A new type of cancer that is transmitted from animal to animal was identified by scientists in freshwater fish in North America.
This is only the fourth known type of contagious cancer, the first recorded in fish, and the first in a freshwater environment.
The disease affects brown bullhead catfish of the species Ameiurus nebulosus in Lake Memphremagog, which spans between the state of Vermont in the US and Canada. The fish exhibit black tumors on their skin, mouth, and sensory barbels that they use to locate their food.
The study, which was conducted by researchers at the University of Vermont and published in the scientific journal Nature, showed that the tumors are not caused by a virus, bacterium, or pollution. Instead, the cancer cells themselves act as an infectious agent and pass from one fish to another.
In ordinary cancers, tumor cells originate from the organism in which they grow and have a close genetic relationship with it. In this case, however, researchers found that the tumors of different fish were genetically much more similar to each other than to the animals that carried them.
This means that the cancer likely began from one fish and has since been spreading as a living line of cancer cells. Within a few years, it had affected up to 40% of the brown bullhead catfish in the lake, while genetic analyses indicate that the same disease has also appeared in other areas of the US and Canada.
How it spreads from lake to lake
Researchers estimate that the cancer has a wide geographical spread, without however appearing in all lakes where this specific fish species lives. In the region monitored by the Vermont biologists, brown bullhead catfish exist in nearly 90 lakes, but the disease has been identified in only one.
This leads scientists to the hypothesis that the spread depends not only on the environment or the presence of the fish, but primarily on the transfer of cancer cells from one water body to another. Human activity, for example the movement of water, bait, or fish by anglers, is considered a likely mechanism.
The same form of cancer has been identified in fish from Maine, New Hampshire, Massachusetts, Pennsylvania, and other US states, as well as in provinces of Canada. However, it is not yet clear whether all cases are linked to a single original focus.
The tumors can affect the ability of the fish to feed, as they often appear around their mouth and sensory barbels. Despite the high frequency of the disease in Lake Memphremagog, the brown bullhead catfish population remains large for now, and the rate of cases appears to have stabilized.
Scientists consider it likely that there are other contagious cancers in the wild that have not yet been recognized. Their identification requires genetic comparison of tumors with the organisms that carry them, a process that is rarely applied systematically to wild animals.
Issue of protective measures remains open
Current regulations prohibit anglers from transferring bait and water from Lake Memphremagog. They were enacted to prevent the spread of invasive species and not specifically the transfer of cancer cells.
Researchers estimate that these rules may need to be reexamined, particularly if it is proven that the disease is transferred via fishing equipment, water, or live fish. For now, however, there are no relevant warning signs at boat launch points at the lake.
To identify new cases, the research team is even examining photos uploaded by amateur anglers on social media applications. Images of fish with characteristic black tumors can help scientists map the spread of the disease in areas where systematic sampling is not conducted.
The discovery reinforces the assessment that contagious cancers may not be as rare as previously thought, but simply remain difficult to identify in the wild.
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