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Pacific pink salmon (Oncorhynchus gorbuscha) were deliberately introduced to rivers surrounding the White Sea and has spread to Norway and several other countries surrounding the North Atlantic Ocean. In August 2021, a female pink salmon displaying pale gills and abnormal behaviour was captured in River Lakselva in Northern Norway and later submitted to the Norwegian Veterinary Institute (NVI) for post-mortem examination. Histological examination of organ samples revealed structures indicative of systemic ichthyophoniasis, caused by Ichthyophonus sp. The parasites appeared to be especially abundant in the heart and skeletal musculature, and local tissue responses were assessed to be absent or very mild. Sequences of the ribosomal 18S rRNA and the mitochondrial cytochrome oxidase 1 (CO1) genes confirmed the diagnosis and identified the pathogen as Ichthyophonus sp. The CO1 sequence further established that the isolate from pink salmon was most similar to sequences of Ichthyophonus sp. from Atlantic salmon, Salmo salar, from the Atlantic Ocean on the east coast of the US and from Atlantic herring, Clupea harengus, from Iceland. We here report the first detection of Ichthyophonus sp. in pink salmon in the North Atlantic Ocean., (© 2024 The Author(s). Journal of Fish Diseases published by John Wiley & Sons Ltd.)

Paramoeba perurans causes amoebic gill disease (AGD), which is a major problem in aquaculture worldwide. The parasite can be cultured in vitro, but to this date, no method for long-term storage of the clones exists. In this study, we describe a method for cryopreservation of Paramoeba perurans. The method was successfully employed on four out the five clones we tested. The thawing success rate, that is the percentage of successfully thawed vials relative to the total number of vials that were thawed, differed for the clones and ranged from 25% to 100%. The age of the clones seemed to have a negative impact on the ability to survive cryopreservation., (© 2020 The Authors. Journal of Fish Diseases published by John Wiley & Sons Ltd.)

We report the development of ddPCR assays for single and simultaneous detection of the bacterial pathogens Flavobacterium psychrophilum and Yersinia ruckeri in water from land-based recirculation aquaculture systems (RAS), producing Atlantic salmon (Salmo salar) smolt. The method was tested and verified for use in water analyses from RAS production sites, and proved to be specific and with sensitivity 0.0011 ng DNA for F. psychrophilum and 1.24 ng for Y. ruckeri. These bacteria are important fish pathogens that have caused reoccurring salmonid infection disease in RAS. Monitoring pathogen levels in water samples could be a useful alternative surveillance strategy to evaluate operational risk assessment connected to stress factors. Water quality is essential for fish health and growth in RAS production in general, and high or increasing levels of these pathogens in the RAS water may generate an early indication of unfavourable conditions in the RAS environment, and give directions to operational actions. This approach may reduce fish mortality, reduce production loss, and offer more effective and targeted preventive measures within RAS production., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Author(s). Published by Elsevier B.V. All rights reserved.)

The myxozoan genus Parvicapsula contains 14 species infecting fish, some of which are known to cause severe disease in farmed and wild salmonids. Parvicapsula pseudobranchicola infections were first reported from seawater-reared Atlantic salmon, Salmo salar, in Norway in 2002 and have since then been an increasing problem. The present study describes a Taqman real-time PCR assay for specific detection of P. pseudobranchicola. The Taqman assay targets the 18S rRNA gene of P. pseudobranchicola and is able to detect as few as ten copies of the target sequence. Using the described assay, P. pseudobranchicola was detected in both farmed and wild salmonids, indicating that wild Atlantic salmon, sea trout, Salmo trutta, and Arctic char, Salvelinus alpinus, may be natural hosts of the parasite. Parvicapsula pseudobranchicola was found in samples from wild salmonids in the far south and the far north of Norway, displaying a wide geographic range of the parasite. Farmed salmonids showed P. pseudobranchicola infection levels many folds higher than that observed for wild sea trout, indicating that farmed Atlantic salmon are subjected to an elevated infection pressure compared with wild salmonids., (© 2011 Blackwell Publishing Ltd.)

