Introduction and temporospatial tracing of piscine orthoreovirus-1 (PRV-1) in Norwegian farmed Atlantic salmon (Salmo salar) after local fallowing.

Piscine orthoreovirus-1 (PRV-1) is a prevalent agent in Atlantic salmon (Salmo salar) and the causative agent of heart and skeletal muscle inflammation (HSMI), an important disease in farmed Atlantic salmon. Investigations into the introduction and dissemination routes of PRV-1 in a field setting have been limited. This study aimed to better understand PRV-1 infections and HSMI-associated mortality under field conditions. We tracked introduction and spread of PRV-1 over one production cycle in a geographically isolated region in Norwegian aquaculture. From five sites, a total of 32 virus isolates were sequenced and genogrouped. The results indicated multiple introductions of PRV-1 to the area, but also revealed a high level of genetic homogeneity among the virus variants. The variants differed from that of the previous production cycle at two out of three sites investigated, suggesting that synchronized fallowing can be a useful tool for preventing dissemination of PRV-1 between generations of fish. Exposure to PRV-1 at the freshwater stage was identified as a potential source of introduction. A low level of HSMI-associated mortality was observed at all sites, with the onset of mortality showing some variation across PRV-1 genogroups. However, the study highlighted the complexity of associating viral genogroups with mortality in a field setting. Overall, this study contributes valuable insights into PRV-1 dynamics in a real-world aquaculture setting, offering potential strategies for disease management and prevention.

Analysing mortality patterns in salmon farming using daily cage registrations.

This study describes the patterns of mortality and investigates the sources of variation in mortality during the marine phase of commercial salmon farming. The study included daily mortality records from stocking to harvest of 21 million salmon from ten hatcheries in 136 fish-groups (fish in the same cage from the same hatchery). The fish was stocked in 2017-2018 at 21 marine farms within two Norwegian companies. The sources of variation in mortality were investigated using multilevel linear regression models with 'fish-group' nested within 'farm' as a random effect, cross-classified with 'hatchery'. In the final model, 'fish-group' was the source of most variation (70%). Furthermore, the mortality categories 'smolt-related mortality', 'infectious diseases' and 'handling and treatment' were responsible for 10%, 17% and 29% of the total number of dead fish respectively. Overall, the study shows that smolt-related mortality is one of the major causes of death in the first part of the production, while handling and treatment was the dominating cause of mortality in total. Mortality varied by fish-group to a large extent. This means that targeted preventive strategies to decrease mortality for individual fish-groups might be more effective than overall measures at farm or hatchery level.

A cohort study of gill infections, gill pathology and gill-related mortality in sea-farmed Atlantic salmon (Salmo salar L.): A descriptive analysis.

Gill disease is an important cause of economic losses, fish mortality and reduced animal welfare in salmonid farming. We performed a prospective cohort study, following groups of Atlantic salmon in Western Norway with repeated sampling and data collection from the hatchery phase and throughout the 1st year at sea. The objective was to determine if variation in pathogen prevalence and load, and zoo- and phytoplankton levels had an impact on gill health. Further to describe the temporal development of pathogen prevalence and load, and gill pathology, and how these relate to each other. Neoparamoeba perurans appeared to be the most important cause of gill pathology. No consistent covariation and no or weak associations between the extent of gill pathology and prevalence and load of SGPV, Ca. B. cysticola and D. lepeophtherii were observed. At sea, D. lepeophtherii and Ca. B. cysticola persistently infected all fish groups. Fish groups negative for SGPV at sea transfer were infected at sea and fish groups tested negative before again testing positive. This is suggestive of horizontal transmission of infection at sea and may indicate that previous SGPV infection does not protect against reinfection. Coinfections with three or more putative gill pathogens were found in all fish groups and appear to be the norm in sea-farmed Atlantic salmon in Western Norway.

Genetic grouping and geographic distribution of Piscine orthoreovirus-1 (PRV-1) in farmed Atlantic salmon in Norway.

