Contagious cancers are a scary idea to begin with, but scientists have made some startling new discoveries about them – they are likely more common in nature than originally thought, and some can even spread between species.

A new study has found a contagious, leukemia-like disease appears to be widespread among shellfish with hinged shells, called bivalves, such as clams, mussels and cockles. And, for the first time, researchers have found evidence of the cancer spreading from one species to another.

Environment Canada's Jim Sherry, seen here sampling mussels in P.E.I., studied soft-shell clam die-offs in P.E.I. linked to a contagious cancer. That eventually let him and his collaborators to see if they could find similar diseases in other shellfish. (Courtesy Jim Sherry)

Mussels living off the coast of British Columbia, along with cockles and golden carpet shell clams in Spain, are all prone to the contagious cancer, similar to one that has devastated soft-shell clam farming operations in P.E.I., the Canadian, U.S. and Spanish scientists reported Wednesday in Nature.

"It may be more widespread in nature than we know," said Jim Sherry, an Environment Canada scientist based in Burlington, Ont., who co-authored the study.

Because cancers originate from the mutated cells of an individual and are genetically similar to that individual, genetic analysis of the cancer cells allows scientists to figure out their origin.

'Wildly different from the host'

Genetic analysis showed the cancer cells found in the golden carpet shell clams were "wildly different from the host," said Stephen Goff, a professor of microbiology at Columbia University who also co-authored the new paper. "It wasn't even the same species."

Genetic analysis showed that the cancer cells found in the golden carpet shell clams, above, originated from another species called the pullet carpet shell. It's the first known case of cross-species cancer transmission. (David Iglesias)

The cancer cells were from a related species called the pullet carpet shell.

"This had to be a case of cross-species transmission," Goff said. That has never been seen before.

However, he said it's unlikely the cancer could spread between species that weren't closely related.

Eating shellfish with the disease should be perfectly safe (although, in general, sick or dying shellfish are not good to eat.)

In humans, cancers originate within their host's body and, as far as we know, can't spread to other people, except in rare cases, such as through organ transplants or pregnancy.

In animals, only a few cancers are known to be transmissible between individuals of the same species, including a sexually transmitted cancer that causes grotesque tumours in dogs, a facial cancer that has devastated Tasmanian devil populations, and a deadly leukemia-like cancer that has killed off huge numbers of soft-shell clams in some areas of Prince Edward Island.

"It's wiped out aquaculture operations," said Sherry, who studied clam die-offs linked to the disease several years ago. "We could often find beds or areas where most of the clams were dead from it, particularly if the summer was hot and the disease gets going, there's a high fatality rate."

Canadian scientists collected mussels in West Vancouver, above, and Esquimalt, B.C. They then took them back to the lab and screened them for cancer. (Annette F. Muttray/University of British Columbia)

Sherry was interested to see whether environmental stresses such as pesticides from nearby potato farming operations might be linked to the high cancer rates in some areas, and whether viruses might also play a role. His colleague, Carol Reinisch, suggested collaborating with Goff, a shellfish virus expert.

Funding for the P.E.I. project was cancelled under Stephen Harper's Conservative government, but the Environment Canada scientists continued to work with Goff, as they were interested to know if they could find this kind of contagious cancer in other shellfish.

Mussel blood test

Sherry worked with Reinisch and scientists at the University of British Columbia to collect mussels in West Vancouver and Esquimalt, B.C. Then they took them back to the lab and screened them for cancer.

They used syringes to draw samples of hemolymph, the shellfish equivalent of blood, and examined the hemocytes, which are similar to white blood cells. In healthy animals, there aren't a lot of hemocytes, and the few there are will flatten out when put on a glass slide, Sherry said. In animals with leukemia, hemolymph "will be nearly chock-a-block" full of rounded cells that don't flatten out.

The researchers used syringes to draw samples of hemolymph, the shellfish equivalent of blood, and examined the hemocytes, which are similar to white blood cells. Samples containing cancer, such as these cells from a cockle, were sent to Stephen Goff's lab for further analysis. (Columbia University)

Researchers in Spain did similar screening with cockles and golden carpet shell clams.

Samples that tested positive for leukemia were sent to Goff and his postdoctoral researcher Michael Metzger, lead author of the new paper, for genetic analysis.

That analysis showed that not all the mussels with leukemia had a contagious cancer – in some cases, the cancer had developed from an individual's own cells, as is typically the case.

But contagious cancers were found in all three species, and were typically clones from a single individual.

In cockles, two types of cancer cells that originated from two different individuals were found. That suggests that new contagious cancers are arising "more often than you might think," Goff said.

Stephen Goff, a professor of microbiology at Columbia University who also co-authored the new paper, is interested in finding out what mutations allowed the transmissible cancer to spread to other individuals. That could potentially help us understand the spread of cancer to other organs in humans. (Lucky Tran/Columbia University)

The researchers think that bivalves may be prone to catching contagious cancers because:

They live in the ocean, which can keep cancerous cells alive for quite a while and transport them long distances

They feed by sucking in huge quantities of water and filtering out food particles

They have an immune system that's very different than that of other animals.

"They don't seem to have any ability to sense this tumour," Goff said.

He is now interested in finding out what mutations arose in the transmissible cancers to allow them to spread to other animals. He added, "Those genes could have implications for our understanding of metastases (cancer spread) within humans."