Instances of cancerous tumors transferring between individuals are quite uncommon. However, a recent genetic analysis indicates that melanoma tumors may be spreading among catfish populations in Quebec and the northeastern United States.
This finding marks the first documented case of transmissible cancer in fish and is among the few instances of such cancers identified overall, as noted in a study published in Nature on Wednesday.
Julie Dragon, who co-led this research, pointed out that only three other types of transmissible cancers have been recognized to date: in Tasmanian devils, dogs, and shellfish.
“Such occurrences are extraordinarily rare in the natural world,” she remarked. “This indicates that some underlying factors must be facilitating this phenomenon.”
Tumors emerged following a major flood
In 2012, fishermen in Lake Memphremagog, which lies along the Quebec-Vermont border, began reporting catches of brown bullhead catfish displaying unusual black spots and growths.
These turned out to be melanoma tumors, a type of skin cancer that can also affect humans.
Currently, approximately one-third of the brown bullheads in the lake are affected.
The extent of illness in the fish remains uncertain. In some instances, the cancer has spread to vital organs such as the brain and liver. However, many fish with skin lesions appear to be relatively healthy, and some older specimens possess them, indicating they may coexist with the condition for a while.
Given the abrupt emergence of these black lesions in Lake Memphremagog following a significant flood triggered by post-tropical storm Irene, local residents expressed concerns that the tumors were linked to carcinogens introduced into the lake by floodwaters.
Cancer researcher Julie Dragon, right, leads the dissection and tissue collection of catfish with melanoma skin cancer at the Gateway Center on the shores of Lake Memphremagog in Newport, Vermont. (Josh Brown)
Although tests for carcinogens have been inconclusive, a study released in May detected elevated levels of seven metals, including zinc and the carcinogen arsenic, in the skin of the affected fish.
In 2014, Dragon, an associate professor of microbiology and molecular genetics at the University of Vermont’s Larner College of Medicine, was commissioned by Vermont’s Fish & Wildlife Department to investigate the potential involvement of a virus.
Findings from genetic testing
Genetic assessments found no evidence of viral presence.
Dragon explored another hypothesis: that the catfish in the lake were inbred and possessed a mutation making them more prone to cancer. She proceeded to analyze the genetics of both healthy and affected fish, as well as the tumors.
The black color and lump on this brown bullhead catfish is a melanoma skin cancer tumor. (Vermont Fish & Wildlife)
The results indicated that the fish were not inbred; in fact, they exhibited a significant degree of genetic diversity.
The most astonishing finding? Unlike typical cancer cells, which usually mutate from their host’s cells and bear resemblance to them, “these cancer cells were more alike to one another than to the fish [they were found on],” Dragon explained.
Feeling puzzled, Dragon and her team consulted with other scientists for their insights. Sam Hart, a researcher at the University of Washington and co-author of the new study, had previously examined the transmissible cancer in shellfish. He proposed that something similar might be affecting the catfish.
Could it be sexually transmitted?
Dragon noted that the tumors seem to easily shed cells: “They tend to disintegrate when touched.”
Since the tumors only affect adult fish, one theory suggests the cancer might be transmitted during spawning, when they gather in large groups and could come into contact with each other’s spines, located behind their pectoral fins.
“They … interact with one another extensively, and we suspect that they may poke each other, allowing cells to transfer between fish,” Dragon remarked.
Up to this point, the researchers have not demonstrated tumor transmission from one fish to another, even in controlled environments, though they plan to investigate this further later in the year.
Due to this uncertainty, Vicki Blazer, the lead author of the May study that identified increased toxin levels in the affected fish, remains skeptical about the classification of this as transmissible cancer.
“I have reservations about labeling something as transmissible when there is no confirmed transmission,” stated Blazer, a retired scientist from the U.S. Geological Survey.
These images illustrate the various ways black lesions can manifest on different fish, as shared by researcher Vicki Blazer from the U.S. Geological Survey. Her analysis revealed elevated levels of seven toxins, including arsenic, in fish displaying these lesions. (Vicki Blazer)
She is currently examining infected fish for PFAS, another potential contaminant that may increase their cancer risk.
Stephen Goff, a researcher from Columbia University who participated in the shellfish cancer research but not this study, expressed his belief that the brown bullheads are indeed affected by a transmissible cancer, asserting that the genetic findings are “entirely convincing.”
What should you do if you catch a fish with tumors?
The research received funding from various U.S. and Vermont government organizations, including the U.S. National Institutes of Health and the Great Lakes Fishery Commission. It also received support from Quebec’s Ministry of the Environment, which assisted in collecting the brown bullheads for analysis.
Quebec’s Ministry of the Environment advises against consuming fish with tumors, despite the absence of evidence showing harm to human health. Dragon mentioned that some people in Vermont do consume them, and she believes they are safe for eating.
Tumor-bearing catfish have also been discovered in Quebec’s Lake Magog and several lakes in Maine and New Hampshire. Anglers are encouraged to send photos of any they catch in other water bodies in Quebec to the Ministry of the Environment for monitoring purposes.
