Exposures to hydrogen peroxide impact healing, redox and oxygen transport differentially from immune mechanisms in the gill of Atlantic Salmon (Salmo salar)

Hydrogen Peroxide is an oxidative chemical used as a topical treatment via bath exposures in salmonid aquaculture to mitigate external infections such as sea lice and amoebic gill disease. Protocols can depend on each individual site treatment, which varies in environmental temperature, dose concentration used and exposure time. Treatments such as this, in addition to environmental insults, increasing ocean temperatures and other farm management practices occurring simultaneously and/or chronically can lead to Complex Gill Disease (CGD). The purpose of this study was to assess the effects of hydrogen peroxide on the gills of Atlantic salmon (Salmo salar) following acute and chronic repeated exposure and dose and temperature dependent exposure. Our focus was to examine biological responses influencing gill function such as immune, healing, and redox reactions while understanding the cellular damage occurring in the gill. Histology and targeted gene expression through RT-qPCR analysis was used to examine these effects. Results showed minor to severe morphological changes in the gill, increasing damage in a linear relationship with increasing temperature and dose. The main morphological changes observed in the gill were lamellar epithelial lifting, lamellar edema, and lamellar fusion. Gene expression analysis revealed upregulation of genes associated with healing, redox, oxygen transport, cell death and DNA repair whereas suppression of immune responses genes was evident for both experimental groups. Differential regulation of genes associated with apparent adaptation to repeated hydrogen peroxide treatment could be useful in identifying markers to differentiate acute and chronic impact of treatment(s) that may not always de discerned through standard histological assessment.