Laboratory efficacy of florfenicol against Streptococcus iniae infection in sunshine bass.

An experimental feeding trial was performed to evaluate the efficacy of florfenicol (FFC) in controlling Streptococcus iniae infection in sunshine bass (female white bass Morone chrysops x male striped bass M. saxatilis). Five dosage levels of FFC in medicated feed were administered daily: 0, 5, 10, 15, and 30 mg of active ingredient/kg of fish body weight. Treatment was started within 22-24 h postchallenge by waterborne exposure to virulent S. iniae. The FFC medication was continued for 10 consecutive days, followed by a 25-d posttreatment observation. At the conclusion of the experiment, FFC treatment significantly increased the survival of S. iniae-challenged sunshine bass from 4.2% in the nonmedicated (positive control) group to 69.2% in the 5-mg/kg dosage group, 86.7% in the 10-mg/kg group, and 94.2% in the 15- and 30-mg/kg groups. Survival was significantly higher in the 15- and 30-mg/kg treatment groups than in the 5-mg/kg treatment group; differences among the 10-mg/kg and higher dosage groups were not significant. Survival curve analysis using a log-rank test indicated no significant difference between curves for the 10- and 15-mg/kg groups but a significant difference between curves for the 5- and 10-mg/kg groups. At the end of the experiment, no carriers were detected in any challenged group receiving an FFC-medicated diet, but the bacterium was recovered from the nonmedicated challenged survivors of the infection. The results of the experiment suggest that the optimum therapeutic daily dose of FFC is between 10 and 15 mg/kg body weight for 10 d.