Contagious cancer cells travel hundreds of miles, infecting several species of clams

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Contagious blood cancers have been found in several species of clams around the world, including soft-shell clams, the most popular commercial clam species. picture by GRID-Arendal/Flickr

BANGOR, Maine, Jan. 27 (UPI) — European scientists have identified a new type of contagious blood cancer capable of infecting several species of clams.

the recent discovery suggests that leukemia-like diseases and interspecies transmission are not as rare as once thought, and that they may spread more easily across the world’s oceans with the help of human activities.

This poses risks to coastal ecosystems and shellfish industries.

“It could lead to a reduction in mussels, clams or cockles,” Alicia Bruzos, a doctoral student at the University of Santiago de Compostela in Spain, told UPI. “There are a lot of people who make a living from harvesting and growing them.”

In the 1980s and 1990s, outbreaks of contagious blood cancer along the coasts of New England and Prince Edward Island were identified among clams, Mya arenaria, and up to 90% local populations in some places were infected and died.

Although blood cancer isn’t the main cause, clam stocks are rapidly dwindling in places like Maine, the nation’s top steamboat producer.

“Hemic neoplasia has had a significant impact in certain limited areas of a few cities in Maine,” Denis-Marc Nault, a biologist with the state Department of Marine Resources, told UPI.

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

However, when outbreaks occur, state fisheries managers work with local clams to limit the spread until the disease subsides.

“DMR and municipalities are carefully monitoring areas of documented disease presence for any mass mortality events,” Nault said.

“Basic management and conservation methods and the timing of clam transplants can limit the spread of the disease. Hemic neoplasia is ubiquitous on the Maine coast, but appears to be more common or endemic in some specific coves,” said he declared.

Same cancer, different clams

Searching for the genetic mutations that allow cancers to jump from one clam to another, scientists realized they had sequenced a new form of cancer in the clam species warty venus.

“In the lab, often you’re looking for something, but you find something else, something unexpected,” Bruzos said.

Deploying a method called whole genome sequencing, Bruzos and his research partners determined that the cancer originated in a single clam and spread between two clam populations hundreds of miles apart.

“The most surprising thing is that we found cancer in the Atlantic Ocean and the Mediterranean Sea,” Bruzos said.

When sequencing cancer cells from both warty venus populations, the scientists also found genetic data belonging to a second, unidentified bivalve species.

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

How cancer spreads

When cancer metastasizes, cells break away from the original tumor mass and cause 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 bites and sexual intercourse, respectively. In the ocean, the mode of transmission is less obvious.

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

“Initially, no one knew they were the cause, only that there would be an occasional cancer outbreak,” Michael Metzger, a researcher at the Pacific Northwest Research Institute who pioneered the discovery of contagious cancers in humans, told UPI. the bivalves.

Genomic data from 2015 first proved that these cancers traveled from one clam host to another, and within a few years scientists discovered a handful of blood cancers spreading among multiple types of clams. bivalves, including mussels, clams and cockles.

Cancers have been around for a long time

Although they are still trying to understand the genetic and biological mechanisms that make possible the transmission of cancer from host to host, the researchers assume that at some point in the history of evolution, cancer cells in the blood have acquired the ability to spread and survive in the ocean.

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

“We think many of these cancers are probably quite old and have been happening for a long time,” said Metzger, who was not involved in the most recent research. “I think it’s just the fact that we can see it now.”

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

A tumor on the toe of a 1.7 million year old human relative, discovered in South Africa, suggests cancer in hominins is even older. Paleontologists have even identified malignant growths on reptiles from the Triassic period, around 250 million years ago.

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

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

“This could have potential implications for the treatment of human cancers,” Metzger said.

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

No major epidemics yet

Although there have not yet been any documented outbreaks among farmed bivalves, Metzger and Bruzos said there is potential for spread to aquaculture populations.

“Anytime you have a large, dense population that is genetically homogeneous, that increases the potential for a pathogen to spread faster,” Metzger said.

“If cancer cells are present in wild populations that live nearby, these cells could potentially infect farmed mussels,” Bruzos said.

Mussel farming is more common than clam farming, but clams are regularly reseeded with clams. Wild transplants and hatchery-reared clams can be used for seeding.

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

The human connection

Unlike clams, which burrow in sand, mussels attach themselves to various marine surfaces, such as seaweed, ropes, docks and cargo ships – which explains why a cancer lineage has been found in several species of different molds all over the world.

“The evidence that humans are involved in the spread of these cancers is growing,” Metzger said.

Scientists are eager to find out how exactly this is involved.

“The more we understand about these transmissible cancers, the more we will be able to ensure that aquaculture practices are designed to be resistant to these cancers,” Metzger said.

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

“Although it is cancer, it behaves like a parasite, we have to treat it as such and check it,” she said.

As scientists strive to better understand the risks of contagious cancers for bivalves, both wild and farmed, there is good news for seafood lovers: cancer cells growing inside the clams – and float in the 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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