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.

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.

Mortality and weight loss of Atlantic salmon, Salmon salar L., experimentally infected with salmonid alphavirus subtype 2 and subtype 3 isolates from Norway.

Pancreas disease (PD) caused by salmonid alphavirus (SAV) has a significant negative economic impact in the salmonid fish farming industry in northern Europe. Until recently, only SAV subtype 3 was present in Norwegian fish farms. However, in 2011, a marine SAV 2 subtype was detected in a fish farm outside the PD-endemic zone. This subtype has spread rapidly among fish farms in mid-Norway. The PD mortality in several farms has been lower than expected, although high mortality has also been reported. In this situation, the industry and the authorities needed scientific-based information about the virulence of the marine SAV 2 strain in Norway to decide how to handle this new situation. Atlantic salmon post-smolts were experimentally infected with SAV 2 and SAV 3 strains from six different PD cases in Norway. SAV 3-infected fish showed higher mortality than SAV 2-infected fish. Among the SAV 3 isolates, two isolates gave higher mortality than the third one. At the end of the experiment, fish in all SAV-infected groups had significantly lower weight than the uninfected control fish. This is the first published paper on PD to document that waterborne infection produced significantly higher mortality than intraperitoneal injection.

Serum enolase: a non-destructive biomarker of white skeletal myopathy during pancreas disease (PD) in Atlantic salmon Salmo salar L.

Diseases which cause skeletal muscle myopathy are some of the most economically damaging diseases in Atlantic salmon, Salmo salar L., aquaculture. Despite this, there are limited means of assessing fish health non-destructively. Previous investigation of the serum proteome of Atlantic salmon, Salmo salar L., during pancreas disease (PD) has identified proteins in serum that have potential as biomarkers of the disease. Amongst these proteins, the enzyme enolase was selected as the most viable for use as a biomarker of muscle myopathy associated with PD. Western blot and immunoassay (ELISA) validated enolase as a biomarker for PD, whilst immunohistochemistry identified white muscle as the source of enolase. Enolase was shown to be a specific marker for white muscle myopathy in salmon, rising in serum concentration significantly correlating with pathological damage to the tissue.

The serum proteome of Atlantic salmon, Salmo salar, during pancreas disease (PD) following infection with salmonid alphavirus subtype 3 (SAV3).

Salmonid alphavirus is the aetological agent of pancreas disease (PD) in marine Atlantic salmon, Salmo salar, and rainbow trout, Oncorhynchus mykiss, with most outbreaks in Norway caused by SAV subtype 3 (SAV3). This atypical alphavirus is transmitted horizontally causing a significant economic impact on the aquaculture industry. This histopathological and proteomic study, using an established cohabitational experimental model, investigated the correlation between tissue damage during PD and a number of serum proteins associated with these pathologies in Atlantic salmon. The proteins were identified by two-dimensional electrophoresis, trypsin digest and peptide MS/MS fingerprinting. A number of humoral components of immunity which may act as biomarkers of the disease were also identified. For example, creatine kinase, enolase and malate dehydrogenase serum concentrations were shown to correlate with pathology during PD. In contrast, hemopexin, transferrin, and apolipoprotein, amongst others, altered during later stages of the disease and did not correlate with tissue pathologies. This approach has given new insight into not only PD but also fish disease as a whole, by characterisation of the protein response to infection, through pathological processes to tissue recovery. BIOLOGICAL SIGNIFICANCE: Salmonid alphavirus causes pancreas disease (PD) in Atlantic salmon, Salmo salar, and has a major economic impact on the aquaculture industry. A proteomic investigation of the change to the serum proteome during PD has been made with an established experimental model of the disease. Serum proteins were identified by two-dimensional electrophoresis, trypsin digest and peptide MS/MS fingerprinting with 72 protein spots being shown to alter significantly over the 12week period of the infection. The concentrations of certain proteins in serum such as creatine kinase, enolase and malate dehydrogenase were shown to correlate with tissue pathology while other proteins such as hemopexin, transferrin, and apolipoprotein, altered in concentration during later stages of the disease and did not correlate with tissue pathologies. The protein response to infection may be used to monitor disease progression and enhance understanding of the pathology of PD.

A comparative study of marine salmonid alphavirus subtypes 1-6 using an experimental cohabitation challenge model.

