Plasma protein changes in Atlantic salmon, Salmo salar L., with infectious salmon anaemia

Moderate to severe anaemia and hypoproteinaemia were reported in a Canadian outbreak of 'haemorrhagic kidney syndrome' in Atlantic salmon, later shown to be caused by a variant of infectious salmon anaemia virus (ISAV). The progressive anaemia associated with ISA has been previously reported, but hypoproteinaemia in salmon infected with European isolates of ISA virus has not been well documented. The present study showed a very significant positive correlation between decreasing haematocrit values and total plasma protein concentrations in Atlantic salmon infected with two Canadian and two Norwegian ISA viral isolates. However, variations in the concentration of individual plasma proteins, typical of acute phase responses in higher vertebrates, were not observed.     

Assessment of infectious salmon anaemia virus prevalence for different groups of farmed Atlantic salmon, Salmo salar L., in New Brunswick

Abstract Infectious salmon anaemia (ISA) virus (ISAV) has been causing disease in New Brunswick since 1996. As a control measure, all fish in an outbreak cage are killed. The objective of this study was to compare ISAV prevalence in cages experiencing an outbreak with healthy cages from the same farm, neighbouring farms and distant farms. Atlantic salmon from five different groups were tested using an RT-PCR test. Groups included moribund fish from a cage experiencing an outbreak (A), healthy fish from an outbreak cage (B), healthy fish from a negative cage from a farm experiencing an outbreak in a different cage (C), healthy fish from a negative farm near an outbreak farm (D), and healthy fish sampled at a negative farm located in an area with only negative farms (E). Apparent prevalences (standard error) for the different groups (A-E) were 0.94 (+/-0.026), 0.41 (+/-0.062), 0.29 (+/-0.040), 0.08 (+/-0.037) and 0.08 (+/-0.037), respectively. All groups were significantly different (P < 0.002) from each other except for groups B and C and groups D and E. Because the prevalence of the virus was significantly higher in the outbreak cage (B) compared with other sites, early harvest of outbreak cages will remove one source of virus. However, ISA negative cages (C) that remain on the positive farm may potentially act as a viral reservoir.     

Estimation of infectious dose and viral shedding rates for infectious pancreatic necrosis virus in Atlantic salmon,Salmo salar L., post‐smolts

Infectious dose and shedding rates are important parameters to estimate in order to understand the transmission of infectious pancreatic necrosis virus (IPNV). Bath challenge of Atlantic salmon post‐smolts was selected as the route of experimental infection as this mimics a major natural route of exposure to IPNV infection. Doses ranging from 10² to 10^?4 50% end‐point tissue culture infectious dose (TCID₅₀) mL^?1 sea water were used to estimate the minimum infectious dose for a Scottish isolate of IPNV. The minimum dose required to induce infection in Atlantic salmon post‐smolts was <10^?1 TCID₅₀ mL^?1 by bath immersion (4 h at 10 °C). The peak shedding rate for IPNV following intraperitoneal challenge using post‐smolts was estimated to be 6.8 × 10³ TCID₅₀ h^?1 kg^?1 and occurred 11 days post‐challenge. This information may be incorporated into mathematical models to increase the understanding of the dispersal of IPNV from marine salmon sites.     

Dietary phosphorus requirement of juvenile Atlantic salmon, Salmo salar L.

The objective of this study was to determine the dietary phosphorus (P) requirement of juvenile Atlantic salmon, Salmon salar L. Triplicate groups of fish (mean initial weight 1.4 g) were fed semipurified, casein-gelatine-based diets containing one of five levels of P (4, 8, 10, 15 and 25 g kg⁻¹) from Ca(H₂PO₄)₂·H₂O, or a commercial feed (17 g kg⁻¹ P) for 9 weeks. Weight gains did not differ significantly among treatment groups fed the experimental diets but were slightly less than gains in fish fed the commercial feed. Feed efficiency (wet weight gain/dry feed consumed) was similar in all groups, averaging 1.45. Availability of dietary P, estimated from apparent retention and apparent digestibility, was 86%. Whole-body P concentrations declined in fish fed diets containing less than 10 g kg⁻¹ P. Fitting a logistic curve to dietary P vs. whole-body P concentrations indicated that a minimum of 11 g kg⁻¹ dietary P (9 g kg⁻¹ digestible P) was required by juvenile Atlantic salmon to maintain whole-body P concentrations at initial levels. Calculation of a dietary requirement using a simple factorial model which incorporated measurements of P availability, feed efficiency and normal whole-body P concentration indicated that the dietary requirement was approximately 10 g kg⁻¹. The dietary requirement established in this study (10–11 g kg⁻¹) is higher than previously reported for Atlantic salmon or other fishes. Possible reasons for the wide range of reported dietary P requirements in fishes are discussed.     

