Scientists discover first known transmissible cancer in fish

Scientists have identified the first known transmissible cancer in fish, after discovering that melanoma tumours are spreading between brown bullhead catfish in lakes across Quebec and the northeastern United States.

The findings, published in the journal Nature, suggest the disease is one of only a handful of contagious cancers ever discovered.

Study co-leader Julie Dragon, from the University of Vermont's Larner College of Medicine, said to CBC News: "These conditions are incredibly rare in nature. So something has to be happening to allow this to happen."

The unusual tumours were first reported by anglers in Lake Memphremagog, which lies on the border of Quebec and Vermont, in 2012.

Researchers estimate that about one-third of the lake's brown bullhead catfish now have the disease.

Scientists initially investigated whether pollution, viruses or inbreeding were responsible. However, genetic testing found no viruses and showed the fish had strong genetic diversity.

Dragon said the research team's biggest discovery came when they compared the tumours with healthy fish.

"These cancer cells were more similar to each other than they were to the fish [they were found on]," she said.

The findings led researchers to conclude the cancer itself is being transmitted between fish, making it the first confirmed case of a contagious cancer in a fish species.