New Transmissible Fish Cancer Spreads in North American Waterways

Scientists have uncovered an unprecedented form of transmissible cancer affecting freshwater fish populations in North America. The discovery, detailed in a recent paper published in the journal Nature by researchers from the University of Vermont, highlights a significant environmental mystery emerging in water bodies across Canada and the United States.

The initial alarm was raised when anglers and biologists noticed brown bullhead catfish with unusual black lesions in Lake Memphremagog, which straddles Quebec and Vermont. Since 2012, these observations have escalated to one in three fish being affected by the condition. The University of Vermont team initially suspected a pollution-related pathogen but later identified the lesions as melanoma, a type of skin cancer typically caused by sunlight exposure.

Upon closer inspection, researchers found that the cancer cells were genetically distinct from their host fish's healthy cells and more closely related to one another, indicating an ability to transmit between hosts. This is highly unusual, with only three known transmissible cancers previously identified in Tasmanian devils, dogs, and mollusks—none of which have been observed in freshwater species until now.

The research suggests that transmission likely occurs during spawning behavior when fish congregate closely together in shallow waters. The lack of scales on these bottom-dwelling fish makes them particularly vulnerable to sediment-borne cancer cells. While the long-term impact on affected populations remains uncertain, some infected fish have been observed surviving for years after initial infection.

The University of Vermont team emphasizes that there is no risk to human health from this cancer, and drinking water from Lake Memphremagog remains safe. However, they advise against consuming fish showing signs of the disease. Initial investigations into potential pollution sources were inconclusive, as contamination levels did not correlate with higher incidences of cancer along the lake.

Instead, researchers have identified high arsenic levels in the cancer cells, a naturally occurring element in the region's soil. This finding has led scientists to speculate that this transmissible cancer may have existed for an extended period but was previously undetected. Historical records from naturalist Henry David Thoreau provide early evidence of similar conditions in catfish as far back as 1852.

By studying how these cancers survive and spread outside their original hosts, researchers hope to gain insights into the mechanisms that keep cancer contained within organisms and what happens when those boundaries break down. This discovery offers a unique opportunity to explore the evolution of cancer and its transmission dynamics in aquatic ecosystems.

The implications of this research extend beyond immediate concerns about fish populations, as it raises broader questions about environmental health and the potential for similar transmissible diseases to emerge in other species or regions.