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An Urban Experience
the infection; this does not necessarily mean that the bacteria are resistant to the antimicrobial, as the high levels following topical treatment frequently exceed the MIC. Antibiotic sensitivity data can be used to predict the efficacy of systemic drugs, although penetration to the ear tissues is often low and high doses are needed.
Topical and systemic therapy
Topical therapy is preferred wherever possible.
Systemic antimicrobial therapy may be less effective in erythroceruminous otitis externa as bacteria are present only in the external ear canal and cerumen. Systemic treatment may be more useful in suppurative otitis externa and/or otitis media where there is an inflammatory discharge with concurrent tissue infection. Systemic treatment is indicated when the ear canal cannot be treated topically (e.g. stenosis, compliance problems or topical adverse reactions) and in otitis media.
Topical antimicrobials
Polymixin B, fusidic acid, florfenicol, gentamicin, enrofloxacin and marbofloxacin are suitable for most bacterial infections. Polymixin B and miconazole
have synergistic activity against Pseudomonas and other Gram-negative organisms, and fusidic acid
and framycetin show synergistic activity against staphylococci. Fluoroquinolones, gentamicin and polymixin B are usually effective against Pseudomonas. Fusidic acid and florfenicol are effective against
MRSA and MRSP. Neomycin is less potent that other aminoglycosides, although it is usually effective against Gram-positive bacteria. It is important to use an adequate volume – 1ml is sufficient for most ears.
Topical therapy achieves high local concentrations that persist. Concentrations of gentamicin were 3-15x and concentrations of miconazole are 1.2-2x the MIC90 for canine otic isolates of staphylococci and Malassezia respectively 10 days after a five day course of Easotic®. Levels of florfenicol and terbinafine are at least 1000x MIC90 for staphylococci and Malassezia for the duration of treatment with two doses of Osurnia®.
Ear cleaning
Removal of debris and purulent material improves the efficacy of topical antibiotics, especially aminoglycosides and polymixin B. Cleaners with chlorhexidine, acids, and/or alcohols are most effective against Malassezia and bacteria. Acidic ear cleaners may inactive aminoglycosides and fluoroquinolones) although the
ear canal pH rapidly returns to normal. Ceruminolytic and ceruminosolvent cleaners should be used with dry waxy debris, surfactant based cleaners with seborrhoeic discharges, and mild aqueous cleaners with purulent material and/or a ruptured tympanic membrane. Techniques include manual cleansing, bulb syringes and ear flushing.
Systemic antimicrobials (see notes on Responsible Antimicrobial use in Treating Pyoderma)
Penetration of antibiotics with a low volume of distribution into ear tissues is limited. Fluoroquinolones have a high volume of distribution and penetrate well. However, these should be used at high dose in end-stage otitis externa and media (e.g. 10-20mg/kg for enrofloxacin).
Pseudomonas otitis
Pseudomonas are inherently resistant to many antibiotics and readily develop further resistance if treatment is ineffective. Other anti-Pseudomonas antibiotics are often expensive, not licensed for animals and have to been given parenterally if used systemically.

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