A comparative challenge study of six marine isolates representing subtypes 1-6 of salmonid alphavirus (salmon pancreas disease virus, Genus Alphavirus, Family Togaviridae) was conducted in Atlantic salmon in a fresh water cohabitation trial. Histopathological lesions typical of pancreas disease were observed with all subtypes, and virus was re-isolated from serum of cohabitant fish in each case. Using a virus neutralization (VN) test neutralizing salmonid alphavirus (SAV) subtype 1 strain F93-125, VN antibodies were detected in all challenge groups, consistent with serological cross-reactivity between these subtypes. Using real-time RT-PCR, SAV RNA was detected in heart tissue from 2 to 3 weeks post-challenge (wpc) in all cohabitant groups excluding controls. The results obtained suggested differences in the dynamics of infection between strains of SAV and potentially between subtypes. Results for SAV subtypes 1 and 3 suggested essentially synchronous infection of cohabitant fish. These two study groups also had the highest virus load in heart tissue as measured by quantitative RT-PCR and also had the most extensive histopathological changes. In contrast, results for SAV subtypes 2 and 6 strains were consistent with asynchronous infection in the cohabitant fish and were characterized by slow spread, low virus loads and mild histopathological changes. The SAV subtype 4 and 5 strains occupied an intermediate position in this regard. Despite the use of concentration procedures, it was not possible to detect SAV RNA in water samples from selected study tanks. However, testing of faeces from the SAV subtypes 1, 3 and 6 challenge groups found positive signals in each beginning at 1-3 wpc and remaining detectable for a further 2-3 weeks. Parallel testing of mucus samples found these became positive at 2-3 wpc and remained positive for a further 1-3 weeks. These results demonstrate for the first time that shedding and transmission of virus may occur by both these routes and suggest that dispersal in these matrices should be included in any disease transmission models.

Prospective longitudinal studies of salmonid alphavirus infections on two Atlantic salmon farms in Ireland; evidence for viral persistence.

Prospective longitudinal studies of two outbreaks of pancreas disease in Atlantic salmon (AS), Salmo salar L., in Ireland were conducted. Both outbreaks occurred during the marine phase of production, with one caused by salmonid alphavirus subtype 1 (SAV1) and the other by SAV4. In addition to screening a range of tissues by real-time reverse transcriptase polymerase chain reaction (RRT-PCR), virological, serological and histopathological examinations were performed along with partial genome sequencing and results were related to environmental and production data and farm history. On Farm 1 (marine sampling only), infection was detected within 3 weeks of smolts being placed on the farm, while on Farm 2 (freshwater and marine sampling), infection was first detected 315 days after transfer to sea. In both outbreaks, RRT-PCR signals were detected in a range of tissues including gill, heart, kidney, pancreas/pyloric caeca, brain and serum. Persistence of signal was longest in gill and heart (> or =265 days on both farms) and shortest in serum. Mortalities on the two farms varied from 10.9% to 30%. In both cases, partial genome sequence of the causative viruses were identical to SAV strains detected in previous populations of AS on each of the study farms, including populations with which the study populations overlapped in time and space.

Alphavirus infections in salmonids--a review.

The first alphavirus to be isolated from fish was recorded in 1995 with the isolation of salmon pancreas disease virus from Atlantic salmon, Salmo salar L., in Ireland. Subsequently, the closely related sleeping disease virus was isolated from rainbow trout, Oncorhynchus mykiss (Walbaum), in France. More recently Norwegian salmonid alphavirus (SAV) has been isolated from marine phase production of Atlantic salmon and rainbow trout in Norway. These three viruses are closely related and are now considered to represent three subtypes of SAV, a new member of the genus Alphavirus within the family Togaviridae. SAVs are recognized as serious pathogens of farmed Atlantic salmon and rainbow trout in Europe. This paper aims to draw together both historical and current knowledge of the diseases caused by SAVs, the viruses, their diagnosis and control, and to discuss the differential diagnosis of similar pathologies seen in cardiomyopathy syndrome and heart and skeletal muscle inflammation of Atlantic salmon.

Pancreas disease in farmed Atlantic salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), in Norway.

