Multiple Aeromonas strains isolated from Atlantic salmon (Salmo salar) displaying red skin disease signs in Scandinavian rivers

Since 2014, Atlantic salmon (Salmo salar L.) displaying clinical signs of red skin disease (RSD), including haemorrhagic and ulcerative skin lesions, have been repeatedly observed in Swedish rivers. Although the disease has since been reported in other countries, including Norway, Denmark, Ireland and the UK, no pathogen has so far been conclusively associated with RSD. In this study, the presence of 17 fish pathogens was investigated through qPCR in 18 returning Atlantic salmon with clinical signs of the disease in rivers in Sweden and Norway between 2019 and 2021. Several potential pathogens were repeatedly detected, including a protozoan (Ichthyobodo spp.), an oomycete (Saprolegnia spp.) and several bacteria (Yersinia ruckeri, Candidatus Branchiomonas cysticola, Aeromonas spp.). Cultivation on different media from ulcers and internal organs revealed high concentrations of rod-shaped bacteria typical of Aeromonadaceae. Multilocus phylogenetic analysis of different clones and single gene phylogenies of sequences obtained from the fish revealed concurrent isolation of several bacterial strains belonging to the species A. bestiarum, A. piscicola and A. sobria. While these bacterial infections may be secondary, these findings are significant for future studies on RSD and should guide the investigation of future outbreaks. However, the involvement of Aeromonas spp. as putative primary etiological agents of the disease cannot be ruled out and needs to be assessed by challenge experiments.

Genotyping tool for salmonid gill pox virus (SGPV) obtained from farmed and wild Atlantic salmon (Salmo salar)

Poxviruses are common viruses found in vertebrate species. In 2006, the first poxvirus associated with salmon, salmonid gill poxvirus (SGPV), was identified during an outbreak of gill disease at a smolt production site in northern Norway and at two marine farms in western Norway. Poxviruses had previously been detected in ayu (Plecoglossus altivelis) and koi carp (Cyprinus carpio). In all three fish species, poxviruses are associated with gill disease. It has not been possible to culture SGPV from Norway, and little is known about its virulence. However, the association between SGPV and gill disease in salmon has shown the need for molecular tools to identify reservoirs and transmission routes. Sequencing the genome of a second isolate of SGPV has made it possible to compare variable regions between two strains of the virus, showing the presence of a large number of variable regions that exhibit both variable numbers of tandem repeats and intra-ORF variation. We present eight regions that are suitable for distinguishing strains of SGPV and determining their phylogenetic relationship, and these were used to compare SGPV isolates obtained from both farmed and wild salmon in fresh and sea water. The prevalence of the virus was found to be higher in wild salmon in rivers than in returning wild salmon collected from traps in Norwegian fjords. Genotyping based on the eight selected variable regions, suggests the presence of geographically distinct isolates in freshwater among both farmed and wild salmon, while SGPV from marine farms shows high local diversity and a wide geographical distribution of similar strains of the virus.

Wild and farmed salmon (Salmo salar) as reservoirs for infectious salmon anaemia virus, and the importance of horizontal- and vertical transmission

The infectious salmon anaemia virus (ISAV) is an important pathogen on farmed salmon in Europe. The virus occurs as low- and high virulent variants where the former seem to be a continuous source of new high virulent ISAV. The latter are controlled in Norway by stamping out infected populations while the former are spreading uncontrolled among farmed salmon. Evidence of vertical transmission has been presented, but there is still an ongoing discussion of the importance of circulation of ISAV via salmon brood fish. The only known wild reservoirs are in trout (Salmo trutta) and salmon (Salmo salar). This study provides the first ISAV sequences from wild salmonids in Norway and evaluates the importance of this reservoir with respect to outbreaks of ISA among farmed salmon. Phylogenetic analyses of the surface protein hemagglutinin-esterase gene from nearly all available ISAV from Norway, Faeroe Islands, Scotland, Chile and wild salmonids in Norway show that they group into four major clades. Including virulent variants in the analysis show that they belong in the same four clades supporting the hypothesis that there is a high frequency of transition from low to high virulent variants in farmed populations of salmon. There is little support for a hypothesis suggesting that the wild salmonids feed the virus into farmed populations. This study give support to earlier studies that have documented local horizontal transmission of high virulent ISAV, but the importance of transition from low- to high virulent variants has been underestimated. Evidence of vertical transmission and long distance spreading of ISAV via movement of embryos and smolt is presented. We recommend that the industry focus on removing the low virulent ISAV from the brood fish and that ISAV-free brood fish salmon are kept in closed containment systems (CCS).

