Molecular tracing confirms that infection with infectious pancreatic necrosis virus follows the smolt from hatchery to grow-out farm

Infectious pancreatic necrosis (IPN) is an important restraint to production of salmonids in aquaculture globally. In order to implement efficacious mitigation strategies for control of this disease, it is important to understand infection routes under current production systems. IPN virus has been shown to be transmitted vertically in Rainbow trout, from broodstock to fingerlings in hatcheries, and there is circumstantial evidence suggesting that vertical transmission can also occur in Atlantic salmon, in addition to horizontal transmission between grow-out fish in farms. In this study, we show that the smolt carries infection with IPN from hatchery to the marine farm. We do this by comparing sequences from fish groups taken both in hatcheries and on corresponding marine grow-out farms. We use statistical analysis to prove that sequences obtained from the same fish group in both hatchery and marine farm are more similar than sequences obtained from random fish groups on hatcheries and marine farms.

Hypoxia tolerance and responses to hypoxic stress during heart and skeletal muscle inflammation in Atlantic salmon (Salmo salar)

Heart and skeletal muscle inflammation (HSMI) is associated with Piscine orthoreovirus (PRV) infection and is an important disease in Atlantic salmon (Salmo salar) aquaculture. Since PRV infects erythrocytes and farmed salmon frequently experience environmental hypoxia, the current study examined mutual effects of PRV infection and hypoxia on pathogenesis and fish performance. Furthermore, effects of HSMI on hypoxia tolerance, cardiorespiratory performance and blood oxygen transport were studied. A cohabitation trial including PRV-infected post-smolts exposed to periodic hypoxic stress (4 h of 40% O₂; PRV-H) at 4, 7 and 10 weeks post-infection (WPI) and infected fish reared under normoxic conditions (PRV) was conducted. Periodic hypoxic stress did not influence infection levels or histopathological changes in the heart. Individual incipient lethal oxygen saturation (ILOS) was examined using a standardized hypoxia challenge test (HCT). At 7 WPI, i.e. peak level of infection, both PRV and PRV-H groups exhibited reduced hypoxia tolerance compared to non-infected fish. Three weeks later (10 WPI), during peak levels of pathological changes, reduced hypoxia tolerance was still observed for the PRV group while PRV-H performed equal to non-infected fish, implying a positive effect of the repeated exposure to hypoxic stress. This was in line with maximum heart rate (f_Hmax) measurements, showing equal performance of PRV-H and non-infected groups, but lower f_Hmax above 19°C as well as lower temperature optimum (T_opt) for aerobic scope for PRV, suggesting reduced cardiac performance and thermal tolerance. In contrast, the PRV-H group had reduced hemoglobin-oxygen affinity compared to non-infected fish. In conclusion, Atlantic salmon suffering from HSMI have reduced hypoxia tolerance and cardiac performance, which can be improved by preconditioning fish to transient hypoxic stress episodes.

PCR survey for Paramoeba perurans in fauna, environmental samples and fish associated with marine farming sites for Atlantic salmon (Salmo salar L.)

Amoebic gill disease (AGD) caused by the amoeba Paramoeba perurans is an increasing problem in Atlantic salmon aquaculture. In the present PCR survey, the focus was to identify reservoir species or environmental samples where P. perurans could be present throughout the year, regardless of the infection status in farmed Atlantic salmon. A total of 1200 samples were collected at or in the proximity to farming sites with AGD, or with history of AGD, and analysed for the presence of P. perurans. No results supported biofouling organisms, salmon lice, biofilm or sediment to maintain P. perurans. However, during clinical AGD in Atlantic salmon, the amoeba were detected in several samples, including water, biofilm, plankton, several filter feeders and wild fish. It is likely that some of these samples were positive as a result of the continuous exposure through water. Positive wild fish may contribute to the spread of P. perurans. Cleaner fish tested positive for P. perurans when salmon tested negative, indicating that they may withhold the amoeba longer than salmon. The results demonstrate the high infection pressure produced from an AGD-afflicted Atlantic salmon population and thus the importance of early intervention to reduce infection pressure and horizontal spread of P. perurans within farms.

