OTITIS externa is a common reason for presentation of both cats and dogs at veterinary practices. The condition is distressing to both pet and owner and often accompanied by behavioural signs in the pet associated with pain and discomfort.
Familiarity with the condition means that many of us treat otitis as a routine diagnosis. There are many tried and tested treatment protocols, not all of which produce a long-lasting solution. There are, however, ways to structure our thinking about otitis that open up a number of new approaches.
Although the principles are nearly 20 years old, the idea of staging ear disease is increasingly advocated when developing treatment plans. This recognises the contribution individual factors play in the development of ear disease. Primary causes include foreign bodies, ear mites, tumours, polyps and generalised skin conditions.
Predisposing causes change the microenvironment in the ear and can help trigger ear disease: anatomical stenosis of the ear canals, as seen in the Shar Pei breed is a good example; while pendulous pinnae is one of the most common predisposing causes seen in practice.
Perpetuating causes aggravate an existing otitis once it has developed and include bacterial and yeast infections.
All too often it is the perpetuating factors that receive the focus of our attention and they are the easiest problems to address through therapy, so it is therefore not surprising that many cases of otitis recur. Addressing the relative contribution of each element, a short- and long-term approach to treatment and prevention can be developed.
Atopic dermatitis is estimated to affect 10% of the dog population and the understanding of this disease has developed considerably.
It has been estimated that 55% of dogs with atopy also have otitis and in 3% of cases it is the only clinical sign.
It has been suggested that atopy is not an allergic reaction caused by inhaling allergens, as originally thought, since intranasal challenge does not worsen the appearance of skin lesions.
Epi-cutaneous exposure (and to a lesser extent oral exposure) is now thought to be the most important route and topical treatment aimed at removing allergen from the coat can ameliorate clinical signs.
Food allergy can also present with similar clinical signs and atopic dermatitis and food allergy can occur concurrently. Food allergies are reported to be much less common a cause compared to atopy.
Cleaning and flushing
Cleaning the ears is an important first step in the management of otitis externa. Removing debris allows better visualisation of the ear structure and provides an exposed surface for the application of topical treatments.
The clinician’s biggest concerns are likely to be whether the tympanic membrane is still intact and the best course of action when ear canals are stenosed due to inflammation.
Prior to cleaning, samples for cytology and, if suggested, culture and sensitivity should be taken. In the case of hyperplastic change a biopsy may be indicated. Debris and cerumen buildup may make visualisation of the tympanic membrane prior to flushing or cleaning impractical and the characteristic signs of otitis media – head tilt, ataxia and difficulty swallowing – are sometimes absent.
Examination under sedation or general anaesthesia and flushing of the ear may still preclude visualisation of the tympanic membrane using a standard otoscope, and it has been noted that otitis media can occur even when the membrane remains intact. If rupture is suspected or cannot be eliminated it is useful to flush any traces of residual ear cleaner from the ear using saline to avoid ototoxicity.
Ear cleaners typically contain ceruminolytics, although the efficacy can be variable and it may be necessary to wait up to 15 minutes after application for the debris to be sufficiently softened. Surfactants and emoillients aim to dissolve and break up cerumen and urea-based products have a foaming action.
Some surfactants and urea peroxide can be irritating to the ear and saline washes should be considered after flushing, with less irritating solutions prescribed for application by the client.
Ear cleaners containing astringents help dry the ear canal. Some ear cleaners also have antimicrobial activity. It is also worth thinking about using a cleaner containing triz-EDTA, a chelating agent that can be used 15 minutes before topical antibiotic administration. The solution increases bacterial cell wall permeability and increases the efficacy of the antibiotic.
Dealing with stenosis
Stenotic ear canals can occur due to acute or chronic inflammation. Those cases where there is fibrosis and glandular hyperplasia may require surgery but medical therapy may improve patency marginally, helping to resolve infection and reduce pain.
More acute cases can benefit from ear cleaning and an antibacterial/ anti yeast flush, followed by systemic steroid therapy (prednisolone 1mg/ kg/day per os) for two weeks and a further four weeks on every other day therapy.
Topical steroid therapy may also be indicated after the initial two week period.
Polyvinyl acetate (PVA) ear wicks have been widely used in people, and latterly in dogs, as a means to deliver antibiotics on the ear. These are soft, highly absorbent sponges, inserted under general anaesthesia. They are soaked in a suitable solution of antibiotics, steroid or triz-EDTA used in combination. Antibiotic is applied daily to the wick.
After 3-10 days the animal is re-examined, the wick removed and, if necessary, replaced. Wicks are said to be well tolerated and are particularly suitable for use in dogs where their owners have difficulty applying topical treatments. Steroidsoaked wicks have been advocated for the treatment and prevention of stenosis of the ear canals.
In the absence of otitis media, ear canal ulceration or Pseudomonas infection, topical antibiotics are the preferred means to deliver antibiotics in otitis externa.
As well as the action of individual antibiotics, some preparations make use of the synergistic effects. For example, polymixin A and miconazole have syngeristic effects on Gramnegative bacteria (such as Pseudomonas) and Malassezia.
Nevertheless, Pseudomonas presents a particular challenge for any antibiotic and clients should be advised that treatment can be complicated by antibiotic resistance, that systemic as well as topical antibiotics may be indicated and that recurrence is common.
Malassezia pachydermatitis is a common pathogen in the ear and topical preparations often contain suitable antifungals such as nystatin, clotrimazole or miconazole.
McTaggart, D. (2008) In Practice 30: 450-458.
Paterson, S. The use of ear wicks in the therapy of otitis externa, as quoted at www.dermapet.com/articles/ wicks.html.
Marsella, R. (2006) Atopic Dermatitis: a new paradigm, Hills Symposium on Dermatology.
Nuttall, T. (2009) Treatment of otitis externa, BSAVA Congress Scientific Proceedings.
Paterson, S. Therapy of ear canal hyperplasia and stenosis as quoted at www.dermapet.com/articles/art_ther_e ar.html.