For more than a decade, brown catfish in Lake Memphremagog, on the border between the United States and Canada, have been displaying mysterious black spots on their skin. What seemed like an environmental or viral problem has turned out to be something much more surprising: a cancer that is transmitted from one fish to another like a parasite.
A team from the University of Vermont has confirmed that it is the primer cáncer transmisible documentado en una especie de agua dulce and the fourth known in the entire animal kingdom. The finding, published in the journal Nature, raises new questions about the spread of these diseases in nature and their potential impact on ecosystems.
A melanoma that jumps from one fish to another
The first cases were detected in 2012, when fishermen and biologists began noticing that many catfish had strange, black, velvety bumps. By 2014, almost one in three catfish in the lake showed these lesions. Initially, a virus or disease was suspected. marine and aquatic pollution from a nearby landfill, but by sequencing the complete genome of the tumors and comparing it with healthy fish tissue, scientists discovered something unexpected: las células cancerosas estaban mucho más emparentadas entre sí que con el pez que las albergabaThat is the unmistakable signature of a transmissible clonal cancer, where the tumor cells themselves act as an infectious agent.
The study, led by geneticist Julie Dragon, revealed that the tumors share hundreds of thousands of genetic variants that do not appear in healthy fish. This can only be explained if all the cancer cells descend from a single common ancestor that, at some point, managed to jump from one fish to another and establish itself as an independent lineage. “I said: We’re wrong. This can’t be right,” Dragon recalls upon seeing the initial results. After repeating the analyses, the pattern was confirmed.
The fourth known transmissible cancer
Until now, science had only documented three examples of transmissible cancers in animals: canine venereal tumor, which has circulated among dogs for thousands of years; Tasmanian devil facial tumor, which has decimated that species since the 1990s; and several forms of contagious leukemia in bivalve mollusks such as clams and mussels, which have also been detected in European waters. Brown catfish melanoma joins this list as the fourth case and the first in a freshwater fish. Este descubrimiento amplía el mapa de los cánceres contagiosos and suggests that the aquatic environment could be especially conducive to the transmission of tumor cells between individuals.
Researchers emphasize that, unlike Tasmanian devil tumors, which are fatal within months, affected catfish can survive for years with melanoma. The long-term ecological impact is still unknown, but the fact that the disease is spreading through entire populations necessitates close monitoring of its evolution.
How is it spread?
One of the biggest unknowns is the exact transmission mechanism. The authors suggest that the infection could occur during spawning season, when catfish congregate in shallow waters and maintain intense physical contact. "It seems to only happen in large, reproductively mature fish," explains Mark Henderson, a biologist and co-director of the study. Las mordeduras, los arañazos y el roce directo podrían facilitar el paso de células tumorales from one fish to another. In addition, as scaleless fish that live on the bottom, they constantly interact with the sediment, where cancer cells could temporarily survive.
The study also indicates that naturally occurring arsenic in New England soils could weaken the fish's immune system, making them more vulnerable to colonization by tumor cells. However, researchers rule out plastic pollution or nearby landfills as the cause, since the cancer appears uniformly throughout the lake and in other lakes with clear water.
No risk to humans
Scientists have emphasized that this cancer poses no danger to humans. Lake Memphremagog provides drinking water to more than 175.000 people, and tests indicate that the tumor cells cannot infect other species. "Humans have a highly efficient immune system, capable of recognizing foreign cells such as cancerous fish cells; the probability of transmission is virtually zero," says Beata Ujvari, a professor at Deakin University and a reviewer of the study. La transmisión de tumores entre humanos es extremadamente rara and it has only been documented in very specific cases, such as mother-to-child transmission or in transplant recipients.
However, researchers recommend not consuming catfish with visible tumors as a precaution. "Personally, I wouldn't eat the ones with tumors," admits Henderson, who notes that about 30% of the fish in the lake are affected.