Presence of selected pathogens on the gills of five wrasse species in western Norway

The objective of this study was to identify gill pathogens in Labridae (wrasse) species used as cleaner fish to control salmon louse in western Norwegian aquaculture. Wrasse are often moved over long distances, raising issues of fish health, welfare and pathogen transmission. Histological examination and real-time RT-PCR analysis of the gills from Centrolabrus exoletus, Ctenolabrus rupestris, Labrus bergylta, L. mixtus and Symphodus melops revealed several pathogens: a new species of Ichthyobodo, Paramoeba perurans, microsporidia, trichodinids, Hatschekia spp., Candidatus Similichlamydia labri and 2 putative new species of Chlamydiae. Cand. S. labri or closely related bacteria were present on most wrasse specimens. Epitheliocysts on the gills of L. mixtus contained large inclusions (120 µm) with actiniae radiating from the inclusion membrane. A possible member of the Candidatus family Parilichlamydiaceae was present at a high prevalence on the gills of L. mixtus, L. bergylta and C. rupestris. Sequencing the 16S rRNA gene showed 93.9% similarity to Cand. S. labri and 96.8% similarity to Cand. Parilichlamydia carangidicola from the gills of Seriola lalandi. This bacterium probably represents a new species within the order Chlamydiales, family Cand. Parilichlamydiaceae. The other Chlamydiae detected on gills of S. melops could represent a new species in Cand. genus Syngnamydia. Ichthyobodo sp. and Paranucleospora theridion were detected on the gills of nearly all individuals, while Paramoeba spp. were detected on the gills of L. bergylta and L. mixtus. Trichodinids, microsporidia and parasitic copepods had low prevalence. Viral haemorrhagic septicaemia virus was not detected.

Infection dynamics and tissue tropism of Parvicapsula pseudobranchicola (Myxozoa: Myxosporea) in farmed Atlantic salmon (Salmo salar)

Background

The myxosporean parasite Parvicapsula pseudobranchicola commonly infects farmed Atlantic salmon in northern Norway. Heavy infections are associated with pseudobranch lesions, runting and mortality in the salmon populations. The life-cycle of the parasite is unknown, preventing controlled challenge experiments. The infection dynamics, duration of sporogony, tissue tropism and ability to develop immunity to the parasite in farmed Atlantic salmon is poorly known. We conducted a field experiment, aiming at examining these aspects.

Methods

Infections in a group of Atlantic salmon were followed from before sea-transfer to the end of the production (604 days). Samples from a range of tissues/sites were analysed using real-time RT-PCR and histology, including in situ hybridization.

Results

All salmon in the studied population rapidly became infected with P. pseudobranchicola after sea-transfer medio August. Parasite densities in the pseudobranchs peaked in winter (November-January), and decreased markedly to March. Densities thereafter decreased further. Parasite densities in other tissues were low. Parasite stages were initially found to be intravascular in the pseudobranch, but occurred extravascular in the pseudobranch tissue at 3 months post-sea-transfer. Mature spores appeared in the pseudobranchs in the period with high parasite densities in the winter (late November-January), and were released (i.e. disappeared from the fish) in the period January-March. Clinical signs of parvicapsulosis (December-early February) were associated with high parasite densities and inflammation in the pseudobranchs. No evidence for reinfection was seen the second autumn in sea.

Conclusions

The main site of the parasite in Atlantic salmon is the pseudobranchs. Blood stages occur, but parasite proliferation is primarily associated with extravascular stages in the pseudobranchs. Disease and mortality (parvicapsulosis) coincide with the completion of sporogony. Atlantic salmon appears to develop immunity to P. pseudobranchicola. Further studies should focus on the unknown life-cycle of the parasite, and the pathophysiological effects of the pseudobranch infection that also could affect the eyes and vision.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2583-9) contains supplementary material, which is available to authorized users.

