Histology, immunocytochemistry and qRT-PCR analysis of Atlantic salmon, Salmo salar L., post-smolts following infection with infectious pancreatic necrosis virus (IPNV)

Isolation of a New Infectious Pancreatic Necrosis Virus (IPNV) Variant from Genetically Resistant Farmed Atlantic Salmon (Salmo salar) during 2021–2022

Infectious pancreatic necrosis (IPN), caused by IPNV, affects several species of farmed fish, particularly Atlantic salmon, and is responsible for significant economic losses in salmon aquaculture globally. Despite the introduction of genetically resistant farmed Atlantic salmon and vaccination strategies in the Chilean salmon industry since 2019, the number of IPN outbreaks has been increasing in farmed Atlantic salmon in the freshwater phase. This study examined gross and histopathological lesions of IPNV-affected fish, as well as the IPNV nucleotide sequence encoding the VP2 protein in clinical cases. The mortality reached 0.4% per day, and the cumulative mortality was from 0.4 to 3.5%. IPNV was isolated in the CHSE-214 cell line and was confirmed by RT-PCR, and VP2 sequence analysis. The analyzed viruses belong to IPNV genotype 5 and have 11 mutations in their VP2 protein. This is the first report of IPN outbreaks in farmed Atlantic salmon genetically resistant to IPNV in Chile. Similar outbreaks were previously reported in Scotland and Norway during 2018 and 2019, respectively. This study highlights the importance of maintaining a comprehensive surveillance program in conjunction with the use of farmed Atlantic salmon genetically resistant to IPNV.

Chlorella vulgaris Extracts as Modulators of the Health Status and the Inflammatory Response of Gilthead Seabream Juveniles (Sparus aurata)

This study aimed to evaluate the effects of short-term supplementation, with 2% Chlorella vulgaris (C. vulgaris) biomass and two 0.1% C. vulgaris extracts, on the health status (experiment one) and on the inflammatory response (experiment two) of gilthead seabream (Sparus aurata). The trial comprised four isoproteic (50% crude protein) and isolipidic (17% crude fat) diets. A fishmeal-based (FM), practical diet was used as a control (CTR), whereas three experimental diets based on CTR were further supplemented with a 2% inclusion of C. vulgaris biomass (Diet D1); 0.1% inclusion of C. vulgaris peptide-enriched extract (Diet D2) and finally a 0.1% inclusion of C. vulgaris insoluble fraction (Diet D3). Diets were randomly assigned to quadruplicate groups of 97 fish/tank (IBW: 33.4 ± 4.1 g), fed to satiation three times a day in a recirculation seawater system. In experiment one, seabream juveniles were fed for 2 weeks and sampled for tissues at 1 week and at the end of the feeding period. Afterwards, randomly selected fish from each group were subjected to an inflammatory insult (experiment two) by intraperitoneal injection of inactivated gram-negative bacteria, following 24 and 48 h fish were sampled for tissues. Blood was withdrawn for haematological procedures, whereas plasma and gut tissue were sampled for immune and oxidative stress parameters. The anterior gut was also collected for gene expression measurements. After 1 and 2 weeks of feeding, fish fed D2 showed higher circulating neutrophils than seabream fed CTR. In contrast, dietary treatments induced mild effects on the innate immune and antioxidant functions of gilthead seabream juveniles fed for 2 weeks. In the inflammatory response following the inflammatory insult, mild effects could be attributed to C. vulgaris supplementation either in biomass form or extract. However, the C. vulgaris soluble peptide-enriched extract seems to confer a protective, anti-stress effect in the gut at the molecular level, which should be further explored in future studies.

