Otitis externa is an acute or chronic inflammation of the epithelium of the external ear canal which may also involve the pinna. It is a common disorder which may present as a primary complaint or concurrently with other problems. Occurs in 3.5% of all dogs, 10-20% of dogs presenting to DVM.
I. Otitis externa (syndrome not diagnosis)
1. Highest incidence is between 5 to 8 years age.
2. Long eared dogs predominate, especially Spaniels, Retrievers, and German shepherds.
3. Feline otitis is less common.
4. Normal cerumen is made of sebaceous and apocrine secretions. Apocrine secretions greatly increase with an otitis externa
B. Primary Causes: Conditions that initiate the inflammatory process within the ear canal
1. Foreign Bodies(especially foxtails in the west), primary vs. secondary
2. Parasites: Otodectes cynotis, Demodex and other parasites.
a. Feline demodex may cause chronic mild ceruminous otitis with no cutaneous lesions.
b. Otodectes-nonburrowing mite. May cause mechanical irritation and hypersensitivity reaction. Pet with hypersensitivity will show very low numbers (<3/canal). Asymptomatic carriers may occur and require all contact animals be treated. Allergic reaction(Type III and Type I) and purulent discharge may kill mites, always check both ears.
3. Pemphigus and other autoimmune skin diseases causing pinnal lesions with secondary otitis.
4. Allergic diseases(especially food and inhalant allergies)
a. Allergic disease may present initially as unilateral disease. 40-55% of atopic disease have otitis, as much as 5% of atopics have only ear disease. Water may especially aggravate or initiate allergic otitis. Early atopic otitis has erythema of pinnae and vertiical canal while horizontal canal is WNL.
b. 80% of food allergic dogs have allergic otitis. Cocker spaniels may require 8 week trials.
c. Contact allergic reaction to otic preps may have lesions anterior to pinnae, side of face. Propylene glycol and neomycin are most common sensitizers.
5. Extension of pinnae disorder
6. Neoplasia-polyps, ceruminous gland carcinoma, SCC, papilloma
7. Metabolic diseases
a. Hypothyroidism and sex hormone imbalance are most common endocrine disorders. Hypothyroidism must be carefully evaluated as many chronic otitis cases have sick euthyroid syndrome.
b. Keratinization disorders and glandular disorders affect the ear canal. Abnormal lipid production are broken down by bacteria and yeast may yield inflammatory mediators. Seborrheic otitis, Zinc and Vitamin A responsive dermatosis, Sebaceous adenitis Apocrine hypertrophy and inflammation(hidradenitis) is often present.
C. Predisposing Causes: Facilitate the inflammation by permitting an environment conducive to survival of perpetuating factors.
1. Conformation and moisture of the ear canal-shave medial pinnae in pendulous ears
2. Hair in the ear canal
3. Breed predisposition, example Shar PEI stenotic canals
4. Immune deficiency
5. Endocrine disorders
6. Overtreatment of ear disorders
D. Perpetuating Causes: Sustain and aggravate the inflammatory disease. The most common cause of recurrence. If > 4 weeks, 50% chance of tympanic membrane rupture.
1. Occlusion of the canal-progressive pathologic changes( hyperkeratosis, hyperplasia, acanthosis, hyperpigmentation, apocrine glandular hyperplasia/adenitis, edema and fibrosis).
2. Alteration of pH
3. Focus of infection (otitis media)-difficult to diagnose, may have no clinical signs. Tympanic membrane thickens with chronic inflammation. Will lead to rupture or dilate into middle ear cavity. Ruptured membrane heals very quickly in dog, but may heal on itself.
5. Bacterial and Yeast infections-80% of normal ear canal have coagulase positive Staph. S. intermedius is most common pathogen from acute otitis, Pseuodmonas and Proteus are more common in chronic ear. Malasezzia sp. is primary yeast pathogen especially common with atopic disease. Yeast fermentation products have been shown to be severely inflammatory.