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 25-28 September, 2018 | Singapore
resistant to many antimicrobials through low cell wall permeability, β-lactamases, clavulanate-resistance and efflux pumps. They readily develop further resistance
if treatment is ineffective as they have a large genome to express resistance genes and mutations, and are capable of plasmid, transposon and bacteriophage transfer. Fluoroquinolones and gentamicin are usually effective against first time Pseudomonas infections. Fluoroquinolones, which have a high volume of distribution and penetrate well into most tissues, may have better efficacy in infections otherwise susceptible to other antimicrobials. Once fluoroquinolone resistance is established other anti-Pseudomonas antimicrobials are indicated; these are often expensive, not licensed for animals and have to been given intravenously if used systemically. (Table 3)
· † Reconstituted solution stable for up to seven days at 4°C or one month if frozen
· # Silver sulfadiazine shows additive activity with gen- tamicin and fluoroquinolones (although synergy has not been proven)
Potential Toxicity of Antimicrobial Agents
Tobramycin and amikacin are potentially ototoxic and should be used with care if the tympanic membrane
is ruptured. Enrofloxacin, marbofloxacin, ceftazidime and silver sulfadiazine appear to be safe in the middle ear. There is potential for systemic toxicity with silver sulfadiazine and aminoglycosides in extensively ulcerated ears, although this is unlikely in practice as the total body dose will be low except in very small animals. The ototoxicity of gentamicin appears to depend on
the preparation, and topical application of injectable solutions of gentamicin appears to be safe. Systemic aminoglycosides can be nephrotoxic and renal function should be monitored. Fluoroquinolones can cause cartilage damage in dogs under 12 months old (18 months in giant breeds) and neurotoxicity at high doses.
TrizEDTA damages bacterial cell walls and increases
antimicrobial efficacy which can overcome partial resistance. It is best given 20 to 30 minutes before
the antimicrobial but can be co-administered. It is
well tolerated and non-ototoxic. TrizEDTA shows
additive activity with chlorhexidine, gentamicin and fluoroquinolones at concentrations of 35.6/9.4 mg/ml, but there is no evidence of synergy and efficacy at lower concentrations. Solutions of 0.6 per cent enrofloxacin, 0.2 per cent marbofloxacin, 0.3 per cent gentamicin,
0.1 per cent amikacin and 1.7 per cent ceftazidime in trizEDTA are effective against many multi-drug-resistant bacteria including Pseudomonas.
Otitis media may need three to four weeks (and possibly longer) systemic treatment, which is a problem if parenteral drugs are used. Pseudomonas infections, however, usually clear quickly once effective cleansing, antimicrobial treatment and control of the primary cause are established.
5. Topical glucocorticoids
Mild inflammation responds rapidly to low potency topical glucocorticoids, but progressively more severe inflammation requires longer courses of more potent products (Table 4). Very potent products should be avoided in severe bacterial infections, particularly Pseudomonas, as they may suppress neutrophil activity.
Once the infection has resolved, topical glucocorticoids should be used at the lowest frequency that controls the inflammation. Systemic treatment is necessary if there
is stenosis, severe fibrosis or mineralisation, or if topical therapy cannot be safely administered. It is usually possible to switch to topical therapy once the ear canals have opened. Dogs better tolerate topical therapy once the pain and inflammation has decreased.
This table should be used for guidance only, as the relative potency of topical glucocorticoids also varies with the concentration, formulation and preparation. Topical therapy is safer than systemic therapy but adverse effects can be seen, for example, the hypothalamic-pituitary-adrenal (HPA) axis can be affected for up to two to four weeks after otic administration
of dexamethasone. Hydrocortisone aceponate and mometasone furoate show less local atrophy and systemic absorption than other glucocorticoids. Atrophic effects can be useful in reversing fibrosis and stenosis early in treatment, but may later interfere with epidermal migration allowing debris and desquamated cells to accumulate in the ear canals.

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