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Simple Summary: Aquaculture is an important source of animal protein. However, in recent years, with an intensification of farming, the risk of infectious diseases is increasing, especially the diseases caused by monogeneans, resulting in huge economic losses to aquaculture. Currently, the prevention and control of monogenean in fish mainly rely on some chemical drugs, such as formaldehyde, rotenone, and praziquantel. However, prolonged and frequent use of these chemicals leads to the occurrence of drug resistance, some adverse environmental impacts, and even contamination of fish products with drug residues. Therefore, it is urgent to develop efficient and environment-friendly drugs for the control of monogeneans in aquaculture. Natural plant-derived medicines are one of the alternative options. Therefore, in this study, anthelmintic efficacy of various essential oils was evaluated. The results indicated that palmarosa oil and curcuma oil showed potent anthelmintic activity against monogenean, and curcuma oil also had an anesthetic effect on monogenean. Moreover, these two essential oils displayed a high safety for fish. Therefore, palmarosa oil and curcuma oil could be viable alternatives for the prevention and control of monogenean infection in aquaculture. Monogeneans are a serious threat to the development of aquaculture due to the severe economic losses they cause. The prevention and treatment of this disease are increasingly difficult because of the environmental and health concerns caused by the use of chemical anthelmintics and the emergence of drug resistance. It is thus necessary to search for effective alternatives for the treatment of monogenean infections. In the current study, anthelmintic efficacy of 16 selected essential oils (EOs) was investigated using the goldfish (Carassius auratus)–Gyrodactylus kobayashii model. The screening experiment indicated that palmarosa oil and curcuma oil had satisfactory anthelmintic activity against G. kobayashii with EC100 values of 10 and 12 mg/L after 24-h exposure, respectively. The in vivo and in vitro assays indicated anthelmintic efficacy of palmarosa oil against G. kobayashii was in a time and dose-dependent manner. Interestingly, curcuma oil showed an anesthetic effect on G. kobayashii, and its anthelmintic activity was dose-dependent rather than time-dependent in the concentration range tested in this study. Additionally, the 24-h LC50 (50% lethal concentration) against goldfish of these two EOs was 8.19-fold and 5.54-fold higher than their corresponding EC50 (50% effective concentration) against G. kobayashii, respectively. Moreover, exposure to these two EOs at 100% effective concentration against G. kobayashii had no serious physiological and histopathological influence on goldfish. These results demonstrated a high safety for goldfish of these two EOs. Overall, palmarosa oil and curcuma oil could be potential candidates for the treatment of G. kobayashii infections in aquaculture. [ABSTRACT FROM AUTHOR]

Gyrodactylus spp. are common monogenean ectoparasites that may lead to significant fish mortality. To find effective anthelmintic agents with lower toxicity, a series of natural saponins were obtained and evaluated for their anthelmintic activity against Gyrodactylus kobayashii and acute toxicity to goldfish (Carassius auratus). Among all tested compounds, six compounds (1, 2, 3, 8, 10, and 13) shown higher anthelmintic activity and safety than widely used formaldehyde-based parasiticides, especially compound 1 having 100% anthelmintic efficacy against G. kobayashii at 0.3 mg/L and a therapeutic index of 16.6. Also, the three-dimensional quantitative structure-activity relationship (3D-QSAR) studies of these saponins have been performed to explore the structural features reasonable for the anthelmintic activity against G. kobayashii. These models demonstrated that the hydroxyl group at C-17 position and the sugar moieties at C-3 position, especially the hydroxyl groups of the sugar moieties, were critical to the anthelmintic activity. The QSAR studies could provide useful information for further rational design and optimization of novel saponins for the control of gyrodactylosis. [ABSTRACT FROM AUTHOR]

Monogenean parasites pose a significant challenge to aquaculture, leading to adverse effects on fish health and yields. Current anthelmintic treatments for monogeneans have demonstrated limited efficacy and are further complicated by potential issues, which emphasize the necessity for effective and safe therapeutic strategies to manage monogeneans in aquaculture. In this study, in vivo and in vitro anthelmintic activity of febantel against Gyrodactylus kobayashii on goldfish (Carassius auratus), as well as its toxicity to goldfish were evaluated. In vivo assays indicated that febantel exhibited potent anthelmintic activity against G. kobayashii with an EC 50 value of 0.03 mg/L and 100 % anthelmintic efficacy at 0.1 mg/L after 48 h of exposure. Moreover, in vivo trials also revealed a notable post-treatment effect of febantel, where infected goldfish transferred to drug-free water after short 6-h exposure could still result in full eradication of the worms, indicating febantel might induce persistent perturbations in parasite physiology. In vitro assays showed a negative correlation between febantel concentrations and the survival of G. kobayashii. However, increasing the febantel concentration to 2.0 mg/L did not result in the complete death of all worms. Oral administration of febantel demonstrated limited anthelmintic activity, with only 49 % efficacy at a dosage of 200 mg/kg body weight daily over five days. Acute toxicity assays revealed the 48-h LC 50 value of febantel was 5.47 mg/L, which was 182.23 times higher than the 48-h EC 50 value, indicating that febantel has a favorable safety profile. However, febantel exposure potentially interfered with hepatic metabolism and oxidative status, as indicated by variations in SOD, GST, and P450 gene expression. In conclusion, treatment with 0.1 mg/L febantel for 24 h completely eradicated G. kobayashii infection on goldfish, demonstrating febantel's potent anthelmintic activity. Coupled with its safety profile and extended post-treatment effectiveness, febantel is a promising candidate for controlling Gyrodactylus infections in aquaculture. • Febantel showed potent anthelmintic efficacy against Gyrodactylus kobayashii with an EC 50 value of 0.03 mg/L. • A notable post-treatment effect of febantel against G. kobayashii was observed. • Febantel has a favorable safety profile. • Febantel is a promising candidate for controlling Gyrodactylus infections in aquaculture. [ABSTRACT FROM AUTHOR]

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 bioassay-guided 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 EC 50 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 EC 50 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. • The methanol extract of P. polyphylla showed 100% anthelmintic activity against G. kobayashii at 20 mg/L. • Dioscin, polyphyllin I and gracillin exhibited high anthelmintic efficacy against G. kobayashii. • The methanol extract of P. polyphylla is a relatively safe and beneficial agent to treat gyrodactylosis in aquaculture. [ABSTRACT FROM AUTHOR]