Are systemic and/or topical antimicrobial therapy alone likely to be curative in this case?
No. Pseudomonas aeruginosa is a drug-resistant, opportunistic pathogen that becomes established only when there is significant pathology and when previous antimicrobial therapy has eliminated "easy to kill", commensal organisms. In this case there is a breed predisposition for otitis (due to ear conformation) and there may be other predisposing causes such as atopy, hypothyroidism, hyperadrenocorticism, etc. Systemic and/or topical antimicrobial therapy alone is unlikely to be effective unless the underlying pathology is addressed.

If you were to include topical therapy as part of your therapeutic regimen, which of the following agents would you expect to be most efficacious? (Tetracycline, TMP/sulfa, Polymyxin B, Gentamicin, or Penicillin)
C. For topical therapy, among the most effective antimicrobial against Pseudomonas is polymixin B. It is often overlooked, as it is systemically toxic and often is not included on susceptibility profiles. Polymixin B's activity - as well as that of other topical antimicrobials - can be enhanced by pretreatment of the ear canals with synergistic agents. Polymyxin B is found in a number of otitis formulations.

The sensitivity profile suggests that there are few options for systemic therapy in this case. Is this unusual for Pseudomonas otitis?
No. Pseudomonas rapidly becomes resistant to many antimicrobials. While typically susceptible to the fluoroquinolones initially, mutations in porin channels allows the bacteria to block uptake of the fluoroquinolones and many other drugs as well. This results in a susceptibility profile showing resistance to drugs that were never administered and severely limiting treatment options. Systemic antimicrobial therapy is only necessary when the otitis externa is so severe that topical antimicrobials can not reach the infection site or when otitis media or interna is present.

If you wished to add a systemic antibiotic to the regimen, which of the following would be most appropriate in this case? (Amoxicillin, amikacin, enrofloxacin, tobramycin, or gentamicin)
C. The fluoroquinolones are the best systemic treatment choices due to their spectrum of activity and their ability to penetrate and have activity in inflamed, infected tissues. But once Pseudomonas becomes resistant to the fluoroquinolones, systemic treatment choices are restricted as the aminoglycosides and anti-pseudomonal penicillins are limited by parenteral administration, poor penetration and systemic toxicity.

If this case fails to respond to medical therapy, the best recommendation to the owner is:
E. Lateral (or vertical) ear resection should be considered for otitis that fails to respond to medical therapy. Both procedures aid medical therapy by improving drainage of exudate from the ear and by making the ear easier to clean and medicate. About 50% of affected dogs will have resolution of their otitis following this procedure. In breeds that are predisposed to chronic otitis, the procedure should be performed before irreversible changes occur in the external ear canal, tympanic membrane and tympanic bulla. Some dogs ultimately will require total ear canal ablation with/without lateral bulla osteotomy. Chronic inflammation of the ear canal secondary to otitis externa causes irreversible hyperplastic changes that eventually progress to mineralization of the annular and auricular cartilages. Nonpliable external ear canals are a sign of end-stage otitis externa, and the otoscopic exam usually reveals completely obstructed ear canals. The goal of surgery is to remove all of the pathological, infected tissues, and amazingly, most dogs retain their hearing even without an external ear canal.

KEY POINTS
Pseudomonas aeruginosa is a drug-resistant, opportunistic pathogen that becomes established only when there is significant pathology and when previous antimicrobial therapy has eliminated "easy to kill", commensal organisms. In this case there is a breed predisposition for otitis (due to ear conformation) and there may be other predisposing causes such as atopy, hypothyroidism, hyperadrenocorticism, etc. Systemic and/or topical antimicrobial therapy alone is unlikely to be effective unless the underlying pathology is addressed. Pseudomonas rapidly becomes resistant to many antimicrobials. While typically susceptible to the fluoroquinolones initially, mutations in porin channels allows the bacteria to block uptake of the fluoroquinolones and many other drugs as well. This results in a susceptibility profile showing resistance to drugs that were never administered and severely limiting treatment options. For topical therapy, the most effective antimicrobial against Pseudomonas is polymixin B. It is often overlooked, as it is systemically toxic and often is not included on susceptibility profiles. It is found in a number of otitis formulations. Systemic antimicrobial therapy is only necessary when the otitis externa is so severe that topical antimicrobials can not reach the infection site or when otitis media or interna is present. The fluoroquinolones are the best systemic treatment choices due to their spectrum of activity and their ability to penetrate and have activity in inflamed, infected tissues. But once Pseudomonas becomes resistant to the fluoroquinolones, systemic treatment choices are restricted as the aminoglycosides and anti-pseudomonal penicillins are limited by parenteral administration, poor penetration and systemic toxicity. Lateral (or vertical) ear resection should be considered for otitis that fails to respond to medical therapy. Both procedures aid medical therapy by improving drainage of exudate from the ear and by making the ear easier to clean and medicate. About 50% of affected dogs will have resolution of their otitis following this procedure. In breeds that are predisposed to chronic otitis, the procedure should be performed before irreversible changes occur in the external ear canal, tympanic membrane and tympanic bulla. Some dogs ultimately will require total ear canal ablation with/without lateral bulla osteotomy. Chronic inflammation of the ear canal secondary to otitis externa causes irreversible hyperplastic changes that eventually progress to mineralization of the annular and auricular cartilages. Nonpliable external ear canals is a sign of end-stage otitis externa, and the otoscopic exam usually reveals completely obstructed ear canals. The goal of surgery is to remove all of the pathological, infected tissues, and amazingly, most dogs retain their hearing even without an external ear canal.

For additional educational opportunities please plan to attend the 78th Western Veterinary Conference February 19 - 23, 2006. Relevant links from the 2006 WVC Program: Diagnostic Approach to the Problem Ear (V215) Systemic Therapy for Otitis Externa & Otitis Media (V219) Managing the Perpetuating Causes of Otitis (V214)