This retrospective study of 100 dogs investigated the possible associations between signalment, history, and clinical and laboratory findings and the various primary, secondary, and perpetuating causative factors of ear canal inflammation in dogs with otitis externa (OE). The predisposing factors for OE identified were pendulous ear pinnae (35/100), presence of hair (18/100), water in the ear canals (4/100), and congenitally stenotic (1/100) or obstructive (1/100) ear canals. Allergic dermatitis was the most common primary cause of OE (43 of 100 cases). The parasitic causes of OE (13 of 100 dogs) were otodectic mange (n = 7), demodectic mange (n = 4), and sarcoptic mange (n = 2). Grass awns were the cause of OE in 12 cases. The remaining primary causes of OE included pemphigus foliaceus-like disease, hypothyroidism, and ceruminous gland adenoma (1 case each). Primary causes of OE could not be found in 32 dogs, while more than 1 cause was documented in 3 dogs. Cytologic evaluation of the ear canal indicated that secondary causative factors of OE included Malassezia species (66/100), cocci (38/100), and rods (22/100). OE-induced changes of the local microenvironment that enhance proliferation of normal flora, introduction of new organisms carried by foreign bodies, and probably auricular self-trauma and microbial synergism may explain the presence of mixed microbial populations. Ear canal stenosis (38/100), cartilage mineralization (3/100), and tympanic membrane rupture-otitis media (25/100) were the perpetuating factors identified on otoscopic and radiologic examination. Results of this study documented a higher prevalence of atopic dermatitis and OE associated with adverse food reactions in females and in dogs with a history of pruritic skin disease. Grass awn-induced OE was more common in cocker spaniels and acute cases. Tympanic membrane perforation was negatively associated with atopic dermatitis or OE associated with adverse food reactions and positively associated with the presence of grass awns and rods. A positive association between cocci overgrowth and ear canal stenosis was observed.

COMMENTARY: This study has several major take-home messages. Allergic dermatitis is the most common cause of OE, especially chronic otitis externa. Bilateral otitis externa is more common than unilateral OE. Over 90% of cases of OE have secondary bacteria or yeast infections, with the latter being more common. At least one third of cases have mixed bacterial flora on cytologic examination. Ear canal stenosis is present in approximately 40% of cases, and at least 25% of dogs examined for OE have tympanic membrane rupture/otitis media. This study confirms what we clinically suspect: When presented with a dog with chronic OE, allergy-especially atopy-is the most likely underlying cause and it is in the best interest of the patient to aggressively pursue diagnosis and treatment of otitis externa or media due to bacteria and/or yeast.
PS: This is yet another study documenting the fact that cocker spaniels are prone to ear disease!

Aetiology of canine otitis externa: A retrospective study of 100 cases. Saridomichelakis MN, Farmakit R, Leontides LS, Koutinast AF. VET DERMATOL 18:341-347, 2007.