Farmed Atlantic salmon likely passed the virus on to wild Pacific salmon

P.specific salmon (Oncorhynchus spp.) are important to the fishing industry, indigenous peoples and endangered local populations of killer whales (Orcinus orca), but some species of salmon have declined to extinction. Farmed salmon are widely used to meet the demand for fish consumption, but aquaculture is a known contributor to diseases that infect wild populations. In a study published on May 26 in Advances in science, Researchers show that piscine orthoreovirus (PRV) – various strains that cause anemia, jaundice, cardiomyopathy, and death in fish – was likely transmitted from farmed Atlantic salmon to wild Pacific salmon (Salmo Salar) in the late 1980s.

“The paper convincingly shows that there is repeated exchange or transmission between wild and domesticated species,” says Martin Krkosek, an ecologist at the University of Toronto who was not involved in the work. “We went through the last year of COVID, most likely due to intensification [of interactions] between human and wildlife populations and global transport of the virus. The same two interaction scales are highlighted in this article, ”he adds. “We are talking about fish in this article, but I think it speaks for the broader issue of how humans interact with the natural environment and how we create opportunities for infectious diseases, humans, our health, and in this case our natural environment and ours Food production systems. “

At the beginning of his postdoc at the University of British Columbia, Gideon Mordecai initially worked more extensively on the discovery of salmon viruses. After hearing other researchers at conferences on salmon farming and health, there was “no evidence of virus transmission between salmon farms and wild salmon,” he recalls. “And those kinds of statements always annoyed me because I was like, ‘Okay, maybe there is no evidence, but that doesn’t mean it won’t happen.'”

He worked with Kristi Miller-Saunders, director of the salmon genetics program at Pacific Biological Station in Canada, and others who use genome sequencing to study how viruses move between different populations. In the new study, researchers used similar strategies to track the movement of PRV through salmon populations.

A global transmission map of the Piscine Orthoreovirus (PRV). In addition to likely being transmitted from Europe to North America in the 1980s, the virus appears to have moved to Chile, where salmon farming is popular but there are no native species. A strain of the virus also likely moved to Iceland from Norway, where Atlantic salmon are raised and found in the wild.

SCI ADV, DOI: 10.1126 / sciadv.abe2592, 2021.

They sequenced the genomes of 392 PRV strains isolated from wild and farmed salmon around the world between 1988 and 2018. Based on their phylogenetic analyzes, the team estimated that a strain seen in the Northeast Pacific, PRV-1a, deviated from the Atlantic form in 1989. This indicated that the strain was introduced to the Pacific in the late 1980s, possibly with the importation of Atlantic salmon eggs from Europe for salmon farming at the time.

Researchers also looked at the prevalence of PRV in Atlantic salmon on farms in the northeast Pacific, most of which are along wild salmon migration routes in the Pacific. Most farmed salmon live in net pens that allow water to be freely exchanged with the surrounding ocean. The team found that almost all of the farmed salmon they tested were infected with PRV. The researchers then evaluated the infection rates in wild Pacific salmon and found that the closer the wild fish were to aquaculture sites, the more likely they were to be infected with PRV. The results suggest that PRV infections are much higher in farmed fish than in wild fish, that freshwater hatcheries can be a source of PRV, and that the main direction of transmission is from farmed fish to wild salmon.

The study describes “something that has. . . previously suspected: that PRV on the west coast of Canada and the United States [comes from] Atlantic salmon farm that got there around the 1980s, ”says Espen Rimstad, a fish virologist at the Norwegian University of Life Sciences who did not participate in the work. “When you have a virus. . . In the case of the Atlantic salmon, it can be very different with the Pacific salmon, ”he adds. “A natural consequence would be an experimental infection with different PRV variants. . . and see how they behave with the Pacific salmon species. “

Pacific salmon populations have been declining since the early 1990s. Research has pointed to a wide variety of likely culprits, including climate change, habitat destruction, and overfishing. To what extent PRV and other diseases played a role is still open.

“There are many reasons why salmon populations have declined over the past few decades,” admits Mordecai. “I’m not saying viruses rule the world and do everything. But it’s one thing that we control because we are farming. “

Researchers suspect that many other pathogens have taken a similar path. “PRV is just the tip of the iceberg,” says Krkosek. “It is one of dozens, if not hundreds, of types of viral and bacterial pathogens that we believe can be passed back and forth between farmed and wild salmon,” he adds. “Trying to find out which ones are likely to show up and cause problems in either the fish farms or the wild salmon is a real problem.”

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