Full Breakdown
Transmissible Melanoma Identified in Lake Memphremagog Catfish
7/23/2026, 7:59:16 PM
Core Event: First Naturally Occurring Transmissible Cancer in a Freshwater Species
Researchers led by Julie Dragon of the University of Vermont published a study on July 22 describing a melanoma that spreads between brown bullhead catfish (*Ameiurus nebulosus*) in Lake Memphremagog, which straddles Vermont and Quebec. Whole-genome sequencing showed tumor cells from different fish share hundreds of thousands of genetic variants, forming a single clonal lineage distinct from the host genomes—the genetic signature of a transmissible cancer.
Background & Context
Anglers first reported black, velvety skin lesions on bullheads in 2012. By 2014, surveys indicated roughly one-third of the lake’s brown bullhead carried the lesions. Genetic analyses ruled out a virus and identified the lesions as melanoma, a skin cancer rarely seen in catfish. Transmissible cancers are known in dogs, Tasmanian devils, and marine bivalves; this is the first documented case in a freshwater fish.
Data & Statistics
- Prevalence: 2014-2017 surveys found 25 %–33 % of brown bullhead in the lake exhibit the lesions.
- Arsenic association: Cancer cells contain elevated arsenic levels, and the disease overlaps regions with naturally high arsenic in surrounding soils.
- Human risk: Agencies state there is no known risk to humans, though they advise against eating visibly diseased fish.
Why It Matters
The discovery expands the known taxonomic range of transmissible cancers to freshwater ecosystems, offering a model for studying how cancer cells evade immune detection. Infected fish have been observed surviving for years, unlike the near-extinction seen in Tasmanian devils.
Official Statements & Responses
- Mark Henderson, co-lead of the study, suggested spawning aggregations provide the close contact needed for tumor cells to move between fish.
- The U.S. National Institutes of Health and the Great Lakes Fishery Commission funded the research; Quebec’s Ministry of the Environment participated in sample collection and recommends not consuming tumor-bearing fish.
Criticism & Opposition
Vicki S. Blazer of the U.S. Geological Survey cautioned that “I guess my first concern is to call something a transmissible anything when you haven’t transmitted it,” highlighting the lack of laboratory-confirmed transmission.
On-the-Ground Reports
Local anglers described handling infected fish as “a black tar-like substance” that clung to their hands.
Conflicting Reports & Gaps
- Transmission pathway: Hypotheses include direct contact during spawning, sediment-borne cells, or arsenic-mediated immune suppression, but no experimental proof has yet been demonstrated.
- Arsenic causality: Elevated arsenic is documented in tumor cells, yet researchers have not established a causal link between environmental arsenic and cancer emergence.
Verbatim Quotes
- “At first, we thought this disease might be a virus, but that wasn’t panning out,” — Julie Dragon
- “And, personally, I wouldn’t eat the ones with tumors,” — Mark Henderson
- “I guess my first concern is to call something a transmissible anything when you haven't transmitted it,” — Vicki S. Blazer
What’s Next
The team is expanding sampling to additional lakes and ponds across Vermont, New Hampshire, Maine, and New Brunswick, and plans laboratory experiments later this year to test whether tumor cells can infect healthy fish under controlled conditions.
