First-known freshwater contagious cancer spreading among catfish in Vermont lake
Brown Bullhead is a species of catfish, here caught on a link of sausage in a freshwater pond. (Judy Darby / Getty Images)
The first-ever transmissible cancer to spread among fish in a freshwater setting has been discovered among brown bullhead catfish in Lake Memphremagog.
According to a study published in the journal Nature, since 2012, these catfish have shown high rates of raised black lesions, leading researchers to investigate if the lesions were cancer that could be spread among species.
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Lake Memphremagog is located in both Vermont and Quebec, Canada, and is a glacial lake.
A study determined these raised black lesions to be malignant melanomas and observed these lesions in 23 to 37% of brown bullheads sampled between 2014 and 2017.
The high number of melanomas led scientists to believe that the cancer was caused by a contaminant or contagion, thus beginning the testing to determine what caused the cancer, as Lake Memphremagog serves as drinking water for 175,000 people.
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One hypothesis scientists had was that Tropical Storm Irene, which caused extensive flooding in 2011, led to reduced water quality of the lake because of runoff pollution.
The sun sets on Lake Memphremagog, which stands the United States and Canadian borders on November 8, 2021. (Josh Kuckens for The Washington Post / Getty Images)
To identify the cause of the cancer, researchers conducted multiple studies, including genomic analyses to determine if there were differences between diseased and healthy fish.
Preliminary genomic analysis showed that the tumors were distinct from healthy tissue. This suggested the cancer may be transmissible, spreading between hosts like a contagious parasite.
The study noted that transmissible cancers have only even been noted in three species: dogs, Tasmanian devils and multiple species of bivalves like clams, oysters, mollusks and scallops.
Brown Bullhead (Ameiurus nebulosus), Ictaluridae. (DeAgostini / Getty Images)
In transmissible cancers, tumor genomes across different hosts match each other more closely than they match their host’s own DNA — showing they all share a single clonal origin, the study said.
To test if the cancer had a clonal origin, scientists took tissue from healthy fish along with normal and tumor tissues from fish with confirmed lesions from all stages, from mild to severe.
They found evidence to suggest the cancer was indeed transmissible, and the first-known transmissible cancer in fish species as well as in freshwater.
Lake Memphremagog, northern Vermont, looking north into Quebec. (Marli Miller/UCG/Universal Images Group / Getty Images)
Scientists said they aren't sure exactly how the cancer transmits, but using clues from other transmissible cancers in animals as well as the biology of the brown bullhead catfish, inferred that reproduction or aging had a role in the transmission.
The study said future sampling of populations of affected brown bullhead catfish will address the age and origin of this clonal cancer, which is also essential for understanding the evolution and the way it impacts the brown bullhead population.