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T. Nuttall1
1University of Edinburgh, Royal Dick School of Veterinary Studies, Roslin, United Kingdom
Dr Tim Nuttall
Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Campus, Roslin, UK.
Treatment of acute otitis externa
Individual bouts of otitis externa can be managed using polyvalent topical ear products with a glucocorticoid (for mild acute inflammation), an antibiotic, and an antifungal (for Malassezia). Cleaning is necessary in cases with large amounts of debris.
Recurrent and chronic otitis
Nearly all ear infections involve commensal (e.g. staphylococci and Malassezia) or environmental (e.g. Pseudomonas) opportunists. The vast majority are secondary to pre-existing inflammation, foreign bodies, obstruction or other primary problems. Ongoing inflammation leads to a cycle of recurrent infection and chronic inflammation leading to progressive pathological changes and end-stage otitis. The chronic inflammation makes each bout of infection harder to treat and repeated antimicrobial use may select for resistance.
Approach to chronic or recurrent otitis
1. Otodectes etc., and check the tympanic membrane (see Diagnostic Approach to Otitis).
2. Identify primary, predisposing and perpetuating cause
3. Cytology should be performed in all cases; it can be particularly useful to identify multiple organisms in mixed overgrowths or infections.
4. Culture if necessary.
Aims of treatment
• Identify and manage the primary cause
• Correct predisposing factors
• Remove debris and discharge
• Manage the secondary infection
• Reverse chronic pathological changes
• Maintain a heathy ear canal environment
The impact of biofilms
Biofilms inhibit cleaning, prevent penetration of antimicrobials and provide a protected reservoir of bacteria. Antibiotics that require bacterial division will
be less effective, as biofilm-forming bacteria are usually
in a quiescent state. Biofims may also enhance the development of antimicrobial resistance - a gradual decrease in antimicrobial concentration means that some bacteria could be exposed to a mutant selection window. This will lead to treatment failure and resistance.
Bacterial culture and sensitivity testing
Using cytology to predict susceptibility patterns
The likely sensitivity of Malassezia and staphylococci
can be predicted using knowledge of local resistance patterns and previous treatment. Gram-negative
bacteria are harder to differentiate on cytology, although Pseudomonas are most common. Their susceptibility is harder to predicate, although most first-time infections will be susceptible to aminoglycosides, polymixin B, silver sulfadiazine and fluoroquinolones. However, they readily acquire multi-drug resistance (see below).
Using bacterial culture and antimicrobial sensitivity testing
Bacterial culture and sensitivity testing can help identify organisms that are hard to differentiate on cytology (e.g. streptococci, enterococci, E. coli, Klebsiella, Proteus and coryneforms). Knowledge of their likely sensitivity patterns can help guide treatment choices.
Understanding breakpoints and resistance
Antimicrobial susceptibility tests do not predict the response to topical treatment. The susceptibility and resistance breakpoints are based on tissue levels following systemic dosing. A resistant result applies to
An Urban Experience

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