New findings reveal a malignant melanoma spreading among catfish in Lake Memphremagog
Category: Health
In a startling discovery, researchers have identified the first-ever transmissible cancer in fish, found among brown bullhead catfish in Lake Memphremagog, which spans the border between Vermont and Quebec. This glacial lake, known for its recreational fishing and as a drinking water source for 175,000 people, has become the center of an unusual health crisis among its aquatic inhabitants. Since 2012, anglers and biologists have reported catfish exhibiting alarming black lesions, prompting an investigation into their cause.
The lesions, which appear as raised black marks on the fish, were initially suspected to be linked to pollution or a viral infection. The reality is far more unsettling: the growths are malignant melanomas that belong to a single cancer lineage capable of spreading from one fish to another. This finding marks a historic moment, as it is the first naturally occurring transmissible cancer documented in fish and the first identified in a freshwater ecosystem.
Brown bullheads, a scaleless species of catfish common across eastern North America, have been particularly affected. Studies conducted between 2014 and 2017 revealed that 23% to 37% of adult brown bullheads sampled from Lake Memphremagog exhibited these black melanoma lesions. Some tumors were found to invade not just the skin but also the muscle and internal organs, including the brain and liver. Surprisingly, many of these affected fish did not die quickly, indicating a chronic condition rather than an acute one.
Julie Dragon, a microbiologist and genomic data scientist at the University of Vermont, led the research team that unraveled the mystery behind these tumors. Through genomic analysis, the team discovered that the tumors clustered together genetically, distinct from the healthy tissue of the fish. "We built these trees, and it turned out that all the tumors clustered together, and all the hosts clustered together," Dragon explained. This pattern pointed to a single cancer lineage that had escaped its original host and was now colonizing others.
Transmissible cancers are exceedingly rare in nature. They have been identified previously in species such as Tasmanian devils, dogs, and various bivalves like clams and oysters. The mechanisms of transmission in these animals often involve direct contact or specific behaviors. In the case of the brown bullhead catfish, scientists are still working to understand how this cancer spreads. The tumors appear to break apart when handled, raising the possibility that they release cells capable of colonizing other fish.
Dragon noted, "These conditions are incredibly rare in nature, so something has to be happening to allow this to happen." The researchers are investigating whether factors such as reproduction or aging may play a role in the transmission of the cancer among the fish population.
The origin of this transmissible cancer has led scientists to explore environmental influences, particularly concerning the flooding caused by Tropical Storm Irene in 2011. This storm resulted in runoff that may have contaminated the lake's waters, leading to a decline in water quality. Vicki Blazer, a retired U.S. Geological Survey fish pathologist, has raised concerns about pollutants, noting higher levels of metals like arsenic in affected fish. Arsenic is a known carcinogen that could potentially weaken the fish's immune systems, making them more susceptible to the cancer.
Blazer stated, "I guess my first concern is to call something a transmissible anything when you haven’t transmitted it." Her caution highlights the complexity of determining the exact role of pollution versus the cancer itself in this outbreak. For now, the cancer poses no known risk to humans, though officials in Quebec recommend against consuming fish with visible tumors.
This discovery opens up new avenues for research into transmissible cancers and their implications for wildlife and ecosystems. Dragon emphasized the importance of this finding for scientific inquiry, asking, "How does it evade the immune system? How old is it? How many similar cancers have gone unnoticed?" These questions are not just academic; they could shed light on the evolution of cancer and its impact on wildlife populations.
The study published in *Nature* suggests that this transmissible cancer could be more widespread than currently recognized, as researchers often overlook conditions they are not actively searching for. The future will see continued sampling of affected populations to gather more data on the age and origins of this clonal cancer.
As scientists work to understand the implications of this transmissible cancer, they also face the challenge of protecting the health of Lake Memphremagog’s ecosystem and its human residents. The situation serves as a reminder of the interconnectedness of environmental health and wildlife, raising questions about how human activities may inadvertently influence the spread of diseases in natural populations.
In a world where climate change and pollution continue to threaten ecosystems globally, this case of transmissible cancer in fish highlights the urgent need for comprehensive environmental monitoring and research into the health of our aquatic systems. The findings from Lake Memphremagog are not just a local issue; they resonate on a broader scale, emphasizing the delicate balance of life in our lakes and rivers.