Identification of a New Infectious Pancreatic Necrosis Virus (IPNV) Variant in Atlantic Salmon (Salmo salar L.) that can Cause High Mortality Even in Genetically Resistant Fish

Infectious pancreatic necrosis (IPN) is an important viral disease of salmonids that can affect fish during various life cycles. In Atlantic salmon, selecting for genetically resistant fish against IPN has been one of the most highly praised success stories in the history of fish breeding. During the late 2000s, the findings that resistance against this disease has a significant genetic component, which is mainly controlled by variations in a single gene, have helped to reduce the IPN outbreaks to a great extent. In this paper, we present the identification of a new variant of the IPN virus from a field outbreak in Western Norway that had caused mortality, even in genetically resistant salmon. We recovered and assembled the full-length genome of this virus, following the deep-sequencing of the head-kidney transcriptome. The comparative sequence analysis revealed that for the critical amino acid motifs, previously found to be associated with the degree of virulence, the newly identified variant is similar to the virus’s avirulent form. However, we detected a set of deduced amino acid residues, particularly in the hypervariable domain of the VP2, that collectively are unique to this variant compared to all other reference sequences assessed in this study. We suggest that these mutations have likely equipped the virus with the capacity to escape the host defence mechanism more efficiently, even in the genetically deemed IPN resistant fish.

Immune Status and Hepatic Antioxidant Capacity of Gilthead Seabream Sparus aurata Juveniles Fed Yeast and Microalga Derived β-glucans

This work aimed to evaluate the effects of dietary supplementation with β-glucans extracted from yeast (Saccharomyces cerevisiae) and microalga (Phaeodactylum tricornutum) on gene expression, oxidative stress biomarkers and plasma immune parameters in gilthead seabream (Sparus aurata) juveniles. A practical commercial diet was used as the control (CTRL), and three others based on CTRL were further supplemented with different β-glucan extracts. One was derived from S. cerevisiae (diet MG) and two different extracts of 21% and 37% P. tricornutum-derived β-glucans (defined as Phaeo21 and Phaeo37), to give a final 0.06% β-glucan dietary concentration. Quadruplicate groups of 95 gilthead seabream (initial body weight: 4.1 ± 0.1 g) were fed to satiation three times a day for 8 weeks in a pulse-feeding regimen, with experimental diets intercalated with the CTRL dietary treatment every 2 weeks. After 8 weeks of feeding, all groups showed equal growth performance and no changes were found in plasma innate immune status. Nonetheless, fish groups fed β-glucans supplemented diets showed an improved anti-oxidant status compared to those fed CTRL at both sampling points (i.e., 2 and 8 weeks). The intestinal gene expression analysis highlighted the immunomodulatory role of Phaeo37 diet after 8 weeks, inducing an immune tolerance effect in gilthead seabream intestine, and a general down-regulation of immune-related gene expression. In conclusion, the results suggest that the dietary pulse administration of a P. tricornutum 37% enriched-β-glucans extract might be used as a counter-measure in a context of gut inflammation, due to its immune-tolerant and anti-oxidative effects.

Antiviral defense in salmonids - Mission made possible?

Viral diseases represent one of the major threats for salmonid aquaculture. Survival from viral infections are highly dependent on host innate antiviral immune defense, where interferons are of crucial importance. Neutralizing antibodies and T cell effector mechanisms mediate long-term antiviral protection. Despite an immune cell repertoire comparable to higher vertebrates, farmed fish often fail to mount optimal antiviral protection. In the quest to multiply and spread, viruses utilize a variety of strategies to evade or escape the host immune system. Understanding the specific interplay between viruses and host immunity at depth is crucial for developing successful vaccination and treatment strategies in mammals. However, this knowledge base is still limited for pathogenic fish viruses. Here, we have focused on five RNA viruses with major impact on salmonid aquaculture: Salmonid alphavirus, Infectious salmon anemia virus, Infectious pancreatic necrosis virus, Piscine orthoreovirus and Piscine myocarditis virus. This review explore the protective immune responses that salmonids mount to these viruses and the existing knowledge on how the viruses counteract and/or bypass the immune response, including their IFN antagonizing effects and their mechanisms to establish persisting infections.