New virus of the family Flaviviridae detected in lumpfish (Cyclopterus lumpus)

In this study, we determined the complete coding sequence of a putative new member of the family Flaviviridae, named “Cyclopterus lumpus virus” (CLuV), which is associated with a serious disease in lumpfish (Cyclopterus lumpus). The virus was present in all tissues tested, but pathology was primarily observed in the liver and kidneys. CLuV shows low but distinct similarity to the unassigned Tamana bat virus (TABV). Unlike other known members of the family Flaviviridae, translation of the entire CLuV polyprotein is dependent on a − 1 ribosomal frameshift in the NS2A region.

A new aquareovirus causing high mortality in farmed Atlantic halibut fry in Norway

A new aquareovirus was isolated from cultured Atlantic halibut (Hippoglossus hippoglossus) fry at a facility where massive mortalities had occurred during the start-feeding phase. The same virus was also detected in juveniles (about 10 grams) of the 2013 generation at two other production sites, but not in larger fish from generations 2007-2012. The virus replicated in BF-2 and CHSE-214 cell cultures and produced syncytia and plaque-like cytopathic effects. This Atlantic halibut reovirus (AHRV) was associated with necrosis of the liver and pancreas, syncytium formation in these tissues, and distinct viroplasm areas within the syncytium in halibut fry. Transmission electron microscopy revealed that the viroplasm contained virions, non-enveloped, icosahedral particles approximately 70 nm in diameter with a double capsid layer, amorphous material, and tubular structures. The RNA-dependent RNA polymerase (RdRp) gene from the AHRV isolates showed the highest amino acid sequence identity (80 %) to an isolate belonging to the species Aquareovirus A, Atlantic salmon reovirus TS (ASRV-TS). A partial sequence from the putative fusion-associated small transmembrane (FAST) protein of AHRV was obtained, and this sequence showed the highest amino acid sequence identity (46.8 %) to Green River Chinook virus which is an unassigned member of the genus Aquareovirus, while a comparison with isolates belonging to the species Aquareovirus A showed <33 % identity. A proper assessment of the relationship of AHRV to all members of the genus Aquareovirus, however, is hampered by the absence of genetic data from members of several Aquareovirus species. AHRV is the first aquareovirus isolated from a marine coldwater fish species and the second reovirus detected in farmed fish in Norway. A similar disease of halibut fry, as described in this paper, has also been described in halibut production facilities in Canada and Scotland.

A new intracellular bacterium, Candidatus Similichlamydia labri sp. nov. (Chlamydiaceae) producing epitheliocysts in ballan wrasse, Labrus bergylta (Pisces, Labridae)

Certain wrasse species (Labridae) are used as cleaner fish in salmon farms on the Norwegian coast, reducing salmon louse intensities. The pathogen repertoire of wrasse in Norway is poorly known, and the objective of the present study is to describe a novel intracellular bacterium detected in Norwegian Labrus bergylta. Histological examination of gill tissues from ballan wrasse, L. bergylta, revealed epitheliocysts occurring basally to the secondary lamellae in the interlamellar epithelium. Ultrastructurally, these had bacteria-filled inclusions with thickened membranes and radiating ray-like structures (actinae). 16S rRNA gene sequences from the gill bacteria showed the highest (97.1 %) similarity to Candidatus Similichlamydia latridicola from the gills of the latrid marine fish Latris lineata in Australia and 94.9 % similarity to Candidatus Actinochlamydia clariae, causing epitheliocystis in the freshwater catfish Clarias gariepinus in Uganda. A total of 47 gill samples from L. bergylta from Western Norway were screened by real time RT-PCR with an assay targeting Candidatus Actinochlamydiaceae 16S rRNA. Prevalence was 100 %. We propose the name Candidatus Similichlamydia labri sp. nov. for this new agent producing gill epitheliocysts in L. bergylta.

Genomic characterization and phylogenetic position of two new species in Rhabdoviridae infecting the parasitic copepod, salmon louse (Lepeophtheirus salmonis)