Immunological interactions between Piscine orthoreovirus and Salmonid alphavirus infections in Atlantic salmon

Heart and skeletal muscle inflammation (HSMI) and pancreas disease (PD) cause substantial losses in Atlantic salmon (Salmo salar) aquaculture. The respective causative agents, Piscine orthoreovirus (PRV) and Salmonid alphavirus (SAV), are widespread and often concurrently present in farmed salmon. An experimental infection in Atlantic salmon was conducted to study the interaction between the two viruses, including the immunological mechanisms involved. The co-infected fish were infected with PRV four or ten weeks before they were infected with SAV. The SAV RNA level and the PD specific lesions were significantly lower in co-infected groups compared to the group infected by only SAV. The expression profiles of a panel of innate antiviral response genes and the plasma SAV neutralization titers were examined. The innate antiviral response genes were in general upregulated for at least ten weeks after the primary PRV infection. Plasma from co-infected fish had lower SAV neutralizing titers compared to the controls infected with only SAV. Plasma from some individuals infected with only PRV neutralized SAV, but heat treatment removed this effect. Field studies of co-infected fish populations indicated a negative correlation between the two viruses in randomly sampled apparently healthy fish, in line with the experimental findings, but a positive correlation in moribund or dead fish. The results indicate that the innate antiviral response induced by PRV may temporary protect against a secondary SAV infection.

Experimental Piscine orthoreovirus infection mediates protection against pancreas disease in Atlantic salmon (Salmo salar)

Viral diseases are among the main challenges in farming of Atlantic salmon (Salmo salar). The most prevalent viral diseases in Norwegian salmon aquaculture are heart and skeletal muscle inflammation (HSMI) caused by Piscine orthoreovirus (PRV), and pancreas disease (PD) caused by Salmonid alphavirus (SAV). Both PRV and SAV target heart and skeletal muscles, but SAV additionally targets exocrine pancreas. PRV and SAV are often present in the same locations and co-infections occur, but the effect of this crosstalk on disease development has not been investigated. In the present experiment, the effect of a primary PRV infection on subsequent SAV infection was studied. Atlantic salmon were infected with PRV by cohabitation, followed by addition of SAV shedder fish 4 or 10 weeks after the initial PRV infection. Histopathological evaluation, monitoring of viral RNA levels and host gene expression analysis were used to assess disease development. Significant reduction of SAV RNA levels and of PD specific histopathological changes were observed in the co-infected groups compared to fish infected by SAV only. A strong correlation was found between histopathological development and expression of disease related genes in heart. In conclusion, experimentally PRV infected salmon are less susceptible to secondary SAV infection and development of PD.

Liquid fat, a potential abiotic vector for horizontal transmission of salmonid alphavirus?

Viral diseases represent serious challenge in marine farming of Atlantic salmon (Salmo salar L). Pancreas disease (PD) caused by a salmonid alphavirus (SAV) is by far the most serious in northern Europe. To control PD, it is necessary to identify virus transmission routes. One aspect to consider is whether the virus is transported as free particles or associated with potential vectors. Farmed salmonids have high lipid content in their tissue which may be released into the environment from decomposing dead fish. At the seawater surface, the effects of wind and ocean currents are most prominent. The aim of this study was primarily to identify whether the lipid fraction leaking from dead infected salmon contains SAV. Adipose tissue from dead SAV-infected fish from three farming sites was submerged in beakers with sea water in the laboratory and stored at different temperature and time conditions. SAV was identified by real-time RT-PCR in the lipid fractions accumulating at the water surface in the beakers. SAV-RNA was also present in the sea water. Lipid fractions were transferred to cell culture, and viable SAV was identified. Due to its hydrophobic nature, fat with infective pathogenic virus at the surface may contribute to long-distance transmission of SAV.

