The aetiology 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, keratinisation disorders, tumours 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 favour 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 buds).