Thursday July 30, 2026
USA Today —
A rare cancer that spreads between fish like an infectious disease has been discovered in catfish in Vermont, marking the first known transmissible cancer in a freshwater fish species.
The disease is only the fourth naturally occurring contagious cancer ever documented in animals. Researchers say the unusual finding could help explain how cancers spread, evolve and survive beyond their original host.
The discovery, published July 22 in the journal Nature, centers on brown bullhead catfish living in Lake Memphremagog, which straddles the Vermont-Quebec border. Scientists found that the tumors are not developing independently in each fish.
Instead, the cancer cells themselves appear to move between animals and grow into new tumors.
How did scientists discover it?
Researchers from the University of Vermont and other institutions investigated after anglers and biologists noticed increasing numbers of catfish with raised black patches on their skin. The lesions first drew attention in 2012.
By 2014, roughly one-third of the catfish sampled in Lake Memphremagog showed signs of the disease.
Initially, scientists suspected pollution, a virus, or another environmental factor was causing the tumors. But genetic testing pointed to a much rarer explanation.