Researchers based in British Columbia have identified a virus that may pose a threat to farmed Pacific oysters and contributed to the mass mortality event of the species in 2020.
Scientists at the University of British Columbia (UBC) published a report on Aug. 4 detailing their findings after an investigation into the large-scale die-off in the province five years ago.
The researchers gathered 33 oysters from two farms in B.C. that experienced the 2020 die-off, along with 26 wild oysters collected from 10 nearby sites.
Ribonucleic acid (RNA) analysis identified a novel virus—Pacific Oyster Nidovirus 1—in the dead or dying oysters. Yet it was not detected in healthy wild oysters, suggesting a correlation between the virus and mortality.
“This discovery highlights how little we know about viruses infecting invertebrates in general and oysters in particular,” senior author and university professor Curtis Suttle said in a news release.
Suttle said investigating the mass die-off to identify the disease was crucial for understanding how to better protect Pacific oysters, a major B.C. industry. Pacific oysters are the main shellfish cultivated in the province and had an estimated value of $16 million in 2023.
“Similar to humans, disease and death is not caused by a single factor, but prevention is not possible until the causes are known,” he said.
Pacific Oyster Nidovirus 1 (PONV1) has one of the largest RNA genomes ever recorded and is the first nidovirus reported to replicate in bivalves, the report says. It also displays a genetic structure featuring protein domains that are absent in other nidoviruses.
Nidoviruses can be found in both animals and humans. One example of this is SARS-CoV-2, a type of nidovirus responsible for COVID-19.
First author Kevin Zhong, a UBC research associate, said the virus possesses an “extraordinarily large” genome. Its scale challenges the current understanding of the maximum size of RNA virus genomes.
“A larger genome may allow the virus to encode more genes or protein domains, potentially expanding or enhancing its ability to interact with hosts,” Zhong said in the news release. “This discovery offers a rare window into the possible evolutionary mechanisms that enable genome expansion in RNA viruses.”
The genetic makeup of the virus is so distinct from that of other nidoviruses that the research team has recommended the creation of a new family, Megarnaviridae, referred to as “large RNA viruses.” The researchers are also suggesting that PONV1 be named Megarnavirus gigas, which translates to “large RNA virus giant.”
These sizable RNA viruses seem to be exclusive to oysters, meaning that humans are not in danger of becoming infected with the virus, Suttle said.
The findings underscore the importance of caution among oyster farmers when transferring juvenile oysters, because there is still much to learn about the causes of disease in bivalve molluscs, he added.
The scientists also stressed the importance of conducting more research to better grasp the association between the virus and mortality in Pacific oysters, and are advocating for ongoing surveillance of oyster populations.
“This research is not a cause for alarm,” said Suttle. “Rather, this is a meaningful step forward in advancing our understanding of oyster health and supporting the long-term sustainability of shellfish aquaculture.”
