"Transmissible tumors have been reported in a handful of animal species, but never before among fish. Is it possible in humans?"

"For over a decade, something strange has been happening in a lake straddling the border of Vermont and Quebec.

Lake Memphremagog is home to brown bullhead catfish, and in 2012, anglers began noticing that some of them were covered in velvety black growths. Wildlife biologists eventually determined that the fish had skin cancer.

So-called melanomas are exceedingly rare in catfish, but more than a third of the bullheads carried them. Now, researchers think they know why: A contagious form of cancer is spreading through the lake, they reported on Wednesday.

The outbreak started with a single catfish, the scientists believe. It developed a tumor, which shed cancer cells into the water. Those cells invaded other catfish, in effect beginning a plague."

Carl Zimmer reports for the New York Times July 22, 2026. [2]