Contagious Cancer Found in North American Catfish

The idea that cancer could behave like a contagious disease may seem absurd; in humans, cancer cannot be transmitted from one person to another. Yet nature is still full of surprises. In exceptional cases, cancer cells themselves can pass from one individual to another and continue to multiply in a new host. This rare phenomenon has been detected in animals such as dogs, Tasmanian devils, and some species of mollusks, and it seems we can add one more to that exclusive club: catfish.

An international team of researchers has identified a transmissible melanoma in the brown catfish (Ameiurus nebulosus)—the first documented case in fish and also the first recorded in a freshwater ecosystem. Their findings were reported this month in the journal Nature.

An Impossible Pattern

The story began in 2012, when fishermen and biologists detected an unusually high number of catfish with black lesions in Lake Memphremagog, a body of water straddling Vermont and Quebec. Previous studies revealed that these spots were malignant melanomas, and that between 23 percent and 37 percent of the specimens examined had these tumors—an unusually high incidence for this species.

“That was surprising,” said Julie Dragon, a researcher at the University of Vermont and co-lead author of the study. “We wanted to know how a bottom-dwelling fish was getting a cancer we associate with exposure to too much sunlight.”

Initially, it was thought that the cause might be some toxic substance or a pathogen linked to pollution. One suspicion was that the flooding caused by Tropical Storm Irene in 2011 had degraded water quality by washing environmental pollutants into the lake. There were also concerns about the health of the lake, which supplies drinking water to more than 175,000 people. However, initial genetic analyses began to point in an unexpected direction.

It was then that the researchers wondered whether the tumors were descended from a single original cancer that had learned to spread among individual fish, rather than arising independently in each fish.

To test this unusual hypothesis, the team sequenced the complete genomes of tumors and healthy tissues from affected fish, and compared the data with that from healthy specimens from different populations.

The results indicated that the tumor cells from different fish were much more closely related to one another than to the animals in which they developed. In other words, the tumors shared their own genetic identity, distinct from that of their hosts. Furthermore, hundreds of thousands of genetic variants appeared repeatedly in the tumors but were absent from the healthy tissues of the same fish.

Such a pattern would be virtually impossible if each melanoma had arisen independently. By way of comparison, the researchers analyzed hundreds of human melanomas and found that nearly all of their mutations were unique to each patient. In contrast, most of the mutations present in the tumors of these fish were shared by numerous individuals—a hallmark of a transmissible clonal cancer.

Another possibility was that a virus or some microorganism was causing the melanomas. But after examining the genetic material in the samples, no viruses or bacteria specifically associated with the tumors were found. Although they cannot rule out the possibility that a pathogen may have played a role in the cancer’s origin or facilitated its spread, the evidence suggests that the transmissible agent is the cancer cells themselves.

Unanswered Questions

It is still unclear how this cancer is transmitted among catfish. The authors propose several possibilities. During reproduction, catfish congregate in confined spaces and maintain close physical contact, which could ease the exchange of cells. It is also possible that tumor cells survive temporarily in the water or in sediments and enter other fish through the skin, gills, or mouth. Another hypothesis is that hormonal changes associated with reproduction alter the immune response and promote the establishment of cancer.

“It seems to only happen in larger fish that are of spawning age,” said biologist Mark Henderson, a co-author of the study, regarding the absence of cancer in younger fish. “Maybe some part of spawning behavior leads to the spreading of the cancer between animals.”

In a release from the University of Vermont, the researchers noted that the tumor cells contained elevated levels of arsenic and that they observed a correlation between the geographic distribution of the cancer and areas with high natural concentrations of this element. They emphasize, however, that this association does not prove that arsenic is the cause of the cancer.

Despite the concerns, the scientists also note that the phenomenon could open up new avenues of research into cancer development, including in humans. “By studying how cancers survive and spread outside their original host, we can learn a great deal about what keeps cancers contained—and what happens when those boundaries break down,” said Dragon. Meanwhile, Henderson had advice for anglers.

“Personally,” he said, “I wouldn’t eat the ones with tumors.”