Antiparasitic efficacy and safety evaluation of metronidazole against Trypanosoma sp. from large yellow croaker in experimentally infected largemouth bass (Micropterus salmoides).

Outbreaks of trypanosomiasis in cage-cultured large yellow croaker in China has caused significant economic losses, however, to date, there are no effective drugs available for the prevention and treatment of this disease. Metronidazole is a nitroimidazole derivative with a wide range of biological activity ranging from antiparasitic to antibacterial, and is especially useful for the treatment of anaerobic protozoan infections. Nevertheless, studies on its efficacy against protozoan parasites in aquatic animals are lacking. In this study, an efficacy trial of metronidazole against trypanosomes isolated from large yellow croaker was conducted, based on the established infection model using largemouth bass, with an intraperitoneal injection dose of 10⁵ cells/mL (100 μL per fish) and maintained at 25°C. The in vivo anti-parasitic efficacy results showed that 300 mg/kg metronidazole significantly reduced the trypanosomes load, resulting in complete parasite clearance in most fish (88.9%) and prevented mortality of fish (95.56%). Acute toxicity test results indicated that 1500 mg/kg metronidazole remained safe for largemouth bass. Meanwhile, histopathological analysis revealed that in the 300 mg/kg metronidazole-treated group, the number of red blood cells increased and their morphology normalized in the liver, spleen, and head kidney. Besides, enzyme activity assays proved the differential expression of alanine aminotransferase (ALT), acid phosphatase (ACP), and total superoxide dismutase (T-SOD) among the positive control (PC), negative control (NC), and metronidazole-treated groups. qPCR results showed that metronidazole can increase the expression level of TNF-α, IFN-γ, and IL-1β, and IL-10. In conclusion, this study demonstrated the promising efficacy of metronidazole against fish trypanosomiasis, laying a solid theoretical and practical basis for controlling aquatic animal parasitic diseases.