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In 2013, a puzzling phenomenon emerged along Lake Memphremagog, a sizable freshwater lake straddling the Vermont-Quebec border. Fishermen and biologists began noticing brown bullhead catfish sporting unusual raised black spots. Subsequent investigations revealed that between 23% and 37% of these catfish samples carried melanoma lesions-a form of skin cancer-according to a recent study published in Nature.
While melanoma in brown bullhead catfish had been reported elsewhere, the prevalence observed in Lake Memphremagog was unprecedented. Initially, researchers suspected environmental factors might be responsible. Tropical Storm Irene had triggered severe flooding in 2011, raising concerns that pollution or carcinogenic contaminants might have elevated cancer rates in the fish population.
However, the new study offers a surprising alternative explanation: the melanoma may be transmissible, with cancer cells passing directly from one catfish to another. This represents the first documented case of a contagious cancer in fish and the first such finding in a freshwater environment.
Can cancer be contagious?
According to the American Cancer Society, cancer is generally not contagious. It develops when an individual’s own cells undergo DNA damage and proliferate uncontrollably, forming tumors. Cancer cannot be transmitted through casual contact, sharing food, or breathing the same air.
Rare exceptions exist, such as cancer transmission through organ or tissue transplants, or exceedingly uncommon pregnancy-related cases. Additionally, some viruses like human papillomavirus (HPV) and hepatitis B can spread between people and increase cancer risk over time-but the viruses themselves, not cancer cells, are contagious.
True transmissible cancers require cancer cells to detach from the original host and continue growing in a new host. While this is extremely rare, the recent study suggests this phenomenon may be occurring in the brown bullhead catfish population of Lake Memphremagog. Yet, the precise mechanisms remain unclear.
Investigating the catfish tumors
Researchers conducted whole-genome sequencing on melanoma tumors and healthy tissue from the catfish. They discovered that the DNA of tumors from different fish was remarkably similar to each other but distinct from the DNA of their hosts. This finding implies the tumors originated from a single lineage of cancer cells spreading between fish, rather than arising independently in each individual.
Brown bullhead catfish are known to congregate closely during spawning, which could facilitate the transfer of cancer cells. However, scientists have not yet confirmed the exact mode of transmission.
Implications for humans and future research
Lake Memphremagog provides drinking water for over 175,000 people, but experts emphasize that there is no evidence the melanoma in catfish poses any risk to humans. The cancer cells appear species-specific and cannot infect people or other local wildlife.
Previously, transmissible cancers have been identified in dogs, Tasmanian devils, and certain shellfish such as clams and mussels. The discovery in catfish adds a new species to this rare category.
This breakthrough offers valuable insights into how cancer cells evade immune defenses, replicate, and survive in foreign hosts. The findings underscore that nature continues to reveal unexpected aspects of cancer biology, which may ultimately enhance our understanding of how malignant cells persist and spread in living organisms.