A Contagious Melanoma in American Brown Catfish
Scientists have confirmed a new form of cancer that can pass between living organisms — the fourth such case verified in nature and the first transmissible cancer ever found in fish. The disease, identified as malignant melanoma in the American brown catfish, is described in a study published in the scientific journal Nature.
The case traces back to 2012, when a catfish with an unusual appearance was pulled from a lake on the border between the US state of Vermont and the Canadian province of Quebec. Its skin was covered in dark, tar-like growths. After years of analysis, researchers at the University of Vermont established that the growths were not an ordinary tumor but cancer cells capable of moving from one fish to another.
Most cancers die with the organism in which they develop. In extremely rare cases, tumor cells survive their host and take root in a new one, making the cancer behave almost like an infectious disease. To do so, the cells must evade the immune system of the new host.
The finding adds fish to a short list of known transmissible cancers: a sexually transmitted tumor in dogs, facial tumor disease in Tasmanian devils, and several lineages found in shellfish. Researchers suspect the real number is higher, because proving a cancer is transmissible requires detailed genetic comparison of tumor and host tissue.
How the Catfish Case Compares With the Other Transmissible Cancers
The Four Known Types and How They Spread
The catfish melanoma joins three previously documented groups. The most ancient is the canine transmissible venereal tumor (CTVT) in dogs, spread during mating. Genetic analysis indicates every case descends from a single dog that lived in northeast Asia between 6,000 and 11,000 years ago — meaning the same tumor cells have survived for millennia after their original host died.
Tasmanian devil facial tumor disease, first recorded in 1996, spreads through bites during fights. Tumors develop on the face and in the mouth, eventually preventing infected animals from eating; affected populations have fallen by more than 80 percent, exceeding 90 percent in some areas. In 2014 researchers found a second, fully independent transmissible cancer in the same species — the only known example of one animal species developing two distinct infectious cancer lines.
Shellfish follow a different route. Malignant cells are released into seawater and taken in by healthy animals as they filter water to feed, so no direct contact is required. Scientists have identified at least ten independent cases across shellfish species, and some tumor lines appear to have crossed between species and even between oceans, possibly via ships' ballast water.
Why the Catfish Case Is Significant
The new finding expands transmissible cancer beyond mammals and mollusks to a vertebrate fish, and it is a melanoma — a cancer type previously unknown to spread between hosts. The source report does not specify how the cancer passes between catfish, leaving the transmission mechanism as the next open question.
A Possibly Under-Counted Phenomenon
Evolutionary geneticists cited in the report argue transmissible cancer may be less rare than believed — merely under-detected. Confirming a case demands proof that tumor cells do not originate from the sick animal itself, a technically demanding comparison. That high detection threshold suggests more examples are likely to emerge, which researchers say could sharpen understanding of how cancers originate, spread and might be treated, in animals and potentially in humans. The human connection, however, remains indirect.
What the Catfish Discovery Means for Cancer Research
For cancer researchers and wildlife biologists, the catfish discovery points to concrete next steps.
- Confirming transmissibility took years of comparing tumor and host genetics; applying the same screening to other species with unusual tumors could reveal whether transmissible cancers are more common than the four known types.
- For wildlife managers, the Tasmanian devil experience — population falls of more than 80 percent — shows how quickly a transmissible cancer can spread once established, making early detection valuable in fish and other species.
- For general readers, none of the four documented types involves humans; each is confined to specific species and transmission routes.
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