Several new viruses have emerged during farming of salmonids in the North Atlantic causing large losses to the industry. Still the blood feeding copepod parasite, Lepeophtheirus salmonis, remains the major challenge for the industry. Histological examinations of this parasite have revealed the presence of several virus-like particles including some with morphologies similar to rhabdoviruses. This study is the first description of the genome and target tissues of two new species of rhabdoviruses associated with pathology in the salmon louse. Salmon lice were collected at different Atlantic salmon (Salmo salar) farming sites on the west coast of Norway and prepared for histology, transmission electron microscopy and Illumina sequencing of the complete RNA extracted from these lice. The nearly complete genomes, around 11,600 nucleotides encoding the five typical rhabdovirus genes N, P, M, G and L, of two new species were obtained. The genome sequences, the putative protein sequences, and predicted transcription strategies for the two viruses are presented. Phylogenetic analyses of the putative N and L proteins indicated closest similarity to the Sigmavirus/Dimarhabdoviruses cluster, however, the genomes of both new viruses are significantly diverged with no close affinity to any of the existing rhabdovirus genera. In situ hybridization, targeting the N protein genes, showed that the viruses were present in the same glandular tissues as the observed rhabdovirus-like particles. Both viruses were present in all developmental stages of the salmon louse, and associated with necrosis of glandular tissues in adult lice. As the two viruses were present in eggs and free-living planktonic stages of the salmon louse vertical, transmission of the viruses are suggested. The tissues of the lice host, Atlantic salmon, with the exception of skin at the attachment site for the salmon louse chalimi stages, were negative for these two viruses.

Characterization of 'Candidatus Syngnamydia salmonis' (Chlamydiales, Simkaniaceae), a bacterium associated with epitheliocystis in Atlantic salmon (Salmo salar L.)

Two Chlamydiales have previously been found to infect Atlantic salmon (Salmo salar L.), Candidatus Piscichlamydia salmonis and Candidatus Clavichlamydia salmonicola. Both develop intracellularly in cyst-like inclusions in gill cells, generally referred to as epitheliocysts. Here, we present evidence for the association of a novel species of Chlamydiales with epitheliocystis in Atlantic salmon. Based on its partial 16S rRNA gene sequence, it is a new member of the family Simkaniaceae, and a 95.7 % identity to the type species Candidatus Syngnamydia venezia suggests inclusion in the candidate genus Syngnamydia. The presence of the bacterium in epitheliocysts in gills of Atlantic salmon was demonstrated by RNA-RNA hybridization. Ultrastructurally, the novel bacterium produces pleomorphic reticulate bodies and elementary bodies (EBs) with a characteristic morphology. The EBs are short rods with a terminal disc-like cap area, a sub-apical spherical vacuole-like electron-lucent structure and a post-equatorial nucleoid. We propose the name Candidatus Syngnamydia salmonis for this new agent from epitheliocysts in seawater-reared salmon .

'Cand. Actinochlamydia clariae' gen. nov., sp. nov., a unique intracellular bacterium causing epitheliocystis in catfish (Clarias gariepinus) in Uganda

Background and Objectives

Epitheliocystis, caused by bacteria infecting gill epithelial cells in fish, is common among a large range of fish species in both fresh- and seawater. The aquaculture industry considers epitheliocystis an important problem. It affects the welfare of the fish and the resulting gill disease may lead to mortalities. In a culture facility in Kampala, Uganda, juveniles of the African sharptooth catfish (Clarias gariepinus) was observed swimming in the surface, sometimes belly up, showing signs of respiratory problems. Histological examination of gill tissues from this fish revealed large amounts of epitheliocysts, and also presence of a few Ichthyobodo sp. and Trichodina sp.

Methods and Results

Sequencing of the epitheliocystis bacterium 16S rRNA gene shows 86.3% similarity with Candidatus Piscichlamydia salmonis causing epitheliocystis in Atlantic salmon (Salmo salar). Transmission electron microscopy showed that the morphology of the developmental stages of the bacterium is similar to that of members of the family Chlamydiaceae. The similarity of the bacterium rRNA gene sequences compared with other chlamydia-like bacteria ranged between 80.5% and 86.3%. Inclusions containing this new bacterium have tubules/channels (termed actinae) that are radiating from the inclusion membrane and opening on the cell surface or in neighbouring cells.

Conclusions

Radiation of tubules/channels (actinae) from the inclusion membrane has never been described in any of the other members of Chlamydiales. It seems to be a completely new character and an apomorphy. We propose the name Candidatus Actinochlamydia clariae gen. nov., sp. nov. (Actinochlamydiaceae fam. nov., order Chlamydiales, phylum Chlamydiae) for this new agent causing epitheliocystis in African sharptooth catfish.

