Scientists discover first contagious cancer in freshwater fish
Scientists have identified the first known contagious cancer in a freshwater fish, finding that a deadly melanoma affecting brown bullhead catfish spreads from fish to fish through the cancer cells themselves rather than developing independently in each animal, Qazinform News Agency correspondent reports.
The discovery comes after researchers investigated unusually high rates of melanoma in brown bullhead catfish living in Lake Memphremagog, which straddles the U.S.-Canada border between Vermont and Quebec. Since 2012, between 23% and 37% of the fish examined in the lake have been found with malignant black skin tumors, far higher than rates reported elsewhere.
To determine the cause, researchers analyzed the genomes of tumor tissue and healthy tissue from affected fish, along with samples from healthy fish from several other locations. They found that tumors from different fish were genetically more closely related to one another than to the fish carrying them. The tumors also shared hundreds of thousands of genetic changes that were absent from their hosts, providing strong evidence that they all originated from a single cancer lineage.
The findings make brown bullhead melanoma the fourth known naturally occurring transmissible cancer in animals, after those found in dogs, Tasmanian devils and several species of shellfish. It is also the first documented case in any fish and the first discovered in a freshwater ecosystem.
Researchers also searched for viruses or other microbes that could explain the outbreak but found no organisms consistently linked to the tumors. While they cannot completely rule out pathogens playing a role in the disease's emergence, the evidence suggests the cancer cells themselves are the infectious agent.
Exactly how the cancer spreads remains unknown. The team suggests it could be transmitted during spawning, when fish gather closely together, or through direct contact with contaminated water or sediment. The rapid rise in infections after 2012 indicates the disease spreads efficiently within the population.
Scientists say it is too early to determine the long-term impact on brown bullhead populations. They also do not yet know whether the cancer exists in other lakes across North America. Further research will focus on how the disease originated, how it spreads, and whether it poses a broader threat to fish populations.
Earlier, Qazinform News Agency reported that a new cancer treatment destroys 99% of cancer cells in laboratory tests.