Otitis Media - Externa in Dogs and Cats

Definition and Overview:

Otitis externa: Inflammation of the external ear canal, often involving secondary bacterial or yeast infections. Critical to identify and address underlying causes to prevent recurrence and antimicrobial resistance.

Otitis media: Secondary to otitis externa, often caused by infection passing through a ruptured tympanic membrane, with middle ear inflammation.

Pathophysiology:

The pathophysiology of these disorders is multifactorial, involving primary, secondary, predisposing, and perpetuating factors that disrupt the normal homeostasis of the ear environment.

Otitis Externa:

In both dogs and cats, otitis externa is characterized by inflammation of the external auditory canal. Primary etiological agents include hypersensitivity reactions (such as atopic dermatitis and adverse food reactions), ectoparasitic infestations (notably Otodectes cynotis), foreign bodies, and keratinization disorders. These primary factors compromise the integumentary defenses of the ear canal, facilitating secondary infections predominantly by Staphylococcus spp. and Malassezia pachydermatis. Predisposing factors, such as anatomical variations (e.g., stenotic ear canals, pendulous pinnae), excessive moisture, and iatrogenic trauma from overzealous cleaning, further exacerbate susceptibility to otitis externa. Perpetuating factors, including epithelial hyperplasia, glandular hyperplasia, and fibrosis, contribute to chronicity by maintaining an environment conducive to microbial proliferation and inflammation.

Otitis Media:

Otitis media often arises as an extension of otitis externa through a compromised tympanic membrane, although it can also occur via ascending infections from the nasopharynx through the auditory tube, particularly in felines with a history of upper respiratory tract infections. In canines, chronic otitis externa is a common precursor to otitis media. The inflammatory process within the middle ear can lead to the accumulation of exudate, resulting in increased intratympanic pressure and potential rupture of the tympanic membrane. Microbial pathogens frequently isolated in otitis media include Pseudomonas aeruginosa, Staphylococcus intermedius, and beta-hemolytic Streptococcus spp. The presence of inflammatory polyps, particularly in cats, can also precipitate otitis media by obstructing normal auditory tube function.

Pathophysiological Mechanisms:

The inflammatory cascade in otitis externa and media involves the release of pro-inflammatory cytokines and chemokines, leading to vasodilation, increased vascular permeability, and leukocyte infiltration. Chronic inflammation induces pathological changes such as epithelial hyperplasia, ceruminous gland hyperplasia, and fibrosis, which narrow the ear canal lumen and impede therapeutic interventions. In otitis media, the accumulation of purulent material can impair auditory ossicle function, leading to conductive hearing loss. Additionally, the proximity of the middle ear to neural structures poses a risk for neurologic complications if the inflammatory process extends beyond the tympanic cavity.

Early signs: Erythema of pinnae, head shaking, otic discharge (ceruminous or purulent), malodor, swelling, and pain.

Chronic progression: Leads to canal stenosis, hyperplasia, fibrosis, and calcification, potentially resulting in otitis media.

Primary Causes of Otitis Externa

Parasites:

Otodectes cynotis: Common in cats (50%) and dogs (5–10%).

Demodex spp.: Rare ceruminous otitis externa.

Otobius megnini: Spinous ear tick prevalent in southwestern USA.

Others: Sarcoptes scabiei, Notoedres cati, Cheyletiella spp., Eutrombicula spp.

Foreign Bodies:

Plant awns (foxtails), dirt, sand, insects.

May rupture the tympanum and result in otitis media.

Hypersensitivity and Allergic Diseases:

Atopic dermatitis: 55% of dogs with atopy have otitis externa; 3–5% present otitis as the sole sign.

Food allergies: Up to 80% of affected dogs have concurrent otitis externa; 25% with only otitis.

Allergic contact dermatitis: Rare; reactions to medications or environmental irritants.

Keratinization Disorders:

Endocrine diseases (e.g., hypothyroidism, hyperadrenocorticism) result in ceruminous otitis externa.

Sebaceous adenitis and idiopathic seborrhea also contribute.

Autoimmune Diseases:

Rare but include pemphigus foliaceus, discoid lupus erythematosus, and bullous pemphigoid.

Predisposing Causes:

Anatomic Factors:

Long pendulous ears, stenotic ear canals, or excessive hair (e.g., Cocker Spaniels, Poodles).

Moisture:

Frequent swimming or bathing disrupts the protective ear barrier, promoting infections.

Iatrogenic Factors:

Trauma (e.g., swabs), irritating solutions, and inappropriate treatments.