Infectious pancreatic necrosis virus triggers antiviral immune response in rainbow trout red blood cells, despite not being infective

Background: Some fish viruses, such as piscine orthoreovirus and infectious salmon anemia virus, target red blood cells (RBCs), replicate inside them and induce an immune response. However, the roles of RBCs in the context of infectious pancreatic necrosis virus (IPNV) infection  have not been studied yet.

Methods: Ex vivo rainbow trout RBCs were obtained from peripheral blood, Ficoll purified and exposed to IPNV in order to analyze infectivity and immune response using RT-qPCR, immune fluorescence imaging, flow cytometry and western-blotting techniques.

Results: IPNV could not infect RBCs; however, IPNV increased the expression of the INF1-related genes ifn-1, pkr and mx genes. Moreover, conditioned media from IPNV-exposed RBCs conferred protection against IPNV infection in CHSE-214 fish cell line.

Conclusions: Despite not being infected, rainbow trout RBCs could respond to IPNV with increased expression of antiviral genes. Fish RBCs could be considered as mediators of the antiviral response and therefore targets of new strategies against fish viral infections. Further research is ongoing to completely understand the molecular mechanism that triggers this antiviral response in rainbow trout RBCs.

Infectious pancreatic necrosis virus infection of fish cell lines: Preliminary analysis of gene expressions related to extracellular matrix remodeling and immunity

The pathogenic infectious pancreatic necrosis virus (IPNV) causes high economic losses in fish farming. This virus can modulate several cellular processes during infection, but little is known about the infection mechanism. To investigate gene activation in response to IPNV, CHSE/F and SHK-1 cell line were infected with a cytopathic Sp field isolate of IPNV, and the expression profiles of proinflammatory, antiviral cytokine, and extracellular matrix markers were analyzed. IPNV induced the production of perlecan, fibulin-1, matrix metalloproteinase-2, 14-3-3β, interleukin-1β, Mx1, and interferon regulatory factors-1, -3, and -9. Interestingly, IPNV-mediated activity was blocked by pharmacological inhibitors of the NF-κB signaling pathway. These results, together with in silico analyses showing the presence of several regulatory consensus-target motifs, suggest that IPNV regulates gene expressions in fish through the activation of several key transcription factors. Collectively, these data indicate that IPNV is a viral regulator of expression for extracellular-matrix and immune markers, even during early infection. Finally, this is the first report in fish to find IPNV modulating the activation of interleukin-1β production primarily through the NF-κB pathway.

Gene expression comparison of resistant and susceptible Atlantic salmon fry challenged with Infectious Pancreatic Necrosis virus reveals a marked contrast in immune response

BACKGROUND

Infectious Pancreatic Necrosis (IPN) is a highly contagious birnavirus disease of farmed salmonid fish, which often causes high levels of morbidity and mortality. A large host genetic component to resistance has been previously described for Atlantic salmon (Salmo salar L.), which mediates high mortality rates in some families and zero mortality in others. However, the molecular and immunological basis for this resistance is not yet fully known. This manuscript describes a global comparison of the gene expression profiles of resistant and susceptible Atlantic salmon fry following challenge with the IPN virus.

RESULTS

Salmon fry from two IPNV-resistant and two IPNV-susceptible full sibling families were challenged with the virus and sampled at 1 day, 7 days and 20 days post-challenge. Significant viral titre was observed in both resistant and susceptible fish at all timepoints, although generally at higher levels in susceptible fish. Gene expression profiles combined with gene ontology and pathway analyses demonstrated that while a clear immune response was observed in both resistant and susceptible fish, there were striking differences between the two phenotypes. The susceptible fish showed marked up-regulation of genes related to cytokine activity and inflammatory response that evidently failed to protect against the virus. In contrast, the resistant fish demonstrated a less pronounced immune response including up-regulation of genes relating to the M2 macrophage system.

