Antibiotic plasma levels in dogs with otitis externa treated routinely with various topical preparations

We aimed to determine whether, and at what levels, topical antibiotics applied to treat Otitis externa in dogs are absorbed systemically, leading to an increased risk of antibiotic resistance. 75 dogs brought to a veterinarian for Otitis externa were recruited for a non-interventional study. Selection criteria included diagnosis of Otitis externa and owner consent.The animals were divided into five groups of 15 dogs each. Each group received one of five commonly prescribed topical medications for up to 14 days according to the labeled instructions. Development and validation of low residue detection methods (HPLC-MS/MS) for all active substances studied was performed. Plasma concentrations were evaluated for gentamicin (Otomax, Easotic), marbofloxacin (Aurizon), orbifloxacin (Posatex) and polymyxin B (Surolan). Low-level plasma concentrations of the topically applied antibiotics were detected after multiple administrations. In several samples, the concentrations detected were less than the limit of detection (LOD) of the corresponding analytical method. However, at the end of the treatment period, mean plasma concentrations were in the low pmol/ml range and exceeded the LOD for gentamicin, marbofloxacin and orbifloxacin. None of the plasma samples examined for polymyxin showed levels above the LOD. After routine topical antibiotic use in the treatment of Otitis externa in dogs, low systemic plasma concentrations are likely to develop.This low-level exposure may facilitate cellular changes that lead to an increased possibility for antibiotic resistance. These findings should provoke veterinary clinicians to optimise therapy for Otitis externa in light of minimising the development of antibiotic resistance.