Canadian and US Fish Found With Rare Contagious Skin Cancer, Scientists Discover

Scientists have identified a rare and unusual form of skin cancer affecting brown bullhead catfish living in Lake Memphremagog, a freshwater lake shared by the United States and Canada. The discovery is significant because researchers found evidence that the cancer can spread directly from one fish to another, making it the first confirmed case of a transmissible cancer in freshwater fish.

The findings, published on July 22 in the journal Nature, provide new insights into how certain cancers can evolve and spread between animals. While transmissible cancers remain extremely uncommon in nature, this research expands scientists' understanding of these rare diseases and could influence future studies on cancer biology.

Scientists Investigate Mysterious Black Skin Lesions

Researchers first became aware of the problem after anglers and wildlife biologists began noticing brown bullhead catfish with unusual black growths and skin lesions in Lake Memphremagog around 2012. The lake stretches across the border of Vermont in the United States and Quebec in Canada.

Initially, scientists believed the condition might be caused by a viral infection, pollution, or another environmental factor. However, extensive testing ruled out several common explanations.

Further laboratory analysis confirmed that the black lesions were actually malignant melanoma, a serious type of skin cancer. The diagnosis surprised researchers because melanoma is generally associated with excessive ultraviolet light exposure, yet brown bullhead catfish spend much of their lives near the lake bottom where sunlight penetration is limited.

Cancer Cells Behaved Like Parasites

One of the most remarkable discoveries came from genetic testing of the tumors.

Instead of matching the DNA of each individual fish, the melanoma cells were found to be genetically much more similar to one another. This indicated that the cancer itself was being transferred between fish rather than developing independently in each affected animal.

Researchers explained that the tumor cells behaved more like parasites than conventional cancers. Rather than remaining confined to one host, the cancer cells appeared capable of surviving long enough to establish new tumors after moving into another fish.

This behavior makes the disease fundamentally different from most cancers, which originate from the host's own cells and cannot spread between individuals.

First Confirmed Transmissible Cancer in Freshwater Fish

According to the study authors, this represents the first documented transmissible cancer in any freshwater fish species and the first confirmed case occurring within a freshwater ecosystem.

Until now, contagious cancers had only been confirmed in a small number of animal groups.

Scientists have previously identified transmissible cancers in:

• Tasmanian devils, which spread facial tumor disease
• Dogs, which transmit canine transmissible venereal tumors
• Several shellfish species that develop leukemia-like diseases

The newly identified melanoma in brown bullhead catfish now joins this rare group of naturally occurring transmissible cancers.

Researchers noted that these diseases were once considered biological impossibilities because the immune system usually recognizes and destroys foreign cells.

How Might the Cancer Spread?

Although scientists have not yet determined the exact transmission method, they have developed several theories based on observations of the affected fish.

The disease appears to occur mainly in mature fish of spawning age.

During breeding season, brown bullhead catfish gather closely together in shallow water. Researchers believe this frequent physical contact may allow living cancer cells to transfer directly between animals.

The study also suggests that naturally occurring arsenic found in the lake environment could potentially contribute to the development or persistence of the disease, although additional research is needed before any conclusions can be made.

Scientists continue to investigate how the melanoma cells survive outside one fish long enough to infect another.

Does the Cancer Pose a Risk to Humans?

Researchers emphasized that there is currently no evidence that this contagious fish cancer poses any threat to humans.

Lake Memphremagog serves as a drinking water source for more than 175,000 people, yet scientists said the disease has not been shown to infect humans or other mammals.

Transmissible cancers are typically highly species specific, meaning they spread only within particular animal species.

There is also no indication that eating properly prepared fish from the lake would transmit cancer to humans.

Why This Discovery Matters

Although the disease affects fish, the discovery carries broader scientific importance.

Studying transmissible cancers helps researchers understand how cancer cells evolve, evade immune systems, and survive outside their original hosts.

These rare diseases provide valuable models for investigating:

• Cancer evolution
• Immune system evasion
• Tumor genetics
• Disease transmission
• Wildlife conservation

The findings may also encourage scientists to investigate whether similar cancers exist in other fish species that have not yet been studied.

As researchers continue examining wildlife diseases, additional transmissible cancers could be identified in aquatic ecosystems around the world.

More Research Is Needed

The study raises several unanswered questions.

Researchers still do not know exactly when the cancer first appeared, how widespread it may become, or whether environmental conditions influence its spread.

Future research will focus on understanding the disease's transmission pathway, identifying possible environmental triggers, and determining whether similar cancers exist in other freshwater fish populations.

Scientists believe this discovery may represent only the beginning of a broader understanding of transmissible cancers in aquatic wildlife.

Final Thoughts

The identification of a contagious melanoma in brown bullhead catfish marks a major milestone in cancer research. As the first confirmed transmissible cancer found in freshwater fish, the discovery challenges long-held assumptions about how cancers behave in nature.

While the disease appears limited to brown bullhead catfish and presents no known risk to humans, it offers researchers an important opportunity to better understand the biology of cancer and its rare ability to spread between individuals.

Continued monitoring of Lake Memphremagog and additional studies involving other fish species may reveal whether transmissible cancers are more widespread than previously believed.

Source: The journal Nature and information released by the University of Vermont research team.

Disclaimer: This article is intended for informational and educational purposes only. It should not be considered medical, veterinary, or scientific advice. Readers should refer to the original peer-reviewed research and consult qualified experts for professional guidance.