In vitro screening and in vivo evaluation of antiparasitic phytochemicals against Cryptocaryon irritans in pompano, Trachinotus ovatus

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.