Low virulent infectious salmon anaemia virus (ISAV-HPR0) is prevalent and geographically structured in Norwegian salmon farming

Infectious salmon anaemia (ISA) is a severe disease in farmed Atlantic salmon Salmo salar that has caused epidemic outbreaks in most salmon-producing countries worldwide. The disease is caused by virulent ISA virus (ISAV). Low virulent variants of the virus, characterised by a full-length sequence in the highly polymorphic region of segment 6 in the virus genome, have been reported with increasing frequencies. These variants of the virus, termed HPR0, have been proposed to be ancestors of virulent ISAV. We examined this idea through studies of the phylogeographic and environmental distribution of ISAV-HPR0, as well as phylogeographic associations between virulent ISAV and ISAV-HPR0. Samples from 232 fish groups were screened for ISAV. Real-time RT-PCR was used for detection of ISAV, and the ISAV haemagglutinin esterase (HE) gene was characterised for positive samples. A Mantel test was used to test phylogeographic associations between pairs of ISAV-HPR0 HE gene sequences. A rank test was used to test associations between HE gene sequences from virulent ISAV and ISAV-HPR0. ISAV-HPR0 was detected in fish groups both in freshwater and marine environments, and in juveniles, on-grown marine salmon and broodstock salmon. Genetic and geographic distances between pairs of ISAV-HPR0 HE gene sequences were positively correlated, suggesting that the population of ISAV-HPR0 is geographically structured. Finally, we found a spatial association between fish groups with virulent ISAV (n = 21) and fish groups with ISAV-HPR0 (n = 27), supporting the hypothesis that ISAV-HPR0 may undergo a transition to virulent ISAV.

Lack of evidence for vertical transmission of SAV 3 using gametes of Atlantic salmon, Salmo salar L., exposed by natural and experimental routes

Pancreas disease (PD) is an important cause of losses in farmed salmonids in Norway, the United Kingdom and Ireland. As the spread of salmonid alphavirus (SAV), the causal agent, to naïve populations is of major concern to the farming industry, it is important to uncover the transmission routes of the virus. This study was conducted to investigate the potential for vertical transmission of SAV subtype 3. Progeny of broodstock with signs of late-stage PD and persistent RT-PCR signals for SAV were followed from fertilization to smoltification in an experimental facility. Fertilized ova were either not disinfected or taken through one of three different disinfection regimes. Also, ova and milt from uninfected broodfish from a different population were exposed to a cell-cultured strain of SAV 3 immediately before fertilization to simulate a viraemic phase in parent fish. A group of uninfected controls were also included in the study. Fertilized ova from bath exposed and negative control groups were double disinfected. Following fertilization, experimental fish went through a normal freshwater phase. However, fry were stressed at first feeding to enhance replication of possibly latent virus. Smoltification was induced by an artificial light regime, and experimental fish were followed to the late smoltification phase. Selected samples were investigated by real-time RT-PCR for SAV, by histology for evidence of PD and by serology for neutralising antibodies against SAV. All analysed samples of progeny were negative. This result shows that SAV 3 is not readily transmitted vertically from parents to offspring. Additional negative PCR results from salmon sampled in commercial hatcheries support these findings. Also, recent studies have shown that risk factors for the horizontal transmission route explain the vast majority of PD outbreaks in Norway. It is concluded that if it happens at all, vertical transmission is of minor importance in the spread of SAV 3.

Characterization of the infectious salmon anemia virus fusion protein

Infectious salmon anemia virus (ISAV) is an orthomyxovirus causing serious disease in Atlantic salmon (Salmo salar L.). This study presents the characterization of the ISAV 50-kDa glycoprotein encoded by segment 5, here termed the viral membrane fusion protein (F). This is the first description of a separate orthomyxovirus F protein, and to our knowledge, the first pH-dependent separate viral F protein described. The ISAV F protein is synthesized as a precursor protein, F0, that is proteolytically cleaved to F1 and F2, which are held together by disulfide bridges. The cleaved protein is in a metastable, fusion-activated state that can be triggered by low pH, high temperature, or a high concentration of urea. Cell-cell fusion can be initiated by treatment with trypsin and low pH of ISAV-infected cells and of transfected cells expressing F, although the coexpression of ISAV HE significantly improves fusion. Fusion is initiated at pH 5.4 to 5.6, and the fusion process is coincident with the trimerization of the F protein, or most likely a stabilization of the trimer, suggesting that it represents the formation of the fusogenic structure. Exposure to trypsin and a low pH prior to infection inactivated the virus, demonstrating the nonreversibility of this conformational change. Sequence analyses identified a potential coiled coil and a fusion peptide. Size estimates of F1 and F2 and the localization of the putative fusion peptide and theoretical trypsin cleavage sites suggest that the proteolytic cleavage site is after residue K276 in the protein sequence.