CONCLUSIONS

While only the susceptible phenotype shows appreciable mortality levels, both resistant and susceptible fish can become infected with IPNV. Susceptible fish are characterized by a much larger, yet ineffective, immune response, largely related to cytokine and inflammatory systems. Resistant fish demonstrate a more moderate, putative macrophage-mediated inflammatory response, which may contribute to their survival.

Individual Monitoring of Immune Response in Atlantic Salmon Salmo salar following Experimental Infection with Infectious Salmon Anaemia Virus (ISAV)

Monitoring the immune response in fish over the progression of a disease is traditionally carried out by experimental infection whereby animals are killed at regular intervals and samples taken. We describe here a novel approach to infectiology for salmonid fish where blood samples are collected repeatedly in a small group of PIT-tagged animals. This approach contributes to the reduction of animals used in research and to improved data quality. Two groups of 12 PIT-tagged Atlantic salmon (Salmo salar) were i.p infected with Infectious Salmon Anaemia Virus (ISAV) or culture medium and placed in 1 m3 tanks. Blood samples were collected at 0, 4, 8, 12, 16, 21 and 25 days post infection. The viral load, immune and stress response were determined in individual fish by real-time quantitative PCR (QPCR) on the blood cells, as well as the haematocrit used as an indicator of haemolysis, a clinical consequence of ISAV infection. "In-tank" anaesthesia was used in order to reduce the stress related to chase and netting prior to sampling. The data were analysed using a statistical approach which is novel with respect to its use in fish immunology. The repeated blood collection procedure did not induce stress response as measured by HSP70 and HSP90 gene expression in the un-infected animals. A strong increase in viraemia as well as a significant induction of Mx and γIP gene expression were observed in the infected group. Interleukin 10 was found induced at the later stage of the infection whereas no induction of CD8 or γ IFN could be detected. These results and the advantages of this approach are discussed.

Immune responses to oral pcDNA-VP2 vaccine in relation to infectious pancreatic necrosis virus carrier state in rainbow trout Oncorhynchus mykiss

The VP2 gene of infectious pancreatic necrosis virus, encoded in an expression plasmid and encapsulated in alginate microspheres, was used for oral DNA vaccination of fish to better understand the carrier state and the action of the vaccine. The efficacy of the vaccine was evaluated by measuring the prevention of virus persistence in the vaccinated fish that survived after waterborne virus challenge. A real-time RT-qPCR analysis revealed lower levels of IPNV-VP4 transcripts in rainbow trout survivors among vaccinated and challenged fish compared with the control virus group at 45 days post-infection. The infective virus was recovered from asymptomatic virus control fish, but not from the vaccinated survivor fish, suggesting an active role of the vaccine in the control of IPNV infection. Moreover, the levels of IPNV and immune-related gene expression were quantified in fish showing clinical infection as well as in asymptomatic rainbow trout survivors. The vaccine mimicked the action of the virus, although stronger expression of immune-related genes, except for IFN-1 and IL12, was detected in survivors from the virus control (carrier) group than in those from the vaccinated group. The transcriptional levels of the examined genes also showed significant differences in the virus control fish at 10 and 45 days post-challenge.

Induction of anti-inflammatory cytokine expression by IPNV in persistent infection