Piscine orthoreovirus-1 (PRV-1) is the causative agent of heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon (Salmo salar). However, it has been shown that PRV-1 variants differ in their ability to induce HSMI. The objective of this work was to identify the PRV-1 variants in Norwegian aquaculture and their geographical distribution. Sequencing and subsequent analysis of the five genomic segments (S1, S4, M2, L1 and L2) putatively linked to virulence, made out the basis of the study. Thirty-seven Norwegian PRV-1 isolates were sequenced, and they grouped into eight genogroups based on combinations of the five analyzed genomic segments. Two groups were defined as high-virulent and two low-virulent, based on comparison with PRV-1 reference isolates with known virulence. The remaining four groups were of unknown virulence. The geographic distribution indicated a higher frequency of the high-virulent isolates in the mid- and northern regions. The present study confirms circulation of both high- and low-virulent isolates of PRV-1 in farmed Atlantic salmon in Norway. To reduce the impact of PRV-1 related disease, detection and differentiation between high- and low-virulent genogroups of PRV-1 could be a targeted approach for reduction of high-virulent variants.

Estimating cage-level mortality distributions following different delousing treatments of Atlantic salmon (salmo salar) in Norway.

This retrospective descriptive study estimates cage-level mortality distributions after six immediate delousing methods: thermal, mechanical, hydrogen peroxide, medicinal, freshwater and combination of medicinal treatments. We investigated mortality patterns associated with 4 644 delousing treatment of 1 837 cohorts of farmed Atlantic salmon (Salmo salar) stocked in sea along the Norwegian coast between 2014 and 2017. The mortality is expressed as mortality rates. We found distributions of delta mortality rate within 1, 7 and 14 days after all six delousing treatments, using mortality rate within 7 days before treatments as baseline. The results show that we can expect increased mortality rates after all six delousing methods. The median delta mortality rates after thermal and mechanical delousing are 5.4 and 6.3 times higher than medicinal treatment, respectively, for the 2017 year-class. There is a reduction in the delta median mortality for thermal and freshwater delousing from 2015 to 2019. There is a wide variability in the mortality rates, in particular for thermal delousing. Our results suggest that the variability in delta mortality for thermal delousing has been reduced from the 2014 to 2017 year-class, indicating an improvement of the technique. However, a significant increase in the number of thermal treatments from 14 in 2015 to 738 in 2018 probably contributes to the overall increased mortality in Norwegian salmon farming.

Variations in mucous cell numbers in gills of Atlantic salmon (Salmo salar) presmolt in commercial freshwater farms in Norway.

Fish gills are heavily exposed to the external milieu and may react against irritants with different cellular responses. We describe variations in mucous cell counts in gills from healthy Atlantic salmon (Salmo salar) presmolts in five recirculating aquaculture system (RAS) farms and one flow-through farm. Based on certain criteria, mucous cells were histologically quantified in a defined lamellar region of the gills and the counts were analysed. Immunohistochemistry (IHC) was used to investigate epithelial responses. The median number of total mucous cells in the defined region was 59 per fish. Between the farms, the medians varied from 31 to 101 with the lowest in the flow-through farm. A regression model was fitted with "total mucous cells" as the dependent variable and with "fish length" and "fish farm" as independent variables. The proportion of variation in mucous cell counts explained by the model was twice as high when "fish farm" was included compared to only "fish length." IHC revealed proliferative responses in coherence with high mucous cell numbers. Conclusively, the variation in mucous cell counts depends on combined farm-related factors. Establishing a baseline for mucous cell counts is fundamental in the development of high-throughput monitoring programmes of gill health in farmed fish.

Natural infection of Atlantic salmon (Salmo salar L.) with salmonid alphavirus 3 generates numerous viral deletion mutants.

Salmon pancreas disease virus (SPDV) also referred to as salmonid alphavirus (SAV) is a virus causing pancreas disease in Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss). Although the virus causes an economically important disease, relatively few full-length genome sequences of SAV strains are currently available. Here, we report full-length genome sequences of nine SAV3 strains from sites farming Atlantic salmon geographically spread along the Norwegian coastline. The virus genomes were sequenced directly from infected heart tissue, to avoid culture selection bias. Sequence analysis confirmed a high level of sequence identity within SAV3 strains, with a mean nucleotide diversity of 0.11 %. Sequence divergence was highest in 6K and E2, while lowest in the capsid protein and the non-structural proteins (nsP4 and nsP2). This study reports for the first time that numerous defective viruses containing genome deletions are generated during natural infection with SAV. Deletions occurred in all virus strains and were not distributed randomly throughout the genome but instead tended to aggregate in certain areas. We suggest imprecise homologous recombination as an explanation for generation of defective viruses with genome deletions. The presence of such viruses, provides a possible explanation for the difficulties in isolating SAV in cell culture. Primary virus isolation was successfully achieved for only two of eight strains, despite extensive attempts using three different cell lines. Both SAV isolates were easily propagated further and concomitant viral deletion mutants present in clinically infected heart tissue were maintained following serial passage in CHH-1 cells.

