Fluoroquinolones are used extensively in veterinary medicine, especially in dermatology cases. Veterinary microbiology laboratories often perform in vitro antimicrobial susceptibility testing of bacteria isolated from clinical samples. Current guidelines of the Clinical and Laboratory Standards Institute recommend separate testing of the fluoroquinolones, but a recent survey of veterinary diagnostic laboratories showed that not all laboratories are following these recommendations. This study was designed to determine whether ciprofloxacin disk diffusion susceptibility test (DDT) results could be used to assess the in vitro susceptibility of otic bacterial organisms to enrofloxacin and determine the effect of concurrent enrofloxacin administration on the DDT results. Dogs with end-stage otitis externa that were scheduled for total ear canal ablation were enrolled in the study. Middle-ear tissue was removed from the dogs during surgery and cultured. Twenty-four the dogs were treated with intravenous doses of enrofloxacin at various doses before surgery; 5 dogs did not receive enrofloxacin and acted as controls. A total of 82 organisms for 10 different genera were identified. Discrepancies were noted in DDT results for enrofloxacin and ciprofloxacin. The percentage of organisms in which the results did not agree ranged from 5.9% for the 10 mg/kg treatment group to 27.8% in the 15 mg/kg treatment group. This study showed that ciprofloxacin DDT results are not an accurate indicator of the in vitro susceptibility of enrofloxacin for bacterial organisms isolated from the middle-ear tissue of dogs and that concurrent treatment with enrofloxacin when obtaining the cultures may increase the chance of discordant results.
COMMENTARY: This is not the first study to show discrepancies in the use of DDT to evaluate susceptibilities to ciprofloxacin and other fluoroquinolones. Discrepancies could result in a choice of doses higher than necessary, increase the cost of treatment, or cause treatment failures. It's worthwhile to check with your laboratory to determine which disks are available for your patients.
Ciprofloxacin as a representative of disk diffusion in vitro susceptibility of enrofloxacin for bacterial organisms from the middle-ear tissue of dogs with end-stage otitis externa. Cole LK, Kwochka KW, Hillier A, et al. Vet Dermatol 17: 128-133, 2006.
