Scientists Discover Mega RNA Virus Linked to Pacific Oyster Die-Offs

In a groundbreaking discovery, scientists have identified a previously unknown mega RNA virus in farmed Pacific oysters, potentially shedding light on mysterious mass die-offs that have plagued aquaculture operations in British Columbia and beyond.

The virus, named Pacific Oyster Nidovirus 1 (PONV1), was detected in 20 out of 33 dead or dying oysters collected during a 2020 mortality event. Notably, the virus was absent in healthy wild oysters from nearby sites, suggesting a strong link between PONV1 and oyster deaths.

What makes this virus extraordinary is its genome size—one of the largest ever recorded for an RNA virus. Researchers believe this expanded genetic blueprint may allow the virus to encode more proteins, enhancing its ability to infect and manipulate host organisms.

PONV1 belongs to a newly proposed viral family called Megarnaviridae, or “large RNA viruses.” It is only the second nidovirus ever found in bivalves, a group that includes clams, mussels, and oysters. Nidoviruses are known to infect a wide range of animals, including humans—SARS-CoV-2, the virus behind COVID-19, is also a nidovirus.

Despite its genetic similarity to other nidoviruses, PONV1 appears to be specific to oysters, posing no known risk to humans. However, its discovery has raised alarms in the aquaculture industry, where oyster farmers often move juvenile oysters across regions and borders. Experts warn that such practices could inadvertently spread the virus and trigger new outbreaks.

Global genetic databases have revealed 15 related viruses in oysters from Europe and Asia, indicating that these pathogens may be widespread but underreported. While no mortality has been linked to those strains yet, the findings underscore the urgent need for enhanced biosecurity and viral surveillance in shellfish farming.

This discovery not only advances our understanding of oyster diseases but also challenges existing theories about RNA virus evolution. Scientists say it opens a rare window into how viral genomes can expand and adapt—potentially reshaping how we approach disease prevention in marine ecosystems.