Vertebral column deformity with curved cross-stitch vertebrae in Norwegian seawater-farmed Atlantic salmon, Salmo salar L

Pathological changes in the vertebral column of farmed Atlantic salmon in Norway have been reported since the 1990s. Based on the characteristic radiographic findings, we here present a vertebral column deformity named "curved cross-stitch vertebrae" that mainly affects the middle aspect of the vertebral column. Sixty fish, from the west/northwest coast of mid-Norway, were sampled at slaughter and examined by radiography, computed tomography (CT), necropsy, macrophotography, and histology. The vertebral deformities were radiographically graded as mild, moderate, or marked. The main differences between these grades of changes were defined by increased curving of the peripheries of endplates, reduced intervertebral spaces, and vertical displacement of the vertebrae. The curved rims of endplates were located peripheral to a continuous and approximately circular borderline. The CT studies revealed small, multifocal, hypo-attenuating, round to crescent-shaped areas in the notochord, compatible with the presence of gas. Additionally, histology revealed that the axial parts of endplates had circular zones with perforations, through which either notochordal tissue prolapsed into the vertebrae or vascularized fibrochondroid proliferations extended from the vertebrae into the notochord. Inflammation was present in many vertebral bodies. To the best of our knowledge, this is the first report of gas in the notochord of fish.

Melanized focal changes in skeletal muscle in farmed Atlantic salmon after natural infection with Piscine orthoreovirus (PRV)

Melanized focal changes in skeletal muscle of farmed Atlantic salmon (Salmo salar) are a major quality problem. The aetiology is unknown, but infection with Piscine orthoreovirus (PRV) has been associated with the condition. Here, we addressed the pathogenesis of red and melanized focal changes and their association with PRV. First, a population of farmed fish (PRV-negative prior to sea transfer) was sequentially investigated throughout the seawater period. The fish were autopsied and tested for PRV infection. Muscular changes were described by macroscopy and histology, and a classification system was established. Second, in an experimental infection trial, PRV was injected intramuscularly to induce changes. The farmed fish was gradually infected with PRV. Red focal changes occurred throughout the observation period with a low prevalence regardless of PRV status. Melanized changes were highly diverse and their prevalence increased during the trial. Changes of low macroscopic grade and histological category were more prevalent in PRV-negative fish. Diffuse granulomatous melanized changes only occurred after PRV infection. No muscular changes were observed in the experimentally challenged fish. Our studies do not indicate that PRV infection causes red focal changes, but seems important in the development of granulomatous melanized changes.

Morphological and functional development of the interbranchial lymphoid tissue (ILT) in Atlantic salmon (Salmo salar L)

The interbranchial lymphoid tissue (ILT) of Atlantic salmon originates from an embryological location that in higher vertebrates gives rise to both primary and secondary lymphoid tissues. Still much is unknown about the morphological and functional development of the ILT. In the present work a standardized method of organ volume determination was established to study its development in relation to its containing gill and the thymus. Based on morphological findings and gene transcription data, the ILT shows no signs of primary lymphoid function. In contrast to the thymus, an ILT-complex first became discernible after the yolk-sac period. After its appearance, the ILT-complex constitutes 3-7% of the total volume of the gill (excluding the gill arch) with the newly described distal ILT constituting a major part, and in adult fish it is approximately 13 times larger than the thymus. Confined regions of T-cell proliferation are present within the ILT. Communication with systemic circulation through the distal ILT is also highly plausible thus offering both internal and external recruitment of immune cells in the growing ILT.