The present paper describes, for the first time, clinical signs and pathological findings of pancreas disease (PD) in farmed Atlantic salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), in sea water in Norway. Similarities and differences with reports of PD from Ireland and Scotland are discussed. Samples of 68 rainbow trout from disease outbreaks on 14 farms and from 155 Atlantic salmon from outbreaks on 20 farms collected from 1996 to 2004 were included in the present study. The histopathological findings of PD in Atlantic salmon and rainbow trout in sea water were similar. Acute PD, characterized by acute necrosis of exocrine pancreatic tissues, was detected in nine Atlantic salmon and three rainbow trout. Salmonid alphavirus (SAV) was identified in acute pancreatic necroses by immunohistochemistry. Most fish showed severe loss of exocrine pancreatic tissue combined with chronic myositis. Myocarditis was often but not consistently found. Kidneys from 40% and 64% of the rainbow trout and Atlantic salmon, respectively, had cells along the sinusoids that were packed with cytoplasmic eosinophilic granules. These cells resembled hypertrophied endothelial cells or elongated mast cell analogues. Histochemical staining properties and electron microscopy of these cells are presented. SAV was identified by RT-PCR and neutralizing antibodies against SAV were detected in blood samples.

Experimental infection of Atlantic salmon Salmo salar pre-smolts by i.p. injection with new Irish and Norwegian salmonid alphavirus (SAV) isolates: a comparative study.

Atlantic salmon Salmo salar L. pre-smolts were experimentally infected with 2 different isolates of salmonid alphavirus (SAV): a Subtype 1 isolate from Ireland and a Subtype 3 isolate from Norway. Sequential samples of tissue and blood were collected during a period of 20 wk post injection and subjected to virus isolation from kidney tissue and serum, detection of viral nucleic acid in heart tissue and serum by real-time RT-PCR, detection of specific antibodies by virus neutralisation assay, and histopathological examination. Successful reproduction of pancreas disease (PD) was obtained by intraperitoneal (i.p.) injection of both isolates. No mortality was observed post infection in either group, but typical PD histopathological lesions in heart and pancreas tissue were observed with both isolates. The prevalence and severity of lesions in the pancreas, heart, skeletal muscle and brain were similar in both groups with only subtle differences recorded. Re-isolation of virus from kidney tissue was performed at 7 and 14 d post infection (d p.i.) only and was positive for both test groups at both sampling points. Isolation of virus from sera from both groups was positive at 4 to 14 d p.i., but was negative at later sampling points when antibody production had begun. Virus may be detected only during the acute phase using both methods. Specific neutralising antibodies could be detected for both test groups from Day 21 p.i. until the end of the experiment at 140 d p.i. Peak antibody titres were seen 70 d p.i. Using real-time RT-PCR, pancreas disease virus (PDV)-specific RNA was detected frequently in serum samples up to 14 d p.i. and occasionally thereafter. In contrast, viral RNA could still be detected in the heart tissue of fish from both groups for at least 140 d p.i.

Serological, virological and histopathological study of an outbreak of sleeping disease in farmed rainbow trout Oncorhynchus mykiss.

A prospective longitudinal survey for sleeping disease (SD) was carried out over a 20 wk period on a caged freshwater population of farmed rainbow trout Oncorhynchus mkyiss. Pancreas, heart and red and white skeletal muscle were examined histologically and the presence and severity of lesions recorded. Sera were tested for viraemia with Salmonid Alphavirus (SAV) and for virus neutralizing (VN) antibodies. Viraemia was detected for 4 wk, beginning at Week 6 and with a peak prevalence of 57.9% at Week 7. Clinical signs and mortalities appeared at Week 8. Total mortality in the study cage from Week 6 onward was 6.3 %, but other cages at the site had mortality levels of up to 47.2%. VN antibodies were first detected at Week 9, with seroprevalence increasing to 80% by the end of the study (Week 20). Geometric mean antibody titres peaked at 1/89.4 at Week 17. Histological lesions were first detected at Week 7 (pancreas only), before increasing in prevalence and severity to peak at Weeks 9 and 10. The majority of lesions were resolved by Week 15.

Sub-clinical infection of farmed Atlantic salmon Salmo salar with salmonid alphavirus--a prospective longitudinal study.

A prospective longitudinal study of salmonid alphavirus infection in farmed Atlantic salmon Salmo salar L. was initiated in post-transfer smolts on a UK farm in July 2004 and continued for 320 d. Sampling was concentrated on a single caged population (C4) with serum and tissue samples collected and tested for viraemia, virus neutralising (VN) antibodies and viral nucleic acid by real time RT-PCR and by histopathology; 380 sera collected between Days 0 (DO) and 139 (D139) were consistently negative for both viraemia and VN antibodies. The first evidence of infection was detected on D146, when 4 out of 20 fish were found to be viraemic and 1 of 20 to be antibody-positive. On D153 only 2 of 20 fish was viraemic and 1 antibody positive. At the next sampling (D158) no viraemic or antibody positive fish were detected. Thereafter, one or two viraemic fish were detected on 6 occasions, including on D320. The prevalence of antibody-positive fish remained low (0 to 5%) until D192 after which time it rose irregularly to a peak of 57.9% on D320. Real time RT-PCR testing of sera was more sensitive than screening for viraemia, detecting a peak of 35 % positive on D153 before declining. Histological lesions diagnostic for pancreas disease (PD) were observed at D146 and D153 only. In addition, mild cardiac and to a lesser extent brain lesions were frequently found after virus was detected, but not in earlier samples. No clinical signs or mortalities attributable to PD occurred throughout the study. This is the first detailed report of sub-clinical infection and highlights the usefulness of longitudinal surveys and the detection of virus and antibodies as diagnostic and epidemiological tools.

