LAUSR

Riemerella anatipestifer can cause septicemia and death in ducks and geese, leading to significant economic losses to animal farms. The emergence of resistance of R. anatipestifer to commonly used antibiotics increases the difficulty of treating R. anatipestifer infection. The aim of this study was to evaluate the utility of antibiotic combination to restrict mutant selection of multidrug-resistant (MDR) R. anatipestifer isolates. Pharmacokinetics of florfenicol and chlortetracycline in Pekin ducks were evaluated using both noncompartmental analysis and population pharmacokinetic models. The areas under the curve of florfenicol and chlortetracycline after single 20 and 10 mg/kg oral administration were 49.3 and 6.84 mg*h/L, respectively. Chlortetracycline exhibited high apparent clearance and low systemic exposure. Minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC) values of the two antibiotics were determined in 10 and 2 MDR R. anatipestifer isolates, respectively, to derive fTMSW (the fraction of time over 24 hours wherein the free drug concentration was within the mutant selection window [MSW]) and fT>MPC (the fraction of time that the free drug concentration was above the MPC). Both fTMSW and fT>MPC were estimated from simulated concentration-time profiles relative to MIC and MPC. Florfenicol and chlortetracycline combination have additive activities against R. anatipestifer in majority of isolates and could significantly decrease monotherapy MPC of florfenicol and chlortetracycline, as well as optimize both fTMSW and fT>MPC parameters, provided that the bioavailability of chlortetracycline is improved. The application of pharmacokinetic/pharmacodynamic analyses to MPC concepts to restrict selection of mutant bacterial strains can help improve short- and long-term outcomes of antibiotic treatment in animal farms.