Dogs, devils, clams and catfish: When cancer is contagious

Infectious melanoma cells can grow anywhere on a bullhead catfish’s body, including in the mouth. In a handful of animals, tumors can be transmitted like infectious diseases or parasites, defying science’s tidy definitions Human beings — at least for now — have just one world. And the same is true for most cancers, trapped as they are in the body from which they arose and multiplying until their resources run out.

Then, says Joel Brown, an evolutionary ecologist at the Moffitt Cancer Center in Tampa, Florida, “they die with their planet.” But a few cancers have made the giant leap, colonizing new hosts and spreading through animal populations as contagious parasites. These “transmissible cancers” have been found in Tasmanian devils, dogs, clams and, most recently, catfish. Blood, poop and carcasses: How feasting bugs are helping map biodiversity The lucrative bird-singing contests that are pushing species to the brink Researchers once doubted whether transmissible cancers could even exist.

Now they are starting to uncover how such entities survive and evolve. And as examples accumulate, many researchers are wondering how rare they really are. Cancers develop when a cell that is part of a larger creature abandons its cooperative role and starts behaving as “a new little single-cell beastie,” says Brown.

It proliferates and evolves, treating the body’s tissues as habitats. The immune system fights back, but it is at a disadvantage because the cancer still resembles the well-behaved cells it descended from. But things should be different if a cancer cell ends up in a different individual: It should be recognized as foreign and destroyed.

Immune systems are good at recognizing foreign cells; that’s why people have to take immune-suppressing drugs to receive organ transplants. Researchers are learning that transmissible cancers have a plethora of strategies, only partly understood, to foil their hosts’ defenses. For example, a type of clam cancer throttles the activity of many genes involved in immune signaling, hinting at how it evades the host’s immune system, according to research published in 2025.

And cancers that infect dogs and Tasmanian devils can suppress their own molecular ID badges, the major histocompatibility complex class I (MHC-1) proteins, that help vertebrates distinguish their own cells from invaders. The first transmissible cancer ever discovered was that of the dog. It is known as canine transmissible venereal tumor (CTVT), and it grows as cauliflower-like tumors on dog genitals.