Pharmacodynamics of florfenicol for calf pneumonia pathogens

The antimicrobial properties of florfenicol were investigated for the bovine respiratory tract pathogens, Mannheimia haemolytica and Pasteurella multocida . Three in vitro indices of efficacy and potency were determined; minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and in vitro time-kill curves for six pathogenic strains of each organism. Each was monitored in two matrices, Mueller Hinton broth (MHB) and calf serum. MBC:MIC ratios were low, 1.8 : 1 for M haemolytica in both MHB and serum and 2.4 : 1 and 2.1 : 1 for P multocida in MHB and serum, respectively. The killing action of florfenicol had the characteristics of concentration dependency against M haemolytica and codependency (on time and concentration) against P multocida . Modelling of the time-kill data after 24 hours exposure was undertaken to quantify three levels of activity for the ratio, area under concentration-time curve over 24 hours (AUC 24h )/MIC; bacteriostatic action (no change in bacterial count), 3log 10 reduction and 4log 10 reduction in bacterial count. Mean AUC 24h /MIC values for P multocida in MHB (and serum) were 22.0 (23.3) hour, 34.5 (39.9) hour and 45.8 (50.4) hour, respectively. Similar numerical values were obtained for M haemolytica. For both bacterial species , interstrain variability was low; coefficients of variation ( per cent) in serum for 3log 10 and 4log 10 reductions in count were, respectively, 14.3 and 24.1 for P multocida and 7.8 and 11.4 for M haemolytica . These data form a rational basis for dosage selection for treatment of calf pneumonia caused by M haemolytica or P multocida .