Piscine orthoreovirus (PRV) in red and melanised foci in white muscle of Atlantic salmon (Salmo salar)

Melanised focal changes (black spots) are common findings in the white skeletal muscle of seawater-farmed Atlantic salmon (Salmo salar). Fillets with melanised focal changes are considered as lower quality and cause large economic losses. It has been suggested that red focal changes (red spots) precede the melanised focal changes. In the present work, we examined different populations of captive and wild salmon for the occurrence of both types of changes, which were investigated for the presence of different viruses by immunohistochemistry and RT-qPCR. The occurrence of red or melanised foci varied significantly between the populations, from none in wild fish control group, low prevalence of small foci in fish kept in in-house tanks, to high prevalence of large foci in farm-raised salmon. Large amounts of Piscine orthoreovirus (PRV) antigen were detected in all foci. No other viruses were detected. Red focal changes contained significantly higher levels of PRV RNA than apparently non-affected areas in white muscle of the same individuals. Some changes displayed a transient form between a red and melanised pathotype, indicating a progression from an acute to a chronic manifestation. We conclude that PRV is associated with the focal pathological changes in the white muscle of farmed Atlantic salmon and is a premise for the development of focal melanised changes.

Molecular detection and genotyping of Aphanomyces astaci directly from preserved crayfish samples uncovers the Norwegian crayfish plague disease history

Aphanomyces astaci causes crayfish plague in European freshwater crayfish, but most historical epizootics lack agent isolation and identification. Although declared as crayfish plague outbreaks by the Norwegian Competent Authorities, only presumptive diagnoses without agent isolation exist from Norwegian epizootics until 2005. Molecular methods now allow both A. astaci detection and genotype determination from preserved samples. We therefore aimed to (1) investigate molecularly if A. astaci was involved in a selection of mass-mortality events in Norwegian noble crayfish populations from 1971 to 2004, and (2) determine the eventually involved A. astaci genotype groups both from these historical and also more recent mass-mortality events. DNA was extracted directly from presumptively infected crayfish tissues, and screened by A. astaci specific qPCR. A representative selection of positive samples was confirmed by ITS-sequencing. Finally, genotype determination was performed with microsatellite markers that distinguish all known A. astaci genotype groups. The molecular examination detected A. astaci in crayfish materials from all examined mass-mortality events. The first event in 1971-1974 was caused by the A. astaci genotype group A, presumably the first genotype group that entered Europe more than 150 years ago. All later outbreaks were caused by the A. astaci genotype group B which was introduced to Europe by importation of signal crayfish in the 1960s. The results suggest that molecular methods can verify the involvement of A. astaci in the vast majority of observed crayfish mass mortalities in Europe whenever preserved materials exist. Moreover, microsatellite genotyping can reveal at least parts of the underlying epidemiology.

'Candidatus Branchiomonas cysticola' is a common agent of epitheliocysts in seawater-farmed Atlantic salmon Salmo salar in Norway and Ireland

The prevalence and geographical distribution of the recently described endosymbiont 'Candidatus Branchiomonas cysticola' in Atlantic salmon Salmo salar gill epithelial cell cysts was investigated in seawater-farmed fish suffering proliferative gill inflammation (PGI). To this end, we developed a specific and sensitive real-time PCR assay for detection of the bacterium. 'Ca. B. cysticola' was found to be highly prevalent in Atlantic salmon gills sampled over 7 yr and from 17 geographically distant seawater locations in Norway and Ireland. 'Ca. B. cysticola' was found in significantly greater quantities in fish with large numbers of epitheliocysts, and fluorescence in situ hybridization confirmed its localisation within cysts. 'Ca. Piscichlamydia salmonis', a bacterium previously linked to epitheliocysts, was identified at relatively low levels of infection, apparently independent of epitheliocyst prevalence. These results suggest that 'Ca. B. cysticola' is the main cyst-forming bacterium in seawater-farmed Atlantic salmon in the studied countries. Our results also suggest a relationship between load of 'Ca. B. cysticola' and extent of pathological changes. Taken together with a previously described association between epitheliocyst load and severity of PGI in Norwegian salmon, the results could indicate a role for 'Ca. B. cysticola' in gill diseases such as PGI.

