Should equine practitioners still prescribe chloramphenicol for treating horses?

Summary Background Clinical breakpoints for chloramphenicol are not currently available to test susceptibility of bacteria isolated from horses. Without these breakpoints, testing the susceptibility of bacterial isolates from horses cannot be interpreted properly. Objective Our objective was to develop antimicrobial susceptibility testing breakpoints for testing bacteria isolated from horses and to determine if the use of chloramphenicol in horses can be justified. Study design Our design incorporated a search of the pharmacokinetic literature to identify pharmacokinetic data from horses and perform pharmacokinetic‐pharmacodynamic (PK‐PD) analysis on those data. We combined the analysis with minimal inhibitory concentration (MIC) data for bacteria that cause infections in horses. Methods Dose‐exposure data for oral administration of chloramphenicol to horses, along with the PK‐PD index of unbound drug area under the concentration time curve in plasma over a 24‐h dosing period ( f AUC 24 ) divided by a bacterial minimal inhibitory concentration (MIC) was used to predict the efficacy of chloramphenicol administered by the oral route to horses. Results An analysis of multiple pharmacokinetic studies from oral administration of chloramphenicol to horses generated LS mean values and associated variability for pharmacokinetic parameters. Ten thousand Monte Carlo simulations were generated to determine the MIC at which 90% of the simulated population would achieve the PK‐PD target. Main limitations This analysis was performed using published pharmacokinetic data and MIC values collected from a monitoring program. We cannot ensure that our analysis reflects the performance of chloramphenicol for treating a clinical infection. Conclusions Based on this analysis, a suggested clinical breakpoint for susceptible pathogens is 0.5 μg/mL. This value is well below the MIC distribution of Escherichia coli, Staphylococcus aureus and Streptococcus equi (subspecies equi and zooepidemicus) chloramphenicol MIC values that are likely to be encountered in horses. Chloramphenicol is unlikely to be effective in the treatment of bacterial infections in horses.