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Tris-EDTA & Otitis in Dogs

Clinician's Brief (Capsule)


|March 2014

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Pseudomonas aeruginosa is the most common Gram-negative cause of otitis externa/media in dogs. Multi-drug resistant P aeruginosa is particularly difficult to treat. Tris-buffered EDTA has been shown to inhibit the growth of this pathogen both in vitro and in vivo and to potentiate the action of some antibiotics. In this study, the impact of Tris-EDTA on the minimal bactericidal concentrations (MBC) and minimal inhibitory concentrations (MIC) of two commonly used injectable antibiotics (ie, marbofloxacin, gentamicin) was evaluated. Eleven multi-drug resistant P aeruginosa isolates were incubated alone with each antibiotic and in combination with Tris-EDTA. The Tris-EDTA alone was bacteriostatic but not bactericidal. However, the addition of Tris-EDTA significantly reduced the MBC and MIC of both marbofloxacin and gentamicin.


Multi-drug resistant P aeruginosa otitis externa/media in dogs is always difficult to treat. Not only does one have to address the infections but also with the underlying trigger, often allergy and/or a primary disorder of keratinization are challenging without the complication of a multi-drug resistant otitis. If these cannot be successfully treated, total ear canal ablation is often indicated. Further complicating the situation is that systemic antibiotics cannot reach high enough tissue concentrations within the ear canal to resolve these infections. Topical antibiotic solutions are the only means to ensure high antibiotic concentrations at infection site. If the ear canal is obstructed, an aggressive course of oral prednisone may be indicated to decrease swelling and obstruction, otherwise topical ear cleaners and otic medications are useless. If the canal is stenotic, surgery is the best option for good quality of life. If the ear canal is open but filled with exudate, topical medications will not work in the presence of debris. Severe pain may be caused by obstruction and/or the infection and must be addressed, and veterinarians need to ensure that asking owners to medicate a dog’s ear will not put the owners at risk for injury.—Karen A. Moriello, DVM, DACVD


Tris-EDTA significantly enhances antibiotic efficacy against multidrug-resistant Pseudomonas aeruginosa in vitro. Buckley LM, McEwan NA, Nuttall T. VET DERMATOL 24:519-E122, 2013.

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