Otitis externa
Otitis externa is a very common diagnosis in small animal practice. In a recent study, otitis or infection with the ear mite Otodectes cynotis was the final diagnosis in 5% of all canine consultations and 2% of all feline consultations.1 Although otitis externa is often presented, it can (as with many types of skin disease) be a challenging and sometimes frustrating disease to manage. Successful management can involve long term, or even lifelong, treatment which requires a high level of owner commitment and a good relationship between the owner and veterinary surgeon.
1 = Hill, P.B. et al. Veterinary Record (2006); 158: 533-539
What causes otitis externa?
The etiology of otitis externa is multifactorial, and involves differing combinations of primary and secondary causes of inflammation, along with perpetuating and predisposing factors. This classification reflects the latest thinking among veterinary dermatologists.
Primary causes of inflammation include allergic skin diseases such as atopic dermatitis and food hypersensitivity, ectoparasites, foreign bodies, keratinization disorders, tumors or polyps of the ear canal and immune mediated and autoimmune diseases.
Secondary causes of inflammation are bacterial and yeast infections. Infections arise because of a change within the microclimate of the ear canal brought about by primary causes of inflammation plus perpetuating and predisposing factors that favor the growth of micro-organisms.
Perpetuating factors are a consequence of otic inflammation and prevent resolution of otitis. Examples include loss of the ear’s ‘self-cleaning mechanism’ and progressive pathological changes to the ear canal. These factors alter the environment of the ear canal for example by narrowing the lumen, reducing air circulation, increasing humidity and providing a food supply and a ‘safe haven’ for microbes. These pathological changes can become irreversible if the otitis is inappropriately treated. Predisposing factors increase the risk of an animal developing otitis, but will not cause otitis in isolation – underlying primary triggers will always be present.
Predisposing factors include conformational abnormalities (e.g. abnormally narrow ear canals, pendulous pinnae or excessively hairy ear canals), excessive moisture (e.g. from swimming), endocrinopathies such as hypothyroidism and iatrogenic factors (e.g. trauma from use of cotton swabs).
Microbial involvement
The skin of the normal ear canal is not sterile. The normal flora consists primarily of Gram positive bacteria, but numbers are low. Yeasts are also present in small numbers. Therefore bacteria and yeasts are not primary factors in the development of otitis, since they will not cause otitis in normal ears. However, changes in the microenvironment of the ear canal can result in overgrowth of these commensal microbes or a shift to non-commensal pathogenic Gram negative bacteria. If this happens, bacterial and yeast infections can act as secondary causes of otic inflammation.
A shift to Gram negative organisms such as Pseudomonas is most commonly associated with chronic otitis externa. Stopping acute cases from becoming chronic, via correct initial treatment and ongoing preventative management, can help to prevent infection with this difficult-to-treat bacterium. This table outlines the prevalence of different micro-organisms isolated from over 3,200 canine ears affected by otitis externa in 15 different clinical studies.2 Staphylococci and Malassezia were the most common microbes isolated. Gram negative organisms, such as Pseudomonas, were isolated much less frequently.
| Staphylococci | 51.6% |
| Malassezia pachydermatis | 41.6% |
| Streptococci | 15.1% |
| Pseudomonas | 13.5% |
| Proteus | 10.9% |
| Coliforms | 8.8% |
2 Harvey, R.G.et al Ear Diseases of the Dog and Cat (2001); 37 Manson Publishing
Clinical signs
Clinical signs of otitis include pruritus, head shaking, an aural discharge, malodor, and pain or discomfort. Often the owner will detect one or more of these clinical signs, and this will prompt them to seek veterinary attention for their pet.
Diagnosis
A diagnosis of otitis externa can be made by reviewing the clinical signs and performing a general clinical examination and an otoscopic examination. Ideally cytology and parasitology should also be undertaken in all cases so that appropriate therapy can be selected. If rod bacteria are detected on cytology, samples of otic discharge should be sent for culture and sensitivity testing.
Otitis cases can be classified as acute, subacute, chronic or recurrent/relapsing, depending on how long the clinical signs have been present for and how the case has previously responded to treatment.
| Acute | Clinical signs present for ≤7 days |
| Subacute | Clinical signs present for >7 but ≤30 days |
| Chronic | Clinical signs present for >30 days |
| Recurrent/Relapsing | Episodes cured after proper treatment but re-appear on a regular basis |
There isn’t time to take a full dermatological history in a 10 minute consultation, but fortunately, a detailed dermatological history and investigation is probably not necessary for a 1st or 2nd presentation acute otitis externa case. However if the otitis is recurrent or chronic, then a much more detailed dermatological history and investigation will be required. These cases may require referral.
Treatment
Symptomatic therapy of acute otitis externa on the first or second presentation in an individual animal is quite reasonable. Topical treatments that can be used include ear cleansers, such as MalAcetic® Ultra Otic, TrizUltra™+Keto or MalAcetic™ Otic Cleanser, and medicated ear treatments, such as Animax® Ointment (nystatin-neomycin-sulfate-thiostrepton-triamcinolone-acetonide-ointment).
Whatever topical treatment is selected, it is important to follow-up the case to ensure that the ear canal is clean and repeat cytology to ensure that the infection has resolved. It should be emphasized to the owner that, in many cases, prophylactic measures such as continued ear cleaning will be necessary to help prevent recurrence.
Any cat or dog that presents with an acute episode of ear disease has the potential to develop into a recurrent or chronic case, and so correct treatment of these early episodes, and appropriate follow-up and ongoing preventative care is vital.