The efficacy of amoxicillin sodium against streptococcosis in cultured olive flounder Paralichthys olivaceus and its pharmacokinetics.

The efficacy of amoxicillin sodium for controlling field and experimental Streptococcus iniae and S. parauberis infections in olive flounder (Paralichthys olivaceus) was evaluated after a single intramuscular administration. Furthermore, the minimal inhibitory concentrations (MIC) against 21 Streptococcus strains were determined. In addition, the pharmacokinetics and residue depletion in olive flounder were investigated. Single intramuscular doses of amoxicillin sodium at 20, 40, 80, and 160 mg/kg b.w. fish significantly reduced cumulative mortality rates to 18.8-31.3% (P < 0.05) for S. iniae and to 5.0-15.0% (P < 0.01) for S. parauberis, whereas the S. iniae- and S. parauberis-infected positive control groups showed cumulative mortality rates of 68.8% and 60.0%, respectively. In a S. parauberis outbreak, amoxicillin sodium reduced the cumulative mortality rate to 7.5% and 4.8% at 20 and 40 mg/kg b.w. fish, respectively, whereas that of the untreated control group was 35.2%. Peak plasma concentrations (Cmax ) following a single intramuscular dose of 40 and 80 mg/kg b.w. fish were 62.64 (Tmax , 1.59 h) and 87.61 (Tmax , 3.02 h) μg/mL, respectively, with large AUC0-t /MIC and Cmax /MIC ratios, and sufficient T > MIC (time for maintaining plasma drug concentration greater than MICs) for S. iniae and S. parauberis. The estimated withdrawal period of amoxicillin sodium from muscle of olive flounder was about 8 days at 40 mg/kg b.w. fish (at 22 ± 1 °C). These results demonstrated a single intramuscular administration of amoxicillin sodium to be effective against streptococcosis in olive flounder.