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

Assessment of a semiquantitative scoring system for mild-to-moderate gill lesions in Atlantic salmon reared in recirculating aquaculture systems in Norway

Compromised gill health is a critical cause of forfeited welfare in Atlantic salmon farming. Detecting and quantifying the early onset of gill disease is important to reveal initial inflicting stimuli. We collected gill samples of 45 Atlantic salmon from 2 commercial recirculating aquaculture systems (RASs) spanning fry-to-market-size fish with no clinical signs of gill disease. Gill samples were assessed histologically by 3 independent raters with different levels of experience. Semiquantitative scoring for 7 types of gill changes was carried out for 10 filaments per gill (450 filaments total) over 3 rounds on anonymized samples. Scores were summarized for each type of gill change. The assumed clinical relevance for each change was transformed into a category score, followed by an assessment of agreement within (intra) and between (inter) raters. A generalized linear model estimated the difference in score levels between raters. For each rater, intra-rater agreement was high for 6 gill changes and moderate for 1 gill change. Inter-rater agreement was moderate to almost-perfect, except for 2 gill changes; generalized linear model regression revealed systematic differences in score usage between the raters. Our scoring protocol worked satisfactorily for mucous cell amount, lamellar clubbing, lamellar hypertrophy and/or hyperplasia, and aneurysms, despite different levels of expertise in histologic evaluation. Intra-rater agreement was consistent, but differences existed for interlamellar hypercellularity, lamellar inflammation, and degeneration. Scoring subclinical gill changes is a challenge, and our scoring system for mild-to-moderate lesions may enable early intervention to limit the detrimental effects of poor gill health in RAS farming.

Gill Health in Fish Farmed in Recirculating Aquaculture Systems (RAS): A Review

Recirculating Aquaculture Systems (RAS) have been proposed as the future of aquaculture, because they can be used anywhere regardless of access to water, they offer high level of control over farming environment, including biosecurity, and are considered to be sustainable. However, despite of continuous development, there can be still issues with water quality affecting gill health of fish farmed in these systems. This review provides an overview of fish gill structure and gill immune response, and discusses the known impacts of RAS on gill health. Several experimental studies have inadequately reported conditions, particularly water quality, making it difficult to determine if the observed effects were due to water quality issues or RAS system itself. It is crucial for studies investigating the impact of RAS on fish to report water quality during the study. Furthermore, assessments of RAS effects on gill health should include sufficient independent replicates and flow through controls using a common water source. Various methods have been used to assess gill health in RAS, including gill histology, presence of pathogens, gene expression in the gills and gill microbiome analysis. Differences in gill health in fish from RAS and a flow through system have been shown for a number of freshwater and marine fish species. However, these results have been inconsistent across studies, and some results have been challenging to interpret as indicators of gill health. Holistic studies including a number of different methods to assess fish gills would give more conclusive results. More research is needed, in particular, on brackish and marine RAS, to fully understand their impacts on gill health.

Diseases of marine fish and shellfish in an age of rapid climate change

A recurring trend in evidence scrutinized over the past few decades is that disease outbreaks will become more frequent, intense, and widespread on land and in water, due to climate change. Pathogens and the diseases they inflict represent a major constraint on seafood production and yield, and by extension, food security. The risk(s) for fish and shellfish from disease is a function of pathogen characteristics, biological species identity, and the ambient environmental conditions. A changing climate can adversely influence the host and environment, while augmenting pathogen characteristics simultaneously, thereby favoring disease outbreaks. Herein, we use a series of case studies covering some of the world's most cultured aquatic species (e.g., salmonids, penaeid shrimp, and oysters), and the pathogens (viral, fungal, bacterial, and parasitic) that afflict them, to illustrate the magnitude of disease-related problems linked to climate change.

Effectiveness of functional ingredients to enhance gill disease in Atlantic salmon (Salmo salar, L.)