Identification of an interferon antagonist protein encoded by segment 7 of infectious salmon anaemia virus

Infectious salmon anaemia virus (ISAV) is an orthomyxovirus and member of the genus Isavirus, which contains eight genomic segments coding for ten viral proteins. This study focussed on identifying the function of the largest protein encoded by ISAV genomic segment 7 (7i), which like influenza A segment 7 encodes two proteins, one of which is based on removal of an intron from the primary transcript. Using two independent methods, an Mx1 promoter-driven reporter system and real-time PCR of FACS-sorted transfected cells, we demonstrate that the non-structural ISAV 7i protein is an interferon-signalling antagonist. Other transfection studies indicated a predominantly cytoplasmic localisation of the expressed protein, which is consistent with this role. The demonstration that ISAV segment 7 encodes a putative non-structural IFN system antagonist reveals a difference with influenza A virus, where segment 7, which shares a similar coding strategy, encodes the structural matrix proteins.

Infectious salmon anemia virus (ISAV) genomic segment 3 encodes the viral nucleoprotein (NP), an RNA-binding protein with two monopartite nuclear localization signals (NLS)

Infectious salmon anemia virus (ISAV) is the type species of the genus Isavirus belonging to the Orthomyxoviridae, and causes serious disease in Atlantic salmon (Salmo salar). This study presents the expression and functional analysis of the ISAV genome segment 3, and provides further evidence that it encodes the viral nucleoprotein (NP). The encoded protein was expressed in a baculovirus system, and Western blot analysis showed that it corresponds to the 66-71 kDa structural protein previously found in purified ISAV preparations. RNA-binding activity was established by the interaction of viral and recombinant NP with single-stranded RNA transcribed in vitro. Immunofluorescence studies of infected cells showed the ISAV NP to be an early protein. It locates to the nucleus of infected cells before it is transported to the cytoplasm prior to virus assembly. A similar localization pattern was observed in cells transfected with the NP gene, confirming that the encoded protein has an intrinsic ability to be imported into the nucleus. Two monopartite nuclear localization signals (NLS) at amino acids (230)RPKR(233) and (473)KPKK(476) were identified by computer analysis, and validated by site-directed mutagenesis. In contrast to other orthomyxovirus-NPs, that have several NLSs that function independent of each other, both NLSs had to be present for the ISAV NP protein to be transported into the nucleus, indicating that these motifs cooperate to target the protein to the nucleus.

Identification and characterization of viral structural proteins of infectious salmon anemia virus

Infectious salmon anemia virus (ISAV) is an unclassified Orthomyxovirus that has been shown to contain a segmented genome with eight single-stranded RNA species coding for 10 viral proteins. Four major structural proteins were characterized in the present study: two glycosylated proteins with estimated molecular masses of 42 and 50 kDa, one 66-kDa phosphoprotein, and one 22-kDa protein. Examination of lysed virions revealed the two glycoproteins and the 22-kDa protein in the soluble fraction, while the 66-kDa phosphoprotein and a minor part of the 22-kDa protein were found in the pelleted fraction. Immunofluorescence staining of infected cells demonstrated that the 22-kDa protein was a late protein accumulating in the nucleus. We conclude that the 66-kDa protein is the nucleoprotein, the 22-kDa protein is the matrix protein, and the 42- and 50-kDa proteins are the surface proteins. Radioimmunoprecipitation analysis of the 42-kDa glycoprotein, which was previously shown to represent the ISAV hemagglutinin, indicated that this protein exists at least as dimers. Further, by labeling of purified ISAV with [1,3-(3)H]diisopropyl fluorophosphate, it was also demonstrated that the viral esterase is located with the hemagglutinin. This finding was confirmed by demonstration of acetylesterase activity in affinity-purified hemagglutinin preparations. Finally, the active-site serine residue could be tentatively identified at position 32 within the amino acid sequence of the hemagglutinin of ISAV strain Glesvaer/2/90. It is proposed that the ISAV vp66 protein be termed nucleoprotein, the gp42 protein be termed HE protein, and the vp22 protein be termed matrix protein.

