Page 250 - WSAVA2018
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 25-28 September, 2018 | Singapore
M. Burrows1
1Animal Dermatology Clinic Perth Murdoch University Division of Veterinary and Biomedical Science Murdoch, Western Australia
Cases of chronic or recurrent otitis are challenging. Successful treatment requires that all the underlying factors leading to persistence or recurrence of the otitis are identified and managed. In particular, it is important to recognise the role of inflammation in otitis. Most owners and clinicians recognise ear infections, which are then successfully managed. However, the ongoing inflammation is often missed. This leads to a cycle of recurrent infection and chronic inflammation leading
to progressive pathological changes and end-stage
otitis that requires surgical intervention. The chronic inflammation makes each bout of infection harder to treat and repeated antimicrobial use may select for resistance.
In recurrent otitis, the treatment that is prescribed
is effective but the condition recurs after therapy is withdrawn. In refractory otitis, the infection persists even when treatment is being administered.
Where to start?
Some cases of refractory otitis externa develop infection with Gram negative organisms such as Pseudomonas aeruginosa. These most commonly follows treatment of a pre-existing ear infections with various antimicrobial agents. These organisms can be resistant to many commonly used antibiotics and can be difficult to treat.
1. Perform cytology and culture and sensitivity
There is substantial controversy about whether to perform bacterial culture and sensitivity of infected ears. In my opinion, when there is chronic disease where rods or mixed populations are found on cytology, culture is useful.
Detecting rods on cytology is consistent with Gram negative bacteria such as Pseudomonas species, Proteus species, Escherichia coli. Pseudomonas are the most common. Bacterial culture and sensitivity testing definitively identifies the bacteria involved
in the infection. This can be useful for less common organisms that are hard to differentiate on cytology, eg, streptococci, enterococci, Escherichia coli, Klebsiella, Proteus and coryneforms.
The susceptibility pattern of these bacteria is hard
to predict, although most first-time infections are susceptible to topical aminoglycosides, polymyxin B, silver sulfadiazine and fluoroquinolones. However, Pseudomonas species readily acquire resistance and most isolates from recurrent infections will be multi-drug resistant. Knowledge of their likely sensitivity patterns can then help guide treatment choices.
Antimicrobial susceptibility data is less useful for topical otic drugs because concentrations in the ear canal are much higher than those used with in vitro tests predict. The response to treatment is best assessed using clinical criteria and cytology. Antimicrobial sensitivity data can be used to predict the efficacy of systemic drugs, although the concentration in the ear tissues is often low and high doses are needed. For example, enrofloxacin would need to be given at 20 mg/kg to treat Pseudomonas isolates with an MIC of 0.5 μg/ml (middle of the susceptible range) in chronic otitis.
2. Systemic corticosteroids
As otitis progresses, the inflammation created by primary factors leads to hyperplasia of the stratified squamous epithelial lining of the canal, resulting in narrowing of
the lumen and glandular hyperplasia, leading in turn to an increased production of cerumen and hidradenitis. These chronic changes, often in tandem with recurrent courses of topical antibiotics, lead to the development
of a more resistant population of bacteria, especially Gram-negative bacteria, such as Pseudomonas species. Such cases require careful management as the canal is often ulcerated and painful and otitis media is a common sequel. Where the disease process becomes chronically irreversible, the lumen may be completely obliterated and in some of these cases ossification of the soft
tissue may also take place. Once the damage to the ear has become this extensive, medical therapy is rarely effective. The most important perpetuating factors are listed in Table 1.
Reducing pruritus, swelling, exudation and tissue proliferation is a key goal of therapy. Glucocorticoids (particularly dexamethasone) also reverse the ototoxic effect of Pseudomonas infections. Prednisolone (1 to 2 mg/kg every 12 to 24 hours) is sufficient to control inflammation and stenosis in most cases. Patients
with severe fibrosis and stenosis, however, may respond better to dexamethasone (10 times as potent

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