Quality of raw and smoked fillets from clinically healthy Atlantic salmon, Salmo salar L., following an outbreak of pancreas disease (PD)

Effect of pancreas disease vaccines on infection levels and virus transmission in Atlantic salmon (Salmo salar) challenged with salmonid alphavirus, genotype 2

Salmonid alphavirus (SAV) causes pancreas disease (PD), which negatively impacts farmed Atlantic salmon. In this study, fish were vaccinated with a DNA-PD vaccine (DNA-PD) and an oil-adjuvanted, inactivated whole virus PD vaccine (Oil-PD). Controls were two non-PD vaccinated groups. Fish were kept in one tank and challenged by cohabitation with SAV genotype 2 in seawater. Protection against infection and mortality was assessed for 84 days (Efficacy study). Nineteen days post challenge (dpc), subgroups of fish from all treatment groups were transferred to separate tanks and cohabited with naïve fish (Transmission study 1) or fish vaccinated with a homologous vaccine (Transmission study 2), to evaluate virus transmission for 26 days (47 dpc). Viremia, heart RT-qPCR and histopathological scoring of key organs affected by PD were used to measure infection levels. RT-droplet digital PCR quantified shedding of SAV into water for transmission studies. The Efficacy study showed that PD associated growth-loss was significantly lower and clearance of SAV2 RNA significantly higher in the PD-DNA group compared to the other groups. The PD-DNA group had milder lesions in the heart and muscle. Cumulative mortality post challenge was low and not different between groups, but the DNA-PD group had delayed time-to-death. In Transmission study 1, the lowest water levels of SAV RNA were measured in the tanks containing the DNA-PD group at 21 and 34 dpc. Despite this, and irrespective of the treatment group, SAV2 was effectively transmitted to the naïve fish during 26-day cohabitation. At 47 dpc, the SAV RNA concentrations in the water were lower in all tanks compared to 34 dpc. In Transmission study 2, none of the DNA-PD immunized cohabitants residing with DNA-PD-vaccinated, pre-challenged fish got infected. In contrast, Oil-PD immunized cohabitants residing with Oil-PD-vaccinated, pre-challenged fish, showed infection levels similar to the naïve cohabitants in Transmission study 1. The results demonstrate that the DNA-PD vaccine may curb the spread of SAV infection as the DNA-PD vaccinated, SAV2 exposed fish, did not spread the infection to cohabiting DNA-PD vaccinated fish. This signifies that herd immunity may be achieved by the DNA-PD vaccine, a valuable tool to control the PD epizootic in farmed Atlantic salmon.

Effects of a DNA and multivalent oil-adjuvanted vaccines against pancreas disease in Atlantic salmon (Salmo salar) challenged with salmonid alphavirus subtype 3

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Efficacy of a DNA- and conventional vaccines against pancreas disease is compared.

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Higher neutralization antibody levels in the DNA vaccine group compared to controls.

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Significantly lower viremia levels in the DNA vaccine group than the controls.

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Efficacy against disease-induced growth loss and damage in target organs is shown .

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Mortality levels low and not significantly different from the control group.

Salmonid alphavirus (SAV) causes pancreas disease (PD) in Atlantic salmon (Salmo salar). In seawater-farmed salmonids in the southern part of Norway SAV subtype 3 (SAV3) is dominating. PD continues to cause significant economic and fish health concerns in this region despite years of extensive use of oil-adjuvanted vaccines (OAVs) containing inactivated whole virus (IWV) antigens. In the current study, three commercially available PD vaccines were tested. Group A got a DNA vaccine (DNAV) injected intramuscularly (i.m.) plus an OAV without a PD component injected intraperitoneally (i.p.). Groups B and C got different OAV IWV vaccines injected i.p., respectively. The control group was i.p. injected with saline. Approximately 12 weeks after vaccination, the post smolt groups were challenged in seawater with SAV3 by cohabitation. Samples were collected pre-challenge, and at 19, 54 and 83 days post-challenge (dpc). There were no differences in growth or visible intraperitoneal side effects between the immunized groups prior to challenge. Fish in group A had significantly higher SAV3 neutralizing antibody titers than the other groups pre-challenge and significantly lower SAV3 viremia levels than the control group at 19 dpc. Fish in group A had significantly more weight gain than the other groups measured at 54 and 83 dpc. Prevalence and severity of heart necrosis at 19 dpc and loss of exocrine pancreas tissue at 54 and 83 dpc were significantly lower in groups A and B compared to group C and controls. The cumulative mortality in the control group during the challenge period was 10.5%. Group A experienced the lowest mortality (6.4%) albeit not statistically different from the controls. The results suggest that DNAV may reduce the clinical and economic impact of PD by improved protection against SAV3-induced changes in pancreas tissue and growth impairment.

