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

## TL;DR

This study shows that fenbendazole, a common veterinary drug, can safely and effectively treat a parasitic infection in goldfish, offering a new solution for aquaculture.

## Contribution

The study demonstrates the repurposing of fenbendazole for treating Gyrodactylus kobayashii in goldfish with optimized dosing and safety evaluation.

## Key findings

- Oral fenbendazole at 20 mg/kg for three days removed 96.28% of parasites in goldfish.
- A 6-hour bath at 0.06 mg/L fenbendazole eliminated all parasites in goldfish.
- Fish showed mild, reversible health effects and recovered by 15 days post-treatment.

## Abstract

Parasitic infections are a major problem in fish farming, often causing illness and financial losses. In particular, tiny worms from the genus Monogenean can harm fish by attaching to their skin and gills. Currently, fish farmers have very few safe and effective medicines to treat these parasites, and repeated use of existing drugs may cause resistance. This study explored whether a common veterinary medicine called fenbendazole, usually used in terrestrial animals, could be an effective treatment for infected fish. The results showed that fenbendazole was highly effective at killing parasites, especially when given as a short bath or added to feed. Oral treatment for just three days removed over 96% of parasites, and the fish recovered well with only mild and temporary effects on their health. These findings suggest that fenbendazole could be a promising, practical, and safe option for treating fish parasites. Using this already-approved drug for a new purpose could offer a quicker, cheaper way to improve fish health and support sustainable aquaculture.

This preclinical study investigated the efficacy and safety of fenbendazole, a broad-spectrum benzimidazole anthelmintic, for the treatment of Gyrodactylus kobayashii in goldfish (Carassius auratus). 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 G. kobayashii in goldfish. This study underscores the potential of drug repurposing as an effective strategy for developing novel anthelmintic agents in aquaculture.