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.     

Observation of a rickettsia-like organism in Atlantic salmon, Salmo salar L., in Ireland

Abstract. A rickettsia-like organism was observed in tissues of farmed Atlantic salmon, Salmo salar L., experiencing mortalities in the West of Ireland. Gross pathology of affected fish included haemorrhages in the skeletal muscle, splenomegaly, congestion of pyloric caeca and yellow mottling of livers, which appeared in ring formations in some cases. Histopathology was most obvious in the spleens, livers, kidneys and hearts, with widespread tissue necrosis and numerous basophilic granules present in the cytoplasm and cytoplasmic vacuoles of a variety of cells. Transmission electron microscopy revealed micro-organisms within membrane-bound cytoplasmic vacuoles similar in appearance to rickettsia.     

Nocardiosis in tank-reared Atlantic salmon, Salmo salar L.

Nocardiosis is an infection caused by aerobic Gram-positive, branching, filamentous rods of the genus Nocardia . The organism is resident in both soil and plants ( Austin & Austin 1993 ; Frerichs 1993 ), and is closely related to Mycobacterium spp. Nocardiosis caused by Nocardia asteroides and N. seriolae (previously N. kampachi ) has been reported in several finfish species, both freshwater and marine. The first reported incidence was recorded by Valdez & Conroy (1963) in neon tetras, Hyphessobrycon innesi (Myers), and subsequently in other fish species, some of commercial significance including rainbow trout, Oncorhynchus mykiss (Walbaum) ( Snieszko, Bullock, Dunbar & Pettijohn 1964 ), brook trout, Salvelinus fontinalis (Mitchill), ( Campbell & MacKelvie 1968 ) and yellowtail, Seriola quinqueradiata (Temminck & Schlegel; Kubota, Kariya, Nakamura & Kira 1968 ). Despite two of these incidences occurring in salmonids, infection by Nocardia spp. in Atlantic salmon, Salmo salar L., has not previously been reported.     

Reproductive strategies in Atlantic salmon, Salmo salar L.

Abstract. Life-history strategies are means by which animals solve the problems of successful reproduction in varying environments. Their development patterns are consequences of responses to the opportunities the environment offers them. Understanding them requires an understanding of the way they evolved, their ontogenetic development, their physiological control, and their adaptive value. The present paper views the salmonids as marine fishes, which have radiated into fresh water through using river beds as protected spawning grounds. It also takes the view that the maturation process has priority over somatic growth in fish, and that it has already been initiated by the time of first feeding. Its completion is environmentally dependent, and can be arrested annually. Whether or not it will be arrested depends on the status of the energy stores of the individual at particular critical times of year. This mechanism has adaptive value both for immediate reproductive success — adequate energy to provision the next generation — and for later overwinter survival, ensuring that if energy stores are inadequate for reproduction they are spared. Atlantic salmon show variation in their reproductive patterns, and examples are given from laboratory and aquaculture experiments to demonstrate some environmental controls which result in these variations. A hypothetical model is presented to account for the operation of these controls.     

Suspected myocardial necrosis in farmed Atlantic salmon, Salmo salar L.: a field case

Arteriosclerosis of the coronary artery has been described as a 'fact of life' for Pacific and Atlantic salmonids due to the high prevalence in spawning fish. The lesions are believed to be the result of overstretching of the highly distensible bulbus arteriosus whereby the endothelium of the main coronary vessel becomes mechanically damaged and a smooth muscle proliferation ensues with resultant partial occlusion of the vessel. The physiological significance for the function of the heart has yet to be demonstrated, but experimental studies show that, for example, swimming performance is compromised in fish in which the coronary artery has been ligated. This paper describes a case of myocardial necrosis in harvest-size Atlantic salmon during and after transportation to the slaughterhouse. Mortality during this process reached 10% in some of the transports and affected fish showed characteristic signs of congestive cardiac failure. Histology revealed extensive myointimal proliferation in the coronary artery and patchy necrosis of the compact ventricular myocardium. Several unfavourable factors such as high water temperature, skeletal malformations and crowding all probably contributed to extra cardiac workload. To the best of our knowledge, this is the first reported field case showing a link between coronary lesions and severe cardiac pathology.     