Haemorrhagic smolt syndrome (HSS) in Norway: pathology and associated virus-like particles

Atlantic salmon Salmo salar pre-smolt, smolt and post-smolt, with clinical signs of haemorrhagic smolt syndrome (HSS) have been found in several locations along the Norwegian coast (Rogaland to Troms). Affected fish had pale gills and bleeding at the fin bases, but seemed to be in good physical condition with no obvious weight loss. The internal organs and body cavity showed distinct bleedings. Petechiae were found on the gastrointestinal tract, swim bladder and peritoneum, visceral adipose tissue, heart and somatic musculature. The liver was bright yellow and sometimes mottled with petechiae and ecchymoses. Acitic fluid was found in the visceral cavity and fluid was also present in the pericardial cavity. Histological examination revealed haemorrhage in most organs. The glomeruli were degenerated and the renal tubules were filled with erythrocytes. The aims of this study were to describe the pathology and discover, if possible, the aetiology of the HSS. Tissues were collected for light and transmission electron microscopy (TEM), immunofluorescence (IFAT), reverse transcription (RT)-PCR diagnostics (screening for infectious salmon anaemia virus [ISAV], viral haemorrhagic septicaemia virus [VHSV], salmon pancreas disease virus [SPDV], sleeping disease virus [SDV] and infectious haematopoetic necrosis virus [IHNV]), and tissue homogenates (heart, liver, kidney and spleen) were sterile-filtered and inoculated into cell cultures. Homogenates made from several tissues were also injected intraperitoneally into salmon and rainbow trout Oncorhynchus mykiss. The diagnostic tests revealed no consistent findings of any pathogens, with the exception of TEM which showed 2 types of virus-like particles: Type I was 50 to 60 nm in diameter and Type II about 50 nm in diameter. These virus-like particles were found in salmon from all farms affected by HSS and screened by TEM. Several different cells, blood vessel endothelial cells, endocardial cells, heart myofibres, and leukocytes were associated with the 2 virus-like particles. The Type I particle seems to be an infectious pancreatic necrosis (IPN)-like virus, while (based on the number of target cells, particle morphology, budding and uptake into target cells) Type II particle could be a togavirus.

Isolation and characterisation of segment 1 of the infectious salmon anaemia virus genome

The isolation and characterisation of the largest genomic segment of infectious salmon anaemia virus (ISAV) is reported. Following identification of ISAV-specific clones from a cDNA library, a rapid amplification of cDNA ends-PCR strategy was designed to obtain the sequence of the full length mRNA transcript. The full length open reading frame (ORF) of this gene was shown to be 2169 nucleotides in length, encoding a putative protein of 722 aa. This sequence was demonstrated by RT-PCR to be specific to ISAV-infected cell cultures. The start codon of this ORF was preceded by the ISAV consensus sequence 5' GCTAAGA 3' indicating the full 5' end of the gene to have been obtained. Based on protein size and amino acid composition, this protein was shown to be similar to the PB2 protein of other orthomyxoviruses. Furthermore, a bipartite nuclear localisation signal was identified in the C-terminus of the protein as is found on all of the influenza virus P proteins. Expression of the putative PB2 as a green fluorescent marker protein-fusion protein confirmed that this protein exhibited nuclear localisation in a fish cell line. Sequences of the ISAV segment 1 gene were obtained from Scottish, Norwegian and Canadian ISAV isolates. Analyses confirmed the close genetic relationship between Norwegian and Scottish ISAV and indicated that this segment was among the most conserved of the ISAV genes identified to date. Thus, this evidence strongly suggests that the genomic segment 1 of ISAV encodes a polymerase protein which is thought to be analagous in function to the PB2 protein of influenza viruses.

