Antioxidative, histological and immunological responses of rainbow trout after periodic and continuous exposures to a peracetic acid-based disinfectant

Peracetic acid (PAA)-based disinfectants are considered as sustainable alternatives in aquaculture due to their harmless residues from spontaneous decay. The key components of PAA-based disinfectants are PAA and hydrogen peroxide (H2O2). Little is known if the exposure to exogenous PAA and H2O2 from PAA-based disinfectants interferes with endogenous reactive oxygen species (ROS) regulation and innate immunity in fish. In the present study, rainbow trout (Oncorhynchus mykiss) were exposed to a PAA-based disinfectant for 6 weeks in flow-through systems by either periodic (pulse) or continuous modes. For the periodic exposure, a single dose of 1 mg L−1 PAA (and 1.4 mg L−1 H2O2) was applied biweekly and spontaneously degraded within hours. For the continuous exposure, a constant drip was applied which aimed to maintain 0.2 mg L−1 PAA (and 0.28 mg L−1 H2O2) in the inflow water but was counteracted by escalating degradation by the second week. The exogenous PAA/H2O2 elevated the level of endogenous total free radicals (TFR), probably through diffusion at the gill, as well as stress-activated endogenous generation (by periodic exposure only at the higher PAA/H2O2 dose). In response, the total antioxidant capacity (TAC) in gill and serum was significantly elevated. Liver showed no significant changes in the levels of TFR and TAC. Epidermal mucous cell density was significantly lower in response to persistent scarce exogenous PAA/H2O2 of the continuous exposure compared to the control. This sensitive response was absent in the periodic exposure probably due to rapid epidermal recovery during the exposure intervals. In contrast, the branchial structural adjustment of trout in response to exogenous ROS was less sensitive. Minimal hyperplasia of lamellae was present only in the periodic exposure. Enzymes responsible for innate cutaneous and humoral immunity including lysozyme, alkaline phosphatase, myeloperoxidase and esterase were not influenced by either exposure. However, ceruloplasmin was highly sensitive to exogenous PAA/H2O2; it was significantly elevated in fish skin, independent of the exposure modes, suggesting potential antioxidant protection of ceruloplasmin in fish skin. In serum, ceruloplasmin reduction was positively correlated to the reduction of antiprotease activity which indicates that ceruloplasmin may have anti-proteolytic function in fish blood. In conclusion, both exposure modes of the PAA-based disinfectant triggered mild antioxidant defenses in rainbow trout. To minimize the risk of oxidative damage while applying PAA-based disinfectants, sufficient intervals between periodic applications might be a better option than the persistent low PAA/H2O2 concentration of the continuous application