Espen Rimstad

Heart and skeletal muscle inflammation of farmed salmon is associated with infection with a novel reovirus

Atlantic salmon (Salmo salar L.) mariculture has been associated with epidemics of infectious diseases that threaten not only local production, but also wild fish coming into close proximity to marine pens and fish escaping from them. Heart and skeletal muscle inflammation (HSMI) is a frequently fatal disease of farmed Atlantic salmon. First recognized in one farm in Norway in 1999[1], HSMI was subsequently implicated in outbreaks in other farms in Norway and the United Kingdom[2]. Although pathology and disease transmission studies indicated an infectious basis, efforts to identify an agent were unsuccessful. Here we provide evidence that HSMI is associated with infection with piscine reovirus (PRV). PRV is a novel reovirus identified by unbiased high throughput DNA sequencing and a bioinformatics program focused on nucleotide frequency as well as sequence alignment and motif analyses. Formal implication of PRV in HSMI will require isolation in cell culture and fulfillment of Kochs postulates, or prevention or modification of disease through use of specific drugs or vaccines. Nonetheless, as our data indicate that a causal relationship is plausible, measures must be taken to control PRV not only because it threatens domestic salmon production but also due to the potential for transmission to wild salmon populations.

Infectious salmon anaemia virus.

Infectious salmon anaemia virus (ISAV) is a commercially important orthomyxovirus causing disease in farmed Atlantic salmon. The cumulative mortality in a net pen during an outbreak may vary from insignificant to more than 90%. The infection is spread by management activity such as well‐boat traffic, but possibly also through contact with wild fish. In many of its aspects, including the structure of the virus particle and replication strategy, the ISAV is similar to the influenza viruses. Variations between ISAV and the influenza viruses can mostly be related to differences in the temperature at which replication occurs and the immune response of their respective host animals. ISAV shows both haemagglutinating and receptor‐destroying activity. The variability of the ISAV haemagglutinin molecule is concentrated around a small domain close to the transmembrane region. The function of this variable region is unknown, but it may be related to a recent or ongoing crossing of a species barrier. Alignment studies based on genetic data indicate that the phylogenetic relationship to the influenza viruses is distant, and that ISAV therefore could possibly warrant a new genus within Orthomyxoviridae.

A novel totivirus and piscine reovirus (PRV) in Atlantic salmon (Salmo salar) with cardiomyopathy syndrome (CMS).

BackgroundCardiomyopathy syndrome (CMS) is a severe disease affecting large farmed Atlantic salmon. Mortality often appears without prior clinical signs, typically shortly prior to slaughter. We recently reported the finding and the complete genomic sequence of a novel piscine reovirus (PRV), which is associated with another cardiac disease in Atlantic salmon; heart and skeletal muscle inflammation (HSMI). In the present work we have studied whether PRV or other infectious agents may be involved in the etiology of CMS.ResultsUsing high throughput sequencing on heart samples from natural outbreaks of CMS and from fish experimentally challenged with material from fish diagnosed with CMS a high number of sequence reads identical to the PRV genome were identified. In addition, a sequence contig from a novel totivirus could also be constructed. Using RT-qPCR, levels of PRV in tissue samples were quantified and the totivirus was detected in all samples tested from CMS fish but not in controls. In situ hybridization supported this pattern indicating a possible association between CMS and the novel piscine totivirus.ConclusionsAlthough causality for CMS in Atlantic salmon could not be proven for either of the two viruses, our results are compatible with a hypothesis where, in the experimental challenge studied, PRV behaves as an opportunist whereas the totivirus might be more directly linked with the development of CMS.

Characterization of the Infectious Salmon Anemia Virus Genomic Segment That Encodes the Putative Hemagglutinin

ABSTRACT The genomic segment encoding the putative hemagglutinin of infectious salmon anemia virus (ISAV) is described. Expression of the putative hemagglutinin in a salmon cell line demonstrated hemadsorptive properties of the protein for salmon erythrocytes. The polypeptide was recognized by an ISAV-specific monoclonal antibody. Nucleotide sequencing indicated the occurrence of a variable region in the hemagglutinin gene.

