Heart inflammation and piscine orthoreovirus genotype-1 in Pacific Canada Atlantic salmon net-pen farms: 2016–2019

Abstract

Piscine orthoreovirus genotype-1 (PRV-1) is a virus commonly associated with Atlantic salmon aquaculture with global variability in prevalence and association with disease. From August 2016 to November 2019, 2,070 fish sampled at 64 Atlantic salmon net-pen farm sites during 302 sampling events from British Columbia, Canada, were screened for PRV-1 using real-time qPCR. Nearly all populations became PRV-1 positive within one year of seawater entry irrespective of location, time of stocking, or producer. Cohorts became infected between 100–300 days at sea in > 90% of repeatedly sampled sites and remained infected until harvest (typically 500–700 days at sea). Heart inflammation, which is sometimes attributed to PRV-1, was also assessed in 779 production mortalities from 47 cohorts with known PRV status. Mild heart inflammation was common in mortalities from both PRV + and PRV- populations (67% and 68% prevalence, respectively). Moderate and severe lymphoplasmacytic heart inflammation was rare (11% and 3% prevalence, respectively); however, mainly arose (66 of 77 occurrences) in populations with PRV-1. Detection of PRV-1 RNA was also accomplished in water and sediment for which methods are described. These data cumulatively identify that PRV-1 ubiquitously infects farmed Atlantic salmon in British Columbia during seawater production but only in rare instances correlates with heart inflammation.

British Columbia freshwater salmon hatcheries demonstrate minimal contribution to piscine orthoreovirus (PRV) regional occurrence with no evidence for nonendemic strain introductions

“…It therefore does not appear that freshwater Atlantic salmon facilities used to seed regional seawater net-pens have played a meaningful role in PRV-1 persistence in the region in recent years even though they may have acted as a reservoir historically within the region. Further, given that some Atlantic salmon producers rely on PRV-1 infected broodstock for gametes (Polinski et al 2022), it also appears that current iodine-based disinfection strategies employed by the industry are effective at deterring egg-associated transmission of PRV-1 as previously suggested (Polinski et al 2020).…”

“…Sixty fish were targeted in each instance to ensure 95% confidence in identifying at least 5% prevalence of PRV at the population level using the USDA APHIS animal sample size calculator (https://www.aphis.usda.gov) assuming 95% diagnostic test sensitivity based on performance of similar qPCR assays (Polinski et al 2021a;Yang et al 2022;Delphino et al 2023). Fish were sampled from the general population at all locations as neither morbidity nor outward visual appearance/behavior have shown correlation with PRV infection prevalence or load (Takano et al 2016;Wessel et al 2017;Polinski et al 2019Polinski et al , 2022Marty et al 2020). Samples were collected at all 12 licensed commercial freshwater salmon facilities in British Columbia: 9 that exclusively rear Atlantic salmon and 3 that exclusively rear Chinook salmon (Supplemental material 1).…”

“…At DFO-PBS, nucleic acids were purified from tissues either manu-ally using Trizol (Thermo-Fisher Scientific; 448 samples) or via a semiautomated process involving a Kingfisher Flex and MagMAX™ Pathogen RNA/DNA kit (Thermo-Fisher Scientific; 4771 samples). Eluted RNA (up to 1 μg) from either method was then reverse transcribed using a High-Capacity cDNA Reverse Transcription Kit (Applied Biosystems) and screened for PRV-1 (Polinski et al 2022). A detailed protocol for nucleic acid extraction, reverse transcription, and PRV-1 qPCR screening is provided in Supplementary material 2.…”

“…However, occurrence of PRV-1a (or any other genotype/subgenotype) in freshwater juvenile anadromous salmon within the region is far less clear. It is apparent that nearly all net-pen farmed salmon in British Columbia, Canada, become infected with PRV-1 within the first 18 months at sea (Polinski et al 2022), and cumulative data from multiple wild Pacific salmon surveys have identified an overall regional prevalence in ocean-caught Chinook and coho salmon between 2013 and 2019 of approximately 6% (Polinski et al 2020). Historically, there has been at least some evidence that PRV-1 has occurred in commercial Canadian salmon hatcheries rearing juvenile fish (Marty et al 2015;Garver et al 2016), although distribution and temporal changes in prevalence within freshwater facilities are unknown.…”

Response to “Assessing the role of Piscine orthoreovirus in disease and the associated risk for wild Pacific salmon”

“…Indeed, we would have had difficulty choosing a salmon virus with a more supportive literature for low virulence than the PRV isolate we used. Specifically, our PRV isolate (i) has not been known to cause disease in sockeye salmon even under circumstances of persistent high-load systemic infection, host saltwater adaptation, or viral co-infection [ 3 , 8 ], (ii) has failed to cause disease in at least 4 other related salmon species in multiple saltwater and freshwater challenge trials conducted in the region [ 9 – 12 ], (iii) has shown minimal association (< 0.3%) and lack of direct causation for morbidity or mortality in commercial Atlantic salmon net-pen production in its endemic region [ 11 , 13 ], (iv) is considered of low virulence to Norwegian Atlantic salmon and less impactful relative to endemic PRV in that region [ 14 ], and (v) has failed to induce respiratory consequences in Atlantic salmon [ 10 ] similar to what we identified in Sockeye. In consideration of cumulative evidence and literature, a formal risk assessment commissioned by the government of Canada has further concluded the risks posed to wild sockeye salmon from regionally endemic PRV used in our study to be less than minimal [ 15 ].…”