Science Advisory Report 2019/022

Advice from the assessment of the risk to Fraser River Sockeye Salmon due to piscine orthoreovirus (PRV) transfer from Atlantic Salmon farms in the Discovery Islands area, British Columbia

Summary

Piscine Orthoreovirus (PRV-1) Transfer Risk Assessment

  • PRV-1 released from Atlantic Salmon (Salmo salar) farms operating in the Discovery Islands area was assessed to pose minimal risk to Fraser River Sockeye Salmon (Oncorhynchus nerka) abundance and diversity. The uncertainties ranged from high certainty to high uncertainty (see below).
  • The assessment was conducted using relevant scientific information related to piscine orthoreovirus (PRV-1), including farm-related fish health data, current fish health management practices, surveys and studies from enhanced and wild salmon in British Columbia.
  • The risk assessment was conducted based on the findings that all active farms likely become positive for PRV-1 and Fraser River Sockeye Salmon are susceptible to PRV-1 infection.
  • The risk assessment assumed:
    • continuous PRV-1 shedding from farms, that the exposure time and dose are sufficient to cause infection in at least one Fraser River Sockeye Salmon when they migrate through the Discovery Islands; and
    • that results for laboratory studies on the impact of PRV-1 infection in juvenile Sockeye Salmon are indicative of what occurs in marine environments.
  • The overall Likelihood Assessment concluded that at least one Fraser River Sockeye Salmon, at either the juvenile or adult stage, becoming infected with PRV-1 attributable to Atlantic Salmon from in Discovery Islands area is very likely with the uncertainties for the different steps ranging from high certainty to high uncertainty.
  • The Consequence Assessment concluded that the potential magnitude of consequences to Fraser River Sockeye Salmon abundance and diversity is negligible with reasonable certainty for juveniles and reasonable uncertainty for adults. The levels of the uncertainty of this conclusion were discussed and participants came to different conclusions on the applicability and abundance of the data to support uncertainty estimates.
  • The main uncertainties in this risk assessment are:
    • the high uncertainty related to the likelihood of infection of Fraser River Sockeye Salmon with PRV-1 from infected Atlantic Salmon farms given the lack of data to estimate the concentration of PRV-1 attributable to infected Atlantic Salmon farms, and given that the exposure duration required for infection with PRV-1 to occur and the minimum PRV-1 infectious dose for Sockeye Salmon are unknown; and
    • the reasonable uncertainty related to the consequence assessment given the applicability of proxy data and laboratory studies to estimate consequences for adult Sockeye Salmon.

This risk assessment was informed by a summary of the current state of knowledge related to piscine orthoreovirus (PRV) and associated diseases (Polinski and Garver, 2019) of which the key elements are summarized below.

Characterization of Piscine Orthoreovirus (PRV) and Associated Diseases

  • Of the three known PRV genogroups, PRV-1 occurs in BC, Norway and other areas. The variant of PRV-1 found in British Columbia is the subject of the risk assessment.
  • Data from Norway suggests there is variation in the virulence among strains of PRV-1. BC PRV-1 has less genetic variability and less virulence for Atlantic Salmon than Norwegian PRV-1.
  • PRV-1 is ubiquitous and highly prevalent in marine net-pen farmed Atlantic and Chinook salmon of British Columbia. PRV-1 has a wide geographic distribution among wild Pacific salmon (BC, Alaska and Washington) but at a lower prevalence than farmed salmon and with species/stock-specific variation.
  • Evidence of infection for PRV-1 relies on the use of molecular methods which detect the presence of PRV-1 genetic material; however, this does not indicate infective PRV-1.
  • PRV-1 infects red blood cells. In laboratory challenge trials with juvenile Atlantic or Sockeye salmon, high loads of PRV-1 have been reported. However, it was not predictive of development of disease.
  • Farmed and wild salmon of British Columbia appear most likely to become infected with PRV-1 in seawater, although infections in juvenile salmon have been reported in freshwater.
  • Sockeye Salmon appear less susceptible to infection relative to Atlantic Salmon in British Columbia following experimental exposure.
  • In marine net-pens, PRV-1 has been associated with severe heart inflammation in farmed Atlantic Salmon and jaundice/anemia syndrome in farmed Chinook Salmon in British Columbia; but a causal relationship has not been established.
  • In laboratory challenge trials with juvenile Atlantic Salmon, when high viral loads were generated, the BC variant of PRV-1 enhanced prevalence of minor to moderate heart lesions, without any fish mortality, clinical signs or anaemia.
  • In four laboratory challenge trials with juvenile Sockeye Salmon, high viral loads of PRV-1 were generated without any fish mortalities, clinical signs or anaemia. The histopathology results could not be attributed to PRV-1.
  • No impairment of respiratory function has been demonstrated in BC PRV-1 infected juvenile Atlantic or Sockeye salmon under experimental conditions.
  • Based on observations described above, current evidence does not support the conclusion that BC PRV-1 causes disease or mortality in Sockeye Salmon.

This Science Advisory Report is from the January 28-30, 2019 National Peer Review Meeting on Assessment of the risk to Fraser River Sockeye Salmon due to piscine orthoreovirus (PRV) transferred from Atlantic Salmon farms located in the Discovery Islands area, British Columbia. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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