Molecular and functional characterization of two infectious salmon anaemia virus (ISAV) proteins with type I interferon antagonizing activity

In this study we characterize two proteins encoded by the two smallest genomic segments of the piscine orthomyxovirus infectious salmon anaemia virus (ISAV). Both proteins, encoded by the un-spliced ORF from genomic segment 7 (s7ORF1) and the larger ORF from segment 8 (s8ORF2), are involved in modulation of the type I interferon (IFN) response. The data suggests that the s7ORF1 protein is collinearly encoded, non-structural, contains no nuclear localisation signals, localises mainly to the cytoplasmic perinuclear area and does not bind single- or double-stranded RNA. On the other hand, genomic segment 8 uses a bicistronic coding strategy and the encoded s8ORF2 protein is a structural component of the viral particle. This protein contains two nuclear localisation signals, has a predominantly nuclear localisation, binds both double-stranded RNA and poly-A tailed single-stranded RNA, but not double-stranded DNA. In poly I:C stimulated salmon cells both ISAV proteins independently down-regulate the type I IFN promoter activity. Thus, ISAV counteracts the type I IFN response by the action of at least two of its gene products, rather than just one, as appears to be the case for other known members of the Orthomyxoviridae.

Piscine orthoreovirus (PRV) infects Atlantic salmon erythrocytes.

Piscine orthoreovirus (PRV) belongs to the Reoviridae family and is the only known fish virus related to the Orthoreovirus genus. The virus is the causative agent of heart and skeletal muscle inflammation (HSMI), an emerging disease in farmed Atlantic salmon (Salmo salar L.). PRV is ubiquitous in farmed Atlantic salmon and high loads of PRV in the heart are consistent findings in HSMI. The mechanism by which PRV infection causes disease remains largely unknown. In this study we investigated the presence of PRV in blood and erythrocytes using an experimental cohabitation challenge model. We found that in the early phases of infection, the PRV loads in blood were significantly higher than in any other organ. Most virus was found in the erythrocyte fraction, and in individual fish more than 50% of erythrocytes were PRV-positive, as determined by flow cytometry. PRV was condensed into large cytoplasmic inclusions resembling viral factories, as demonstrated by immunofluorescence and confocal microscopy. By electron microscopy we showed that these inclusions contained reovirus-like particles. The PRV particles and inclusions also had a striking resemblance to previously reported viral inclusions described as Erythrocytic inclusion body syndrome (EIBS). We conclude that the erythrocyte is a major target cell for PRV infection. These findings provide new information about HSMI pathogenesis, and show that PRV is an important factor of viral erythrocytic inclusions.

Immunohistochemical detection of piscine reovirus (PRV) in hearts of Atlantic salmon coincide with the course of heart and skeletal muscle inflammation (HSMI)

Aquaculture is the fastest growing food production sector in the world. However, the increased production has been accompanied by the emergence of infectious diseases. Heart and skeletal muscle inflammation (HSMI) is one example of an emerging disease in farmed Atlantic salmon (Salmo salar L). Since the first recognition as a disease entity in 1999 it has become a widespread and economically important disease in Norway. The disease was recently found to be associated with infection with a novel reovirus, piscine reovirus (PRV). The load of PRV, examined by RT-qPCR, correlated with severity of HSMI in naturally and experimentally infected salmon. The disease is characterized by epi-, endo- and myocarditis, myocardial necrosis, myositis and necrosis of the red skeletal muscle. The aim of this study was to investigate the presence of PRV antigens in heart tissue of Atlantic salmon and monitor the virus distribution in the heart during the disease development. This included target cell specificity, viral load and tissue location during an HSMI outbreak. Rabbit polyclonal antisera were raised against putative PRV capsid proteins μ1C and σ1 and used in immunohistochemical analysis of archived salmon heart tissue from an experimental infection. The results are consistent with the histopathological changes of HSMI and showed a sequential staining pattern with PRV antigens initially present in leukocyte-like cells and subsequently in cardiomyocytes in the heart ventricle. Our results confirm the association between PRV and HSMI, and strengthen the hypothesis of PRV being the causative agent of HSMI. Immunohistochemical detection of PRV antigens will be beneficial for the understanding of the pathogenesis of HSMI as well as for diagnostic purposes.

