Melanoma formed by living cancer cells reached brown bullheads, crossed a 48-kilometer lake, and surprised researchers.
An unusual scientific discovery revealed that a type of cancer can pass directly from one fish to another.
The case was identified in brown bullheads found in Lake Memphremagog, located between Vermont, USA, and Quebec, Canada.
Researchers from the University of Vermont concluded that the tumors did not arise separately in each animal.
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The discovery represents the fourth transmissible cancer known to science, as well as being the first recorded in fish and in a freshwater environment.
Fisherman found fish covered with tar-like spots
The first case drew attention in 2012, when a fisherman pulled a brown bullhead from the lake covered with dark growths.
The lesions looked like a layer of tar spread over the animal’s skin.
The fish was handed over to Peter Emerson, a biologist from the Vermont Department of Fish and Wildlife.
Emerson had been monitoring Lake Memphremagog since 2009 but had never observed anything similar.
Less than three years later, the tumors were already appearing in a significant portion of the bullheads analyzed.
Pollution and microorganisms were investigated
Initially, the researchers suspected environmental contamination.
Brown bullheads are considered sentinel species because they can develop tumors in waters contaminated by polycyclic aromatic hydrocarbons.
Tropical storm Irene hit the region in 2011 and caused intense flooding.
Biologists considered the possibility that the water had displaced toxic substances into the lake.
Subsequent analyses showed, however, that more contaminated lakes housed apparently healthy fish.
Viruses, bacteria, and other microorganisms were also searched for over the years.
No conventional infectious agent was linked to the disease.
Genetic sequencing revealed clonal cancer
The team began analyzing tissue samples through the bioinformatics core at the University of Vermont.
Julie Dragon, professor of Microbiology and Molecular Genetics, decided to look for shared mutations among the tumors.
Common tumors usually have DNA similar to the animal in which they arose.
The catfish melanomas showed completely different behavior.
All the tumors formed a single genetic group, while the host fish belonged to another set.
Hundreds of thousands of mutations appeared in the tumors of different animals.
These changes were not present in the DNA of the catfish themselves.
The analysis confirmed that the melanoma was clonal.
Therefore, the cancer itself functioned as a transmissible agent.
Disease crossed lake over 48 kilometers
The team returned to monitor the lake in 2017.
Four points were in Vermont, while another three were located in Quebec.
The prevalence of the disease was similar in all the areas analyzed.
This result indicated that the cancer had spread across almost the entire binational lake in a few years.
Genetic tests suggested, however, that the tumor lineage may have originated elsewhere.
The tumors were closer to healthy catfish found in the states of Maine and New Hampshire.
Cases also appeared in New Brunswick, Nova Scotia, Massachusetts, Pennsylvania, Wisconsin, and New York.
Cancer appears in few lakes
The region monitored by Emerson has approximately 90 lakes.
Catfish live in almost all of them, but cancer was identified in only one.
This distribution suggests that the disease can survive in the animals.
The main barrier seems to be transportation between different bodies of water.
Human activities can contribute to this movement, especially through water, equipment, and live bait.
The behavior is similar to that observed in the spread of invasive species.
Tumors affect the mouth and “whiskers” of fish
Melanomas can appear in different external parts of the body.
Lesions were found in the mouth and barbels, popularly known as the “whiskers” of catfish.
These structures help the animals locate food at the bottom of the lake.
Advanced tumors make the barbels smaller and can impair feeding.
Lake Memphremagog has abundant food, reducing the immediate risk of starvation.
The catfish population remains numerous, while the prevalence of the disease seems to have stabilized.
Scientists are still studying how to prevent transmission
No specific method has been defined to control the disease in the lake.
Local rules already prohibit the transport of water and live bait to other environments.
These regulations were created to prevent the spread of invasive species.
The same measure can, however, reduce the risk of carrying cancerous cells to other lakes.
Photographs published by anglers on the Fishbrain app also help researchers locate new cases.
Monitoring shows how images sent by citizens can enhance the observation of diseases in wild animals.
How many other transmissible cancers might be circulating among little-studied species without science noticing? Share your opinion!