A sequential study of incomplete Freund's adjuvant-induced peritonitis in Atlantic cod

Development of diagnostic and prophylactic methodologies is dependent on knowledge of the host's defence system and reaction to different vaccine adjuvants. Here we present a sequential morphological study of peritonitis and inflammatory cell processing of incomplete Freund's adjuvant (IFA) in intraperitoneally injected Atlantic cod. The peritoneal tissue responses were characterised using necropsy, histology and electron microscopy. An extensive inflammatory response as characterised by leukocyte morphology and contents of enzymes, presence of apoptotic cells and IFN-γ-expressing cells was observed. Three days post injection, IFA droplets were surrounded by different types of inflammatory cells and two different patterns could be discerned. The first was characterised by flattened and concentrically arranged interdigitating cells connected by desmosomes and with macrophage-like cells (MLCs) predominant in the periphery. The second type possessed four stratified layers with an inner layer containing many apoptotic MLCs; a second layer containing flattened and shrunken cells and outer layers comprising moderately flattened cells and an outermost layer of mononuclear cells expressing IFN-γ. Oil was detected both inside and outside MLCs. The two types of processes, of which the second was clearly stratified, were similar to those observed in other teleosts, indicating a variety of reaction modes or alternatively sequential process development. The numerous dead MLCs contributed to inflammation.

Exophiala angulospora causes systemic inflammation in atlantic cod Gadus morhua

Species of Exophiala are opportunistic fungal pathogens that may infect a broad range of warm- and cold-blooded animals, including salmonids and Atlantic cod. In the present study, we observed abnormal swimming behaviour and skin pigmentation and increased mortality in cod kept in an indoor tank. Necropsy revealed foci of different sizes with a greyish to brownish colour in internal organs of diseased fish. The foci consisted of ramifying darkly pigmented fungal hyphae surrounded by distinct layers of inflammatory cells, including macrophage-like cells. In the inner layer with many hyphae, the macrophage-like cells were dead. We observed no apparent restriction of fungal growth by the inflammatory response. A darkly pigmented fungus was repeatedly isolated in pure culture from foci of diseased fish and identified as Exophiala angulospora using morphological and molecular characters. This species has not been previously reported to cause disease in cod, but has been reported as an opportunistic pathogen of both marine and freshwater fish. Based on the morphology and sequence analysis presented here, we conclude that E. angulospora caused the observed chronic multifocal inflammation in internal organs of cod, leading to severe disease and mortality.

Presence and interaction of inflammatory cells in the spleen of Atlantic cod, Gadus morhua L., infected with Francisella noatunensis

Serious infectious diseases, accompanied by macrophage-dominated chronic inflammation, are common in farmed Atlantic cod. To increase knowledge relating to morphological aspects of such inflammatory responses, cod were challenged with Francisella noatunensis, an important bacterial pathogen of this fish species. Tissue and cell dynamics in the spleen were examined sequentially over 60 days. Small clusters of mainly macrophage-like cells (MLCs) staining for non-specific esterase and acid phosphatase developed with time. These foci were transiently infiltrated by pleomorphic proliferating cells of unknown nature and by granulocyte-like cells (GCLCs) staining for peroxidase and lysozyme. The latter cell type, which appeared to be resident in the red pulp of control fish, migrated into the inflammatory foci of infected fish. Cells expressing genes encoding IFN-γ and IL-8 increased in number during the study period. Bacteria were detected only in the MLCs and their number increased despite the extensive inflammation. Our results demonstrate an intimate spatial relationship in inflammatory foci between at least three cell types. The presence of GCLCs, together with MLCs, suggests pyogranulomatous inflammation as a more appropriate descriptive term than granulomatous inflammation.