Infectious Pancreatic Necrosis Virus (IPNV) is the agent of a well-characterized acute disease that produces a systemic infection and high mortality in farmed fish species but also persistent infection in surviving fish after outbreaks. Because viral persistence of susceptible mammal hosts appears to be associated with the modulation of anti-inflammatory cytokine expression, in this study we examined the expression levels of key pro- and anti-inflammatory cytokines in kidney and spleen of trout, as well as humoral immune response (IgM and IgT) during experimental persistent viral infection and in the acute phase of infection as a comparison. IPNV infection in rainbow trout resulted in a distinct profile of cytokine expression depending on the type of infection, acute or persistent. Levels of early pro-inflammatory cytokines, IL-1β and IL-8, did not increase in the head kidney of the fish with persistent asymptomatic infection but increased in some of the symptomatic infected fish. The antiviral cytokine IFNα was not significantly induced in any of the infected fish groups. The level of expression of the Th1-related cytokine IL-12 was significantly higher in trout with persistent asymptomatic infection than in symptomatic fish. This was also accompanied by an increase in IFNγ. The anti-inflammatory cytokines IL-10 and TGF-β1 had distinct expression profiles. While IL-10 expression increased in all infected fish, TGF-β1 was only up-regulated in fish with persistent infection. All infected fish had significantly lower total IgM levels than the non-infected fish whereas IgT levels did not change. Specific and neutralizing antibodies against IPNV were not observed in acute and persistent infection except in the group of fish with the lowest degree of clinical signs. Interestingly, the lack of humoral immune response could be associated with the high expression of anti-inflammatory cytokines, which might inhibit antibody production. The balance between pro-inflammatory Th1 type cytokines and the regulatory cytokines could explain the high percentage of survival and the resolution of the inflammatory response in the IPNV-infected fish but also the establishment of viral persistence.

Innate immune responses of salmonid fish to viral infections

Viruses are the most serious pathogenic threat to the production of the main aquacultured salmonid species the rainbow trout Oncorhynchus mykiss and the Atlantic salmon Salmo salar. The viral diseases Infectious Pancreatic Necrosis (IPN), Pancreatic Disease (PD), Infectious Haemorrhagic Necrosis (IHN), Viral Haemorrhagic Septicaemia (VHS), and Infectious Salmon Anaemia (ISA) cause massive economic losses to the global salmonid aquaculture industry every year. To date, no solution exists to treat livestock affected by a viral disease and only a small number of efficient vaccines are available to prevent infection. As a consequence, understanding the host immune response against viruses in these fish species is critical to develop prophylactic and preventive control measures. The innate immune response represents an important part of the host defence mechanism preventing viral replication after infection. It is a fast acting response designed to inhibit virus propagation immediately within the host, allowing for the adaptive specific immunity to develop. It has cellular and humoral components which act in synergy. This review will cover inflammation responses, the cell types involved, apoptosis, antimicrobial peptides. Particular attention will be given to the type I interferon system as the major player in the innate antiviral defence mechanism of salmonids. Viral evasion strategies will also be discussed.

Tissue distribution of infectious pancreatic necrosis virus serotype Sp in naturally infected cultured rainbow trout, Oncorhynchus mykiss (Walbaum): an immunohistochemical and nested-PCR study

This study investigates the occurrence and distribution pattern of infectious pancreatic necrosis virus (IPNV) within the pancreas, liver, kidney and spleen of naturally infected cultured rainbow trout, Oncorhynchus mykiss (Walbaum), using immunohistochemistry (IHC). A nested PCR was also employed to confirm the presence of the virus in the pooled tissues of the specimens. All the examined tissues except spleen were immunohistochemically positive for IPNV, but staining intensity and distribution pattern varied. The kidney tubules had the most intense and widespread staining by IHC, indicating a specific tissue tropism at least for this particular serotype. The nucleotide sequence had the greatest identity with the Sp serotype confirming the presence of the nucleic acid of IPNV in the pooled tissues. Based on the present findings, it could be concluded that the absence of lesions consistent with infectious pancreatic necrosis (IPN) disease in the H&E-stained sections cannot rule out the presence of the IPNV, and the use of an alternative rapid confirmatory method such as IHC with formalin-fixed, paraffin-embedded tissue sections is helpful for the final diagnosis of IPN in rainbow trout.

