout New England, and there are very high concentrations in northeastern Vermont and Maine, where we've also seen evidence of this cancer," Henderson said.
Interestingly, the first description of this cancer may date back to 1852, when philosopher and naturalist Henry David Thoreau wrote in his journal about a catfish with "a large velvety black spot" in the Concord River. Six years later, he described another case. Scientists are now exploring rivers and ponds in Massachusetts to determine if the cancer may have originated there.
No risk to humans
One of the most frequently asked questions is whether this cancer could affect humans. Lake Memphremagog provides drinking water to more than 175.000 people, but researchers are adamant: there is no known risk to humans . "Cancer cells cannot infect or survive in other species," explains Dragon. "The fish are safe to handle and study." Beata Ujvari, from Deakin University, adds that "humans have a highly efficient immune system, capable of recognizing foreign cells. The probability of transmission from catfish to humans is virtually zero."
In humans, tumor transmission is extremely rare and limited to very specific cases such as mother-to-child transmission (0,1% of pregnancies) or cancer derived from donors in organ transplants (0,04% of recipients). This is nothing like what happens in these fish.
The long-term ecological impact remains uncertain. While Tasmanian devil cancer has been devastating, canine cancer has coexisted with its hosts for millennia. In Lake Memphremagog, severely ill catfish can survive for years, but it's unknown how this will affect populations. What is clear, however, is that this finding opens a new window into understanding cancer evolution. "By studying how cancers survive and spread outside their original host, we can learn a great deal about what keeps them contained and what happens when those barriers break down," concludes Dragon. These fish, unwittingly, have become a natural laboratory for oncology.