Diseases of farmed Atlantic salmon Salmo salar associated with infections by the microsporidian Paranucleospora theridion

The microsporidian Paranucleospora theridion was discovered in Atlantic salmon Salmo salar suffering from proliferative gill disease in a marine farm in western Norway in 2008. The parasite develops in cells of the reticuloendothelial system, cells important for normal immune function. The aim of this study was to see if P. theridion could play a part in some of the diseases with unclear causes in salmon production in Norway, i.e. proliferative gill disease (PGI), pancreas disease (PD), heart and skeletal muscle inflammation (HSMI) and cardiomyopathy syndrome (CMS). P. theridion was present in all areas with salmon farming in Norway, but high prevalence and densities of the parasite in salmon and salmon lice were only seen in southern Norway. This region is also the main area for PGI and PD in Norway. Quantification of pathogens associated with PGI, PD, HSMI and CMS diagnoses showed that P. theridion levels are high in southern Norway, and may therefore play a role in susceptibility and disease development. However, among the different diagnoses, fish with PGI are particularly heavily infected with P. theridion. Therefore, P. theridion appears as a possible primary agent in cases with high mortality in connection with PGI in western Norway.

Characterization of 'Candidatus Clavochlamydia salmonicola': an intracellular bacterium infecting salmonid fish

The phylum Chlamydiae contains obligate intracellular bacteria, several of which cause disease in their hosts. Morphological studies have suggested that this group of bacteria may be pathogens of fish, causing cysts in epithelial tissue - epitheliocystis. Recently, the first genetic evidence of a chlamydial aetiology of this disease in seawater reared Atlantic salmon from Norway and Ireland was presented, and the agent was given the name 'Candidatus Piscichlamydia salmonis'. In this article we present molecular evidence for the existence of a novel Chlamydiae that also may cause epitheliocystis in Norwegian salmonids. This novel Chlamydiae has been found in salmonid fish from freshwater, and based on its partial 16S rRNA gene, it may constitute a third genus in the family Chlamydiaceae, or a closely related sister family. By using whole-mount RNA-RNA hybridization we demonstrate how infected cells are distributed in a patchy manner on a gill arch. The morphology of the novel Chlamydiae includes the characteristic head-and-tail cells that have been described earlier from salmonid fish suffering from epitheliocystis. We propose the name 'Candidatus Clavochlamydia salmonicola' for this agent of epitheliocystis in freshwater salmonids.

Transmission of infectious salmon anaemia virus (ISAV) in farmed populations of Atlantic salmon (Salmo salar)

In the present study, 24 smolt production sites were screened for the presence of infectious salmon anaemia virus (ISAV) with the help of a specific real-time RT PCR assay, and 22 of these sites had smolts that were positive. If these smolt production sites are representative for the prevalence of ISAV in Norwegian smolts, then most marine production sites must be considered to be positive for ISAV. In addition, 92 European ISAV isolates have been genotyped based on the hemagglutinin-esterase gene (HE), and their distribution pattern was analysed. This pattern has been coupled to information about the origin of smolt, eggs, and broodfish in those cases where it has been possible to obtain such information, and with information about ISAV in neighbouring farms. The pattern suggests that an important transmission route for the ISAV could be that the salmon farming industry in Norway is circulating some of the isolates in the production cycle, i.e. some sort of vertical or transgenerational transmission may occur. It has also been shown that avirluent ISAV isolates are fairly common in Norwegian farmed salmon. Based on this, it is hypothesized that the change from avirulent to virulent ISAV isolates is a stochastic event that is dependent on the replication frequency of the virus and the time available for changes in a highly polymorphic region (HPR) of the HE gene to occur. This, and the possibility that only avirluent ISAV isolates are vertically transmitted, may explain why ISA most often occurs at marine sites and why no more than about 15 farms get ISA every year in Norway.

