A Catfish With a Tar-Like Skin Lesion Reveals a Contagious Melanoma
A brown bullhead catfish pulled from a lake straddling the Vermont-Quebec border in 2012 had skin so discolored it looked as if it had been coated in tar. This month, biologists at the University of Vermont reported in the journal Nature that the lesions were an infectious melanoma — a cancer whose tumor cells can pass from one animal to another. It is only the fourth transmissible cancer known to science and the first documented in fish.
Cancer is normally an evolutionary dead end: when the host dies, its tumor dies with it. In a handful of cases over the history of life, however, tumor cells have escaped that fate. They detached from the body in which they arose and became independent, living, infectious cell lineages that move between hosts. The catfish melanoma is the latest such example.
The finding broadens a small but striking list of contagious cancers. The canine transmissible venereal tumor (CTVT) is the oldest known, traced to a single dog that lived in northeastern Asia between 6,000 and 11,000 years ago. Tasmanian devils have two separate transmissible cancers, and at least ten independent transmissible cancers have been found in shellfish. Counting all independent origins, researchers now recognize at least 13 such events.
How the catfish cancer spreads between fish is not detailed in the report, and no immediate threat to humans is indicated. Biologists say the discovery suggests transmissible cancers may be less rare than previously thought — and that more are likely to be found as genome sequencing becomes more common in wildlife research.
Why the Catfish Cancer Joins an Extremely Rare Club
The Known Roster of Contagious Cancers
The catfish case joins a short list built over more than a century. In 1876, Russian veterinarian Mstislav Novinski experimentally transferred a tumor from one dog to another, the first experimentally confirmed transmission of any tumor. That cancer — CTVT — has since been shown to descend from a single dog in northeastern Asia, and genome sequencing indicates it has survived for thousands of years, making it the oldest known cancer and the oldest continuously living mammalian cell line.
Tasmanian devils tell a more alarming story. Devil facial tumor disease, first recorded in 1996, is spread through biting during fights over food and mates. It has reduced devil populations by more than 80 percent overall and by more than 90 percent in the worst-affected areas. In 2014, researchers found a second, fully independent transmissible cancer in the species — the only species known to have produced two. Both devil cancers arise from Schwann cells and evade immune detection by switching off MHC molecules, the cell-surface markers that normally mark tissue as 'self'.
Shellfish add a different dimension. A leukemia-like disease has emerged independently at least ten times across at least ten species of clams and related mollusks, with cancer cells spreading through seawater as healthy animals filter-feed. In one striking case on Europe's Atlantic coast, tumor cells found in golden carpet shells did not match that species at all; their DNA belonged to Venerupis corrugata, a different species living in the same sandy shallows. That is a documented example of a transmissible cancer jumping species. Some shellfish lineages appear to have crossed oceans, likely in ballast water or on ship hulls; the oldest known lineage, infecting Mya arenaria on the North American coast, is estimated to be more than two centuries old.
What Makes a Cancer 'Parasitic'
For a tumor to become transmissible, it must overcome the immune systems of genetically distinct hosts. That is a far harder task than the contagious sarcoma that spread through laboratory hamsters in past experiments; those hamsters were so genetically uniform they were effectively clones. The catfish, devil, shellfish and dog lineages are therefore not accidents of laboratory conditions but genuine evolutionary outliers.
In the dog lineage, researchers have also seen a striking adaptation: CTVT appears to capture healthy mitochondria from the dogs it infects, replacing damaged ones — a mechanism the source reports has occurred at least five times. In devils, rapid evolution is visible too: within a few generations, some wild populations have developed measurable resistance, and the outlook has shifted from extinction scenarios toward coexistence with the disease.
Why the Catfish Discovery Matters
The source article stresses that the count of 'four' transmissible cancers refers to four disease types, not the number of independent escapes; the true count is at least 13. Finding them requires sequencing both the tumor and its host and proving their DNA does not match — an analysis rarely performed on wild animals. That suggests the known cases are probably an undercount.
'I think these cases are not as rare as we believe,' Andrew Storfer, an evolutionary geneticist at the University of Washington who studies Tasmanian devil disease, told Forbes. The Vermont catfish now joins that unusual club — and, on current evidence, it is unlikely to be the last.
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