Contagious Cancer Discovered in Freshwater Catfish

Scientists in Vermont have discovered the first contagious cancer in a freshwater animal, identifying a transmissible melanoma affecting brown bullhead catfish (Ameiurus nebulosus) in Lake Memphremagog and other New England and Canadian waters. Published on July 27, 2026, in Nature, whole-genome sequencing revealed that the tumor cells behave like parasites, passing directly between fish and sharing hundreds of thousands of genetic variants absent from the host catfish. The research team, co-led by scientists Julie Dragon and Mark Henderson from the University of Vermont, initially suspected viruses or pollution from nearby landfills such as the Coventry Landfill, but genetic and water quality analyses ruled out industrial contamination. Instead, the geographical distribution correlates with elevated arsenic concentrations in regional soil, though exact transmission mechanisms and ecological impacts remain under investigation. This marks only the fourth recognized type of transmissible cancer in the animal kingdom, joining facial tumor disease in Tasmanian devils, canine transmissible venereal tumors, and leukemia-like cancers in bivalve mollusks. Historical journals from Henry David Thoreau in 1852 and 1858 describing similar dark lesions on catfish suggest this transmissible cancer may have a long and unrecognized history in regional waters. Researchers emphasize that the disease poses no known threat to humans, as the cancer cells cannot infect or survive in other species, and affected fish waters supply drinking water to over 175,000 people safely. Scientists are currently surveying Massachusetts ponds and rivers to map the distribution, understand cancer evolution, and uncover the biological boundaries that normally keep cancers confined to single animals.