Effects of thermal and mechanical delousing on gill health of farmed Atlantic salmon (Salmo salar L.)

Widespread resistance to medicinal delousing compounds has resulted in non-medicinal delousing methods being the preferred treatment option for sea lice infestation in Norwegian aquaculture. Thermal delousing involves submerging fish in water with a temperature of 28-34 C for 20-30 s, while mechanical delousing entails removing the lice with flushing, brushing or turbulence. As mechanical and thermal delousing became increasingly common, injuries and increased mortality were often reported in association with treatments and currently mechanical injuries sustained during non-medicinal delousing are considered to be an important cause of mortality and reduced fish welfare. Gill injuries have been reported after non-medicinal delousing and it has been shown that increased temperatures can lead to differential gene expression in the gill tissue. It was therefore of interest to explore if thermal and mechanical delousing can compromise gill health. The objective of this study was to determine if thermal and mechanical delousing under a commercial setting can lead to gill damage, differential gene expression, and changes in pathogen prevalence and load in the gill tissue of farmed Atlantic salmon. To assess this, gill tissue from presumed healthy fish was collected prior to and at two time points post thermal or mechanical delousing and examined using a detailed histopathological assessment protocol. Further, for gill tissue from the thermal delousing site, mRNA expression of markers of heat shock responses, hypoxia, inflammation, and repair were assessed by RT-qPCR analysis. RT-qPCR analysis for gill pathogens Neoparamoeba perurans, Candidatus Branchiomonas cysticola, Desmozoon lepeophtherii and salmon gill poxvirus (SGPV) was also conducted. Microscopic examination showed an increased percentage of gill tissue with vascular and hyper plastic lesions and an increase in the prevalence of putative gill pathogens observed post-treatment. Gene expression analysis revealed differential expression of genes involved in pathways of cell stress, inflammation, repair, and proliferation in the gill tissue after thermal delousing. Lastly, RT-qPCR analysis showed increased pathogen load of the putative gill pathogen Ca. B. cysticola after thermal delousing. The percentage of gill tissue affected was low, generally estimated to be < 2%, and thus the clinical impact of these lesions remains to be established. However, the observed vascular and hyperplastic lesions will reduce overall gill capacity.