A prospective longitudinal study was performed on three cages at each of three Norwegian Atlantic salmon seawater sites that experienced outbreaks of pancreas disease (PD). Once salmonid alphavirus (SAV) ribonucleic acid (RNA) was detected by real-time RT-PCR (Rt RT-PCR) at a site, it became detected in all studied cages and was persistently found until the end of the study period up to 19 months after first detection. SAV-specific antibodies were detected at all sites until the end of the study period and were also found at a high prevalence in broodfish at the time of stripping. No evidence of increased viral activity was detected in these broodfish. One site tested negative over several months prior to the first detection of SAV by Rt RT-PCR and SAV-specific antibody, which occurred 1 month prior to clinical manifestations of PD. Moribund fish or thin fish/runts that were sampled after the first PD diagnosis had almost twice the risk of testing positive by one or more diagnostic tests compared to that of randomly selected apparently healthy individuals. This paper describes the first detailed investigation of the disease development of PD at site and cage level in Norway, as well as an assessment of the performance and agreement of the commonly used diagnostic tests.

Extensive mortality in Atlantic salmon fry was reported in the River Aelva from 2002 to 2004. Dead fish were collected in late summer 2006, and live fish were sampled by electrofishing in September the same year. At autopsy and in histological sections, the fish kidneys were found to be pale and considerably enlarged. Proliferative lesions with characteristic PKX cells were seen in a majority of the fish. DNA from kidney samples of diseased fish was subjected to PCR and sequencing, and the amplified sequences matched those of Tetracapsuloides bryosalmonae. We concluded that this myxozoan transmitted from bryozoans was the main cause of the observed mortality in salmon fry in 2006. Results from quantitative electrofishing in 2005 and 2006, combined with the observed fry mortality from 2002 to 2004, show that the smolt production in the river is severely reduced and that T. bryosalmonae is the most likely explanation for this decline. The present study is the first to report a considerable negative population effect in wild Atlantic salmon due to proliferative kidney disease (PKD). It also represents the northernmost PKD outbreak in wild fish. The river is regulated for hydroelectric power purposes, causing reduced water flow and elevated summer temperatures, and the present PKD outbreak may serve as an example of increased disease vulnerability of northern fish populations in a warmer climate.

Within the literature, most discussion of sampling protocols for monitoring aquatic parasites is based on the assumptions of simple random sampling. Recent research has shown that in monitoring parasite abundance on fish farms composed of discrete cages, care must be taken to properly account for the clustering which naturally occurs. This paper illustrates the effect of clustering in the context of monitoring ectoparasitic sea lice Lepeophtheirus salmonis and Caligus elongatus in salmon farms. The degree of clustering of sea lice infections in fish within cages is measured using the intraclass correlation coefficient (ICC). A wide range of ICC values from sites in Scotland and Norway were estimated for the chalimus and mobile stages of L. salmonis, and for C. elongatus mobiles. The analyses indicate that significant clustering of lice infections within cages occurs across lice species and stages on both Scottish and Norwegian farms. A Monte-Carlo simulation using two sets of data from Scottish farms with ICC values for adult L. salmonis of 0.35 [0.08-0.73, 95% CI] and for adult C. elongatus of 0.39 [0.16-0.69, 95% CI] were used to illustrate the implications of clustering. The protocols simulated reflect those typically used across a range of countries and production environments in which salmon are currently reared. The findings demonstrate that the "few fish from many cages" approach results in a marked improvement in precision when sampling aquatic one-host parasites in cage-based production systems.

