A previously unknown virus in farmed Pacific oysters may hold the key to mass die-offs in the multimillion-dollar industry, according to research from the University of British Columbia (UBC) in Vancouver, Canada.
"We've recently seen annual mass die-offs in B.C. and elsewhere of Pacific oysters," Kevin Zhong, a research associate in UBC's Department of Earth, Ocean, and Atmospheric Sciences, said. "Often, we don't know the cause."
Pacific oysters are the primary shellfish species grown in B.C., accruing an estimated commercial value of USD 16 million (EUR 13.7 million) in 2023, but the mass die-offs were putting a dent in that industry’s profitability. Now, the researchers said they may have identified the reason behind the annual event.
Researchers collected 33 oysters from two farms in B.C. during a mass die-off in 2020, along with 26 wild oysters from 10 nearby sites. Analysis of their RNA revealed the presence of a previously unknown virus, now dubbed Pacific Oyster Nidovirus 1 (PONV1), in 20 of the dead and dying farmed oysters but not in healthy wild oysters, suggesting a link between the virus and mortality.
"The discovery of a previously unknown RNA virus was not the original objective but, rather, an unexpected and significant outcome of this comprehensive investigation," Zhong said. "Ultimately, the presence of this novel virus in diseased and dying oysters from farms, combined with its absence in healthy oysters from nearby naturalized beach areas, suggested a strong link between the virus and the observed oyster die-offs."
RNA, or ribonucleic acid, is a crucial molecule that acts as a messenger, carrying genetic information from DNA to the ribosomes – cellular structures responsible for protein synthesis.
"Interestingly, through analysis of publicly available genetic datasets from oysters around the world, we found that this virus is also present in Pacific oysters from various locations in China and France," Zhong said. "This suggests that the virus is geographically widespread. However, in these cases, its presence was not associated with any reported mass mortality events."
Nidoviruses like this one are found in a wide range of animals, including humans. The nidovirus SARS-CoV-2, for example, is the source of the Covid-19 pandemic.
The newly discovered virus is different genetically from other nidoviruses, enough so that the research team has proposed a new family: Megameaviridae, or large RNA viruses. PONV1, which the team is proposing to name Megamavirus gigas, or large RNA virus giant, and its relatives appear to be specific to oysters, so humans are not at risk from contracting the virus, Curtis Suttle, a UBC professor of Earth, ocean, and atmospheric sciences, botany, microbiology, and immunology, said.
"The virus itself appears to specifically infect Pacific oysters and not other species of oysters, and there is no evidence to suggest that this virus poses any risk to people or other animals who may consume these oysters," Zhong added.
The virus can be traced back to Pacific oyster samples from China included in a study published in 2012.
"This suggests that the virus has been present in oyster populations for over a decade but remained undetected until now," Zhong said.
Given that the illness has been present for at least a decade, the next question is what has made it suddenly potentially deadly for Pacific oysters on farms. Zhong said determining whether changing climate conditions such as warming ocean temperatures are a contributing factor is the next question – but other evidence points to a correlation.
"While further research is needed to establish a direct link, environmental stressors like rising temperatures have been shown to influence disease dynamics in oysters, as seen in past disease outbreaks due to other microbial pathogens such as Vibrio spp. and Perkinsus marinus,” Zhong said. “It's possible that similar stressors could play a role in facilitating the emergence or impact of this virus, and this is an area that warrants further investigation."
Suttle said that investigation is important so that future outbreaks can be prevented.
"Similar to humans, disease and death are not caused by a single factor, but prevention is not possible until the causes are known,” Suttle said.
Regardless of the cause for virus becoming deadly, its discovery serves as a reminder to B.C. oyster farmers to minimize movement of the species if possible. Growers in the region, like growers elsewhere, often import oyster seed and juvenile oysters, or spat, from domestic and international hatcheries.
"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 cause of disease and mortality,” Suttle said.
Zhong further explained the research began in the summer of 2020 and is still ongoing.
"We hope to continue and expand this work if we secure funding," he said.
Reporting by Margaret Bauman
