In 2012 an angler pulled a brown bullhead out of a lake on the Vermont-Quebec border with what looked like tar smeared across its skin. This month biologists at the University of Vermont reported in Nature that the black melanoma is contagious, the tumor cells themselves passing from one fish to the next. It is the fourth kind of transmissible cancer known to science, and the first in a fish.
Cancer is supposed to be a dead end. When the host dies, its cancer dies too. A few times in the history of life a cancer cell has slipped that fate, breaking free of the body that made it and spreading to new hosts as a living, contagious lineage of its own. The catfish melanoma is the newest of these. Here are the other three.
Dogs: The Oldest Cancer On Earth
The cancer in dogs has been around the longest, by a wide margin. Canine transmissible venereal tumor, or CTVT, is a genital cancer that dogs pass to one another during mating. Every case on the planet, from Buenos Aires to Kathmandu, descends from a single dog that lived somewhere in northeastern Asia between roughly 6,000 and 11,000 years ago. That makes CTVT the oldest known cancer, and the oldest continuously living mammalian cell line of any kind.
Early estimates, drawn from a few fast-changing stretches of DNA, had put the lineage at only a few centuries old. Sequencing its entire genome corrected that. The mutations it carries could only have piled up over thousands of years. The founder was an inbred animal, husky-like, and long dead. Its tumor cells outlived it by millennia. It was also the first cancer anyone proved could spread at all: in 1876 the Russian veterinarian Mstislav Novinsky, a founder of experimental oncology, transplanted it from one dog to another, the earliest experimental transmission of any tumor.
For most of that history the cancer stayed local. Then, about five centuries ago, it went global, carried around the world by dogs aboard European trading and colonial ships. Along the way it solved a problem that should have killed it.
A cell line that only ever copies itself slowly fills with mutations it can never shed, and its mitochondria are especially exposed. Mitochondria are the tiny power plants that supply a cell’s energy, and they carry their own small loop of DNA, separate from the rest. CTVT escapes the reckoning by capturing fresh mitochondria from the dogs it infects, swapping failing power plants for working ones, a transfusion it has performed at least five times. It is also, mercifully, mild. Most dogs clear it, through their own immune response or a short course of a common chemotherapy drug.
Tasmanian Devils: The Cancer That Emptied An Island
The cancer in Tasmanian devils is the opposite story. Devil facial tumor disease appeared in 1996 in Tasmania’s northeast, grotesque growths on the face and mouth that the animals spread by biting one another in their squabbles over food and mates. It is nearly always fatal, and it moved fast. The Tasmanian devil population has fallen by more than 80 percent, and by over 90 percent in the hardest-hit populations. The species is now endangered, extinct on the Australian mainland for three thousand years and besieged on the island that is its last refuge.
The strangest thing about the devil disease is that it happened twice. A second, unrelated cancer from a different founder devil turned up in 2014, the only case of a single species producing two distinct cancer lineages. Both arise from Schwann cells, the sheathing around nerves, and both hide from the immune system by switching off their MHC molecules. MHC, short for major histocompatibility complex, are the surface flags a cell uses to identify itself as part of the body, the same molecules that make a transplant match or fail. Stripped of them, the tumor no longer reads as foreign. The devils are adapting. In only a handful of generations some populations have begun evolving resistance, and the crash many expected to end in the extinction of the species now looks more like an uneasy coexistence.
Shellfish: The Cancer That Crossed The Ocean
The strangest cancer of all lives in shellfish. Clams, mussels and cockles suffer a leukemia-like cancer. Like human leukemia, it is a cancer of the blood: the clam’s own circulating cells, its hemocytes, turn malignant and multiply until they choke its watery blood, the hemolymph, and the animal dies. Those cancerous cells travel from one mollusk to the next through seawater, drawn in when a healthy animal filters its food. No contact is needed. A dying clam seeds the water, and its neighbors draw the cancer in.
Here the phenomenon stops looking like a fluke. Bivalve cancer has arisen independently at least ten separate times, in as many species, and the count keeps rising as biologists look. One lineage jumped the species barrier entirely. Along the Atlantic coast of Europe a cancer riddles the golden carpet shell clam, Polititapes aureus, yet genotyping showed its cells are not that clam’s at all. They carry the DNA of a different species, the pullet carpet shell Venerupis corrugata, which lives buried in the same sandy shallows. The cancer arose in one clam, crossed into the other, and now flourishes in its new host, while the pullet carpet shell in those same waters shows no trace of the disease. Others have crossed whole oceans, spreading between hemispheres by a route scientists think is probably shipping, the cancer carried in ballast water or on the hulls of ships. The oldest of these clam cancers infects the soft-shell clam, Mya arenaria, of the North American coast. By reading the mutations stacked up in the tumor’s genome as a kind of clock, researchers estimate the lineage has been dividing and passing from clam to clam for more than two centuries, outlasting countless generations of its hosts.
How Many More Transmissible Cancers Are Out There?
The count of four is misleading, because it tallies kinds of cancer rather than the number of times cancer has escaped its host. Counted the second way, the total is at least thirteen: once in dogs, twice in Tasmanian devils, and at least ten times in shellfish. The tally is low because catching one is painstaking work, requiring sequencing the genomes of both the tumor and the host and showing the two do not match, which almost no one does in wild animals.
Decades ago, though, biologists caught a hint of all this. In the 1960s a contagious reticulum cell sarcoma spread through a colony of laboratory hamsters, passing from animal to animal and even by the bite of a mosquito. But those hamsters were so inbred as to be near-clones, and an immune system does not reject cells it cannot tell from its own. In the wild every new host is a genetic stranger, its immune system built to destroy foreign tissue. A cancer that spreads there has cleared a bar the hamster tumor didn’t face, which is exactly what makes the four real ones so strange, and so important for further research.
“I don’t think these are as rare as we think,” Andrew Storfer, an evolutionary geneticist at Washington State University who studies the devil disease, told me.
The catfish in that Vermont lake has joined a very strange club. I doubt it will be the last one biologists find, however.
