Effectiveness assessment of plant mixtures against Ichthyophthirius multifiliis in grass carp Ctenopharyngodon idella

Ichthyophthirius multifiliis, a ciliated parasite, results in "white spot disease" of freshwater fish. The outbreak of this disease causes a significant economic loss in the aquaculture industry. The present study aimed to screen the plants mixture with high antiparasitic efficacy against I. multifiliis and assess the toxicity of plants mixture to grass carp. The screening study indicated that ethanol extracts of Cynanchum atratum, Psoralea corylifolia, Zingiber officinale, Sophora flavescens, Homalomena occulta, Curcuma longa, and Morus alba showed antiparasitic efficacy against I. multifiliis. Twenty-one combinations composed of every two of the seven plants were prepared to investigate their additive, synergistic, and antagonistic effects as well as indifference using the checkerboard arrays method. Nine combinations with synergistic and additive effects were screened for the further in vitro, in vivo, and acute toxicity experiments. Finally, combination 4 (Com 4) containing C. atratum and S. flavescens showed high efficacy in treating grass carp infected with I. multifiliis. Com 4 at a concentration of 6 mg/L with a continuous usage for 10 days cured the infected grass carp. The 96 h median lethal concentration (LC50) of Com 4 to grass carp was 173.4 mg/L with a safe concentration (SC) of 47.4 mg/L. Thus, Com 4 has a low toxicity potential candidate to prevent and control I. multifiliis in grass carp.