Cryptocaryon irritans , one of the most devastating holotrichous ciliated protozoans, causes massive economic losses in the pompano, Trachinotus ovatus , mariculture industries. Considering the many advantages of phytochemicals, such as low cost, low residuals, and low pollution, they can be a good alternative for the prevention and control of C. irritans infestations. In this study, 14 phytochemicals were evaluated for their antiparasitic activity against the theront of C. irritans , and nine showed significant antiparasitic activity. Nerolidol was the most promising compound, causing 100% mortality of theronts with 30-min exposure at a concentration of 10.0 μl/L. The half-maximal inhibitory concentrations ( IC 50 s) of nerolidol on C. irritans theronts, protomonts, and tomonts were 2.1, 13.3, and 7.1 μl/L, respectively. Abnormal morphologies of the theronts, trophonts, and tomonts were observed after exposure to 10.0, 30.0, and 10.0 μl/L nerolidol solution for 15 min, 30 min, and 480 min, respectively. A diet supplemented with 2.3 ml/kg of nerolidol reduced the number of trophonts on the gills of pompano by 95.7% decrease compared with the control groups and the theront chemotaxis to pompano mucus and sera, while increased the survival rate of infected pompanos by 24.44%, and had no significant effect on pompano hindgut and serum myeloperoxidase. These results suggest that nerolidol is a new potential drug for controlling fish cryptocaryonosis.
Ziqing Zhong,Xiping Wu,Mindong Bai,Xiaodian Huang,Qilin Zheng,Chunxiang Ai
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 (•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 •OH solution at concentrations of 0.5 mg/L within 2 min. •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 •OH; as a result, its metabolism and life activities were disturbed. The •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 •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 •OH treatment. Additionally, the gill of E. coioides in the •OH group exhibited slight and reversible transformation compared with the uninfected control group. Compared with •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, •OH could be applied effectively to control C. irritans infection without affecting the normal physiological condition of E. coioides .
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