Introduction
Gill diseases are an important challenge for Atlantic salmon (Salmo salar) aquaculture worldwide. Complex gill disorder (CGD) is a multifactorial and multiaetiological condition that is thought to be a consequence of the interaction of a number of factors including environment, management practices and pathogenic microorganisms. The main infectious agents associated with cases of CGD in Atlantic salmon are Candidatus Branchiomonas cysticola, Desmozoon lepeophtherii and salmon gill poxvirus (SGPV) (Herrero et al. 2018). The individual roles and possible interactions between these pathogens have not yet been fully elucidated and studies on infectious gill disease agents in Scottish aquaculture, other than Neoparamoeba perurans, are scarce. In the absence of in vivo or in vitro experimental models, prospective longitudinal studies can help to clarify any associations between exposure to a potential aetiology and the development of disease.
Material and methods
A prospective longitudinal study was designed to investigate the infectious dynamics of the putative pathogens of CGD for one year, starting from the late freshwater stage of the production cycle and continuing through the subsequent marine stage. The protozoan N. perurans, which causes a specific disease known as amoebic gill disease (AGD), was also included in the monitoring of the study. The relative loads of the four pathogens, estimated using real-time reverse transcriptase polymerase chain reaction (RT-rtPCR) Ct values, were correlated with a semi-quantitative histological gill scoring system derived from the samples. Generalised additive models (GAMs) were used to represent changes over time in the Ct values of the different infectious agents in the gills of salmon at the various sampling timepoints in the two farms, and to represent the variation of the gill score across time and farms. Linear regression models were used to study the possible associations between gill score and different explanatory parameters such as variations in the pathogens’ loads, environmental and farm management factors.
Results and discussion
The two marine farms screened in this study were positive for the three main pathogens associated with CGD by RT-rtPCR, which shows that the detection of D. lepeophtherii, Ca. B. cysticola, and SGPV are common in marine Scottish salmon farms. Ca. B. cysticola and D. lepeophtherii were the most prevalent agents, whilst SGPV was detected irregularly throughout the study. In addition, Ca. B. cysticola and SGPV were first detected in the late freshwater stages of both populations suggesting that these pathogens were carried from the freshwater to the marine farm.
In the marine phase, Farm B showed significantly higher gill scores compared to Farm A (p< 0.05). Farm A experienced minimal to mild unspecific gill changes during the study whilst fish in Farm B had gill pathology characterised by AGD lesions and low to occasionally moderate vascular, proliferative and inflammatory pathology, from late summer until early winter. Gill score was significantly higher during the autumn on both farms compared to other seasons (p< 0.05). The strong link between the gill score and seasonality may indicate that water temperature is an important risk factor in terms of gill health compromise. In this study, the load variation of SGPV was not significantly associated with changes in the gill score (p> 0.05) and typical pathology associated with SGPV was not detected. As fish seemed to be first infected during the freshwater stage, it could be that the virus becomes latent and can be re-activated at a later stage, for example during episodes of immunosuppression, contributing to the gill pathology. An increase in Ca. B. cysticola load was associated with an increase in the gill score (p< 0.05) when only Farm B was used in the statistical analyses. Ca. B. cysticola was shown to be associated with gill pathology by in situ hybridization, for CGD events occurring in the absence of visible epitheliocysts, showing that the effects of the bacteria can be easily overlooked by common histological methods (Gjessing et al., 2021). In this study, the inflammatory and necrotic cell reaction in the gills was not marked (changes associated with Ca. B. cysticola), and the low level of epitheliocysts detected in the histopathology suggests the bacterium was not a major cause of the gill disease present in Farm B although it could have contributed to some of the changes present. Higher loads of D. lepeophtherii were significantly associated with an increased gill score in Farm B (p< 0.05) but not in Farm A (p> 0.05), and characteristic D. lepeophtherii-type microvesicles were minimal and not significant in the gills of fish examined from both farms, which suggests the significant associations could be the result of the parasite developing in more affected gills (with higher gill score) rather than D. lepeophtherii being a causative agent of the gill pathology observed. An increase in Neoparamoeba perurans loads was significantly associated with an increase in gill score (p< 0.05) on both farms and an outbreak of amoebic gill disease was one of the main causes of the gill disease observed in Farm B.
When using only Farm B in the linear model, parameters such as a lower number of days since last net cleaning and a lower number of days since the last hydrogen peroxide treatment were significantly associated with an increase in the gill score (p< 0.05). In situ net-pen pressure washing cleaning of the nets and the mechanical trauma caused during the treatments performed likely contributed to the gill damage observed in Farm B (Østevik et al., 2021, 2022).
References
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Herrero, A., Thompson, K. D., Ashby, A., Rodger, H. D., & Dagleish, M. P. (2018). Complex gill disease: an emerging syndrome in farmed Atlantic salmon (Salmo salar L.). Journal of Comparative Pathology, 163, 23–28. https://doi.org/10.1016/j.jcpa.2018.07.004
Østevik, L., Stormoen, M., Evensen, Ø., Xu, C., Lie, K.-I., Nødtvedt, A., Rodger, H., Skagøy, A., Manji, F., & Alarcón, M. (2022). Effects of thermal and mechanical delousing on gill health of farmed Atlantic salmon (Salmo salar L.). Aquaculture, 552, 738019. https://doi.org/10.1016/j.aquaculture.2022.738019
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