Q&A with Grieg’s veterinarian Dr. Patrick Whittaker – the PRV debate


Dr. Patrick Whittaker has been Grieg Seafood BC’s veterinarian since 2016. He graduated from the Western College of Veterinarian Medicine in Saskatoon in 2004 and practiced in both Alberta and New Zealand.

Although he began his vet experience with cows and other farm animals, Dr. Whittaker’s relationship with salmon goes back to his childhood, as his dad was an avid fresh and saltwater sport fisherman on Vancouver Island. Dr. Whittaker grew up in the Comox Valley and loves to fish in his spare time.

We spoke to Dr. Whittaker about his research and findings on BC’s strain of Piscine orthoreovirus (known as PRV-1a) since he began with Grieg.


  1. What is PRV-1a?

Piscine orthoreovirus (PRV) is a common virus of salmon and can infect both Atlantic and Pacific salmonids. The vast majority of infections do not result in disease. Science has shown most of the time that the virus is contained within the host fish, it doesn’t transmit to infect other fish/organisms. Research suggests infected salmon may test positive but are not always infectious. There is a narrow time window when a salmon may transmit PRV-1a to other salmon, but outside of that window, it will not impact other fish. For example, at our Barnes Bay farm, our fish were infected in 2019 and now in 2020 currently test positive, but we know that they will not infect the wild salmon outmigration this year.


  1. Was PRV-1a brought to the Pacific Ocean from the Atlantic? What is the history of the virus?

The oldest known detection of PRV-1a was from BC wild Pacific salmon from 1977, more than a decade before fish farming began in BC.  A variety of theories are out there as to the origins of PRV-1a, but one theory is related to all the stocking of Atlantic Salmon and Brown trout by the provincial government nearly a century ago when they were trying to establish recreational fishing.


  1. Good animal husbandry requires that salmon farmers need to decrease the transfer of pathogens between wild and farmed salmon, but what is your opinion on the transfer of PRV-1a and its impact on both wild and farmed salmon?

All our salmon are stocked into the ocean pens without PRV-1a.  Like all farm animals living in the natural environment, they are exposed to viruses existing in the water and on wild fish and become infected at some point in their time from some natural reservoir, potentially wild salmon.  There is no impact to our salmon of this infection, the only way to know is to routinely test.  Research has shown that once infected, and like many pathogens, there is an infectious phase, then a return to a latent or dormant phase.  More work needs to be done to fully understand the dynamics involved.  So far, work at Fisheries & Oceans Canada (DFO), the University of British Columbia and Washington State University’s Aquatic Health Section suggests that in the lab Pacific salmon can be experimentally infected with no consequence or effect.  Despite the absence of salmon farming in Alaska, the PRV-1a virus has been found in the river systems in that state, in addition to Washington state and British Columbia.


  1. There is a lot of speculation that salmon farmers do not screen their fish for PRV-1a and other diseases before transferring them to the ocean. Is this true?

No, it’s not true that we don’t screen for this virus. Since 2016, Grieg BC has been routinely screening every batch of smolts produced in our hatchery, as well as external contract hatcheries and supplemental smolt purchases all before they are transferred to our ocean pens.  To this date every test has been negative.


  1. If Grieg’s salmon are put into the ocean without PRV-1a, why do they test positive for it at various lengths of time after?

The marine environment is a complex place, with a diverse ecosystem.  We have determined during our testing that our farm fish become positive for PRV-1a at different points in time, and months after we place the fish in the pens.  That time period can vary, and we are not sure why. At one of our Sechelt farms, for example, our PRV-1a-free salmon were placed in pens 12 months ago and recently in 2020 we identified that they had PRV-1a.

Grieg and the other BC salmon farming companies have investigated wild fish species around their farms to determine if they are carrying PRV1-a.  To the date of this interview (February 2020), in BC the only other fish species found to have PRV1-a is Pacific salmon, so this suggests that wild salmon migrating past fish farms could be the source of PRV1-a.


  1. How often do you test your salmon for PRV-1a and other diseases once they’re placed in the farm pens? Why?

We routinely test our farm fish in our lab at minimum every three months.  If there are any changes or concerns, then we visit the source farm immediately to conduct additional in-depth testing.  At Grieg, we are involved in a variety of research projects internally as well as research collaborations with DFO scientists, and other academic institutions with similar marine research interests.  At some of our farm areas we have tested for the absence or presence of PRV-1a on a monthly basis to better pinpoint the timing of this infection.


  1. What are you seeing out at your sites right now? Are there any areas that you are testing and seeing positive results come in? Will all farmed fish eventually get PRV-1a?

So far as is our understanding, yes, all our company’s salmon will eventually become infected with PRV-1a.  DFO’s Canadian Science Advisory Secretariat’s more recent paper on PRV (https://waves-vagues.dfo-mpo.gc.ca/Library/4080530x.pdf) also discusses this infection rate for farm fish. In late summer 2019, our Nootka Sound farms became infected, while in the fall of 2019 one of three of our farms on the Sunshine Coast was identified with the PRV-1a virus.  I expect the other two will likely be infected as well.


  1. What can salmon farmers do to mitigate the transfer of PRV-1a between wild fish and farmed fish?

With continued research to identify and monitor the timing of infection of this virus on farm fish, we hope to be able to pinpoint the source of infection and potentially find ways to mitigate the spread from wild to farmed.


Dr. Patrick Whittaker is Grieg Seafood BC’s veterinarian and an avid fisherman.