Summary (tl;dr)

Scientists have identified the first known transmissible cancer in freshwater fish, a melanoma spreading among brown bullhead catfish in lakes across the northeastern United States and Canada.

Essential Background

For over a decade, anglers and biologists have reported an increasing number of brown bullhead catfish with distinctive black skin lesions, particularly noticeable in Lake Memphremagog, which spans the border between Vermont and Quebec, since at least 2012. Initial investigations into these mysterious growths focused on potential causes like viruses or environmental pollutants, as brown bullhead catfish are often used as indicators of water quality. Historically, similar "inky-black" spots on catfish were documented by Henry David Thoreau in Massachusetts in 1852, suggesting this phenomenon may have a long, unexamined history. Prior to this discovery, only three other types of transmissible cancers had been identified in animals: in dogs, Tasmanian devils, and certain bivalve mollusks.

The Full Story

Researchers from the University of Vermont have now confirmed that these black lesions are, in fact, melanoma, a form of skin cancer. A study published in the journal Nature revealed that the cancer cells from different fish were genetically more similar to each other than to the fish they inhabited, leading to the conclusion that the cancer itself is transmissible and spreads between individuals much like a parasite. This groundbreaking finding marks the first-ever discovery of a transmissible cancer in any fish species and the first in a freshwater environment. Currently, approximately one-third of the brown bullhead catfish population in Lake Memphremagog is affected. The transmissible cancer has also been found in other lakes and ponds across New England and Canada, including Lake Magog in Quebec, and locations in Maine, New Hampshire, and New Brunswick, even in areas with pristine water quality, indicating that local pollution is not the direct cause of its spread.

Why It Matters

This discovery is crucial for understanding the broader biology and evolution of cancer, offering a unique natural system to study how cancers can survive and spread outside their original host. While the long-term ecological impact on brown bullhead catfish populations is still being investigated, some heavily diseased fish have been observed to survive for years. Importantly, researchers stress that the disease poses no known threat to humans, and the cancer cells cannot infect or survive in other species, making the fish safe for handling. However, the Ministry of the Environment in Quebec advises against consuming fish found with tumors. The findings suggest that transmissible cancers may be more prevalent in nature than previously recognized, often going undetected.

Geographic Location

• Lake Memphremagog, Vermont, United States and Quebec, Canada (discovery and study of transmissible melanoma in brown bullhead catfish)
• Newport, Orleans County, Vermont, United States (location of University of Vermont research and tissue collection related to the study)
• Lake Magog, Quebec, Canada (detection of transmissible melanoma in brown bullhead catfish)
• Maine, United States (detection of transmissible melanoma in brown bullhead catfish)
• New Hampshire, United States (detection of transmissible melanoma in brown bullhead catfish)
• New Brunswick, Canada (detection of transmissible melanoma in brown bullhead catfish)
• Concord River, Middlesex County, Massachusetts, United States (historical observation of similar lesions in catfish by Henry David Thoreau in 1852)