DAO 117:13-20 (2015)  -  DOI: https://doi.org/10.3354/dao02925

ABSTRACT: Infection by Ichthyophthirius multifiliis, a ciliated protozoan parasite, results in high fish mortality and causes severe economic losses in aquaculture. To find new, efficient anti-I. multifiliis agents, cynatratoside-C was isolated from Cynanchum atratum by bioassay-guided fractionation in a previous study. The present study investigated the anti-theront activity, determined the toxicity of cynatratoside-C to grass carp Ctenopharyngodon idellus and mammalian blood cells, and evaluated the protection of cynatratoside-C against I. multifiliis theront infection in grass carp. Results showed that all theronts were killed by 0.25 mg l^-1 of cynatratoside-C in 186.7 ± 5.8 min. Cynatratoside-C at 0.25 mg l^-1 was effective in treating infected grass carp and protecting naive fish from I. multifiliis infestation. The 96 h median lethal concentration (LC₅₀) of cynatratoside-C to grass carp and 4 h median effective concentration (EC₅₀) of cynatratoside-C to theront were 46.8 and 0.088 mg l^-1, respectively. In addition, the hemolysis assay demonstrated that cynatratoside-C had no cytotoxicity to rabbit red blood cells. Therefore, cynatratoside-C could be a safe and effective potential parasiticide for controlling I. multifiliis.

KEY WORDS: Cynatratoside-C · Cynanchum atratum · Ctenopharyngodon idellus · Antiparasitic activity · Toxicity · Ichthyophthirius multifiliis