Researchers have discovered a contagious cancer in catfish, just the fourth example of tumor cells spreading naturally from one animal to another.

The others are a sexually transmitted tumor in dogs, two distinct lines of cancer that threaten Tasmanian devils (Sarcophilus harrisii) and numerous independent tumor lineages affecting bivalves, including clams, cockles, and mussels.

The latest findings explain the mysterious appearance, more than a decade ago, of black lesions on the skin of brown bullhead catfish (Ameiurus nebulosus) in a lake on both sides of the Canada-United States border, and possibly observations elsewhere dating back to the 19th century.

Anglers and wildlife biologists first noticed the tumors in 2012 in fish from Lake Memphremagog, which stretches from Newport in Vermont to Magog, Canada, and subsequent studies suggested that about a third of the lake’s brown bullhead catfish were affected.¹.

The skin lesions, a form of melanoma, are inky black and sometimes crusty; Some fish have “tumors on top of tumors” and can spread to the brain, liver and other organs, says Julie Dragon, a data scientist at the University of Vermont in Burlington and lead author of a study published July 22 in Nature². But the health effects are unclear, because the tumors appear in young and old mature fish. “We have no evidence of a huge old cliff to suggest it is killing the fish,” adds Dragon.

Contamination was suspected to be the cause, as was a viral infection. Working with biologists from the state of Vermont, Dragon found no evidence of any widespread virus. The team cast a wider net and sequenced the genomes of individual fish, looking for possible cancer-causing DNA mutations that were common in the lake’s brown catfish.

Instead, they found characteristics of a transmissible tumor. Tumor cells from different fish shared hundreds of thousands of genetic variants, forming a single genetic lineage distinct from healthy cells from the same individual fish. “We had no idea this was going to be the case,” says Dragon. “It’s still amazing.”

The fishwives’ tale

Michael Metzger, a biologist at the Pacific Northwest Research Institute in Seattle, Washington, isn’t surprised that another transmissible cancer has been found, but he didn’t expect it in a freshwater species. Metzger’s group has found that transmissible tumor cells from marine bivalves can survive for days in salt water, supporting the theory that they spread via free-floating cells, but die instantly in fresh water.³. He wonders if catfish tumors are transmitted by direct contact, like those that affect dogs and Tasmanian devils.