Microbial and pathological findings in farmed Atlantic salmon Salmo salar with proliferative gill inflammation

Proliferative gill inflammation (PGI) is an important cause of loss in seawater-farmed Atlantic salmon in Norway. Several microbes have been associated with PGI, including the commonly but not exclusively observed inclusions (epitheliocysts) within the gill lamellae related to infection with 'Candidatus Piscichlamydia salmonis'. Atlantic salmon transferred in the spring of 2004 to 12 seawater farms situated in mid- and southwest Norway were sampled throughout that year. Outbreaks of PGI, as evaluated by clinical examination, histology, and mortality data, were diagnosed in 6 of 7 farms in southwest Norway but not in the 5 farms studied in mid-Norway. Generally, mortality started 3 to 5 mo after seawater transfer and outbreaks lasted at least 1 to 3 mo. 'Ca. P. salmonis' was detected by real-time PCR only in fish from PGI-affected farms and our results indicate an association between 'Ca. P. salmonis' load and PGI severity. Likewise, although widely distributed in all 12 farms studied, epitheliocyst prevalence and number per fish as observed by histology appears associated with PGI prevalence and severity. However, the occurrence of epitheliocysts showed no association with molecular detection of 'Ca. P. salmonis', suggesting that at least 1 other organism is responsible for many of the observed inclusions. A microsporidian, Desmozoon lepeophtherii, was identified at high prevalence regardless of fish and farm PGI status, but at higher loads in fish with PGI. Our results support a multifactorial etiology for PGI in which 'Ca. P. salmonis', an unidentified epitheliocyst agent, and the microsporidian are contributing causes. No evidence for the involvement of Atlantic salmon paramyxovirus in PGI development was identified in the present study. High water temperatures and ectoparasites probably exacerbated mortality.

Nodavirus provokes subclinical encephalitis and retinochoroiditis in adult farmed Atlantic cod, Gadus morhua L

Viral nervous necrosis (VNN) caused by beta-nodavirus affects many species of farmed marine fish, in particular juveniles. Apparently healthy, normally feeding, adult farmed Atlantic cod, Gadus morhua, were sampled in a farm 14 months after an outbreak of VNN with clinical signs. Following necropsy, brain and eye tissues were examined by histology, immunohistochemistry and polymerase chain reaction (PCR). Nodavirus-provoked cell death and inflammation was detected in eye and brain, particularly in the retina and cerebellum and differed from that previously described in Atlantic cod during clinical stages of VNN. Virus was detected both by PCR and immunohistochemistry. This is, to the best of our knowledge, the first description of pathological changes associated with chronic subclinical nodavirus infection in Atlantic cod. Our observations suggest that severe infection and pathological changes may go undetected if investigations are restricted to clinical examination and macroscopic evaluation at necropsy.

Molecular characterisation of Atlantic salmon paramyxovirus (ASPV): a novel paramyxovirus associated with proliferative gill inflammation

Atlantic salmon paramyxovirus (ASPV) was isolated in 1995 from gills of farmed Atlantic salmon suffering from proliferative gill inflammation. The complete genome sequence of ASPV was determined, revealing a genome 16,968 nucleotides in length consisting of six non-overlapping genes coding for the nucleo- (N), phospho- (P), matrix- (M), fusion- (F), haemagglutinin-neuraminidase- (HN) and large polymerase (L) proteins in the order 3'-N-P-M-F-HN-L-5'. The various conserved features related to virus replication found in most paramyxoviruses were also found in ASPV. These include: conserved and complementary leader and trailer sequences, tri-nucleotide intergenic regions and highly conserved transcription start and stop signal sequences. The P gene expression strategy of ASPV was like that of the respiro-, morbilli- and henipaviruses, which express the P and C proteins from the primary transcript and edit a portion of the mRNA to encode V and W proteins. Sequence similarities among various features related to virus replication, pairwise comparisons of all deduced ASPV protein sequences with homologous regions from other members of the family Paramyxoviridae, and phylogenetic analyses of these amino acid sequences suggested that ASPV was a novel member of the sub-family Paramyxovirinae, most closely related to the respiroviruses.

