Scientists Identify First Transmissible Cancer in Freshwater Fish
Researchers have confirmed that a melanoma-like tumor spreading among brown bullhead catfish in Lake Memphremagog is a contagious cancer, marking the first known transmissible cancer in a freshwater animal.
Scientists studying brown bullhead catfish in Lake Memphremagog, which spans Quebec and Vermont, have found that blackened skin lesions on the fish are caused by a transmissible form of melanoma. Genetic analysis showed that tumors from different fish share hundreds of thousands of DNA variants not present in the hosts, indicating the cancer itself is spreading between individuals.
The discovery, published in Nature, represents the fourth documented case of a contagious cancer in animals and the first in a fish or any freshwater species. Researchers observed that the lesions have been increasing since 2012, with about one-third of the local catfish affected by 2014.
"Transmissible cancers are a puzzling biological phenomenon that are not expected to exist, with the potential to devastate host populations," the study authors wrote.
Co-lead investigator Julie Dragon of the University of Vermont explained that, unlike typical cancers, the tumors become more genetically similar as more samples are examined, a pattern characteristic of transmissible cancers. Tests ruled out environmental toxins as the primary cause, and similar tumors have now been identified in catfish populations across other New England lakes.
The mode of transmission remains uncertain; possibilities include spread during spawning when fish congregate or uptake of cancer cells from lake sediments, a mechanism seen in some bivalve cancers. Researchers also noted elevated arsenic levels in the tumors, suggesting environmental factors may influence susceptibility.
While the disease poses no known risk to humans, its impact on catfish populations is unclear. Past transmissible cancers have ranged from highly lethal, as in Tasmanian devils, to relatively benign, as in canine transmissible venereal tumor. The team plans further genomic studies to understand how the cancer evades the fish immune system, which could provide insights relevant to human oncology.
"A big [question] is how the cancer is avoiding the fish’s immune system. 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 said.
The findings suggest that other undetected transmissible cancers may exist in wildlife, underscoring the need for broader surveillance.