Obstructive Diseases:

Inflammatory polyps or tumors prevent drainage, predisposing to infections.

Perpetuating Causes:

Bacteria:

Staphylococcus intermedius, Pseudomonas aeruginosa, Proteus mirabilis, etc.

Chronic cases often involve resistant Pseudomonas strains.

Yeast:

Malassezia pachydermatis, occasionally Candida spp.

Otitis Media:

Secondary to otitis externa through tympanic membrane rupture.

Pathologic Changes:

Chronic inflammation leads to fibrosis and mineralization.

Clinical Evaluation:

History:

Detailed dermatologic and general history (e.g., swimming, infections, allergy signs).

Physical Examination:

Assess ear canal erythema, discharge, and ulceration.

Check for concurrent skin lesions to identify systemic diseases.

Diagnostic Tests:

Skin scrapings (e.g., rule out demodicosis).

Cytology and culture for bacterial and fungal pathogens.

Treatment:

Cleaning:

Regular cleaning with veterinarian-recommended solutions to remove debris and exudate.

Topical Therapy:

Antibacterials, antifungals, and anti-inflammatory agents based on cytology results.

Systemic Therapy:

Antibiotics for otitis media or unresponsive infections.

Analgesics and anti-inflammatory medications for pain and inflammation.

Surgical Intervention:

Myringotomy for middle ear drainage or total ear canal ablation (TECA) for chronic cases.

Otic Cytology

General Overview

• Otitis Externa affects 10-20% of dogs and 2-6% of cats. It is not life-threatening but significantly diminishes quality of life.

• Failure to manage primary and perpetuating factors (e.g., infections) leads to chronic disease.

Key Diagnostic Goals

1. Identify and manage primary causes (e.g., atopy, food allergies, parasites like Otodectes cynotis, foreign objects).

2. Identify and manage perpetuating factors (e.g., bacterial/yeast infections, otitis media).

3. Cytology is essential for identifying microbial infections and guiding therapy.

Cytologic Examination

Sample Preparation

• Collect samples from each ear independently before any cleaning agents or therapies.

• Use a cotton-tipped applicator inserted into the horizontal canal (deeper areas are more clinically relevant).

• Roll the swab onto a slide, heat-fix lightly, and stain using a modified Wright’s stain like Diff-Quik.

Normal Cytology

• Normal findings include:

  • Keratinized epithelial cells (lightly stained, sometimes containing melanin granules).

  • Resident bacteria: Coagulase-negative Staphylococcus, Streptococcus spp.

  • Small numbers of Malassezia yeast (e.g., Malassezia pachydermatis).

• Findings suggest disease if bacteria or yeast are present with leukocytes.

Abnormal Cytology

Malassezia Yeast

• Species: M. pachydermatis (non-lipid-dependent), M. furfur, M. globosa (lipid-dependent).

• Clinical Significance: Normal residents in the external canal but can overgrow under certain conditions.

• Diagnostic Criteria:

  • Dogs: ≥5 yeast organisms/HPF = abnormal.

  • Cats: ≥12 yeast organisms/HPF = abnormal.

Bacteria

• Common pathogens: Coagulase-positive Staphylococcus, Pseudomonas spp., Proteus spp., β-hemolytic Streptococcus.

• Overgrowth vs. Infection: Presence of leukocytes and significant bacterial counts (>25 organisms/HPF in dogs, >15 in cats) indicates true infection requiring therapy.

Leukocytes

• Presence indicates inflammation or infection (e.g., neutrophils with phagocytized bacteria suggest active infection).

• Useful for monitoring disease progression and treatment response.

Parasites

• Most common: Otodectes cynotis (ear mites), responsible for up to 50% of feline otitis externa and 5-10% in dogs.

• Diagnosis:

  • Visualize mites via mineral oil preparation.

  • Treat based on clinical signs, even if parasites are not detected in samples.

Other Mites:

• Demodex spp.: Found in hair follicles, causing folliculitis and cellulitis.

• Sarcoptes/Notoedres spp.: Rarely involve the ear canal.

Management Guidelines

1. Perform cytology routinely for all patients with otitis symptoms.

2. Combine cytology with bacterial culture for definitive diagnosis.

3. Use systemic antibiotics only when cytology shows leukocytes or if otitis media is present.

4. Semi-quantitative criteria help assess yeast and bacterial overgrowth.

References:

• Otitis Externa in Animals. Veterinary Merck Manual.