Genetic markers in Atlantic salmon,Salmo salar L., their potential for experimental studies of gene flow and adaptation

Abstract. In autumn 1992, a number of farmed and wild Atlantic salmon, Salmo salar L., spawners were screened for genetic variability in isoenzymes expressed in low-risk tissues. Samples of muscle and fin tissues were taken by biopsy shortly before spawning. In the Daleelv hatchery a male heterozygous for GPI-3*105 allele was crossed with four females homozygous for the common allele from Matre aquaculture station. In the River Hålva a wild male heterozygous for a GPI-1,2*160 allele was crossed with a female homozygous for the common allele. Another female from Matre aquaculture stations, heterozygous for the MDH-3,4*85 allele was crossed with a male homozygous for the common allele. Mortality was recorded during the egg and fry stages. The families were sampled four times during 1993, each individual was genotyped, and length and weight of the various genotypes recorded and compared. In all families the observed genotypic distributions were in close agreement with the expected Mendelian ratios, even in one family where the mortality was over 95%. There were no differences in performance between the two genotypes in any of the families. The inheritance of the observed variability was verified, and it is concluded that the detected alleles have a potential as genetic markers in experimental studies of gene flow and adaptation.     

Genetic variation in Norwegian piscine myocarditis virus in Atlantic salmon,Salmo salar L.

Cardiomyopathy syndrome (CMS) in Atlantic salmon, Salmo salar L., is a severe cardiac disease characterized by a necrotizing myocarditis involving the atrium and the spongious part of the ventricle. The disease is caused by piscine myocarditis virus (PMCV), a double‐stranded RNA virus likely belonging to the family Totiviridae. The objective of this study was to evaluate the genetic variation in Norwegian PMCV isolates focusing on the putative structural proteins encoded by open reading frames (ORFs) 1 and 3. The virus isolates were sampled from a total of 36 farms along the Norwegian coastline. This study represents the first investigation of PMCV genome variation and shows that Norwegian isolates are highly similar, with the most divergent isolates sharing 98.6% nucleotide identity. Interestingly, amino acid sequence diversity within ORF3 is approximately threefold higher than for ORF1. While phylogenetic analysis based on concatenated nucleotide data covering ORF1 and ORF3 revealed four main clusters, the maximum sequence variation of 1.4% at the nucleotide level suggests that all Norwegian isolates belong to a single genogroup. Substantial sequence variation within farms was also observed, which may complicate future molecular epidemiological investigations. The genetic homogeneity among the Norwegian isolates might facilitate development of both diagnostic tools and an efficient vaccine against CMS in the future.     

Haemorrhagic kidney syndrome of Atlantic salmon, Salmo salar L.

This report describes a new syndrome affecting farmed Atlantic salmon on the Canadian east coast that has resulted in increased morbidity and mortality in affected stocks. The major pathological findings are apparent only microscopically and include renal interstitial haemorrhage and acute tubular necrosis and tubular casting. As a result, the disease has become known as haemorrhagic kidney syndrome (HKS). Affected fish are lethargic and anorectic, and lack external lesions. Clinically, HKS fish are anaemic, hypoproteinaemic and hyperosmolalic, with increased serum concentrations of sodium and chloride. At necropsy, internal changes ranged from apparently normal to include one or several of the following: swelling and/or patchy reddening of the kidney, pale gills, exophthalmos, serosanguinous ascites, darkening of the posterior intestine and splenomegaly. Ultrastructurally, viral inclusions were found in the cytoplasm of erythrocytes of HKS fish, and there were unusual electron‐dense inclusions within the tips of renal tubular microvilli of HKS fish. The significance and relevance of the ultrastructural findings to HKS are unknown. Virus isolation was attempted using CHSE, RTG‐2, FH‐10, BB and EPC cell lines; no virus was isolated. Bacteriological analysis failed to reveal significant pathogens. Analysis of tissues for heavy metals and pesticides was negative. Assays for clostridial toxins, lipopolysaccharide and verotoxins were negative. The aetiology of HKS remains unresolved.     

Incidence of hybridization between Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in Ireland

The substantial growth of the farmed salmon industry in Europe since the 1970s has highlighted concerns regarding the genetic impact of escaped farmed salmon on wild salmonid stocks. High incidences of salmon × trout hybrids have been recorded in rivers situated near intensive salmon farming in Norway and Scotland, which may be indicative of a breakdown in reproductive isolation between salmon, Salmo salar L., and brown trout, Salmo trutta L. In the present study, salmonid fry and 0+ parr were collected from rivers in western Ireland. Allozyme and minisatellite DNA analysis were carried out on fry to determine the frequency of F1 hybrids from 10 rivers located within 38 km of salmon farms and three rivers at least 80 km from salmon farms. A total of 49 hybrids were recorded from 4135 salmonid fry (frequency = 1.2%). Mitochondrial DNA analysis showed that all hybrids arose from Atlantic salmon female × brown trout male crosses. Hybrid parr were recorded from one of the low-risk rivers (1.0%), but were present in seven out of the 10 catchments located within 38 km of salmon farms, with frequencies ranging from 0.7% to 3.1%. The results of the present survey, which represents the first extensive record of the levels of salmon-trout hybridization in Ireland, are discussed in relation to the continued growth of salmon farming in this country.     

