Scientists Discover First Contagious Cancer in Fish

Scientists have identified what appears to be the first known transmissible cancer in a wild fish, a discovery that could reshape how researchers study cancer and disease in animals.

The findings come from a new study published in the journal Nature after researchers investigated brown bullhead catfish in Lake Memphremagog, which spans the U.S.-Canada border.

Biologists first noticed unusual black spots on the fish more than a decade ago. Further testing revealed that between 23% and 37% of the catfish sampled had melanoma, a form of skin cancer.

Get our free mobile app

Using DNA sequencing, researchers determined the tumors found on different fish were nearly genetically identical but different from the fish carrying them. That suggests the cancer cells themselves are spreading from fish to fish rather than developing independently in each animal. If confirmed, it would mark the first known transmissible cancer in a fish and the first ever discovered in a freshwater ecosystem.

Scientists say they still do not know exactly how the cancer spreads, but they believe it may occur when catfish gather closely during spawning season or through contact with contaminated lake sediment.

Researchers emphasized that there is no evidence the cancer poses a risk to humans. The cancer cells cannot infect people or other animal species, and there is no indication the lake's drinking water is unsafe.

The discovery joins a very short list of naturally occurring transmissible cancers previously identified only in dogs, Tasmanian devils and certain shellfish. Scientists say studying the disease could provide valuable insight into how cancer cells survive, evade the immune system and spread.