Parvicapsula pseudobranchicola infections in farmed Atlantic salmon in Norway are associated with low-grade to significant mortalities. The parasite is found as mature spores in pseudobranchs, but has also been detected in the gills, liver and kidney. Diagnosis has relied on the detection of Parvicapsula spores, with the pseudobranch being the preferred organ. A better understanding of the epizootiology of this myxosporean is a prerequisite for appropriate management and control. Hence, early detection of infections and life cycle studies are needed. We sequenced the small subunit (ssu) rDNA (18S) from P. pseudobranchicola and developed a sensitive diagnostic PCR protocol. This allowed us to (1) identify appropriate tissues for diagnostic assays, (2) examine the intraspecific variation in ssu rDNA in the parasite's Norwegian range, (3) examine annelid potential primary hosts and (4) obtain additional ssu rDNA sequences of marine Parvicapsula species to perform a phylogenetic study. Primers were constructed targeting the ssu rDNA from P. minibicornis. With these we obtained a partial ssu sequence of the P. pseudobranchicola type isolate. A new set of primers (PCF3/PCR3) was constructed for diagnostic purposes. These were tested against DNA from the host and several myxozoan species infecting Norwegian salmon. The primers give a positive product of 203 bp and pick out P. pseudobranchicola in salmnonids. They also amplify the congeners P. unicornis and P. asymmetrica infecting unrelated fish. The PCR protocol developed showed a greater sensitivity than light microscopy. The pseudobranchs were always positive and are the recommended organ for PCR diagnostics. There was no sequence variation between geographic isolates from farmed salmon. Preliminary examinations of marine polychaetes and oligochaetes collected from farm sites with parvicapsulose problems were negative. A comparison of the sequence of the ssu rDNA from P. pseudobranchicola with that of other myxozoans shows that it groups closely together with P. unicornis and P. asymmetrica. The closest relative to this group is P. minibicornis.

Osmotic cataracts were diagnosed in all of 191 Atlantic salmon Salmo salar L. postsmolts caught during 8 trawl hauls on the western side of the Vøringsplateau, Norwegian Sea, in June 2001. The changes varied from a hazy opacity in the anterior part of the lens to cataracts affecting the whole lens. Severely affected lenses appeared swollen and large vacuoles were visible in the opaque areas. Large vacuoles in otherwise clear lenses were diagnosed in 1 of 4 adult salmon examined. Histologically, widened sutures, vacuolation of lens epithelium and cortex, and proteinaceous lakes subjacent to the epithelium were the most frequent changes, while extensive cortical necroses and epithelial proliferation were seen in a few cases. UV-absorbance of the aqueous humor was determined and levels compared to plasma levels and also to levels in farmed Atlantic salmon of the same developmental stage. Wild salmon generally showed higher levels of protective factors than farmed fish. The osmotic type of cataract diagnosed leads to poor vision and is a potential cause of reduced survival in postsmolts. The cause of the cataracts could not be determined, but defective osmoregulation is suspected.

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.

In the present study, a total of 1,180 samples of muscle or viscera from Norwegian-farmed salmon (Salmo salar L.) were examined for the presence of nematode larvae. The samples represented all salmon-producing counties in Norway. The method applied was based on the degradation of fish soft tissue in an acidified pepsin enzyme solution. None of the samples examined in this study contained nematodes.

Subclinical infections with a herpesvirus were detected by light and electron microscopic examination of juvenile turbot collected during a national surveillance programme. Virions detected in the epidermis of skin and in the epithelium of gills had a morphology consistent with those of Herpesvirus scophthalmi described from turbot in the United Kingdom and Denmark. This is the first report of herpesvirus infection in turbot in Norway.

Farming of Atlantic halibut Hippoglossus hippoglossus is a growing industry in Norway. This study comprises the first description of heart morphology in fish from one Atlantic halibut farm. Fish with no clinical signs of disease were randomly sampled (8 to 12 mo old, weight 1 to 50 g). The hearts were studied by light and electron microscopy and samples were taken from the kidney and pericardial cavity for bacteriological examination. Slight to severe changes were found in most hearts at all samplings. The most serious pericardial and myocardial changes were seen in larger fish (15 to 50 g). In some of the hearts the affected pericardium formed multifocal protrusions. Eight fish had an adhesion between the apex of the ventricle and the parietal wall of the pericardial cavity. Results of bacteriology and immunohistochemistry investigations for common disease agents were negative.