Clostridium difficile has been associated with necrotizing enterocolitis in neonatal foals and typhlocolitis in adult horses. From May, 1993 through March, 1996, toxigenic C difficile were isolated from feces obtained from 238 adult horses. Antimicrobial agents are used as part of the treatment regimen for affected horses. Retrospective data are used to choose an appropriate antimicrobial agent. For this reason, 105 isolates, each from a different horse with diarrhea, were selected for susceptibility testing. Minimal inhibitory concentrations (MICs) of bacitracin, chloramphenicol, metronidazole, rifampin, and vancomycin were determined by the E test and by agar dilution.

E test results agreed with those obtained by agar dilution. All of the isolates were susceptible to <= 4 µg chloramphenicol/ml; 96% were susceptible to <= 0.25 µg rifampin/ml, 93% were susceptible to <= 1 µg vancomycin/ml, 81% were susceptible to <= 8 µg metronidazole/ml, and 98% were resistant to >= 128 µg bacitracin/ml.

Our results show that most equine isolates were susceptible to chloramphenicol, metronidazole, rifampin, and vancomycin. Of concen, is the relatively large number of isolates resistant to > 8 µg metronidazole/ml. Whether bacitracin is a feasible choice for treatment of affected horses is unknown.