The development and application of functional feed ingredients represents a great opportunity to advance fish growth and health, boost the immune system, and induce physiological benefits beyond those provided by traditional feeds. In the present study, we looked at the feasibility of in vitro methods for screening the qualities of functional feed ingredients using the fish cell line RTgill-W1, which has never been used in fish nutrition, and the culture of Paramoeba perurans. Five functional feed ingredients (arginine, β-glucan, vitamin C, and two phytogenic feed additives) were selected to investigate their effects on cell viability and reactive oxygen species production. Three of the selected ingredients (arginine and two phytogenic feed additives) were additionally tested to assess their potential amoebicidal activity. As these functional ingredients are the core of a commercially available feed (Protec Gill, Skretting AS), their beneficial effects were further assessed in a field trial in fish affected by complex gill disease. Here, the analyzed parameters included the evaluation of macroscopic and histopathological gill conditions, pathogen detections, and analyses of plasma parameters. RTgill-W1 cell line assays were a good tool for screening functional ingredients and provided information about the optimal ingredient concentration ranges, which can be helpful for adjusting the concentrations in future feed diets. Through the culture of P. perurans, the tested ingredients showed a clear amoebicidal activity, suggesting that their inclusions in dietary supplements could be a viable way to prevent microbial infections. A three-week period of feeding Protec Gill slowed the disease progression, by reducing the pathogen load and significantly improving gill tissue conditions, as revealed by histological evaluation. The use of diets containing selected functional ingredients may be a feasible strategy for preventing or mitigating the increasingly common gill diseases, particularly in cases of complex gill disease, as documented in this study.

Nasal responses to elevated temperature and Francisella noatunensis infection in Atlantic cod (Gadus morhua)

We report the histological and transcriptomic changes in the olfactory organ of Atlantic cod exposed to Francisella noatunensis. Experimental infection was performed at either 12 °C or 17 °C. Infected fish presented the classic gross pathologies of francisellosis. Nasal morpho-phenotypic parameters were not significantly affected by elevated temperature and infection, except for the number of mucus cells in the 12 °C group seven weeks after the challenge. A higher number of genes were altered through time in the group reared at 17 °C. At termination, the nasal transcriptome of infected fish in both groups was similar to the control. When both infected groups were compared, 754 DEGs were identified, many of which were involved in signalling, defence, transmembrane and enzymatic processes. In conclusion, the study reveals that elevated temperature could trigger responses in the olfactory organ of Atlantic cod and shape the nasal response to F. noatunensis infection.

First detection of Flavobacterium psychrophilum in juvenile Siberian sturgeons (Acipenser baerii) and description of the pathological findings

Flavobacterium psychrophilum affects many cultured fish species and is considered one of the most important bacterial pathogens causing substantial economic losses in salmonid aquaculture worldwide. Here, F. psychrophilum was identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and nested PCR as the aetiological agent causing mortality in diseased juvenile Siberian sturgeons (Acipenser baerii) reared on a freshwater fish farm. Diseased sturgeons were lethargic and displayed dark skin pigmentation, increased mucus production and the presence of skin ulcerations and haemorrhages specially on the ventral side and the base of fins. The histological examination of fish revealed proliferative branchitis, ulcerative and necrotizing dermatitis and myositis, lymphoid tissue atrophy, liver and kidney degeneration and thrombosis. To the best of our knowledge, this is the first report describing the infection of Siberian sturgeons by F. psychrophilum. The detection of F. psychrophilum in diseased Siberian sturgeons and the description of the pathological findings observed during the outbreak may contribute to a better understanding of the bacterium pathogenicity and the range of fish species susceptible to infection.

Emergence of Salmon Gill Poxvirus

The Salmon gill poxvirus (SGPV) has emerged in recent years as the cause of an acute respiratory disease that can lead to high mortality in farmed Atlantic salmon presmolts, known as Salmon gill poxvirus disease. SGPV was first identified in Norway in the 1990s, and its large DNA genome, consisting of over 206 predicted protein-coding genes, was characterized in 2015. This review summarizes current knowledge relating to disease manifestation and its effects on the host immune system and describes dissemination of the virus. It also demonstrates how newly established molecular tools can help us to understand SGPV and its pathogenesis. Finally, we conclude and ask some burning questions that should be addressed in future research.