Comparative virulence of Infectious salmon anaemia virus isolates in Atlantic salmon, Salmo salar L.

Infectious salmon anaemia virus (ISAV) surveillance in the Bay of Fundy has identified the existence of a large number of genetically distinct ISAV isolates which appear to be of variable virulence. Genetically distinct isolates are currently being designated based on sequencing of the hyper polymorphic region (HPR) of genomic segment 6, which encodes the haemagglutinin–esterase protein, but it has been difficult to elucidate a clear association between these molecular variations and variations in virulence. This has hampered the establishment of proactive management decisions regarding infected fish, and ISAV infections, regardless of type, must be treated as one. Field data of ISAV infections is difficult to collect and to compare between infections because of a wide range of confounding factors including time of year, fish stock, cage site location, mitigating factors and stressors. An important tool in determining the relationship between molecular differences and virulence comes from analysis of quarantine studies. The goal of this study was to compare the virulence, by co-habitation and intraperitoneal injection, of four regionally common and recent ISAV isolates in a controlled environment. We found significant differences in mortality between ISAV molecular isolates, and present data showing that survival of ISAV infection confers significant resistance to re-infection with a different ISAV isolate. These findings, if borne out in field studies, will significantly alter the way ISAV infections are managed in the Bay of Fundy and elsewhere.     

Mycobacterium salmoniphilum infection in farmed Atlantic salmon, Salmo salar L

Multiple greyish‐white visceral nodules containing abundant rapidly growing and acid‐fast bacteria, subsequently identified as Mycobacterium salmoniphilum, were detected in moribund and newly dead market‐sized fish during a period of increased mortality in an Atlantic salmon, Salmo salar, farm in western Norway. Isolates cultured from diseased fish were phenotypically consistent with Mycobacterium sp. previously isolated from Atlantic salmon [MT 1890 (= NCIMB13533), MT1892, MT1900 and MT1901] in the Shetland Isles, Scotland. Partial sequences of 16S rDNA, ribosomal RNA internal transcribed spacer (ITS1), 65‐kDa heat‐shock protein (Hsp65) and β subunit of RNA polymerase (rpoB) revealed 97‐99% similarity with M. salmoniphilum type strain ATCC 13758^T. The source of infection was not confirmed. Koch’s postulates were fulfilled following experimental challenge of Atlantic salmon with field isolate NVI6598 ( FJ616988 ). Mortality was recorded in experimentally infected fish; however, the infection remained subclinical in the majority of affected fish over the 131‐day challenge period.     

Erythrocytic inclusion body syndrome virus in wild Atlantic salmon, Salmo salar L

An investigation was undertaken to establish aspects of the epizootiology of erythrocytic inclusion bodies in wild Atlantic salmon, Salmo salar L., in Scotland. From 1992 to 1995, adult and juvenile salmon, from Scottish rivers, were screened for the presence of erythrocytic inclusion bodies and haematological parameters measured. The nature of the inclusion bodies was assessed through transmission electron microscopy, negative-staining and blood smear-staining techniques and was demonstrated to be viral in origin with characteristics similar to a member of the family Togaviridae. Specifically, these were a viral genome of single-stranded RNA, spherical virion morphology with an icosahedral core, average size of 70 nm and a buoyant density of 1.15-1.20 g cm(-3). The cytoplasmic inclusions were either large, single inclusions (1-2 microm) or smaller multiple inclusions (0.5-1 microm). A total of 4.2% (n=48) and 27.7% (n=213) of the parr and adult salmon, respectively, were positive for the presence of the inclusions. The intensity of the inclusions, when present, varied from light in parr to moderate and heavy in adults, when graded according to the number of inclusions per field of view. Neither haematological variations nor clinical disease was associated with the presence or absence of viral inclusions.     

