A Strange, Never-Before-Seen Cancer Seems to Be Spreading Among Catfish in a New England Lake

A troubling pattern in Lake Memphremagog has moved from an unsettling observation to a scientific finding with broad implications for wildlife health. Brown bullhead catfish in the lake have developed striking black growths on their skin, and a new genetic analysis indicates that the disease may be a cancer capable of passing between fish. If confirmed through continued work, it would represent the first transmissible cancer identified in any fish species.

The marks have not appeared only once or in a handful of animals. Fishers and fisheries researchers began noticing them more than ten years ago, when bullheads bearing dark, almost black patches turned up in the New England lake. Their appearance drew attention because the lesions are conspicuous and unlike ordinary changes in a fish's coloration. Peter Emerson, a fisheries biologist at the Vermont Fish & Wildlife Department, described the effect to NPR as resembling “black tar on the skin.”

Early examination established that the patches are melanomas. This form of skin cancer begins when melanocytes, the cells responsible for making the pigment melanin, multiply without normal control. In these catfish, the resulting lesions can cover areas of the body with a deep black color. Melanoma is an unusual diagnosis in catfish, making the frequency of the condition in Lake Memphremagog especially notable.

About one in three brown bullheads in the lake is now thought to carry such lesions. That level turns an isolated veterinary mystery into a population-level concern, even before researchers can answer every question about how the disease moves or how it affects individual fish. It also gives scientists an unusually large natural setting in which to study a cancer that appears to be circulating among animals rather than arising independently in each one.

Emerson and his co-authors reported their latest evidence in Nature on July 22. Genetic testing led the team toward a striking explanation: the cancer itself may be infectious. That conclusion means the tumors may not be independent cases of a rare disease. Instead, the genetic results support the possibility that the cancer is being passed through the population, linking affected fish in a way not normally expected for melanoma.

The distinction matters. Cancer is generally understood as a disease that begins within a single body, when that animal's own cells acquire changes that let them grow out of control. A transmissible cancer would challenge that usual boundary by behaving, in practical terms, as a disease shared within a population. The researchers' conclusion is therefore not merely that many fish have melanoma, but that the cluster may stem from a contagious form of it.

For now, the genetic results point to that interpretation rather than settling every remaining detail. The report says the catfish seem to be infected by a contagious skin cancer, a formulation that leaves room for further investigation into transmission and the course of the disease. Continued monitoring will be important for showing whether the lesions keep spreading, remain concentrated in particular groups of fish, or change in frequency over time.

The discovery also expands the scientific significance of a lake problem that first looked like an alarming surface condition. A disease of brown bullheads can offer clues about how cancers persist and spread in wild populations, while also underscoring the value of long-term observation by people who work on the water and biologists who follow up on unusual sightings. What began with blackened skin on catfish has become evidence of a potentially new category of fish disease.

Lake Memphremagog's bullheads are now at the center of a question with consequences beyond the lake: how common might transmissible cancers be in species that have not been examined closely? The study does not answer that question, and it does not establish that other fish face the same risk. But it provides a reason to look more carefully when rare tumors appear repeatedly in a wild population, particularly when the pattern is as widespread as the one now documented in these catfish.

Source: Smithsonian Magazine