Antimicrobial susceptibility of Riemerella anatipestifer isolated from ducks and the efficacy of ceftiofur treatment

The in vitro susceptibilities of 50 field isolates of Riemerella anatipestifer from ducks to ceftiofur and 16 other commonly used antimicrobials were determined. The MIC50 values (MIC refers to minimum inhibitory concentrations) for the antimicrobials used in this study are as follows: penicillin was 16 mug/ml; ceftiofur was 32 mug/ml; cephalothin, chloramphenicol, flumequine, and kanamycin were 64 mug/ml; nalidixic acid, nitrofurantoin, and sulfamethoxazole were 128 mug/ml; amikacin, ampicillin, gentamicin, lincomycin, spectinomycin, streptomycin, tetracycline, and trimethoprim were greater than or equal to256 mug/ml. The therapeutic efficacy of ceftiofur against a highly lethal experimental R. anatipestifer infection in ducks was also evaluated. All experimental ducks were infected through the infraorbital sinus with 1 ml of 9 x 10(9) CFU of R. anatipestifer. Ceftiofur (0, 0.25, 0.5, 1, and 2 mg/kg) was injected subcutaneously 5 hours after infection. A single dose of 2 mg/kg resulted in 73% survival as compared with 10% survival in the infected, but untreated controls.