Indications for a vertical transmission pathway of piscine myocarditis virus in Atlantic salmon (Salmo salar L.)

Losses due to cardiomyopathy syndrome (CMS) keep increasing in salmon‐producing countries in the North‐Atlantic. Recently, Piscine myocarditis virus (PMCV) has been detected in post‐smolts shortly after sea‐transfer, indicating a possible carry‐over from the hatcheries. In addition, there are reports of prevalences of PMCV as high as 70%–90% in certain groups of broodfish, and a recent outbreak of CMS in the Faroe Islands has been linked to the importation of eggs from a CMS‐endemic area. Thus, there is a need to investigate whether PMCV can be transmitted vertically from infected broodstock to their progeny. In the present study, samples from eggs, larvae, fingerlings and presmolt originating from PMCV‐positive broodstock from two commercial Atlantic salmon producers were tested for PMCV. The prevalence of PMCV in the broodstock was 98% in the hearts, 69% in the roe and 59% in the milt. Piscine myocarditis virus was detected in all stages of the progeny until and including the 40 g stage. Piscine myocarditis virus was also detected in presmolt sampled for tissue tropism. This provides farmers with several options for minimizing the risk of transfer of PMCV from broodstock to progeny, including screening of broodstock and aiming to use only those that are negative for PMCV or have low levels of virus.     

Isolation and structural characterization of hepatic metallothionein from the roach (shape Rutilus rutilus L.)

Metallothioneins (MTs), considered as metal biomarkers by many authors, are very conserved proteins that present great interests in phylogenetical studies. This paper reports the purification of the hepatic MT from the roach (Rutilus rutilus). After extensive reverse-phase HPLC, the sequence of the MT was established by a combination of techniques including automated Edman degradation, mass spectrometry and enzymatic treatment. Sequence comparison with other fishes or mammalian MTs showed considerable identities and a high conservation in cysteine residues positions. MTs of roach and goldfish that both belong to the same family (Cyprinidae) share 97% identity. Ninety five% identity were found between roach and stone loach (Cypriniforms, Cobitidae) MTs. Only 77% identity were found with pleuronectiforms or salmoniforms. This sequence brings a new element in the study of the phylogenetical relationships among teleost species.     

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

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.     

Piscine reovirus,but not Jaundice Syndrome,was transmissible to Chinook Salmon,Oncorhynchus tshawytscha (Walbaum), Sockeye Salmon,Oncorhynchus nerka (Walbaum), and Atlantic Salmon,Salmo salar L.

A Jaundice Syndrome occurs sporadically among sea‐pen‐farmed Chinook Salmon in British Columbia, the westernmost province of Canada. Affected salmon are easily identified by a distinctive yellow discolouration of the abdominal and periorbital regions. Through traditional diagnostics, no bacterial or viral agents were cultured from tissues of jaundiced Chinook Salmon; however, piscine reovirus (PRV) was identified via RT‐rPCR in all 10 affected fish sampled. By histopathology, Jaundice Syndrome is an acute to peracute systemic disease, and the time from first clinical signs to death is likely <48 h; renal tubular epithelial cell necrosis is the most consistent lesion. In an infectivity trial, Chinook Salmon, Sockeye Salmon and Atlantic Salmon, intraperitoneally inoculated with a PRV‐positive organ homogenate from jaundiced Chinook Salmon, developed no gross or microscopic evidence of jaundice despite persistence of PRV for the 5‐month holding period. The results from this study demonstrate that the Jaundice Syndrome was not transmissible by injection of material from infected fish and that PRV was not the sole aetiological factor for the condition. Additionally, these findings showed the Pacific coast strain of PRV, while transmissible, was of low pathogenicity for Atlantic Salmon, Chinook Salmon and Sockeye Salmon.     

Monitoring infection with Piscine myocarditis virus and development of cardiomyopathy syndrome in farmed Atlantic salmon (Salmo salar L.) in Norway

An epidemiological study was carried out in Norway in 2015–2018, investigating the development of infection with Piscine myocarditis virus (PMCV) and development of cardiomyopathy syndrome (CMS) in farmed Atlantic salmon. Cohorts from 12 sites were followed and sampled every month or every other month from sea transfer to slaughter. PMCV was detected at all sites and in all sampled cages, and fish in six sites developed clinical CMS. The initial infection happened between 1 and 7 months post‐sea transfer, and the median time from infection with PMCV until outbreak of CMS was 6.5 months. Generally, fish from sites with CMS had higher viral titre and a higher prevalence of PMCV, compared to sites that did not develop clinical CMS. The virus persisted until the point of slaughter at most (11 out of 12) of the sites. The detection of PMCV in all sites suggests that PMCV is more widespread than previously known. Screening for PMCV as a tool to monitor impending outbreaks of CMS must be supported by observations of the health status of the fish and risk factors for development of disease.     

Prevalence of piscine orthoreovirus and salmonid alphavirus in sea‐caught returning adult Atlantic salmon (Salmo salar L.) in northern Norway

Heart and skeletal muscle inflammation (HSMI) caused by piscine orthoreovirus (PRV) and pancreas disease (PD) caused by salmonid alphavirus (SAV) are among the most prevalent viral diseases of Atlantic salmon farmed in Norway. There are limited data about the impact of disease in farmed salmon on wild salmon populations. Therefore, the prevalence of PRV and SAV in returning salmon caught in six sea sites was determined using real‐time RT‐PCR analyses. Of 419 salmon tested, 15.8% tested positive for PRV, while none were positive for SAV. However, scale reading revealed that 10% of the salmon had escaped from farms. The prevalence of PRV in wild salmon (8%) was significantly lower than in farm escapees (86%), and increased with fish length (proxy for age). Sequencing of the S1 gene of PRV from 39 infected fish revealed a mix of genotypes. The observed increase in PRV prevalence with fish age and the lack of phylogeographic structure of the virus could be explained by virus transmission in the feeding areas. Our results highlight the need for studies about the prevalence of PRV and other pathogens in Atlantic salmon in its oceanic phase.     

Inactivated Piscine orthoreovirus vaccine protects against heart and skeletal muscle inflammation in Atlantic salmon

Heart‐ and skeletal muscle inflammation (HSMI) caused by infection with Piscine orthoreovirus (PRV) is one of the most common viral diseases in farmed Atlantic salmon (Salmo salar) in Norway, and disease outbreaks have been reported in most countries with large‐scale Atlantic salmon aquaculture. Currently there is no vaccine available for protection against HSMI, partly due to the lack of a cell line for efficient virus propagation. Erythrocytes are the primary target cells for PRV in vivo and a potential source for isolation of PRV particles. In this study, PRV was purified from infected erythrocytes, inactivated and used in a vaccination trial against HSMI. A single immunization with adjuvanted, inactivated PRV induced protection against HSMI in Atlantic salmon infected by virus injection 6 weeks later, while a moderate protection was obtained in fish infected through natural transmission, i.e. cohabitation. The PRV vaccine significantly reduced PRV loads and histopathological lesions typical for HSMI compared to the unvaccinated control group. This is the first demonstration of protective vaccination against PRV, and promising for future control of HSMI in Atlantic salmon aquaculture.