Laboratory Efficacy of Amoxicillin for the Control of Streptococcus iniae Infection in Sunshine Bass

An experimental trial was performed to evaluate the efficacy of amoxicillin in controlling Streptococcus iniae infection in sunshine bass (a hybrid of female white bass Morone chrysops × male striped bass M. saxatilis). Minimum-inhibitory-concentration studies of amoxicillin against multiple S. iniae isolates showed a sensitivity range of 0.0156–0.5 μg/mL. The amoxicillin dose levels tested were 30, 50, 80, and 120 mg of active ingredient per kilogram of fish body weight per day. Administration of medicated feed started 1 d after infection by immersion exposure to S. iniae and continued for eight consecutive days; this was followed by an observation at 15 d posttreatment. Amoxicillin increased the survival rate from 1% in the infected, nonmedicated group to an average of 95% in the infected groups receiving the four doses; there were no significant differences among the medicated groups. Survivors of the infection were not found to be carriers of the bacteria (i.e., there was negative bacterial isolation).