Evaluation of Efficacy, Safety, and Withdrawal Period of Enrofloxacin (ENR) in Black Rockfish ( Sebastes schlegelii ) for Possible Treatment of Bacterial Diseases Caused by Streptococcus iniae and Aeromonas salmonicida

Although enrofloxacin (ENR) is an important pharmaceutical in aquaculture for its excellent efficacy, it is mostly utilized through an off‐label prescription system. Thus, the optimal clinical use regimen is not well‐established. A series of studies was performed to obtain pharmacological and safety data needed for the use of ENR, a synthetic fluoroquinolone, in treating bacterial diseases occurring in black rockfish, Sebastes schlegelii . Minimal inhibitory concentrations showed that ENR exerts a potent in vitro efficacy inhibiting the growth of several pathogenic bacteria at concentrations less than 1 mg/L. ENR with oral doses of 5 mg/kg and up for 1 week significantly prevented mortality of black rockfish infected with Streptococcus iniae and Aeromonas salmonicida , demonstrating in vivo efficacy. Complete protection was obtained against these two bacteria by 10 mg/kg ENR in this in vivo infection trial. Black rockfish did not exhibit any severe abnormal signs by ENR up to 20 mg/kg when assessed by basic haematoclinical and histopathological parameters. Efficacy and toxicity study results collectively suggest that oral dose of 10 mg/kg ENR is optimal to treat bacterial infections. Through muscle residue analyses, the required withdrawal period for safe fish consumption was found to be approximately 90 days by measuring clearance of ENR and its metabolite ciprofloxacin (CIP). This study provides necessary information for the use of ENR to treat two frequently‐occurring bacterial infections, Streptococcus iniae and Aeromonas salmonicida in black rockfish. Obtained data can be the basis for an official guideline for aquacultural ENR use.