Antiparasitic efficacy of honokiol against Cryptocaryon irritans in pompano, Trachinotus ovatus

Cryptocaryon irritans, a marine fish ectoparasite, causes mass economic loss in the mariculture and ornamental fish industries. It is important to find effective alternatives that may be both affordable and environmentally acceptable to control the damage of C. irritans. In this study, honokiol from Magnolia officinalis was screened from 36 phytochemical compounds for its significant antiparasitic activity against C. irritans theronts. Honokiol at a concentration of 1.00 μg/mL resulted in 100% mortality of C. irritans theronts within 30 min of exposure and >88.6% inhibition of tomont and protomont transformations post 8-h exposure to 1.00 and 4.00 μg/mL, respectively. Moreover, after exposure to 0.80, 1.00, and 4.00 μg/mL of honokiol solution for 30 min, 8 h, and 8 h, abnormal morphologies of theronts, tomonts, and trophonts were observed, respectively. After consecutively fed a diet containing 400 mg/kg of honokiol for 7 d, pompano (Trachinotus ovatus) were exposed to C. irritans theronts and the results showed that the number of C. irritans trophonts on the pompanos' gills was significantly lower than that in the control group 3 days later, while a similar efficacy was observed at doses above 100 mg/kg 8 days later. In addition, a dose of 200 mg/kg honokiol significantly reduced the chemotaxis of C. irritans theronts to the sera; meanwhile, the chemotaxis of C. irritans theronts to dorsal trunk muscle solutions was significantly reduced at doses above 100 mg/kg. Compared to the control group, a dose of 400 mg/kg honokiol increased the survival rate of the infected pompano by 65 ± 5% (mean ± SE). The results of hemolysis assay showed that the EC1 of honokiol to pompano erythrocytes was 6.36 μg/mL, 6.36 times the concentration that allowed honokiol to completely stop the movement of 100% C. irritans theronts in 30 min. Therefore, honokiol has the potential to be a safe and effective candidate, and could also serve as a leading compound in the development of a commercial drug to control C. irritans in mariculture.