Researchers have identified a contagious form of melanoma in brown bullhead catfish (Ameiurus nebulosus), marking the first time a transmissible cancer has been discovered in a freshwater fish species. The findings, published in the journal Nature, confirm that the black skin lesions observed on catfish in Lake Memphremagog are caused by tumor cells that spread from one animal to another.
Transmissible Cancer Identified in Freshwater Fish
This discovery represents only the fourth type of naturally occurring transmissible cancer identified in the animal kingdom. Other documented cases include a sexually transmitted tumor in dogs, two distinct lineages affecting Tasmanian devils, and independent tumor lineages found in bivalves such as clams, cockles, and mussels.
The Progression of the Disease
Anglers and wildlife biologists first began reporting brown bullhead catfish with raised, inky black skin lesions in Lake Memphremagog—which spans the border between Vermont and Quebec—in 2012. Subsequent surveys indicate that approximately one-third of the lake’s brown bullhead population is afflicted.
Evidence of Transmission
Initial investigations into the cause of the lesions focused on potential viral infections or environmental pollutants, such as toxins leaching from the nearby Coventry Landfill.
The breakthrough came through whole-genome sequencing, which compared the DNA of the tumors to the healthy tissue of the host fish. The team discovered that the cancer cells shared hundreds of thousands of genetic variants that were absent in the host fish. This genetic signature revealed that the tumor cells were more closely related to each other than to the fish they inhabited, functioning essentially as infectious agents rather than conventional cancers that arise from an individual’s own mutated cells.
Potential Modes of Spread
Researchers are currently investigating how the cancer moves between hosts. Because the disease appears primarily in adult fish, scientists suspect that spawning behavior may facilitate transmission. During spawning, brown bullhead catfish cluster tightly in shallow water, creating opportunities for physical contact.
The study also suggests that environmental factors, such as hormonal changes or exposure to pollutants like naturally occurring arsenic, could weaken the immune systems of the fish, potentially making them more susceptible to infection. While the ecological impact is still being studied, the discovery of similar lesions mentioned in 19th-century records by naturalist Henry David Thoreau suggests the condition may be more widespread than previously realized. As noted by the CBC, further investigations in New England and New Brunswick have already identified affected catfish in additional lakes and ponds.
Summary of Known Transmissible Cancers
| Species Affected | Type of Cancer |
|---|---|
| Brown Bullhead Catfish | Melanoma (Skin Cancer) |
| Tasmanian Devils | Facial Tumor Disease |
| Dogs | Venereal Tumor |
| Bivalves (Clams, Mussels) | Leukemia-like Disease |
For now, researchers emphasize the importance of these findings for understanding how cancers survive and spread outside of an original host.
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