Virological, serological and histopathological evaluation of fish strain susceptibility to experimental infection with salmonid alphavirus.

Pancreas disease (PD) of farmed Atlantic salmon Salmo salar L., which is caused by an alphavirus known as salmon pancreas disease virus (SPDV), can have serious economic consequences. An epidemiological survey carried out in Ireland in 2003 indicated that within individual farms there were significant differences in the susceptibility of different strains of farmed Atlantic salmon to infection with SPDV, as measured by levels of clinical disease and mortality. The aim of this preliminary study was to investigate this field observation by comparing lesion development, viraemia and serological responses of 3 commercial strains of Atlantic salmon (A, B and C) experimentally infected with SPDV. Highly significant differences in the severity of lesions in the pancreas at Day 21 post-infection (pi) were detected (p < 0.01), with Group B being more severely affected. There were also significant differences in the prevalence and severity of lesions in heart and skeletal muscle at Day 21 and 35 pi respectively, with Group B results again significantly higher than those from both Groups A and C (p < 0.05). There was no overlap between viraemia and the presence of specific SPDV antibody. Some fish in all groups had no viraemia, lesions or evidence of seroconversion. There were no significant differences seen between the challenged groups in relation to the percentage of viraemic fish at each time point. Viral loads were not determined. Differences between the number of antibody-positive fish in each challenge group were found at Days 28 and 35 pi (p < 0.1). Highly significant differences (p < 0.01) in the geometric mean titres of seropositive fish were detected at Day 28. These results, obtained using a challenge model, confirm that there are strain differences in the susceptibility to experimental SPDV infection in commercial farmed Atlantic salmon.

Longitudinal serological surveys of Atlantic salmon, Salmo salar L., using a rapid immunoperoxidase-based neutralization assay for salmonid alphavirus.

Longitudinal serological surveys for salmon pancreas disease virus (SPDV), the causal agent of pancreas disease (PD), were conducted on multiple caged populations of Atlantic salmon, Salmo salar L., on two farms over a 77-week period (farm 1, freshwater and marine stages) and a 36-week period (farm 2, marine stage only), using a microtitre-based virus neutralization (VN) assay. Collected sera were also screened for viraemia with SPDV, and pancreas, heart and muscle tissues were examined for lesions consistent with PD. Outbreaks of PD occurred during the marine phase on both farms, as demonstrated by seroconversion, the isolation of virus and progressive histopathological changes consistent with a PD outbreak. All populations monitored showed a progressive increase in seroprevalence of 90-100%, typically accompanied by rises in geometric mean antibody titres. With the exception of one caged population, which showed a marked biphasic seroprevalence pattern, the seroprevalence figures in the remaining four monitored populations remained high (> or =70%) until the end of the study period. Peak VN titres of > or =1/1280 were detected on both farms. The results provide essential baseline information for the interpretation of SPDV VN serology results, and indicate that this methodology is suited to both the diagnosis and seroepidemiology of SPDV infections.

Detection and antigenic characterization of salmonid alphavirus isolates from sera obtained from farmed Atlantic salmon, Salmo salar L., and farmed rainbow trout, Oncorhynchus mykiss (Walbaum).

A simple method of detecting the presence of the salmonid alphaviruses (SAVs), salmon pancreas disease virus (SPDV) and sleeping disease virus (SDV), from serum samples is described. Using a 96-well tissue-culture plate format, test sera are diluted in medium and added to chinook salmon embryo (CHSE-214) cells. After incubation for 3 days at 15 degrees C, plates are fixed and stained using a monoclonal antibody (mAb)-based immunoperoxidase (IPX) detection system, and virus-infected cells are observed microscopically by white light. Application of this screening test, which is now used routinely in our laboratory in conjunction with an IPX-based virus neutralization (IPX-VN) test for detecting antibodies to SAVs, has resulted in the recovery of 12 additional isolates from salmon sera and four additional isolates from trout sera. A low level of antigenic variation was detected when these SAV isolates were investigated by indirect immunofluorescence using a panel of mAbs raised to reference SPDV and SDV isolates.

