Antibiotic susceptibility patterns from Irish equines

Abstract

A longitudinal, retrospective investigation was carried out of antimicrobial resistance in bacterial isolates (n = 1060) from horses in Ireland over the period 1990 through 2000. Bacterial isolates that were collected from clinical cases of equine infection from throughout Ireland in 1990 (n = 394), 1996 (n = 323), and 2000 (n = 343), were recultured from archived isolates. Standard antibiotic disk susceptibility tests were performed against the following antibiotics (disk concentration): penicillin, ampicillin, amoxicillin, amoxicillin/clavulanic acid, gentamicin, sulfamethoxazole/trimethoprim, erythromycin, and ceftiofur. In all cases, plates were incubated at 35°C for 18 hours before results were read. Antibiotic resistance rates are shown in the Table (opposite page). Using 2 test for trend, statistically significant rising trends (P < .05) were identified in the resistance of (1) Escherichia coli and gentamicin, (2) hemolytic streptococci and penicillin, (3) hemolytic streptococci and penicillin, (4) hemolytic streptococci, (5) hemolytic streptococci and ampicillin, and (6) hemolytic streptococci and erythromycin. No significant trends were apparent for any of the other organism/drug interactions examined. To date, there has been relatively limited data in the literature regarding antibiotic resistance patterns in horses, and the current study is the first formal report of resistance patterns in horses from Ireland. Ensink et al1 suggested that quantitative antibiotic susceptibility testing of bacterial pathogens in equine infection may serve as a useful means to optimize antibiotic management in the patient. National monitoring of antibiotic resistance patterns in horses is important for several reasons, including (1) optimum employment of licensed anti-infectives for therapeutic purposes, (2) development of a longterm anti-infective strategy between equine veterinarians and the pharmaceutical industry, and (3) examining the potential interaction with human clinical medicine. Currently there has been growing concern over antimicrobial resistance in pathogenic bacterial populations in both human and clinical medicine and a recent report has demonstrated the nosocomial transmission of methicillin-resistant Staphylococcus aureus from humans to horses in a surgical unit in a veterinary teaching hospital.2 The emphasis of antimicrobial resistance in equines differs from other large animals, in that antimicrobials are not used as growth promoters nor is the zoonotic link with humans as strong, given that consumption of horse meat in the British Isles is not common. In conclusion, the main findings of this study were rising trends in resistance by the majority of the isolates tested. Significant differences in resistance levels were found in at least one antimicrobial in each organism isolated. Prospective investigations are needed to identify selection pressures on the development of antimicrobial resistance in bacterial isolates from horses, so that control measures can be put in place to minimize future increases in resistance. Regular surveillance should therefore be encouraged to monitor resistance patterns in healthy, as well as sick, equines.