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Contagious cancer travels hundreds of miles, infecting multiple clam species

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Contagious cancer travels hundreds of miles, infecting multiple clam species
Contagious blood cancers have been found in several clam species around the world, including soft-shell clams, the most popular commercial clam species. Photo by GRID-Arendal/Flickr

BANGOR, Maine, Jan. 27 (UPI) -- Scientists in Europe have identified a new type of contagious blood cancer capable of infecting multiple clam species.

The recent finding suggests that leukemia-like diseases and interspecies transmission aren't as rare as once thought, and that they may be spreading more easily across the world's oceans with the help of human activities.

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That poses risks to coastal ecosystems and shellfish industries.

"This could lead to a reduction of mussels, clams or cockles," Alicia Bruzos, a doctoral student at the University of Santiago de Compostela in Spain, told UPI. "There are many people who make their livelihoods by harvesting and cultivating these."

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In the 1980s and '90s, outbreaks of contagious blood cancer along the coasts of New England and Prince Edward Island were identified among soft-shell clams, Mya arenaria, and up to 90% of local populations in some locations were infected and died.

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Though blood cancer isn't the main cause, soft-shell clam stocks are declining rapidly in places like Maine, the nation's leading producer of steamers.

"Hemic neoplasia has had a significant impact in some limited areas in a few Maine towns," Denis-Marc Nault, a biologist with the state's Department of Marine Resources, told UPI.

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Overall, invasive predators like green crabs and moon snails pose a larger threat to soft-shell clam populations in Maine than contagious cancers, according to Nault.

When outbreaks do occur, however, state fisheries managers work with local clammers to limit spread until the disease subsides.

"DMR and municipalities are keeping a watchful eye out in areas of documented disease presence for any mass mortality events," Nault said.

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"Basic management and conservation methods and timing of transplants of soft-shell clams can limit the spread of the disease. Hemic neoplasia is ubiquitous across the coast of Maine, but appears to be more common or endemic in some specific coves," he said.

Same cancer, different clams

While hunting for the genetic mutations that allow cancers to jump from one clam to another, scientists realized they had sequenced a novel form of cancer in the clam species Venus verrucosa.

"In the lab, many times, you are looking for something, but you find something else, something unexpected," Bruzos said.

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Deploying a method called whole-genome sequencing, Bruzos and her research partners determined that the cancer had originated in a single clam and spread between two clam populations separated by several hundred miles.

"The most surprising thing was that we found the cancer in the Atlantic Ocean and the Mediterranean Sea," Bruzos said.

When sequencing cancer cells from the two Venus verrucosa populations, scientists also found genetic data belonging to a second unidentified bivalve species.

Using genomic databases, the team determined the foreign DNA belonged to Chamelea gallina -- proving the cancer had spread among two species of clams.

How cancer spreads

When cancer metastasizes, cells dispatch from the original tumor mass and spawn cancerous growth elsewhere in the body.

In humans, this type of spread is limited to an individual host, but in some animal species, cancer cells can move from one host to another.

Researchers first identified the phenomenon in the 1970s in Tasmanian devils and dogs, which spread cancer through biting and intercourse, respectively. In the ocean, the mode of transmission is less obvious.

Though scientists have known about cancer in bivalve populations for several decades, evidence of the contagion only surfaced relatively recently -- with the advent of genomic sequencing technologies.

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"Initially, nobody knew that they were the cause, only that there would be the occasional outbreak of cancer," Michael Metzger, a researcher at Pacific Northwest Research Institute who pioneered the discovery of contagious cancers in bivalves, told UPI.

Genomic data in 2015 first proved that these cancers were moving from one clam host to another, and in the span of a few years, scientists have found a handful of blood cancers spreading among several kinds of bivalves, including mussels, clams and cockles.

Cancers have existed for a long time

Though they still are working to make sense of the genetic and biological mechanics that make host-to-host cancer transmission possible, researchers surmise that at some point in evolutionary history, blood cancer cells gained the ability to dispatch and survive in the ocean.

Because bivalves are filter feeders that cluster in large numbers, cancer cells from one host inevitably were ingested by another, sparking a chain of transmission.

"We think that many of these cancers are probably quite old and have been occurring for a long time," said Metzger, who was not involved in the newest research. "I think it's just the fact that we're just being able to see it now."

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The earliest evidence of cancerous growth in modern humans, found in an Egyptian mummy, dates to 1500 B.C.

A tumor on the toe of a 1.7 million-year-old human relative, unearthed in South Africa, suggests that cancer among hominins is even older. Paleontologists have even identified malignant growths on reptiles from the Triassic Period, some 250 million years ago.

Metzger suspects cancers have been infecting bivalves for a long time, too, which explains why some species and populations have been able to recover from outbreaks relatively quickly.

Understanding the genetic and physiological differences that allow some bivalves to fight off blood cancers better than others could provide useful scientific insights, Metzger said, helping scientists better predict disease spread, develop interventions and

"It could have potential implication for treating human cancers," Metzger said.

But while some populations have proven resilient, Metzger and Bruzos both say these contagious leukemias pose risks that must be better understood.

No large outbreaks yet

Though there have yet to be any documented outbreaks among farmed bivalves, both Metzger and Bruzos said the potential exists for spread to aquaculture populations.

"Any time you have a large, dense population that is genetically homogenous, that raises the potential for a pathogen to spread more rapidly," Metzger said.

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"If the cancer cells are around in wild populations that live nearby, those cells could potentially infect farmed mussels," Bruzos said.

Mussel cultivation is more common than clam farming, but clam flats are regularly re-seeded with baby clams. Both wild transplants and hatchery raised clams can be used for seeding.

In Maine, Nault said clam seeds are checked for disease before being transplanted, but Bruzos said it's possible the transplantation of clam seeds from France to Spain could explain how the same blood cancer was found among two distant clam populations.

The human link

Unlike clams, which bury themselves in the sand, mussels tether themselves to various marine surfaces, like seaweed, ropes, docks and cargo ships -- which explains why one cancer lineage has been found in several different mussel species all over the world.

"The evidence that humans are involved in spreading these cancers is growing," said Metzger.

Exactly how involved is something scientists are keen to find out.

"The more we understand these transmissible cancers, the more we'll be able to ensure aquaculture practices are designed to resist these cancers," Metzger said.

In the meantime, Bruzos said regulators need to start developing defenses.

"Although it is a cancer, it behaves like a parasite, we need to treat it as such and check for it," she said.

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While scientists work to better understand the risks of contagious cancers to bivalves, both wild and farmed, there is good news for seafood fans: The cancer cells growing inside clams -- and floating through ocean in search of new hosts -- pose no risk to humans.

"Our immune system will recognize it as foreign, so no problem at all, we can eat clams, mussels and other bivalves, even if they have cancer," Bruzos said.

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