Antimicrobial Resistance of Aerobic Respiratory Isolates From Young New Zealand Horses

REASONS FOR PERFORMING STUDY: Decreased efficacy of veterinary antimicrobials and increased prevalence of multi‐drug resistance (MDR) is of concern, but little is known of antimicrobial resistance encompassing the New Zealand (NZ) equine population. Recent concerns have arisen over the emergence of multi‐resistant bacteria [1], especially on NZ stud farms where antibiotics are frequently used for respiratory disease without veterinary input [2]. OBJECTIVES: To describe bacterial culture and antimicrobial sensitivity results from respiratory samples submitted of young horses (4 weeks to 3 years old). STUDY DESIGN: Retrospective study of clinical pathology records. METHODS: A database search for isolates and sensitivity of respiratory samples from young horses (April 2004–July 2014) was conducted. The results of in vitro sensitivity testing by Kirby‐Bauer disk diffusion were tabulated for major bacterial species isolated. Multiple correspondence analysis was used to describe clustering of multi‐drug resistance (MDR) and selected demographic variables. RESULTS: 237/289 eligible respiratory samples had at least one aerobic bacterial isolate. Most of the 774 bacterial isolates were Gram‐positive (68%). Streptococcus species were the most common genus isolated (40% of isolates). Sensitivity of Streptococcus spp. to penicillin, gentamicin and ceftiofur was >85%, but only 53% to trimethoprim‐sulfamethoxone. Gram‐negative sensitivity to ceftiofur, tetracycline, and trimethoprim‐sulfamethoxone was <75%. MDR was found for 16% of isolates and in 39% of horses. CONCLUSIONS: Penicillin is an appropriate first‐line antimicrobial for use in most NZ young horses with suspected bacterial respiratory infection. However, based on findings of MDR, submission of samples for culture and monitoring of sensitivity should be used to inform antimicrobial selection. Ethical animal research: Not applicable. Sources of funding: Massey University McGeorge Fund; New Zealand Equine Research Foundation. Competing interests: None declared.