Immunomagnetic separation of a Norwalk-like virus (genogroup I) in artificially contaminated environmental water samples

Rabbit polyclonal antibodies were raised against a recombinant capsid protein from a genogroup I Norwalk-like virus (NLV). Magnetic beads coated with these antibodies were used in immunomagnetic separation (IMS) of the NLV. After capture of the NLV and washing of the beads, viral RNA was heat released and detected by RT-PCR. This IMS procedure was shown to have high sensitivity for detection of homologous NLV, while capture of a genogroup II NLV was less efficient. Antigen capture was not influenced by the content of humic acids in the samples. The combination of IMS and heat release was found to be more efficient than organic extraction of RNA from water contaminated with humic acids. The efficacy and simplicity of IMS/heat release render this combination a feasible tool for the preparation of NLV RNA from environmental samples, although the antigenic diversity of NLV may be a complicating factor.

The viral RNA 3'- and 5'-end structure and mRNA transcription of infectious salmon anaemia virus resemble those of influenza viruses.

Summary. The nucleotide sequences of the termini of two of the genomic segments of the negative strand RNA virus infectious salmon anaemia virus (ISAV) were determined. The sequence of the terminal 9 nucleotides at both ends of the viral RNAs was identical, and showed distinctive sequence homology with the conserved terminal sequences found in the orthomyxoviruses. For both ISAV genomic segments a computer-based secondary structure modelling indicated that the terminal 21–24 nucleotides were able to form self-complementary panhandle structures. Comparison with ISAV-derived mRNA sequences showed that ISAV mRNAs have heterogeneous 5′-ends, and are polyadenylated from a signal sequence 13–14 nucleotides downstream of the 5′-end terminus of the vRNA. Furthermore, the in vitro replication of ISAV was hindered by the RNA polymerase II inhibitor α-amanitin. These findings indicate that the mechanisms for replication of ISAV are similar to those of the orthomyxoviruses, and add to the previously reported structural similarities between ISAV and the orthomyxoviruses.

Detection of norovirus genotype I.3b and II.4 in bioaccumulated blue mussels using different virus recovery methods

Noroviruses (NoVs) are the most common non bacterial human pathogens associated with shellfish borne gastroenteritis. Norovirus detection is based on molecular procedures such as reverse transcriptase (RT)-PCR. A variety of methods have been developed to extract viral RNA from complex shellfish matrixes and to reduce the level of RT-PCR inhibitors. The present study had three objectives: 1) Determine the most appropriate sample treatment protocol for detection of NoVs in mussels, 2) Examine whether there is a variation of the binding affinity between a NoV GI and a GII strain to mussel digestive tissue and how this influences the detection sensitivity, 3) Establish an internal control for sample processing and virus detection. Three RNA extraction methods were evaluated on extracts from blue mussels (Mytilus edulis) spiked with NoV GII.4. The most efficient RNA extraction method was subsequently used for evaluation of three virus recovery methods of blue mussels bio accumulated with NoV GI.3b and GII.4. Mengovirus was evaluated as an internal process control and TaqMan RT-PCRs were used for virus detection. Elution of the two viruses from shellfish tissue differed, indicating a difference in binding affinities. Only a method based upon Proteinase K digestion followed by NucliSenseasyMAG was able to detect both NoV GI.3b and GII.4 (3.0% and 3.5% recovery respectively). The results show that the processing method influences the possibility to detect different variants of NoV.