The impact of piscine orthoreovirus (PRV) to salmon health, both wild and farmed, has been greatly exaggerated, and the focus on its perceived threat has become a political tool, according to an industry veterinarian.
The fear mongering that PRV elicits is out of proportion to the scope and impact of the virus, fish health expert Hugh Mitchell told IntraFish.
Mitchell's comments were in a response to the findings of a study backed by Canada's Department of Fisheries and Ocean (DFO) that said the transfer of PRV from Atlantic salmon farms poses “minimal risk” to wild Fraser River sockeye.
The report came just a few days after a Canadian federal court made it mandatory to screen for PRV before juvenile farmed salmon can be transferred from land-based smolt facilities to open net pens in British Columbia, Canada.
Mitchell agreed with the findings of the study, adding that other diseases, including infectious hematopoietic necrosis (IHN), infectious pancreatic necrosis (IPN) and infectious salmon anemia (ISA), can cause greater harm to farmed salmon, and are worthier contenders of closer scrutiny.
Veterinarian Mitchell, who is the co-owner of the Washington-based fish health consultancy AquaTactics Fish Health, has more than 25 years experience working with fish diseases and fish vaccines for both wild and farmed fish.
He joined the board directors of the Northwest Aquaculture Alliance, a trade association that represents aquaculture producers and suppliers in the Northwest region, in February this year.Excessive bad press for "disease agent"
The case that ended in the ruling mandating screening for PRV was brought by biologist Alexandra Morton, represented by Ecojustice, and the 'Namgis First Nation, who had challenged the federal policy last year.
They argued PRV was a “disease agent” and it should be illegal for salmon carrying the virus to be transferred into the ocean.
The degree of bad press surrounding PRV is surprising, Mitchell told IntraFish, as "by many standards PRV is not a pathogen" and there is no evidence that any variant of PRV-1 (the strain found in BC waters) has been involved in any massive mortalities of farmed or wild salmon anywhere in the world.
Unlike DNA viruses such as IHN or ISA, PRV-1 is an RNA reovirus that has not yet been isolated or propagated, and has not been shown to produce disease on its own, Mitchell said.
The pathology associated with PRV-infected farmed salmon are "microscopic, transitory lesions of inflammation in the heart and skeletal muscle," called HSMI.
Mortalities are rare, and always associated with additional factors such as stress or other diseases, Mitchell told IntraFish.
In most cases, the virus lives in the fish through its lifespan without any consequence, and is occasionally brought out by stress or other diseases and environmental variables.
Given its limited scope, the general media reportage surrounding PRV's presence in BC waters and its impact on farmed and wild salmon has taken on a political edge, Mitchell said.