Newly discovered melanoma in brown bullhead catfish reveals unique transmission methods and potential ecological impacts
Category: Health
In a surprising twist in the field of oncology, scientists have identified the first known contagious cancer in a freshwater fish species. Brown bullhead catfish in Lake Memphremagog, which straddles the U.S.-Canada border, are now recognized as hosts to a transmissible melanoma that can spread between individual fish. This discovery, published in July 2026 in the journal Nature, marks a landmark moment in cancer research, as it is the first transmissible cancer documented in a freshwater ecosystem and the fourth known transmissible cancer in any animal species.
Researchers from the University of Vermont began their investigation after anglers and biologists reported an alarming increase in catfish exhibiting black lesions. Between 23% and 37% of the examined fish were found to have malignant melanomas, a concerningly high rate for this species. The brown bullhead catfish is considered an indicator species for water quality, prompting initial concerns about pollution or pathogens affecting the lake, which serves as a drinking water source for over 175,000 people.
The study revealed that the melanoma affecting these catfish is not developing independently in each individual. Instead, the cancer cells themselves are being transmitted from one fish to another. Researchers sequenced the complete genomes of tumors and healthy tissues from affected fish, finding that tumor cells from different fish were more closely related to one another than to the fish carrying them. This suggests that the cancer originated from a single ancestral tumor that spread among the population.
“It seems to only happen in larger fish that are of spawning age,” said co-author Mark Henderson, highlighting the role of reproductive behaviors in the transmission of this disease. The research team found hundreds of thousands of shared genetic mutations among the tumors that were absent in healthy tissues, providing strong evidence for the transmissibility of the cancer.
Scientists are still piecing together how this cancer spreads among the catfish. Several theories have emerged: one possibility is that the cancer cells are transmitted during spawning, as catfish gather closely together during this period, allowing for physical contact. Another theory posits that cancer cells might survive in the water or sediment and enter fish through their skin, gills, or mouths. The study notes that the cancer has only been detected in spawning-age fish, indicating that reproduction may play a key role in its spread.
Interestingly, researchers also noted elevated levels of arsenic in tumor cells and observed that cancers appeared more frequently in areas with higher natural arsenic concentrations. Yet, they cautioned that this correlation does not definitively prove that arsenic causes the disease.
This discovery has broad implications for both cancer biology and wildlife conservation. The melanoma was first observed around 2012, but by 2015, approximately 30% of brown bullhead catfish in Lake Memphremagog showed signs of the disease. Scientists are now investigating whether this same lineage of cancer has spread to other freshwater lakes across North America.
Transmissible cancers are rare, with only three other types known to science prior to this discovery. The first is the canine transmissible venereal tumor (CTVT), which spreads among dogs during mating. The second is the devil facial tumor disease, which has devastated the Tasmanian devil population, and the third occurs in several species of marine shellfish, where leukemia-like cancers spread through seawater.
Each of these cancers has unique characteristics and impacts on their respective species. For example, CTVT is relatively mild, with many dogs clearing the cancer through their immune response or treatment. In stark comparison, the facial tumor disease in Tasmanian devils is nearly always fatal, leading to a dramatic decline in their population.
Though the discovery of a contagious cancer in fish is alarming, experts assure that there is no risk to humans. Human cancers are not contagious due to our immune systems' effectiveness at recognizing and destroying foreign cells. The cancer cells in these catfish represent an extraordinary biological exception; they are the infectious agents, not a virus or bacteria. This finding could provide valuable insights into how cancers evolve and evade immune defenses, potentially informing future cancer research.
“By studying how cancers survive and spread outside their original host, we can learn a great deal about what keeps cancers contained—and what happens when those boundaries break down,” said co-lead author Julie Dragon.
As researchers continue to explore the implications of this discovery, they stress the importance of monitoring the health of brown bullhead catfish populations and the potential ecological impacts of this transmissible cancer. The long-term effects on these populations remain uncertain, but the findings could shift our perspective on cancer biology and its implications for wildlife conservation.
For anglers, the advice is clear: “Personally, I wouldn’t eat the ones with tumors,” Henderson cautioned. As scientists work to unravel the mysteries surrounding this contagious cancer, they highlight the need for continued vigilance in monitoring affected fish populations and their environments.
The implications of this discovery extend beyond the immediate health of the fish; it raises questions about the resilience of wildlife in the face of disease and the interconnectedness of ecosystems. As more studies emerge, the scientific community hopes to gain a clearer picture of how transmissible cancers may influence not just individual species but entire aquatic ecosystems.