Fosmidomycin for the Treatment of Canine Otitis Externa: A Randomised, Double‐Blinded, Controlled ‘Split Body’ Clinical Trial

Background Targeted antimicrobial therapy for canine otitis externa ( OE ) represents an opportunity for antimicrobial stewardship. Fosmidomycin selectively inhibits the non‐mevalonate pathway for isoprenoid biosynthesis utilised by canine‐adapted, and not human‐adapted, staphylococci. Objectives To evaluate the safety and efficacy of fosmidomycin for treatment of bacterial OE compared with enrofloxacin. Animals Fifteen client‐owned dogs with bilateral bacterial OE were enrolled. Materials and Methods Fosmidomycin stability in solution for 28 days was confirmed before trial commencement. A ‘split body’ design was used: each ear canal was randomised to receive a solution of fosmidomycin or enrofloxacin applied twice daily for 28 days, combined with a tapering anti‐inflammatory course of oral prednisone. Owners and investigators were blinded to treatments. Dogs were evaluated at Day (D)0, D14 and D28 with clinical scores (0–3 otitis index score [ OTIS3 ], ear cytological results, pain) and owner assessments (pruritus scores for each ear, quality‐of‐life scores and a hearing questionnaire). On D28 , owners and investigators assessed global treatment efficacy for each ear. Results Treatment group did not significantly influence clinical scores; cytological scores, OTIS3 scores and pruritus scores significantly improved for both groups over the trial period. Treatment efficacy for both ears was assessed as good‐to‐excellent by owners and investigators for the majority of dogs. No safety concerns were identified. Conclusions and Clinical Relevance Fosmidomycin and enrofloxacin performed comparably for topical treatment of bacterial otitis in this study. Fosmidomycin is a promising targeted antimicrobial for canine bacterial infections while limiting selection for antimicrobial resistance in human pathogens such as Staphylococcus aureus .