In vitro susceptibilities of recent field isolates of Mycoplasma hyopneumoniae and Mycoplasma hyosynoviae to valnemulin (Econor(R)), tiamulin and enrofloxacin and the in vitro development of resistance to certain antimicrobial agents in Mycoplasma hyopneumoniae

The in vitro activities of valnemulin (Econor(R)) and two other antimicrobial agents were determined against recent field strains of Mycoplasma hyopneumoniae and Mycoplasma hyosynoviae using a broth microdilution method. Valnemulin showed exceptional activity against M hyopneumoniae (MIC90 0.0005 mu g ml(-1)) and M hyosynoviae (MIC range 0.0001 mu g ml(-1) to 0.00025 mu g ml(-1)) field strains. Tiamulin was 100-fold less active (MIC90 0.05 mu g ml(-1)) and enrofloxacin 20-fold less active (MIC90 0.01 mu g ml(-1)) than valnemulin against M hyopneumoniae field isolates and 20-fold to 25-fold less active (MIC range 0.0025 mu g ml(-1) to 0.005 mu g ml(-1)) and 400-fold to 500-fold less active (MIC range 0.05 mu g ml(-1) to 0.1 mu g ml(-1)) respectively against M hyosynoviae field isolates. No significant resistance developed to valnemulin or tiamulin in the type strain of M hyopneumoniae (strain J) or in a recent field isolate (MEVT G23) exposed to 10 in vitro passages in broths containing these antibiotics. Only slight resistance to oxytetracycline was observed. High resistance to tylosin developed in both M hyopneumoniae strains within five to seven in vitro passages in tylosin-containing broth. Providing that similar results are obtained in vivo under field conditions, valnemulin may well prove to be effective in the treatment of enzootic pneumonia and acute polyarthritis in pigs.