Strain variation, based on the hemagglutinin gene, in Norwegian ISA virus isolates collected from 1987 to 2001: indications of recombination

Infectious salmon anemia (ISA) is caused by a virus that probably belongs to the Orthomyxoviridae and was first recorded in Norway in 1984. The disease has since spread along the Norwegian coast and has later been found in Canada, Scotland, the Faroe Islands, Chile, and the USA. This study presents sequence variation of the hemagglutinin gene from 37 ISA virus isolates, viz. one isolate from Scotland, one from Canada and 35 from Norway. The hemagglutinin gene contains a highly polymorphic region (HPR), which together with the rest of the gene sequence provides a good tool for studies of epizootics. The gene shows temporal and geographical sequence variation, where certain areas are dominated by distinct groups of isolates. Evidence of transmission of ISA virus isolates within and between regions is given. It is suggested that the hemagglutinin gene from different isolates may recombine. Possible recombination sites are found within the HPR and in the 5'-end flanking region close to the HPR.

Cloning and identification of the infectious salmon anaemia virus haemagglutinin

Infectious salmon anaemia virus (ISAV) is an orthomyxo-like virus that causes serious disease in Atlantic salmon (Salmo salar). Like the orthomyxoviruses, ISAV has been shown to possess haemagglutinin (HA) activity. This study presents the cloning, expression and identification of the ISAV HA gene, which was isolated from a cDNA library by immunoscreening. The HA gene contained an ISAV-specific conserved nucleotide motif in the 5' region and a 1167 bp open reading frame encoding a protein with a predicted molecular mass of 42.4 kDa. The HA gene was expressed in a baculovirus system. A monoclonal antibody (MAb) shown previously to be directed against the ISAV HA reacted with insect cells infected with recombinant baculovirus. Salmon erythrocytes also adsorbed to these cells and adsorption was inhibited by the addition of either the ISAV-specific MAb or a polyclonal rabbit serum prepared against purified virus, confirming the virus specificity of the reaction. Immunoblot analyses indicated that ISAV HA, in contrast to influenza virus HA, is not posttranslationally cleaved. Sequence comparisons of the HA gene from five Norwegian, one Scottish and one Canadian isolate revealed a highly polymorphic region that may be useful in epidemiological studies.

Use of RT-PCR for diagnosis of infectious salmon anaemia virus (ISAV) in carrier sea trout Salmo trutta after experimental infection

The emergence of infectious salmon anaemia virus (ISAV) in Canada and Scotland and frequent new outbreaks of the disease in Norway strongly suggest that there are natural reservoirs for the virus. The main host for the ISA virus is probably a fish occurring in the coastal area, most likely a salmonid fish. Since sea trout is an abundant species along the Norwegian coast, common in areas where ISA outbreaks occur, and possibly a life-long carrier of the ISA virus, a study was initiated to evaluate reverse transcriptase polymerase chain reaction (RT-PCR) for diagnosis of the virus in experimentally infected trout. Several tissues (kidney, spleen, heart and skin) were collected from the trout during a 135 d period. The following diagnostic methods for detection of the ISA virus were compared: cell culture (Atlantic Salmon Kidney, ASK cells), challenge of disease-free salmon with blood from the infected trout, and RT-PCR. The RT-PCR was the most sensitive of these methods. With the help of this technique it was possible to pick out positive individuals throughout the experimental period of 135 d. Challenge of disease-free salmon were more sensitive than cell culture (ASK cells). These 2 latter methods require use of the immunofluorescent antibody test (IFAT) or RT-PCR for verification of presence of ISA virus.