Preclinical Evaluation of Fenbendazole for Controlling Gyrodactylus kobayashii (Monogenea, Gyrodactylidae) in Goldfish: Dose Optimization and Safety Assessment

This preclinical study investigated the efficacy and safety of fenbendazole, a broad-spectrum benzimidazole anthelmintic, for the treatment of <i>Gyrodactylus kobayashii</i> in goldfish (<i>Carassius auratus</i>). In vivo bath treatments demonstrated potent, dose-dependent anthelmintic efficacy, achieving 98.58% efficacy at a concentration of 0.02 mg/L and a 48 h EC₅₀ of 0.006 mg/L. A short-duration (6 h) bath at 0.06 mg/L, followed by an 18 h recovery period in dechlorinated water, resulted in complete parasite elimination. However, acute toxicity assay indicated a relatively narrow safety margin for prolonged bath treatments, with a 96 h LC₅₀ of 0.039 mg/L, highlighting the need for caution when employing extended bath treatments. Oral administration of fenbendazole at 20 mg/kg body weight for three consecutive days resulted in an efficacy of 83.35%, which increased to 96.28% by seven days post-treatment. Safety evaluations revealed this regimen induced transient oxidative stress and mild, reversible histopathological alterations in the liver and gills. Biochemical and histological markers indicated a recovery trend, approaching baseline levels by 15 days post-treatment. These findings suggested that oral fenbendazole is an effective and relatively safe anthelmintic treatment against <i>G. kobayashii</i> in goldfish. This study underscores the potential of drug repurposing as an effective strategy for developing novel anthelmintic agents in aquaculture.