Immersion challenge with low and highly virulent infectious salmon anaemia virus reveals different pathogenesis in Atlantic salmon,Salmo salar L.

The salmonid orthomyxovirus infectious salmon anaemia virus (ISAV) causes disease of varying severity in farmed Atlantic salmon, Salmo salar L. Field observations suggest that host factors, the environment and differences between ISAV strains attribute to the large variation in disease progression. Variation in host mortality and dissemination of ISAV isolates with high and low virulence (based on a previously published injection challenge) were investigated using immersion challenge. Virus dissemination was determined using real‐time PCR and immunohistochemistry in several organs, including blood. Surprisingly, the low virulent virus (LVI) replicated and produced nucleoprotein at earlier time points post‐infection compared to the virus of high virulence (HVI). This was particularly noticeable in the gills as indicated by different viral load profiles. However, the HVI reached a higher maximum viral load in all tested organs and full blood. This was associated with a higher mortality of 100% as compared to 20% in the LVI group by day 23 post‐infection. Immersion challenge represented a more natural infection method and suggested that specific entry routes into the fish may be of key importance between ISAV strains. The results suggest that a difference in virulence is important for variations in virus dissemination and pathogenesis (disease development).     

Effect of marine Eubothrium sp. (Cestoda: Pseudophyllidea) on the growth of Atlantic salmon, Salmo salar L.

Atlantic Salmon, Salmo salar L., experimentally infected with marine Eubothrium sp. were kept together with uninfected salmon in the laboratory for 11 months in two tanks, 80 infected and 80 uninfected in each tank. The infected fish had a reduced growth rate compared with the uninfected fish. Significant differences in growth between infected and uninfected fish were not observed until several months post-infection. There was no correlation between the number of Eubothrium sp. and fish weight, indicating that even low intensity alters the growth rate of the salmon. The cestode had the same effect on both sexes of salmon. Haematocrit level was found to be significantly lower in infected compared with uninfected salmon in one of the samples during the experiment.     

Unstable release strategies in reared Atlantic salmon, Salmo salar L.

The effects of year, size, sexual maturity and release date on the probability of recapture of tagged Atlantic salmon, Salmo salar L., released from a Swedish hatchery in the Baltic Sea were examined. The probability of recapture varied among years for individuals which were juvenile when released (1988, 13.5%; 1989, 4.4%; 1990, 1.2%) or previously mature males (1988, 1.9%; 1989, 0.5%; 1990, 0.4%). Body size was positively associated with the probability of recapture in each release year for both life-history types. Inter-annual changes in recapture rates were similar for both large and small smolts. There was a significant effect of life-history type on recapture rates in 1988 and 1989, but not in 1990. There was a significant effect of release date on recapture rates in 1988 and 1990, but not in 1989. The maximum recapture rates were associated with different release dates in each year, i.e. 27 May 1988, 6 June 1989 and 21 June 1990.     

Infectious pancreatic necrosis in Atlantic salmon, Salmo salar L

A clinical and histopathological review was carried out of 21 outbreaks of acute infectious pancreatic necrosis (IPN) in Scottish Atlantic salmon, Salmo salar L., farms (13 marine and eight fresh water) during 1991-2004. A distinctive syndrome was evident in both post-smolts in sea water and fry in fresh water, where liver lesions, which had not previously been associated with IPN, became a consistent finding in addition to the more typical pancreatic and intestinal changes. Initial cases were described in post-smolts in Shetland, but by the end of the period of investigation this type of pathology had extended down the West coast of Scotland and into Ireland. Limited viral strain analysis suggested that similar strains were involved in both fresh water and sea water and that these differed from earlier isolates from rainbow trout, Oncorhynchus mykiss (Walbaum). In fresh water, recovered fish frequently developed a greatly distended intestine associated with accumulation of undigested food. In sea water, after the initial, often significant (50% or more), losses, there were many fish which failed to grow and became chronically emaciated and prone to sea louse infection. Although use of transfer diets containing immune enhancers and the selection of IPN resistant broodstock has reduced losses the disease remains a serious cause of economic loss.     

