Otitis externa: a review

The incidence of otitis externa can be as high as 10% of the canine population (Hill et al, 2006). Cats (Figure 1) are much less commonly affected. Ear disease can be either unilateral or bilateral. Otitis (Figure 2) can be very painful for the patient and frustrating for owners and vets alike. In comparison to pruritus, otitis is just a clinical sign, not a final diagnosis.

Otitis is a multifactorial disease and it is important to identify and address all the different factors involved. To facilitate identification of these factors, the PSPP system (which stands for primary, secondary, predisposing and perpetuating factors) has been developed by August (1988) and should be followed in any patient with ear disease.

In addition to the primary disease, which drives the inflammation, secondary microbial overgrowth is a big factor in the aetiology of otitis externa. However, this is just in response to the primary disease initiating changes in the ear canal that make it favourable for microbial growth, such as an increase in temperature or humidity. The most common bacterial species found in cases of otitis are Staphylococcus spp. Other bacteria commonly implicated are Pseudomonas spp., Proteus spp., Enterococcus spp., Streptococcus spp., and Corynebacterium spp. Yeast (Figure 3), such as Malassezia spp. are also commonly associated with cases of otitis.