Contagious Cancer Found in Fish for First Time in New England

Scientists have identified the first instance of contagious cancer in fish, discovered in brown bullhead catfish populations in New England. Researchers from the University of Vermont studied melanoma tumors from fish in Lake Memphremagog, finding the cancerous cells were genetically more similar to each other than to their hosts. This strong evidence indicates the cancer is transmissible, a phenomenon rarely observed in animals and unprecedented in fish.

The discovery was made after local biologists and anglers began reporting an increasing number of brown bullhead catfish with unusual blackened skin lesions in Lake Memphremagog, a body of water straddling the border of Quebec, Canada, and Vermont. By 2014, approximately one-third of the catfish in the area exhibited these symptoms. While initial suspicions pointed to viral infections or environmental toxins, genetic analysis revealed the lesions were melanoma tumors. The prevalence of these tumors, especially in bottom-dwelling catfish with limited sun exposure, prompted further investigation into their origins.

To understand the nature of these tumors, the research team conducted whole-genome sequencing on samples. They discovered hundreds of thousands of genetic variants shared among the tumors that were absent in the healthy fish or other unaffected catfish. "When you look at shared variants in non-transmissible cancers, you see many random variants, and as you compare more and more cancers, they share fewer and fewer variants," explained co-lead researcher Julie Dragon, an associate professor of microbiology and molecular genetics at the University of Vermont. "In the case of transmissible cancers, it is the opposite. The more you sample, the more shared variants you find." This pattern strongly suggests that the cancer cells originated from a single source and have been spreading between individual fish.

Unraveling the Mysteries of Transmissible Cancers

This marks only the fourth type of transmissible cancer ever documented in animals, following similar discoveries in dogs, Tasmanian devils, and various bivalve mollusks. However, it is the first identified in any fish species and the first found in a freshwater animal. The implications for the long-term health of brown bullhead catfish populations remain a significant question. While transmissible venereal tumors in dogs are often treatable, outbreaks in Tasmanian devils have a fatality rate of up to 90%. The precise transmission method for this fish cancer is also unclear. Scientists theorize it may spread during the spawning season when fish congregate, or through cancer cells surviving in the lakebed and being ingested by other fish. The researchers also noted that the tumors often contain high levels of arsenic, a known carcinogen, suggesting that environmental factors like heavy metals or hormonal changes during spawning might increase fish susceptibility.

A critical area of future research will focus on how these cancer cells evade the host's immune system. Understanding this mechanism could provide valuable insights into how cancers operate within human bodies and potentially lead to new therapeutic strategies. "We will be exploring that for sure because it can tell us something about how cancers avoid the immune systems in humans as well," Dragon stated.

The researchers are also investigating the timeline of this cancer's emergence, noting that reports of melanoma in brown bullhead catfish date back to the early 20th century, with even earlier descriptions by Henry David Thoreau in the mid-1800s potentially describing similar conditions. The team believes that more transmissible cancers likely exist undiscovered in the wild. "We think that these may be more widespread, but we are just not noticing them. You often don’t see things you are not looking for intentionally," Dragon commented. The ongoing study aims to answer these pressing questions and further explore the prevalence of the cancer in other fish populations. While these parasitic cancers do not pose a threat to humans, the knowledge gained from studying them could offer significant benefits for human health research. "What I hope folks take away is that cancer goes beyond humans, and genomics is a powerful tool for researching cancer and other diseases," Dragon concluded. "With this project we have a unique opportunity to research melanoma in a new way that could greatly benefit humans."
The discovery highlights the ongoing complexities of cancer biology and the potential for unexpected transmission pathways in the natural world.