Genetic diversity of piscine myocarditis virus in Atlantic salmon Salmo salar L. in Ireland

Piscine myocarditis virus (PMCV) is a double‐stranded RNA virus which has been linked to cardiomyopathy syndrome (CMS) in Atlantic salmon (Salmo salar L.). The first recorded outbreak of CMS in Ireland occurred in 2012. Heart tissue samples were collected in the current study from farmed Atlantic salmon from various marine sites around Ireland, and the open reading frames (ORFs) 1 and 3 were amplified and sequenced in order to examine the genetic diversity of PMCV. Results showed PMCV to be largely homogenous in Irish samples, showing little genetic diversity. However, several amino acid positions within both ORF1 and ORF3 showed consistent variations unique to the Irish PMCV strains when compared with previously published Norwegian strains. The phylogeny generated in the present study suggests that PMCV may have been introduced into Ireland in two waves, both coming from the southern part of PMCV's range in Norway. In addition, over three‐quarters of the PMCV strains which were sequenced came from fish not exhibiting any clinical signs of CMS, suggesting that either PMCV is evolving to become less virulent in Ireland or Irish Atlantic salmon are developing immunity to the disease.     

Spatial and non-spatial risk factors associated with cage-level distribution of infectious salmon anaemia at three Atlantic salmon, Salmo salar L., farms in Maine, USA

The distribution of infectious salmon anaemia (ISA) was examined among 80 cages from three Atlantic salmon grow-out farms in Maine, USA that were stocked with smolts from a single hatchery. Cage-level disease was broadly defined as one or more moribund fish testing positive for infectious salmon anaemia virus (ISAV) by RT-PCR and a second confirmatory test (IFAT, culture or genotype sequence). Spatio-temporal and cage-level risks were explored using logistic regression and survival analysis. Non-spatial risk factors associated with ISA, or shortened survival time to disease, included increased predation, trucking company choice for smolt transfers, a finely-sedimented benthic substrate, and smaller average size of smolts at stocking. Univariable analysis identified the time-dependent spatial factor 'adjacency to newly infected cages' to be predictive of new infection in neighbouring cages 11-12 weeks later. However, none of the spatial factors, or their lags retained relevance in multiple-variable models. The results suggest a diffuse distribution of virus exposure throughout infected sites, with host-susceptibility factors probably influencing disease manifestation in individual cages. The narrow focus of the current study may limit application of the findings to other sites and year-classes. However, these data support the relevance of husbandry efforts to optimize fish health in regions affected by ISAV.     

Improved recapture rate of vaccinated sea-ranched Atlantic salmon, Salmo salar L.

Vaccination of sea-ranched Atlantic salmon was conducted in order to investigate if immuno-prophylactic measures could improve their survival. Fish were either vaccinated by bath or injection. A total of 66 000 fish were reared in fresh water at a hatchery on the island of Bornholm and at the presmolt stage were separated in three groups each comprising of 22 000 fish. One group was vaccinated intraperitoneally with a polyvalent vaccine (containing killed Vibrio anguillarum serotype O1 and O2, Yersinia ruckeri and Aeromonas salmonicida ). A second group was bath vaccinated with the corresponding vaccine-components and the third group was used as a non-vaccinated control. One month after vaccination these groups were allocated to three separate net-pens located 500 m from the coastline of the island. After 4 months in the net-pens, 1000 fish from each cage were tagged with Carlin-tags below the dorsal fin. The fish were then released for a migration period in the Baltic Sea. Following a sea period of 40 months (45 months post-vaccination), the recapture rates of the groups were calculated from the returned tags from fishermen. Recapture of the injection vaccinated group was significantly higher (25%) compared with the bath vaccinated fish (14.7%) and the control group (16.8%).