Effect of a novel DNA vaccine against pancreas disease caused by salmonid alphavirus subtype 3 in Atlantic salmon (Salmo salar)

Pancreas disease (PD) caused by salmonid alphavirus subtype 3 (SAV3) is a serious disease with large economic impact on farmed Norwegian Atlantic salmon production despite years of use of oil-adjuvanted vaccines against PD (OAVs). In this study, two commercially available PD vaccines, a DNA vaccine (DNAV) and an OAV, were compared in an experimental setting. At approximately 1040° days (dd) at 12 °C post immunization, the fish were challenged with SAV3 by cohabitation 9 days after transfer to sea water. Sampling was done prior to challenge and at 19, 54, and 83 days post-challenge (dpc). When compared to the OAV and control (Saline) groups, the DNAV group had significantly higher SAV3 neutralizing antibody titers after the immunization period, significantly lower SAV3 viremia levels at 19 dpc, significantly reduced transmission of SAV3 to naïve fish in the latter part of the viremic phase, significantly higher weight gain post-challenge, and significantly reduced prevalence and/or severity of SAV-induced morphologic changes in target organs. The DNAV group had also significantly higher post-challenge survival compared to the Saline group, but not to the OAV group. The data suggest that use of DNAV may reduce the economic impact of PD by protecting against destruction of the pancreas tissue and subsequent growth impairment which is the most common and costly clinical outcome of this disease.

Correlates of protective immunity for fish vaccines

Vaccination is one of the most effective disease control strategies that has contributed to the significant reduction of disease outbreaks and antibiotics usage in salmonid aquaculture. To date, licensing of fish vaccines is to a limited extent based on in vitro correlates of protection, as done for many mammalian vaccines. This is because the immunological mechanisms of vaccine protection have not been clearly elucidated for most fish vaccines. Herein, we provide an overview of the different steps required to establish correlates of protective immunity required to serve as benchmarks in optimizing vaccine production in aquaculture. We highlight the importance of optimizing challenge models needed to generate consistent results used during vaccine development as a basis for establishing immune correlates of protection. Data generated this far shows that antibodies are potentially the most reliable correlates of protective immunity for fish vaccines. Our findings also show that antigen dose can be optimized to serve as a correlate of protection for fish vaccines. Further, there is need to establish signatures of T-cell protective immunity when antibodies fail to serve as proxies of immune protection, particularly for vaccines against intracellular pathogens. We can anticipate that documentation of efficacy for future vaccines in aquaculture, particularly batch testing will be based on in vitro correlates of protective immunity.

Increased dietary protein-to-lipid ratio improves survival during naturally occurring pancreas disease in Atlantic salmon, Salmo salar L

This study demonstrated that increased dietary protein-to-lipid ratio (P/L-ratio) improved survival of farmed Atlantic salmon naturally affected by pancreas disease (PD). In addition to diet, body weight (BW) and delousing mortality prior to the PD outbreak also contributed significantly (p < 0.05) to explain the observed variation in PD-associated mortality. Subsequent to the PD outbreak, large amount of fish failed to grow and caused thin fish with poor condition (runts). At the end of the trial, significantly (p < 0.05) lower amounts of runt fish and increased amount of superior graded fish where detected among fish fed increased P/L-ratio and within the fish with the largest BWs prior to PD. Diet, BW and delousing mortality contributed significantly (p < 0.05) to explain the variation in the amount of superior graded fish, whereas BW and diet explained the variation in the amount of runt fish. A significant (p < 0.01) negative linear relationship was observed between the amount of superior graded fish and the total mortality, whereas a positive linear relationship was detected between percentage of fillets with melanin and the total mortality. Thus, increased dietary P/L-ratio seem to reduce the mortality and impaired slaughter quality associated with PD.

Atlantic salmon adapted to seawater for 9 weeks develop a robust immune response to salmonid alphavirus upon bath challenge

