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PRV not a salmon killer in B.C.: study

Independent review suggests the virus in this province might be a different strain from PRV in Norway
Review of peer reviewed literature on piscine reovirus and disease suggests the virus in B.C. could be a different strain than the one in Norway, where stronger links between the virus and disease in salmon have been established. | BIV archives

A study by Fisheries and Oceans Canada (DFO) scientists that linked piscine reovirus (PRV) to jaundice and anemia in chinook salmon has been savaged by their own government’s science secretariat.

And an independent review of all the science on PRV – a review requested by the B.C. government – finds no evidence that PRV causes disease in fish in the Pacific the way it appears to in Norway.

That suggests the virus in B.C. might be an innocuous strain.

The two reports’ findings are important because the salmon farming industry faces the threat of being shut down due to concerns about disease transmission from farmed to wild salmon.

In a response to a paper published earlier this year by a team of scientists with the Strategic Salmon Health Initiative (SSHI) that linked PRV and jaundice in chinook salmon, Canada’s Centre for Science Advice (CSA) said the conclusions are unsubstantiated.

The team of 10 scientists involved in the SSHI study included DFO scientists Kristi Miller and Emiliano Di Cicco, the lead author.

“Di Cicco et al. (2018) proposes a cause-and-effect relationship between infection with PRV and the development of jaundice/anemia yet present no direct evidence to support this,” the CSA response states.

Meanwhile, a scientific literature review by BC Centre for Aquatic Health Sciences (BCCAHS) – an independent lab – and the University of British Columbia also challenges the theory that PRV causes disease in salmon in B.C.

It concludes that PRV might not only be endemic to this province – possibly predating the arrival of fish farms – but also benign, at least in B.C.

“By all accounts, PRV in B.C. acts in a benign fashion,” it states.

The federal government requested the Di Cicco paper review as part of its defence against Alexandra Morton, an independent biologist who is suing Ottawa in Federal Court.

Morton is seeking an order declaring it illegal for the federal government to allow the transfer of farmed salmon smolts to open-net pens without first testing them for PRV and heart and skeletal muscle inflammation (HSMI).

The BCCAHS study was produced for the B.C. Ministry of Environment to address concerns about effluent from fish farm processing plants. The ministry received it at the end of March, but the study was not mentioned when the government released an audit of fish processing plants two weeks ago.

It’s a puzzling omission, given that the audit was triggered by concerns that farmed fish processing plants might be pumping PRV into the ocean.

“It’s really an innocuous virus,” said Hugh Mitchell, a veterinarian with AquaTactics Fish Health in Washington. “It very rarely causes disease … and when it does, it is always in association with something else. It is complex, and that is why statements and conclusions of untrained activists or pigeon-holed researchers in a lab do an incredible disservice.”

Jim Powell, head of the BCCAHS, said there is compelling evidence from Norwegian studies that PRV might cause HSMI in farmed Atlantic salmon in Norway and result in mortality rates as high as 20%.

But for reasons unknown, scientists have been unable to induce HSMI in salmon here, even when they develop heavy PRV viral loads.

“There’s a dotted line that people are taking as solid between PRV infection and HSMI,” he said. “In Norway, the evidence is compelling, but it’s not definitive. In B.C., or eastern Pacific waters, there isn’t even a dotted line.”

It is difficult to draw that straight line because PRV is a virus that cannot be cultivated in the lab, and HSMI produces symptoms that can be caused by other pathogens.

So the best workaround is to either inject healthy PRV-free fish with fish blood that is infected with PRV or have the fish live with infected fish.

But when salmon are exposed in this way to PRV found in B.C., they don’t develop any disease symptoms, despite some of them having very high viral loads.

And even if HSMI were induced, it could be that other pathogens in the infected blood injected into fish could also be the cause or a contributing factor.

Some studies in Norway have found that, in fish diagnosed with HSMI, PRV was always present, whereas other suspected pathogens were not, which is why such a strong link has been drawn between it and PRV. But that hasn’t been the case with the B.C. virus.

Gary Marty, fish pathologist with the provincial government Animal Health Centre, points out that there have been cases that look like HSMI in B.C. in which PRV was not present.

That means either the HSMI diagnosis was wrong or that PRV isn’t the cause, because a fundamental disease diagnosis principle is that a suspected pathogen believed to cause a disease must be present in all cases of the disease.

According to Powell, the science to date suggests PRV in B.C. might be a less virulent strain than the one that appears to cause HSMI in Norway. He added that a full genomic sequencing of the virus is needed to determine if it has two variants.

But HSMI isn’t the only disease of concern. Earlier this year, Di Cicco published a study that found a link between PRV and jaundice and anemia in chinook. It concluded: “Chinook salmon may be at more than a minimal risk from disease exposure to PRV occurring on salmon farms.”

But the Centre for Science Advice said those claims are unsubstantiated. It found “deficiencies” with the study’s methods, including an overly broad definition of jaundice.

It also said the scientists didn’t acknowledge that the symptoms of jaundice are shared with other diseases, “provided little consideration of the role of other pathogens” and failed to consider the findings of other important studies.

It concluded, “Our detailed review of this manuscript reveals deficiencies with the data presented and the criteria used to characterize jaundice disease that render the conclusions the authors draw from these data and analyses unsupported.”

Di Cicco is preparing an official response to the criticism, but it was not complete in time for Business in Vancouver’s print publication deadline. This story will be updated online.

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