Cross‐sectional study of histopathology and piscine orthoreovirus during a marine production cycle of farmed Atlantic salmon (Salmo salar L.) in British Columbia, Canada

Abstract

Two cohorts of farmed Atlantic salmon, Salmo salar L., in British Columbia, Canada, were sampled for histopathology (nine organs) and piscine orthoreovirus (PRV‐1) PCR after seawater entry at 2, 4, 6, 8, 10, 13, 16 and 19 months (20 fish per cohort per date). One cohort—from a PRV+ hatchery—remained PRV+ throughout the study (sample prevalence 80%–100%). In an adjacent pen, the other cohort—from a PRV− hatchery—was 0% PRV+ at 78 days, 30% PRV+ at 128 days and ≥95% PRV+ thereafter. Among sample cohorts that were ≥80% PRV+, median Ct values were nominally less among fish sourced from the PRV− hatchery (28.7–33.3) than the PRV+ hatchery (30.8–35.2). No microscopic lesions were associated with PRV Ct value (minimum = 25.6). About 3% of fish in both cohorts had moderate inflammatory heart lesions; among these fish, only one had skeletal muscle inflammation (mild), and PRV Ct values were similar to unaffected cohorts sampled the same day. Also, among 16 moribund or freshly dead fish sampled opportunistically during the study, 14 were PRV+, and none had significant inflammatory heart lesions. These data support the hypothesis that British Columbia PRV‐1 does not contribute to mortality.

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

“…Specifically, low relative quantities (for example, blood Ct values > 25 in this study) suggest that the fish was recently infected – likely within the past few weeks – whereas high relative quantities (i.e., blood Ct values < 20 in this study) suggest that individuals have probably been infected for a considerable time – i.e., a month or more [ 7 , 12 ]. In addition, this study confirms that relative PRV-1 load was not useful as a predictor of morbidity or mortality in a commercial net-pen setting which is similar to results of field study [ 3 ] and previous laboratory challenge trials [ 7 ]. This reiterates that PRV-1 load cannot be used as a disease proxy.…”

“…Of the 779 hearts subjected to histopathology, 126 (16%) included PRV PCR results as part of the case; pathologists examined the other 653 hearts while blinded to information about the PRV-1 infection status. For heart histopathology, thirteen indices were scored as either being none/not present (0), mild (1), moderate (2) or severe (3). This included the two inflammatory indices of lymphohistiocytic epicarditis (EPL) and endocarditis (ENL) that are most commonly associated with a diagnosis of HSMI; we combined these to give a cumulative heart inflammatory score (0 = none; 1-2 = mild, 3-4 = moderate, 5-6 = severe).…”

“…2 U.S. Department of Agriculture National Coldwater Marine Aquaculture Center, Portage Rd, Orono, ME 04469, USA. 3 Animal Health Centre, Ministry of Agriculture and Food, 1767 Angus Campbell Rd, Abbotsford V3G2M3, Canada.…”

“…Nevertheless, several instances of HSMI-like heart inflammation have been identified on Atlantic salmon farms in British Columbia [ 6 , 7 ]. In both instances, affected populations were PRV-1 positive; yet it is unclear as to whether PRV-1’s presence was tangential or integral to these HSMI-like occurrences [ 3 , 7 ]. Disease causation has been confounded in British Columbia in part by a lack of disease transmissibility [ 2 , 7 , 12 ] and in part by the fact that occasionally Atlantic salmon with HSMI-like heart inflammation have been identified without detectable PRV-1 infections [ 14 ].…”

“…Piscine orthoreovirus genotype-1 (PRV-1) has commonly been detected among net-pen farmed Atlantic salmon of British Columbia, Canada for more than a decade [1]. Transfer of PRV-1 infected fish from freshwater hatcheries has likely accounted for at least some historical seawater detections owing to the long-term persistence of the virus in infected individuals and the historic identification of PRV-1 at some freshwater facilities [2][3][4]. Nevertheless, PRV-1 detection in commercial Atlantic salmon freshwater facilities has declined in recent years despite increased screening [5] and PRV-1 has not been reliably detected during Fisheries and Oceans Canada audits of British Columbia freshwater Atlantic salmon facilities since 2018 (Polinski, unpublished data).…”