The economic impact of decreased mortality and increased growth associated with preventing, replacing or improving current methods for delousing farmed Atlantic salmon in Norway.

Impacts of salmon lice is a major concern for a sustainable production of farmed Atlantic salmon in Norway. Most treatment methods for removal of salmon lice have associated increased mortality and decreased growth in a period after delousing, which affects the profitability of the farmer, and causes poor welfare and sustainability. In addition, the variance in mortality and growth, especially after non-medicinal treatment methods, is high, which makes it hard for a farmer to decide which control measure to apply to keep lice levels below the legal limit. In this study, we have applied a stochastic partial budget approach to assess the economic impact of reducing mortality and increasing growth of farmed Atlantic salmon by preventing, replacing and improving current delousing methods in Norway. We have simulated a production cycle of two different smolt-groups to find the outcome (harvested biomass, average end weight of the salmon, number of dead fish and feed consumption) of production cycles without or with two, three or four delousing treatments in the on-growing phase at sea. The results suggest that accounting for the biological losses associated with lice treatments is important when making choices of delousing strategies. The biological costs of increased mortality and decreased growth associated with especially non-medicinal treatments are expected to be high, but varies substantially. Therefore, the economic benefit of preventing or improving can also be high. The calculations imply that salmon producers could invest a considerable amount in measures for prevention or improvement of thermal treatments before break-even. For example could a farmer use on average 535,313 €/cage/ 1-yearling production in measure to prevent four thermal treatments before it is no longer economical beneficial. Depending on the performance of the four thermal treatments a farmer could use from 319,196-737,934 €/cage/ 1-yearling production on measures of improvement. Replacing one thermal treatment with another immediate treatment method has a minor economic benefit. The results further shows that sales value and feed consumption constitutes the largest share of the change in profit between different treatment regimes. The results from this study also show that not taking into account the risk of mortality and reduced growth associated with the different treatment methods of delousing, could lead to underestimating the benefit of improving, preventing and replacing treatments.

Modelling the impact of temperature-induced life history plasticity and mate limitation on the epidemic potential of a marine ectoparasite.

Temperature is hypothesized to contribute to increased pathogenicity and virulence of many marine diseases. The sea louse (Lepeophtheirus salmonis) is an ectoparasite of salmonids that exhibits strong life-history plasticity in response to temperature; however, the effect of temperature on the epidemiology of this parasite has not been rigorously examined. We used matrix population modelling to examine the influence of temperature on demographic parameters of sea lice parasitizing farmed salmon. Demographically-stochastic population projection matrices were created using parameters from the existing literature on vital rates of sea lice at different fixed temperatures and yearly temperature profiles. In addition, we quantified the effectiveness of a single stage-specific control applied at different times during a year with seasonal temperature changes. We found that the epidemic potential of sea lice increased with temperature due to a decrease in generation time and an increase in the net reproductive rate. In addition, mate limitation constrained population growth more at low temperatures than at high temperatures. Our model predicts that control measures targeting preadults and chalimus are most effective regardless of the temperature. The predictions from this model suggest that temperature can dramatically change vital rates of sea lice and can increase population growth. The results of this study suggest that sea surface temperatures should be considered when choosing salmon farm sites and designing management plans to control sea louse infestations. More broadly, this study demonstrates the utility of matrix population modelling for epidemiological studies.

Seroprevalence of Toxoplasma gondii infection in Norwegian dairy goats.

BACKGROUND: Toxoplasma gondii is a major problem for the sheep industry as it may cause reproduction problems. The importance of T. gondii in Norwegian goat herds is uncertain, but outbreaks of toxoplasmosis in dairy goat farms have been recorded. The aim of this study was to describe the prevalence of T. gondii infection in Norwegian dairy goats by using serology. FINDINGS: Goat serum originally collected as part of two nationwide surveillance and control programmes between 2002 and 2008 were examined for T. gondii antibodies by using direct agglutination test. In total, 55 of 73 herds (75%) had one or more serologically positive animals, while 377 of 2188 (17%) of the individual samples tested positive for T. gondii antibodies. CONCLUSIONS: This is the first prevalence study of T. gondii infection in Norwegian goats. The results show that Norwegian goat herds are commonly exposed to T. gondii. Nevertheless, the majority of goat herds have a low prevalence of antibody positive animals, which make them vulnerable to infections with T. gondii during the gestation period.