Prevalence of infectious salmon anaemia virus (ISAV) in wild salmonids in western Norway

Studies of infectious salmon anaemia virus (ISAV), an important pathogen of farmed salmon in Norway, Scotland, the Faeroe Islands, Ireland, Canada, the USA and Chile, suggest that natural reservoirs for this virus can be found on both sides of the North Atlantic. Based on existing information about ISAV it is believed to be maintained in wild populations of trout and salmon in Europe. It has further been suggested that ISAV is transmitted between wild hosts, mainly during their freshwater spawning phase in rivers, and that wild salmonids, mainly trout, are possible carriers of benign wild-type variants of ISAV. Change in virulence is probably a result of deletions of amino acid segments from the highly polymorphic region (HPR) of benign wild-type isolates after transmission to farmed salmon. Hence, it has been suggested that the frequency of new outbreaks of ISA in farmed salmon could partly reflect natural variation in the prevalence of ISAV in wild populations of salmonids. The aims of the present study were to screen for ISAV in wild salmonids during spawning in rivers and to determine the pathogenicity of resultant isolates from wild fish. Tissues from wild salmonids were screened by RT-PCR and real-time PCR. The prevalence of ISAV in wild trout Salmo trutta varied from 62 to 100% between tested rivers in 2001. The prevalence dropped in 2002, ranging from 13 to 36% in the same rivers and to only 6% in 2003. All ISAV were nonpathogenic when injected into disease-free Atlantic salmon, but were capable of propagation, as indicated by subsequent viral recovery. However, non-pathogenic ISAV has also been found in farmed salmon, where a prevalence as high as 60% has been registered, but with no mortalities occurring. Based on the results of the present and other studies, it must be concluded that vital information about the importance of wild and man-made reservoirs for the emergence of ISA in salmon farming is still lacking. This information can only be gained by further screening of possible reservoirs, combined with the development of a molecular tool for typing virulence and the geographical origin of the virus isolates.

Ichthyobodo necator (Kinetoplastida)--a complex of sibling species

Ichthyobodo necator is a parasitic flagellate that attacks fishes, causing disease problems in freshwater worldwide. Findings of similar flagellates in strictly marine fishes have indicated that ichthyobodiosis may be caused by more than 1 flagellate species. We obtained partial small subunit rDNA (ssu rDNA) sequences of 14 Ichthyobodo isolates originating from fishes in Norway, Japan, Singapore, South Africa and Brazil, and identified 8 strains or species, including 2 species infecting cultured salmon in Norway. An Ichthyobodo species isolated from the skin of Atlantic salmon parr in freshwater is suggested to represent L. necator sensu stricto, while another species, showing particular affinity for the gills, infects salmon in both fresh- and seawater. Atlantic cod is infected with a marine Ichthyobodo species unrelated to those infecting salmonids; 2 cyprinids originating from different parts of the world had related Ichthyobodo strains/species, and 2 isolates from unrelated North and South American fishes were also closely related. The phylogenetic relationships of the Ichthyobodo isolates is described, and the implications of the molecular findings on past and future morphological studies of Ichthyobodo spp. are discussed.

Emergence and maintenance of infectious salmon anaemia virus (ISAV) in Europe: a new hypothesis

The present study describes the use of molecular methods in studying infectious salmon anaemia virus (ISAV), an important pathogen of farmed salmon in Norway, Scotland, the Faeroe Islands, Canada, USA and Chile. The nucleotide sequences of the haemagglutinin gene (HA) from 70 ISAV isolates have been analysed for phylogenetic relationship and the average mutation rate of nucleotide substitutions calculated. The isolates constitute 2 major groups, 1 European and 1 North American group. The isolate from Chile is closely related to the North American isolates. The European isolates can be further divided into 3 separate groups reflecting geographical distribution, time of collection, and transmission connected with farming activity. Based on existing information about infectious salmon anaemia (ISA) and new information emerging from the present study, it is hypothesised that: (1) ISAV is maintained in wild populations of trout and salmon in Europe; (2) it is transmitted between wild hosts mainly during their freshwater spawning phase in rivers; (3) wild salmonids, mainly trout, possibly carry benign wild-type ISAV isolates; (4) a change (mutation) in virulence probably results from deletions of amino acid segments from the highly polymorphic region (HPR) of benign wild-type isolates; (5) ISA emerges in farmed Atlantic salmon when mutated isolates are transmitted from wild salmonids or, following mutation of benign isolates, in farmed salmon after transmission from wild salmonids; (6) farming activity is an important factor in transmission of ISAV between farming sites in addition to transmission of ISAV from wild salmonids to farmed salmon; (7) transmission of ISAV from farmed to wild salmonids probably occurs less frequently than transmission from wild to farmed fish due to lower frequency of susceptible wild individuals; (8) the frequency of new outbreaks of ISA in farmed salmon probably reflects natural variation in the prevalence of ISAV in wild populations of salmonids.

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.