COMPARATIVE IN-VITRO ACTIVITY OF DOXYCYCLINE AND OXYTETRACYCLINE AGAINST PORCINE RESPIRATORY PATHOGENS

Minimum inhibitory concentrations of doxycycline and oxytetracycline w ere determined against 55 Pasteurella multocida strains, 59 Actinobaci llus pleuropneumoniae strains and 26 Mycoplasma hyopneumoniae strains isolated from the respiratory tract of pigs, An additional set of 76 P multocida strains isolated from pneumonic pigs was tested for their m inimum inhibitory concentrations of doxycycline, The P multocida and A pleuropneumoniae strains were isolated in France and the minimum inhi bitory concentrations were determined by an agar dilution method, The M hyopneumoniae strains were isolated in the United Kingdom and minimu m inhibitory concentrations were determined by a serial broth dilution method, All the strains tested were susceptible to doxycycline wherea s 15 per cent of the P multocida strains and 22 per cent of the A pleu ropneumoniae strains were resistant to oxytetracycline. Doxycycline co ncentrations inhibiting 90 per cent of strains were 1 mu g/ml for P mu ltocida and 2 mu g/ml for A pleuropneumoniae. The ratio of the minimum inhibitory concentrations of doxycycline and oxytetracycline ranged b etween 1/1 and 1/4 for the oxytetracycline-susceptible strains and bet ween 1/16 and 1/64 for the oxytetracycline-resistant strains. All the M hyopneumoniae strains were susceptible to doxycycline and oxytetracy cline, the concentrations inhibiting 90 per cent of strains being 1 mu g/ml and 2 mu g/ml, respectively. These data confirm that doxycycline has a higher in vitro activity against pig respiratory pathogens than oxytetracycline.