[Susceptibility of bacterial isolates from the equine respiratory tract to trimethoprim, sulfadoxine, sulfadimethoxine and combinations of these compounds]

Using a broth microdilution technique, the in vitro susceptibility of Bacterial isolates from the equine respiratory tract to trimethoprim, sulfadoxine, sulfadimethoxine, and combinations of these compounds was determined. The Bacterial strains (n = 88) isolated recently from horses with respiratory symptoms belonged to the following species: Streptococcus equi subsp. zooepidemicus (n = 34), Streptococcus equi subsp. equi (n = 22), Staphylococcus aureus (n = 9), Klebsiella pneumoniae (n = 7), Rhodococcus equi (n = 4), Pseudomonas spp. (n = 3) and Escherichia coli (n = 3). In addition, two isolates of Enterobacter spp. and one isolate of Streptococcus equisimilis, Staphylococcus intermedius, Proteus mirabilis and Serratia marcescens were examined. For determination of susceptibility of an organism the following minimal inhibitory concentrations (MIC) were fixed as limiting values: Trimethoprim < or = 0.5 microgram/ml, sulfadoxine < or = 32 micrograms/ml, sulfadimethoxine < or = 32 micrograms/ml, trimethoprim/sulfadoxine < or = 0.5/32 micrograms/ml, trimethoprim/sulfadimethoxine < or = 0.5/32 micrograms/ml. As expected, Rhodococcus-equi-isolates were resistant to the antimicrobials tested. However, most of the clinically more common isolates showed a high degree of susceptibility to the combinations. The fractional inhibitory concentration (FIC) indices indicated synergism of the combination-partners in a wide range. According to these in vitro results, application of trimethoprim/sulfonamide combinations for the initial therapy of equine respiratory tract infections can be recommended.