A Contagious Cancer Is Spreading Through Catfish. It's the Fourth Known to Science

Cancers found in a cluster usually have a common cause, a chemical carcinogen or an infection. Brown bullhead are a sentinel species for pollution because they develop liver tumors in water contaminated with polycyclic aromatic hydrocarbons, and walleye carry a retrovirus that causes skin sarcomas.

So, these are the causes the researchers hunted for, hard and for years. Vermont biologists tested the watershed for those hydrocarbons after Tropical Storm Irene tore through in 2011, thinking the flooding had stirred up a point source. Waters with far higher levels held clean fish. Tissue went to the bioinformatics core at the University of Vermont from about 2014 on, yet years of sequencing turned up no virus and no microbe of any kind.

Julie Dragon, the associate professor of microbiology and molecular genetics who directs that core, went looking for a shared mutation instead. In an ordinary cancer each tumor is a close genetic relative of the animal it grew in, and a family tree of cancers and non-cancerous tissue puts every tumor next to its own host. The Memphremagog trees came out the other way around: all the tumors in one cluster and all the fish in another, with hundreds of thousands of mutations shared across cancers in different fish and absent from the fish carrying them.