First cases of amoebic gill disease (AGD) in Norwegian seawater farmed Atlantic salmon, Salmo salar L., and phylogeny of the causative amoeba using 18S cDNA sequences

Amoebic gill disease (AGD) was observed in seawater farmed Atlantic salmon at four geographically distant locations on the western coast of Norway. To the best of our knowledge, these are the first detected AGD outbreaks in Norway. The outbreaks lasted for 7-12 weeks in late autumn 2006 and were for the most part concurrent. The crude, cumulative mortality was in the range of 12-20% at three farms and 82% at a fourth. The histopathology showed uniform parasomal amoebae in lesions characteristic for AGD. Another gill disease, proliferative gill inflammation (PGI), was also present to a variable degree and the distinction between the two gill problems is discussed. Seawater temperatures were 3.5 degrees C higher than average before disease outbreaks, which subsided in early winter. The geographical and time pattern of these outbreaks strongly indicates simultaneous infection from the marine environment. Two contiguous 18S cDNA sequences, obtained by reverse transcriptase PCR from gill tissue with AGD-related lesions, showed highest similarity (99.2%) to a newly recognized species designated Neoparamoeba perurans and maximum likelihood analysis demonstrates that they represent Norwegian strains of this Neoparamoeba lineage.

Myocardial glycogen storage disease in farmed rainbow trout, Oncorhynchus mykiss (Walbaum)

This paper is the first description of a spontaneous glycogen-storage disease in a lower vertebrate, as previous descriptions deal with humans and other mammals, or fish where the condition has been experimentally induced. Affected farmed rainbow trout experienced increased mortality from 60 days post-startfeeding and displayed clinical signs of heart failure with abnormal behaviour, exophthalmia, distended abdomen and ventral skin petechiation. Necropsy revealed alterations in cardiac shape with distended atria and rounded ventricles. Microscopically, the compact wall of the ventricle was absent, uneven or thinner than normal. The cardiac myocytes contained extensive amounts of glycogen in cytoplasmic vacuoles as demonstrated by periodic acid-Schiff staining that was abolished by saliva-diastase pretreatment on serial sections. Associated lesions included conspicuous subepicardial and myocardial vascularization, epicardial thickening and necrosis of the ventricular compactum/spongiosum interphase. The lesions in cardiac myocytes had a striking resemblance to glycogenosis type II (Pompe disease), a rare autosomal recessive lysosomal storage disease in humans. This condition was more severe and mortality was higher in a replicate/parallel fish group treated perorally with 17alpha-methyltestosterone to produce all-female progeny, indicating that the hormone treatment aggravated the condition resulting in earlier and more severe manifestation of the disease in this group.

Atlantic salmon paramyxovirus (ASPV) infection contributes to proliferative gill inflammation (PGI) in seawater-reared Salmo salar

Proliferative gill inflammation (PGI) causes significant losses in farmed Atlantic salmon Salmo salar L. in Norway, especially during the first months following seawater transfer. The aetiology is apparently multifactorial, including infection with chlamydia-like bacteria and Atlantic salmon paramyxovirus (ASPV). In the present study, gills from diseased fish from 3 farms on the western coast of Norway were sampled. The pathological changes were briefly described and the aetiological significance of ASPV studied by immunofluorescent staining of cryosections and by immunohistochemistry on sections of formalin-fixed and paraffin-embedded tissue. The pathological changes were macroscopically characterized by palour of the gills, and histologically by inflammation, circulatory disturbances, cell death and epithelial cell proliferation. ASPV was demonstrated in fish from all farms studied, as immunostaining consistent with ASPV was obtained in lamellar epithelial and endothelial cells of pathologically altered tissues. It is concluded that ASPV is at least a contributing cause of PGI. As far as we know, this is the first demonstration of fish disease related to infection with a paramyxovirus.

