The objective of this study was to verify the supposition that efflux might be involved in the drug resistance of Riemerella anatipestifer isolates. Two broad-spectrum efflux pump inhibitors, carbonyl cyanide 3-chlorophenylhydrazone (CCCP) and Phe-Arg-β-naphthylamide (PAβN), on the contribution of minimum inhibitory concentrations of amikacin, streptomycin, chloramphenicol, tetracycline, ceftriaxone, ceftazidime, nalidixic acid, levofloxacin, enrofloxacin, as well as ciprofloxacin against 69 clinical R. anatipestifer isolates were investigated. We first reported that the two efflux pump inhibitors could restore the antimicrobial susceptibility of R. anatipestifer isolates. It is suggested that active efflux system is possible to be linked with the development of resistance in R. anatipestifer isolates.

The purpose of the study was to investigate the pharmacokinetics of cefquinome in plasma and milk samples of lactating Chinese Holstein following a single intramammary administration into one quarter at the dose of 75 mg. Residue depletion of cefquinome in milk administrated at one quarter following three consecutive infusions at the same dose were also carried out. Cefquinome concentrations in plasma and milk were determined by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method. A non-compartmental analysis was used to obtain the pharmacokinetic parameters of cefquinome. Following the single treatment, cefquinome wasn’t detected in any of the plasma samples. The concentration of cefquinome in milk reached peaked values (Cmax) of (599.00±322.00) μg mL-1 at 2 h after administration (Tmax), elimination half-life (t1/2λz) was (4.63±0.26) h, area under the concentration-time curve (AUC0-∞) was (4 890.19±1 906.98) μg mL-1 h, and mean residence time (MRT) was (6.03±2.27) h. In residue depletion study, cefquinome concentrations in 5 out of 6 milk samples at 72 h were lower than the maximum residue limit fixed by the European regulatory agency (20 μg kg-1 for cefquinome) and cefquinome still could be detected in milk of treated quarters at 120 h post-treatment. The maximum concentration (Cmax) of cefquinome in milk from treated quarters was (486.50±262.92) μg mL-1 and arrived at 6 h after administration (Tmax), elimination half-life (t1/2λz) was (6.30±0.76) h, area under the concentration-time curve (AUC0-∞) was (44747.79±11434.43) μg mL-1 h, and mean residence time (MRT) was (10.09±1.40) h. This study showed that cefquinome has the feature of poor penetration into blood and was eliminated quickly from milk in lactating cows after intramammary administration.