Mysterious oyster deaths linked to newly discovered virus

On Canada’s Pacific coast, something unusual has been unfolding beneath the water’s surface. Mass die-offs of farmed oysters, once dismissed as side effects of warming seas or pollution, now appear linked to a mysterious new virus.

The newly discovered virus emerged from British Columbia’s Pacific oyster farms. During a mass mortality event in 2020, scientists identified a previously unknown pathogen.

This finding adds another layer to the complex mystery behind recurring oyster die-offs.

“We’ve recently seen annual mass die-offs in B.C. and elsewhere of Pacific oysters, the most widely farmed shellfish worldwide. Often, we don’t know the cause,” said Dr. Kevin Zhong, lead author from the University of British Columbia’s Department of Earth, Ocean, and Atmospheric Sciences.

Pacific oysters generated around $16 million in B.C. in 2023. Yet, the industry faces ongoing risks from pathogens and rising sea temperatures. In many cases, scientists cannot pinpoint the exact reason behind the deaths.

New virus found in oyster farms

The researchers gathered 33 oysters from two affected farms and 26 wild oysters from nearby coastal areas. RNA tests revealed a novel virus – Pacific Oyster Nidovirus 1 (PONV1) – in 20 of the dead farmed oysters. Wild oysters, however, showed no signs of the virus.

“This discovery highlights how little we know about viruses infecting invertebrates in general and oysters in particular,” said Dr. Curtis Suttle, senior author and UBC professor. “It’s important to investigate mass die-offs in oysters to determine what causes disease.”

“Similar to humans, disease and death are not caused by a single factor, but prevention is not possible until the causes are known.”

The virus appears strongly linked to oyster mortality. However, researchers caution that other stressors may contribute as well.

A record-breaking genome

PONV1 has one of the largest RNA genomes found so far. It’s only the second known nidovirus in bivalves. Other nidoviruses infect many species, including humans. The virus that causes COVID-19, SARS-CoV-2, also belongs to this group.

“The extraordinarily large genome of this virus makes it particularly fascinating as it pushes the known boundaries of how big RNA virus genomes can get,” Dr. Zhong said.

A larger genome may allow the virus to encode more genes or protein domains, potentially expanding or enhancing its ability to interact with hosts.

“This discovery offers a rare window into the possible evolutionary mechanisms that enable genome expansion in RNA viruses,” he said.

This oversized genome gives researchers clues about how viruses evolve. PONV1 may hold answers about genetic flexibility and host adaptation.

Scientists name viral giant

By searching global databases, the team identified 15 similar viruses in oysters from Europe and Asia. These strains appear widespread but are not linked to die-offs.

The newly discovered virus, PONV1, stands out because its genetic structure is unlike that of any known virus in its group.

After analyzing its RNA sequence, researchers found that it differed significantly from other known nidoviruses. This level of genetic difference was so great that the team concluded it didn’t fit within any existing virus family.

As a result, the scientists proposed creating an entirely new family of viruses to accommodate it. They named this new family Megarnaviridae, which means “large RNA viruses.” The name reflects the unusually large genome carried by the virus – one of the biggest ever recorded in RNA viruses.

To further define this discovery, the team suggested a scientific name for PONV1: Megarnavirus gigas. The term gigascomes from Latin and means “giant,” referring again to the virus’s large genetic size. So, Megarnavirus gigas translates to “large RNA virus giant,” highlighting both its uniqueness and its size.

“Fortunately, the virus does not infect humans. It seems specific to oysters,” said Dr. Suttle.

Virus raises warning about oyster trade

B.C. oyster growers often import juvenile oysters, or spat, from hatcheries both domestically and overseas. This practice might carry hidden pathogens.

The discovery of the virus is a reminder that growers should use an abundance of caution when moving oyster spat internationally and nationally, as we still know very little about what causes disease in oysters. New pathogens may be introduced when spat are imported.

“Ultimately, developing rapid tests for detecting potential pathogens is essential for safely importing oyster seed, and identifying the causes of disease and mortality,” Dr. Suttle said.

This research supports better monitoring systems. With early testing, the industry can reduce the spread of unknown viruses.

A turning point for aquaculture

The team stressed that the discovery is not cause for panic. Instead, it opens the door to more informed oyster health management.

“This research is not a cause for alarm,” said Dr. Suttle. “Rather, this is a meaningful step forward in advancing our understanding of oyster health and supporting the long-term sustainability of shellfish aquaculture.”

With more study, experts hope to understand the virus’s exact role. The goal is to improve disease prevention and ensure safe, thriving oyster farms worldwide.

The study is published in the journal Proceedings of the National Academy of Sciences.

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