Pancreas disease (PD) caused by salmonid alphavirus (SAV) is the most serious viral disease in Norwegian aquaculture. Study of the immune response to SAV will aid preventative measures including vaccine development. The innate immune response was studied in Atlantic salmon infected by either bath immersion (BI) or by intra-muscular (i.m.) injection (IM) with SAV subtype 3, two and nine weeks after seawater transfer (Phases A and B respectively). Phase A results have been previously published (Moore et al., 2017) and Phase B results are presented here together with a comparison of results achieved in Phase A. There was a rapid accumulation of infected fish in the IM-B (IM Phase B) group and all fish sampled were SAV RNA positive by 7 dpi (days post infection). In contrast, only a few SAV RNA positive (infected) fish were identified at 14, 21 and 28 dpi in the BI-B (BI Phase B) group. Differences in the transcription of several immune genes were apparent when compared between the infected fish in the IM-B and BI-B groups. Transcription of the analysed genes peaked at 7 dpi in the IM-B group and at 14 dpi in the BI-B group. However, this latter finding was difficult to interpret due to the low prevalence of SAV positive fish in this group. Additionally, fish positive for SAV RNA in the BI-B group showed higher transcription of IL-1β, IFNγ and CXCL11_L1, all genes associated with the inflammatory response, compared to the IM-B group. Histopathological changes in the heart were restricted to the IM-B group, while (immune) cell filtration into the pancreas was observed in both groups. Compared to the Phase A fish that were exposed to SAV3 two weeks after seawater transfer, the Phase B fish in the current paper, showed a higher and more sustained innate immune gene transcription in response to the SAV3 infection. In addition, the basal transcription of several innate immune genes in non-infected control fish in Phase B (CT-B) was also significantly different when compared to Phase A control fish (CT-A).

Selective precipitation reaction: a novel diagnostic test for tissue pathology in Atlantic salmon, Salmo salar, infected with salmonid alphavirus (SAV3)

While investigating biomarkers for infection with salmonid alphavirus (SAV), the cause of pancreas disease (PD), a selective precipitation reaction (SPR) has been discovered in serum which could be an on-farm qualitative test and an in-laboratory quantitative assay for health assessments in aquaculture. Mixing serum from Atlantic salmon, Salmo salar, with SAV infection with a sodium acetate buffer caused a visible precipitation which does not occur with serum from healthy salmon. Proteomic examination of the precipitate has revealed that the components are a mix of muscle proteins, for example enolase and aldolase, along with serum protein such as serotransferrin and complement C9. The assay has been optimized for molarity, pH, temperature and wavelength so that the precipitation can be measured as the change in optical density at 340 nm (Δ₃₄₀ ). Application of the SPR assay to serum samples from a cohabitation trial of SAV infection in salmon showed that the Δ₃₄₀ in infected fish rose from undetectable to a maximum at 6 weeks post-infection correlating with histopathological score of pancreas, heart and muscle damage. This test may have a valuable role to play in the diagnostic evaluation of stock health in salmon.

Immune gene profiles in Atlantic salmon (salmo salar L.) post-smolts infected with SAV3 by bath-challenge show a delayed response and lower levels of gene transcription compared to injected fish

Salmonid alphavirus (SAV) causes pancreatic disease (PD) in salmonids in Northern Europe which results in large economic losses within the aquaculture industry. In order to better understand the underlying immune mechanisms during a SAV3 infection Atlantic salmon post-smolts were infected by either i.m.-injection or bath immersion and their immune responses compared. Analysis of viral loads showed that by 14 dpi i.m.-injected and bath immersion groups had 95.6% and 100% prevalence respectively and that both groups had developed the severe pathology typical of PD. The immune response was evaluated by using RT-qPCR to measure the transcription of innate immune genes involved in the interferon (IFN) response as well as genes associated with inflammation. Our results showed that IFNa transcription was only weakly upregulated, especially in the bath immersion group. Despite this, high levels of the IFN-stimulated genes (ISGs) such as Mx and viperin were observed. The immune response in the i.m.-injected group as measured by immune gene transcription was generally faster, and more pronounced than the response in the bath immersion group, especially at earlier time-points. The response in the bath immersion group started later as expected and appeared to last longer often exceeding the response in the i.m-injected fish at later time-points. High levels of transcription of many genes indicative of an active innate immune response were present in both groups.

The epidemiology of pancreas disease in salmonid aquaculture: a summary of the current state of knowledge

Pancreas disease (PD) is a viral disease caused by Salmonid alphavirus (SAV) that affects farmed Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss (Walbaum)) in the seawater phase. Since its first description in Scotland in 1976, a large number of studies have been conducted relating to the disease itself and to factors contributing to agent spread and disease occurrence. This paper summarizes the currently available, scientific information on the epidemiology of PD and its associated mitigation and control measures. Available literature shows infected farmed salmonids to be the main reservoir of SAV. Transmission between seawater sites occurs mainly passively by water currents or actively through human activity coupled with inadequate biosecurity measures. All available information suggests that the current fallowing procedures are adequate to prevent agent survival within the environment through the fallowing period and thus that a repeated disease outbreak at the same site is due to a new agent introduction. There has been no scientific evaluation of currently used on-site biosecurity measures, and there is limited information on the impact of available mitigation measures and control strategies.