Ichthyophthirius multifiliis (also called "ich") is an external protozoan parasite that may infest almost all freshwater fish species and caused significant economic damage to the aquaculture industry. Since the use of
malachite green was banned, there have been relatively few effective alternative strategies for controlling I. multifiliis
infections. The present study was designed to screen potential
antiparasitic medicinal plants based on our previous studies, and comprehensively evaluate in vitro and in vivo anti-ich activity of selected
plant extracts. The screening results showed that the
methanol extract of Psoralea corylifolia had the highest activity against I. multifiliis theronts. In vivo theront trials demonstrated that 1.25 mg/L or more concentrations of P. corylifolia
methanol extract caused 100 % mortality during the 4-h exposure period, and the subsequent in vitro trials indicated that the minimum concentration of P. corylifolia
methanol extract that prevented the initial infestation was 2.50 mg/L. Protomonts and encysted tomonts surviving trials suggested that encysted tomonts were less susceptible to P. corylifolia
methanol extract than protomonts, and the
methanol extract of P. corylifolia at a concentration of 5.00 mg/L could kill 100 % of protomonts and 88.89 % of encysted tomonts. It was also observed that after 12-h exposure of protomonts or encysted tomonts to 2.50 mg/L of P. corylifolia
methanol extract, the theronts emerged from encysted tomonts led to more
infection level than the ones in the other groups. The results suggested that whether the protomonts finish encystment is crucial to the survival, reproduction, and theronts infectivity. In addition, our results showed that long duration (24 h) and high concentration (5.00 mg/L) significantly reduced the survival and reproduction of I. multifiliis tomont exited from the fish after in-bath treatment, and it is indicated that P. corylifolia
methanol extract had a potential detrimental effect on I. multifiliis trophont in situ.