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.     

Assessment of flesh colour in Atlantic salmon, Salmo salar L.

The degree of pigmentation in muscle of Atlantic salmon, Salmo salar L., fillets of fish that were fed eight diets fortified with 10, 20, 40, 60, 80.100, 150 and 200 mg astaxanthin kg^?1 and a non-supplemented control diet from 3 to 21 months was assessed using different methods. A tristimulus colorimeter (Minolta Chroma Meter) was used to measure the colour composition of the fillets instrumentally. The colour was also determined using the Roche Colour Card for Salmonids. The concentration of astaxanthin in the muscle was measured by chemical analyses. All measurements for colour were done directly on the epaxial muscle anterior to the dorsal fin. The lightness factor (L *). the red/green chromaticity (a*), the yellow/blue chromaticity (b*) and the saturation C* of the colorimetric readings and the Colour Card scores were compared with the chemical analyses. The astaxanthin concentration in the flesh varied from 1 to 10 mg kg^?1 and the visual appearance of the fillets varied from yellowish-white to red. The relationship between the a*, b* and C* values and the astaxanthin concentration in the muscle was non-linear. Non-linear regression lines were found between the a* value and the astaxanthin concentration in the flesh (r²= 0.974) and the b* value and the astaxanthin concentration in the flesh (r²= 0.984). The instrument was not able to detect differences in astaxanthin concentration at astaxanthin levels above 3-4 mg kg^?1 using the presented method directly on the fillet. The instrument might be useful for rejecting groups of salmon with poor pigmentation. A good linear regression was found between the Colour Card score and the mean astaxanthin concentration in the flesh (r² - 0.992). The Colour Card provided a better prediction of the astaxanthin concentration at higher astaxanthin levels than the Chroma Meter. None of the methods provided a satisfactory prediction of the astaxanthin concentration in the muscle of individual fish using the presented methods.     

Maintenance of genetic variation in hatchery stocks of Atlantic salmon, Salmo salar L.: experiences from the River Bush, Northern Ireland

Abstract. An examination of biochemical-genetic variation at seven polymorphic loci was carried out among five year classes of wild Atlantic salmon, Salar L., in the R. Bush and in a hatchery strain derived from the wild population. Within some of the year classes, gene frequencies at several loci differed significantly between wild and artificially reared salmon. Highly significant temporal variation in gene frequencies was detected among successive year classes of the hatchery strain, while this was less significant among the wild salmon. Samples of wild salmon taken as 0+ and 1 + parr in the river showed no significant temporal variability in allelic frequencies. Heterozygosity levels among the wild and hatchery-reared salmon were comparable, averaging 0·185 and 0·176 respectively. The genetic variability of the artificially reared salmon is discussed in relation to numbers of broodstock and breeding regime used at the hatchery.     

Exophiala salmonis infection in Atlantic salmon Salmo salar L.

Abstract. Infection of Atlantic salmon, Salmo salar L., in seawater with Exophiala salmonis is described. Histological effects consisted principally of granuloma formation, especially in the posterior kidney and spread appeared to occur both by extension and by the liaematogenous route. The nature of the outbreak suggested that the original infection occurred via contaminated food.
A comparison is made between this condition and systemic mycoses in other species.     

Epidemiological observations of pancreas disease of farmed Atlantic salmon, Salmo salar L., in Ireland

Epidemiological investigations into the pancreas disease (PD) of farmed salmon were conducted on populations of Atlantic salmon reared in Ireland during 2003 and 2004. The investigations surveyed all marine salmon farms operating in Ireland through a detailed questionnaire with follow-up farm visits. Information was gathered on 21 populations of fish in 2003 and 14 populations in 2004. Thirteen of the 21 populations suffered PD in 2003 and 12 of the 14 in 2004. The mean mortality due to PD on affected farms was 18.8% in 2003 and 14.8% in 2004 and the loss of growth due to PD was estimated at 11.4% over the 2-year period. The highest risk periods for outbreaks of PD were early summer and early autumn and the farms most seriously affected by PD mortality were in the western counties of Ireland. Factors which showed an indication of association with a PD outbreak or high mortality during a PD outbreak were: livestock movement to another sea site, high feeding rate prior to any PD outbreak, the presence of another PD positive farm in the same water body, greater than 250000 fish on a site, a previous history of PD on a site, a high sea lice burden, and sites located in the western regions of Ireland which reared a specific strain of salmon.