Scientists studying a Vermont lake have confirmed the first known case of contagious cancer in fish, adding a new species to the short list of animals known to transmit cancer cells directly to one another.
The discovery centers on brown bullhead catfish in Lake Memphremagog, which straddles the Vermont-Quebec border and supplies drinking water to more than 175,000 people.
Anglers and biologists first noticed the issue in 2012, when large numbers of brown bullhead catfish began developing raised black growths on their skin. Subsequent testing revealed that the growths were malignant melanomas, and by 2017, nearly one-third of the catfish sampled from the lake had them.
That rate is far higher than the number of melanoma cases reported in brown bullhead populations elsewhere, leading researchers to suspect a common underlying cause rather than random chance.
How genetic testing confirmed contagious cancer in fish
To identify the cause, a team led by Emily E. Curd of the Vermont Biomedical Research Network at the University of Vermont compared the genetic makeup of tumor tissue with that of healthy tissue from the same fish.
In a typical case of cancer, tumor cells closely match the DNA of the fish in which they developed. That was not the case here. Instead, the tumors were genetically more similar to one another than to their individual hosts, sharing hundreds of thousands of mutations that appeared repeatedly in tumor samples but were absent from healthy tissue.
That pattern, the study found, is the hallmark of a single cancer lineage spreading from fish to fish rather than dozens of tumors developing independently.
The findings place brown bullhead melanoma alongside just three other confirmed transmissible cancers in nature: a sexually transmitted tumor in dogs, a facial tumor that has devastated Tasmanian devil populations, and a leukemia-like cancer found in several species of clams and mussels.
Researchers also screened the tumors for viruses and other microbes that might explain the outbreak but found nothing unusual, further supporting the conclusion that the cancer cells themselves are the infectious agent.
Unknown cancer transmission routes in bullhead fish populations
The manner through which the disease spreads between catfish remains unknown. The study noted that brown bullhead fish crowd together during spawning season, increasing the possibility that direct contact or the surrounding water may allow tumor cells to spread to new hosts. Researchers also found that melanoma has not been observed in young brown bullhead, suggesting the disease may develop around the time the fish reach breeding age.
What this discovery means for the future of Lake Memphremagog’s brown bullhead population remains unclear. Some transmissible cancers, such as the one affecting Tasmanian devils, have proven deadly enough to threaten an entire species. Others, including the transmissible tumor found in dogs, tend to disappear on their own.
Further sampling will be needed to determine how long the cancer lineage has existed and whether it has already spread beyond the lake, researchers say.
See all the latest news from Greece and the world at Greekreporter.com. Contact our newsroom to report an update or send your story, photos and videos. Follow GR on Google News and subscribe here to our daily email!