Flow cytometry detection of infectious pancreatic necrosis virus (IPNV) within subpopulations of Atlantic salmon (Salmo salar L.) leucocytes after vaccination and during the time course of experimental infection

In the present study, intracellular infectious pancreatic necrosis virus (IPNV) in salmon leucocytes was detected by flow cytometry after experimental cohabitant challenge. IPNV vaccinated, non-vaccinated and intraperitoneally (i.p.) infected salmon (virus shedders) were analysed at different times throughout the period when mortality occurred. Fish that had survived 61 days post challenge (carriers) were also analysed. In particular, we analysed the presence of IPNV in B-cells (C7G7+cells) and in neutrophils (E3D9+ cells) in head kidney leucocytes (HKL) and in peripheral blood leucocytes (PBL). IPNV was present in HKL and PBL from all challenged fish groups at all samplings, including carriers. IPNV was also found intracellular in other leucocytes than B-cells and neutrophils. During the time course of infection there were changes in proportion of B-cells and neutrophils and in proportions of IPNV+ cells. In vaccinated fish, a delay in the changes observed in the proportion of IPNV+ cells and in the proportions of the two subpopulations was identified. The vaccinated fish were protected against disease as no fish died compared to 30.8% of non-vaccinated cohabitant fish. All i.p. infected fish, except one, survived the challenge. This is consistent with previous studies and confirmed that the routes of infection can influence mortality. The analyses in this study could not identify any factors enlightening this absence of mortality in i.p. infected fish, but both flow cytometry and qRT-PCR showed that i.p. infected fish were carriers of IPNV. The present study also found that IPNV was present in both B-cells and neutrophils as well as in other leucocytes in all carriers after cohabitant challenge. These fish had survived 9 weeks post challenge and 4 weeks after mortality has ceased. The fish harbouring virus within their leucocytes might become life long carriers and represent a risk for disease outbreaks, being virus shedders. Such fish are protected from later infections if the virus exposure has resulted in protective immunity. Flow cytometry was found to be very suitable for detection of intracellular virus after in vivo challenge and the sensitivity was demonstrated by the detection of virus in carriers.

IPNV modulation of pro and anti-inflammatory cytokine expression in Atlantic salmon might help the establishment of infection and persistence

IPNV is the agent of a well-characterized acute disease that produces a systemic infection and high mortality in farmed fish species and persistent infection in surviving fish after outbreaks. Because modulation of the host expression of pro and anti-inflammatory cytokines can help establish persistence, in this study, we examined the expression of IL-1β, IL-8, IFNα1 and IL-10 during acute and persistent IPNV infection of Atlantic salmon. Results showed that IPNV infection induces an increase of the IFNα1 and IL-10 mRNA levels in the spleen and head kidney (HK) of fish after acute experimental infection. Levels of the pro-inflammatory cytokines IL-1β and IL-8 did not rise in the spleen although an increase of IL-1β, but not of IL-8, was observed in head kidney. In carrier asymptomatic salmon, cytokine gene expression of IFNα1 in the spleen and IL-10 in head kidney were also significantly higher than expression in non-carrier fish. Interestingly, a decrease of IL-8 expression was also observed. IPNV infection of SHK-1, which is a macrophage-like cell line of salmon, also induced an increase of expression of the anti-inflammatory cytokine IL-10 with no effects on the expression of IL-1β and IL-8. The effects are induced by an unknown mechanism during viral infection because poly I:C and the viral genomic dsRNA showed the opposite effects on cytokine expression in SHK-1 cells. In summary, IPNV always induces up-regulation of the anti-inflammatory cytokine IL-10 in Atlantic salmon. As this is accompanied by a lack of induction of the pro-inflammatory cytokines IL-1β and IL-8, the anti-inflammatory milieu may explain the high frequency, prevalence and persistence of IPNV in salmon. Effects might be part of the viral mechanisms of immune evasion.

