Therapeutic Effect of Fosfomycin against Edwardsiellosis in Red Sea Bream Pagrus major

Therapeutic effect of fosfonnycin (FOM) was studied in red sea bream experimentally and naturally infected with Edwardsiella tarda. Minimum inhibitory concentration of FOM against 67 strains of E. tarda isolated from red sea bream cultured in Ehime Prefecture ranged from 1 to 4 mu g/mL, and no strains resistant to FOM were observed. In experimental infections, the mortality decreased with the increase in FOM dosage, and a remarkable therapeutic effect of FOM was demonstrated by oral administration at a dose of 40-80 mg/kg body weight/day for 6 days. In field trials, FOM was administered to high and low mortality cases of red sea bream naturally infected with E. tarda orally for consecutive 6 days at a dosage of 40 mg/kg/day. Cumulative mortalities of the FOM-medicated groups were significantly lower than those of the non-medicated groups in both cases. In medicated groups, the administration of FOM was more efficient in the lower mortality case. These results indicate that FOM administration is effective against edwardsiellosis in red sea bream and the treatment in early stages of infection is important to obtain a favorable outcome.