Isolation and partial characterization of a novel paramyxovirus from the gills of diseased seawater-reared Atlantic salmon (Salmo salar L)

A formerly undescribed virus has been isolated from the gills of farmed Atlantic salmon post-smolts in Norway suffering from gill disease. Cytopathic effects appeared in RTgill-W1 cells 9 weeks post-inoculation with gill tissue material. Virus production continued for an extended period thereafter. Light and electron microscopic examination revealed inclusions and replication in the cytoplasm. The viral nucleocapsid consisted of approximately 17 nm thick filaments in a herringbone pattern. Certain areas of the plasma membrane were thickened by the alignment of nucleocapsids on the internal surface and projections of 10 nm long viral glycoprotein spikes on the external surface. Virus assembly and release was achieved by budding through the modified plasma membrane. Negatively stained virions were spherical and partly pleomorphic with a diameter of 150-300 nm as seen by electron microscopy. The virus was sensitive to chloroform, heat and low and high pH, and replication was not inhibited by Br-dU or IdU indicating an RNA genome. Both haemagglutination and receptor-destroying enzyme activity were associated with the virions and the formation of syncytia in infected cultures indicated fusion activity. The receptor-destroying enzyme was identified as neuraminidase. The virus contained five major structural polypeptides with estimated molecular masses of 70, 62, 60, 48 and 37 kDa. Its buoyant density was 1.18-1.19 g ml(-1) in CsCl gradients. From the observed properties we conclude that this new virus belongs to the Paramyxoviridae and suggest the name Atlantic salmon paramyxovirus (ASPV). Furthermore, replication occurred at 6-21 degrees C, suggesting a host range confined to cold-blooded animals.

Infection by Parvicapsula sp. (Myxozoa) is associated with mortality in sea-caged Atlantic salmon Salmo salar in northern Norway

In March 2002, 3 seawater farms in northern Norway experienced high mortality among Atlantic salmon postsmolts. A myxosporean parasite assigned to the genus Parvicapsula was detected in the pseudobranchs of diseased fish, and extensive destruction of this organ was observed. The parasite was also found in the gills, liver and kidney of some fish. Based on host species, spore morphology, and the unusual site preference of the parasite, it is likely that it represents a hitherto undescribed species. The diseased fish had been transferred to seawater in September 2001, and it is believed that the infection took place shortly after exposure to seawater. The source of infection is unknown.

Platyspondyly and shortness of vertebral column in farmed Atlantic salmon Salmo salar in Norway--description and interpretation of pathologic changes

Body malformation due to shortness of the vertebral column, in most cases of unknown cause, has been observed in fish for more than 100 yr. It periodically occurs with high prevalence in farmed Atlantic salmon Salmo salar in Norway, and this paper describes the results of macroscopic, radiographic and histologic examination of parr and seawater-transferred fish. The vertebral bodies in both age groups did not acquire the length that they normally should due to a growth disturbance leading to the condition of platyspondyly and shortness in the column. The pathologic changes became visible at different ages in both groups and the process apparently starts in intervertebral tissues. There was proliferation of connective tissue and blood vessels, and sometimes infiltration with inflammatory cells, around affected vertebrae, especially in seawater-transferred fish. This is the first description of inflammation in abnormally short-spined fish, and it may indicate an infectious etiology, at least in farmed seawater-transferred salmon.

Male pseudohermaphroditism in a cat

A seven-month-old male pedigree cat was brought to the Norwegian College of Veterinary Medicine for routine castration. Visual examination of the external genitalia revealed a wide genital cleft with non-fused bilaterally located testicular pouches. A large clitoris, which was penis-like with small penile spines, was seen protruding dorsally from the ventral commissure of the genital cleft. During an exploratory coeliotomy, no intra-abdominal genital structures of müllerian origin were found. The skin pouches on either side of the vulvar cleft were incised and grossly normal testicles were removed. Histology of the removed gonads showed no or very sparse spermatogenesis. The chromosomal sex was determined by karyotyping to be a normal male 38XY. Based on these findings, the diagnosis of male pseudohermaphroditism was made. The aetiology of the condition in this cat was not determined.