The idea that cancer can spread from one animal to another, much like a contagious disease, might sound far-fetched. But researchers have discovered just that in North American catfish.

In 2012, an unusually high number of brown catfish with black lesions were found in Lake Memphremagog, spanning Vermont and Quebec. Genetic analysis revealed these tumors shared a common genetic identity, suggesting they are transmitted among fish rather than arising independently.

The researchers propose several possible explanations for the transmission: close physical contact during spawning, tumor cells surviving temporarily in water or sediments, or even hormonal changes altering immune responses. Curiously, only larger, older fish appear to be affected.

While the exact means of transmission remain unclear, scientists speculate that studying these transmissible cancers might provide insights into preventing cancer spread in humans. For now, anglers are being advised not to eat infected cats.