What are the most common underlying causes of recurrent otitis externa in dogs?

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Allergic dermatitis is the most common primary cause of recurrent otitis externa in dogs, with atopic dermatitis and food allergy being the predominant drivers; more than half of dogs with allergies develop otitis externa, and in some patients it is the only clinical manifestation of allergy.513 The full etiologic framework is organized into primary, secondary, predisposing, and perpetuating factors. Primary causes — those that initiate the inflammatory cascade — include allergic dermatitis, foreign bodies, ectoparasites, endocrinopathies, keratinization disorders, neoplasia, and autoimmune diseases.57 Allergic otitis externa occurs in the context of atopic dermatitis and food allergy but is not a feature of flea allergy dermatitis.5 Secondary causes are bacterial and fungal infections that arise because of the altered ear canal environment; the dominant pathogens are Staphylococcus pseudintermedius, Pseudomonas aeruginosa, Proteus mirabilis, and Malassezia pachydermatis.69 Infection is always secondary — it does not occur in a normal ear.1 Predisposing factors lower the threshold for disease without causing it directly. These include ear canal conformation (pendulous pinnae, stenotic canals), excessive moisture, and swimming.913 Perpetuating factors sustain and amplify the otitis once established: epithelial and glandular hyperplasia, fibrosis, biofilm formation by Staphylococcus and Pseudomonas spp., progressive stenosis, and concurrent otitis media.367 Biofilms facilitate microbial adherence, promote self-sustaining polymicrobial populations, and substantially reduce antimicrobial efficacy.3 Recurrent cycles of inflammation drive a predictable pathologic progression. Repeated flares select for antimicrobial resistance and can shift the microbial population from Malassezia and gram-positive bacteria toward gram-negative organisms, particularly Pseudomonas spp.3 Chronic inflammation produces irreversible proliferative changes that become perpetuating factors in the