Estimation of pharmacological properties of ceftiofur, an injectable cephalosporin antibiotic, for treatment of streptococcosis in cultured olive flounder Paralichthys olivaceus

The application of injectable antibiotics has not been actively adopted in aquaculture despite some clear advantages over the oral administration under certain circumstances. This study aimed to assess the applicability of intramuscularly (i.m.) injectable ceftiofur sodium (CS), a cephalosporin, to treat streptococcal infection in olive flounder (Paralichthys olivaceus). The efficacy of CS was estimated in vitro with 48 different clinical isolates of Streptococcus parauberis: it was found that growth of 32 isolates (66.7%) was inhibited by 0.25 μg/ml and 47 isolates (97.9%) by 8 μg/ml CS. The in vivo laboratory efficacy showed that 20–40 mg/kg single CS (i.m.) injection completely prevented olive flounder from death while streptococcal infection led to 100% mortality without CS. In a field study, CS was administered at 10 or 20 mg/kg i.m. to naturally infected olive flounder in culture farms, and significant mortality prevention was observed by either dose. Plasma and muscle concentrations of CS were monitored in the laboratory following single CS injections at 10 and 20 mg/kg (i.m.) to estimate pharmacokinetic parameters. Peak plasma concentrations (Cmax) were 7.61 and 13.34 µg/ml, respectively, after the two doses leading to large Cmax/MIC50 and T > MIC50 (time period stayed at above MIC50) values against S. parauberis. Analyses of residue depletion from the muscle demonstrated legitimate withdrawal times of 4.15 days after 10 mg/kg and 4.32 days after 20 mg/kg. These results collectively indicate that a single i.m. injection of CS above 10 mg/kg is acceptably efficacious with reasonable withdrawal periods in streptococcal infection of olive flounder.