Treatments of orange-spotted grouper (Epinephelus coioide) against Cryptocaryon irritans with •OH, ClO2 or HCHO: Survival, physiological and histological response

Cryptocaryon irritans causes one of the most serious diseases in various wild and cultured marine fish, leading to mass mortality and economic loss. In this study, hydroxyl radical (center dot OH) solution produced by strong ionization discharge combined with water jet cavitation effect was injected into orange-spotted grouper (Epinephelus coioides) aquaculture tanks for C. irritans control. The results showed that all C. irritans theronts were inactivated by center dot OH solution at concentrations of 0.5 mg/L within 2 min. center dot OH could induce alteration of shape, the absence of motility and macronucleus dispersion in theronts. A possible explanation was that the macronucleus of C. irritans might be damaged by center dot OH; as a result, its metabolism and life activities were disturbed. The center dot OH treatment increased the survival rate of E. coioides challenged with C. irritans from 64.7 +/- 8.0% (mean +/- SD) to 100% and reduced their infection intensity significantly. Stress response biomarkers such as malonaldehyde, glutathione, glutathione peroxidase, superoxide dismutase (SOD) and catalase levels in the gills of E. coioides at different time points were analysed. The SOD activity in the center dot OH group first decreased and then recovered to the initial level at the end of the experiment. The other stress response biomarkers had no significant difference from that in the uninfected control group after center dot OH treatment. Additionally, the gill of E. coioides in the center dot OH group exhibited slight and reversible transformation compared with the uninfected control group. Compared with center dot OH treatment, chlorine dioxide and formalin treatment reduced the survival rate, induced oxidative damage and changed the histological gill structure in E. coioides. In conclusion, center dot OH could be applied effectively to control C. irritans infection without affecting the normal physiological condition of E. coioides.