Anthelmintic efficacy of 35 herbal medicines against a monogenean parasite, Gyrodactylus kobayashii, infecting goldfish (Carassius auratus)

Species of Gyrodactylus are common monogenean ectoparasites of fish and can result in severe economic losses in aquaculture. To find effective and environmentally acceptable therapies to control gyrodactylosis in aquaculture, anthelmintic efficacy of 35 kinds of herbal medicines was investigated against Gyrodactylus kobayashii infecting goldfish (Carassius auratus), and the promising herbal medicines were further isolated by bioassayguided fractionation. Among the tested extracts, the methanol extract of Paris polyphylla was the most effective and showed 100% anthelmintic activity against G. kobayashii at 20 mg/L, and had a therapeutic index (TI) of 37.9, which is higher than that of widely used formaldehyde-based parasiticides in aquaculture. According to high-performance liquid chromatography (HPLC) analysis, six active compounds were identified: dioscin, polyphyllin I, gracillin, polyphyllin II, polyphyllin VII and polyphyllin VI; among these active compounds, dioscin was the most effective in eliminating Gyrodactylus infections, with EC50 value of 0.17 mg/L and TI value of 8.18, and exhibiting 100% anthelmintic efficacy at 0.5 mg/L. In order of efficacy, polyphyllin I and gracillin had EC50 values of 0.21 and 0.19 mg/L and TI values of 5.35 and 4.97, respectively. These findings suggested that dioscin, polyphyllin I and gracillin are effective and safe in the treatment of G. kobayashii infections and might become leading natural compounds for the synthesis and development of therapeutic agents against monogenean parasites in aquaculture. Interestingly, the methanol extract of P. polyphylla had a higher TI value of 37.9 compared to the abovementioned active compounds, although it required higher concentrations to reach 100% anthelmintic efficacy. The results indicated the methanol extract of P. polyphylla was less toxic to fish than the three compounds mentioned above, and although its anthelmintic activity was inferior to that of these compounds, the methanol extract was a relatively safe and beneficial agent for practical application to treat gyrodactylosis in aquaculture.