A rapid immunoperoxidase-based virus neutralization assay for salmonid alphavirus used for a serological survey in Northern Ireland.

A modified virus neutralization (VN) assay was developed to replace an existing assay read on the presence or absence of virus-induced cytopathic effect (CPE). The modified assay used a monoclonal antibody to salmon pancreas disease virus as the first layer of an immunoperoxidase (IPX)-based immunostaining technique to detect viral growth. The IPX-based VN assay required only 3 days to perform, and the adoption of a 96-well microtitre format facilitated a high throughput of samples requiring small volumes of serum, cells and virus. When 352 sera from farmed salmon and 302 sera from farmed trout were tested by both the modified and the original CPE-based assays, overall correlations of 97.72 and 96.03% were, respectively, obtained (96.94% combined). When the modified assay was used to test 188 sera collected from wild salmonids in freshwater river systems in Northern Ireland, no positive results were recorded.

Production and characterisation of monoclonal antibodies to salmon pancreas disease virus.

Six mouse monoclonal antibodies (mAbs) specific to salmon pancreas disease virus (SPDV) were produced following immunisation with purified virus preparations. These mAbs and 2 mAbs resulting from an earlier investigation were characterised. None of the mAbs possessed virus neutralising activity but all reacted with 4 geographically different SPDV isolates as determined by indirect immunofluorescence. Three mAbs produced positive immunostaining with Western blots of SPDV proteins. The 4H1 mAb reacted with the 53 kDa structural E1 glycoprotein present in virus-infected cells and in gradient-purified virus. Two mAbs, 5A5 and 7B2, which exhibited unusual immunofluorescence staining of the nuclear margin, reacted with a 35 kDa protein, which is present in gradient-purified virus and which is considered to be the capsid protein. A sandwich ELISA, based on the use of mAb 2D9 for capture and a biotinylated conjugate of mAb 7A2 for detection, detected SPDV antigen in virus-infected Chinook salmon embryo-214 cells and gradient-purified virus. These mAbs may be of use in pathogenesis studies and in diagnostic test development.

Aspects of the epizootiology of pancreas disease in farmed Atlantic salmon Salmo salar in Ireland.

A computerised database containing information on over 17.8 million salmon contained within 49 separate marine populations was used to study the epidemiology of pancreas disease (PD) in Ireland. Of the 43 recorded PD outbreaks, 57% occurred in the 3 mo period August to October inclusive (17 to 32 wk post-transfer). Analysis of variance of mortality rates during PD outbreaks occurring on 6 marine sites over a 5 yr period showed that mortality rates vary significantly between sites (p < 0.001) but not between years over this time period. The mortality rate during PD outbreaks ranged from 0.1 to 63%. Mortality rates were significantly higher when PD outbreaks occurred earlier in the year (y = -1.28x + 59, SE of b 0.33). The mean length of a PD outbreak was 112 d (SE = 7.7, n = 37). There was no correlation between PD mortality rate and smolt input weight, initial stocking density and transfer mortality.

Salmon pancreas disease virus, an alphavirus infecting farmed Atlantic salmon, Salmo salar L.

A 5.2-kb region at the 3' terminus of the salmon pancreas disease virus (SPDV) RNA genome has been cloned and sequenced. The nucleotide and predicted amino acid sequences show that SPDV shares considerable organizational and sequence identity to members of the genus alphavirus within the family Togaviridae. The SPDV structural proteins encoded by the 5.2-kb region contain a number of unique features when compared to other sequenced alphaviruses. Based on cleavage site homologies, the predicted sizes of the SPDV envelope glycoproteins E2 (438 aa) and E1 (461 aa) are larger than those of other alphaviruses, while the predicted size of the alphavirus 6K protein is 3.2 K (32 aa) in SPDV. The E2 and E1 proteins each carry one putative N-linked glycosylation site, with the site in E1 being found at a unique position. From amino acid sequence comparisons of the SPDV structural region with sequenced alphaviruses overall homology is uniform, ranging from 32 to 33%. While nucleotide sequence analysis of the 26S RNA junction region shows that SPDV is similar to other alphaviruses, analysis of the 3'-nontranslated region reveals that SPDV shows divergence in this region.