Molecular profiling of the gilthead sea bream (Sparus aurata L.) response to chronic exposure to the myxosporean parasite Enteromyxum leei

The aim of the present work was to investigate the transcriptome response of gilthead sea bream (Sparus aurata) after challenge with the myxosporean Enteromyxum leei, a wide-spread enteric parasite causing heavy economic losses in Mediterranean sparid farms. This parasite causes severe desquamative enteritis which usually leads to death of the fish, and there are no preventative or curative treatments for this enteromyxosis. After 113 days of exposure to parasite-contaminated effluent, fish were classified into three cohorts: control fish not exposed to parasite, those that were exposed and infected, and those that were exposed but not infected. In order to detect target genes that may be candidates for infective status or resistance, a cDNA microarray containing 18,490 cDNA clones enriched in genes differentially expressed after infection was hybridised with head kidney and intestine samples. In infected fish, 371 and 373 genes were differentially regulated at the >1.5-fold level in intestine and head kidney respectively, whereas in non-infected fish 175 and 501 genes were differentially regulated in these tissues, respectively. A global marked gene down-regulation was evident in infected fish, mainly in genes involved in the immune and acute phase response particularly complement and mannose binding lectin. Microarray analysis demonstrated a complex interplay between host and/or parasite derived proteases and protease inhibitors, apoptosis, cell proliferation and antioxidant defence genes in exposed fish. In the head kidney of non-infected fish a marked depression of genes involved in the acute phase response was evident. By contrast, in the intestine of non-infected fish, interferon-stimulated and MHC class II genes involved in antigen processing and presentation were up-regulated, possibly indicating that an active immune response at the local level is important to avoid infection with or proliferation of the parasite.

Aerobic training stimulates growth and promotes disease resistance in Atlantic salmon (Salmo salar)

Improving fish robustness is of utmost relevance to reducing fish losses in farming. Although not previously examined, we hypothesized that aerobic training, as shown for human studies, could strengthen disease resistance in Atlantic salmon (Salmo salar). Thus, we exercised salmon pre-smolts for 6 weeks at two different aerobic training regimes; a continuous intensity training (CT; 0.8bls(-1)) and an interval training (IT; 0.8bl s(-1) 16h and 1.0bl s(-1) 8h) and compared them with untrained controls (C; 0.05bl s(-1)). The effects of endurance training on disease resistance were evaluated using an IPN virus challenge test, while the cardiac immune modulatory effects were characterized by qPCR and microarray gene expression analyses. In addition, swimming performance and growth parameters were investigated. Survival after the IPN challenge was higher for IT (74%) fish than for either CT (64%) or C (61%) fish. While both CT and IT groups showed lower cardiac transcription levels of TNF-α, IL-1β and IL-6 prior to the IPN challenge test, IT fish showed the strongest regulation of genes involved in immune responses and other processes known to affect disease resistance. Both CT and IT regimes resulted in better growth compared with control fish, with CT fish developing a better swimming efficiency during training. Overall, interval aerobic training improved growth and increased robustness of Atlantic salmon, manifested by better disease resistance, which we found was associated with a modulation of relevant gene classes on the cardiac transcriptome.

Liver involvement in post-smolt Atlantic salmon, Salmo salar L., infected with infectious pancreatic necrosis virus (IPNV): a retrospective histopathological study

Histological changes associated with infectious pancreatic necrosis virus (IPNV) infection have historically been described for the pancreas and gut, but any involvement of the liver was poorly acknowledged or described. The aims of this study were to find robust evidence that the reported increase in liver pathology in Atlantic salmon post-smolts in natural outbreaks was effectively related to IPNV infection and retrospectively to report when such a shift in the involvement of the liver had taken place, supported by a histopathological description for a differential diagnosis. The study reports new findings concerning the dynamics of liver pathology development, with apoptosis, demonstrated by histological and immunological techniques, described as the most relevant and particular feature. Immunohistochemical examination of affected liver suggests apoptosis is not only the result of the virus infection itself but triggered through the action of the host's innate immune response. Liver involvement contributes to the nature of infection and becomes an important factor in the disease process. Additionally, it was established that the increase in infectious pancreatic necrosis prevalence is correlated with a new distinct pattern of outbreak distribution throughout the year. The role of smolt category (i.e. S1, S1/2 or S0), hence timing of seawater transfer as a strong correlating factor, is discussed.