The ear canal is lined by skin and as such any skin problem can also affect the ears. A range of primary triggers have been identified as causing otitis externa. These are listed in Table 5.1.

Table 5.1 Primary triggers for otitis externa

5.1 ALLERGY (SEE ALSO CHAPTER 4, SECTION 4.1.1)

5.1.1 Atopy, Food Allergy, Contact Allergy/Irritancy

Clinical Signs

In acute allergy the canal appears erythematous and hyperplastic (Fig. 5.1) with minimal exudation. The pars flaccida may appear swollen and oedematous (Fig. 5.2). In more chronic disease the canal can become narrowed (Fig. 5.3) with increased erosion and ulceration, exudation and secondary infection (see later, section 5.10). Food allergy and atopy cannot be distinguished purely on the grounds of the appearance of the canals. Disease can be unilateral. In cases of contact irritancy/allergy where topical medication is not discontinued the canal can become severely ulcerated.

Figure 5.1 Erythema of the ear canal in early allergy.

Figure 5.2 Oedema of the pars flaccida in an allergic ear.

Figure 5.3 Narrowed hyperplastic allergic ear canal.

5.2 ENDOCRINE DISEASE

5.2.1 Hypothyroidism (See Also Chapter 4, Section 4.2.2)

Clinical Signs

Changes in the canal are none specific with mild hyperplasia and a thick ceruminous discharge (Fig. 5.4). Chronic cases develop bacterial and/or yeast infection and on occasion aural demodicosis.

Figure 5.4 Ceruminous discharge in the ear canal of a hyporthyroid dog.

5.2.2 Hyperadrenocorticism (See Also Chapter 4, Section 4.2.2)

Clinical Signs

Glandular secretions are often reduced in uncomplicated cases, but the immunosuppressive effects of the disease can predispose to aural demodicosis or infection with bacterial or yeast (Fig. 5.5).

Figure 5.5 Pseudomonas infection in a dog with hyperadrenocorticism.

5.3 ECTOPARASITES

5.3.1 Otodectes Cynotis

Aetiology and Pathogenesis

Otodectes cynotis is a relatively large mite (0.3–0.4 mm) that lives predominantly in the ear canal of the dog and cat although it can be found occasionally on other areas of the body, especially around the tail base on cats where they sleep curled up. It is a non-burrowing mite and lives by browsing on surface on skin debris and tissue fluid. It is thought to be able to live off the host for several weeks. Mites have zoonotic potential and have been recorded to cause a popular eruption in contact people.

Clinical Signs

Young dogs appear to be predisposed. Otodectes infestation leads to the production of copious amounts of dry, dark brown, crumbly waxy debris. Some authors describe it as having a ‘coffee grounds’ appearance. Mites can be seen down an otoscope with the naked eye as small, white, moving dots (Fig. 5.6). The degree of erythema and pruritus within the canal is variable, as some animals are thought to mount an allergic reaction to the mites leading to a disproportionate amount of inflammation for the number of mites that are present.

Figure 5.6 Otodectes mites in an ear canal.

Key Diagnostic Tests

• Clinical signs.
  
• Microscopic examination of ear wax (which can be mounted in 10% potassium hydroxide) reveals typical oval mites with short pedicles and suckers on the front four legs (Fig. 5.7).

Figure 5.7 Otodectes cynotis in ear wax.

5.3.2 Demodex Mites

Aetiology and Pathogenesis

See Chapter 4, section 4.7.2.

Clinical Signs

Demodex canis, the follicular demodex mite, is reported as a rare cause of otitis externa in the dog. Clinical signs within the canal are none specific which is usually hyperplastic and waxy (Fig. 5.8). In cats D. gatoi is more commonly associated with a ceruminous otitis externa. Demodectic mites can be recovered from the ear canal either by examination of wax or by skin scrapings taken from the wall of the canal (Figs 5.9, 5.10).

Figure 5.8 Hyperplastic waxy canal in a dog with demodicosis.

Figure 5.9 Demodex mites in ear wax.

Figure 5.10 Demodex partially obscured by debris on a tape stripping.

5.4 KERATINIZATION DISORDERS

5.4.1 Sebaceous Adenitis

Aetiology and Pathogenesis

See Chapter 4, section 4.2.4.

Clinical Signs

Destruction of sebaceous glands in the skin also leads to damage to glandular tissue within the ear canal. Dogs typically have an erythematous, often hyperplastic, canal containing large amounts of dry, crusty material (Fig. 5.11). Over-cleaning of these ears with acidic solutions or potent ceruminolytics can lead to severe Gram-negative infection.

Figure 5.11 Ear canal of dog with sebaceous adenitis.

5.4.2 Primary Idiopathic Seborrhoea

Aetiology and Pathogenesis

See Chapter 4, section 4.2.4.

Clinical Signs

Can be seen in many different breeds, but the Cocker Spaniel appears to be over-represented. Increased wax production in the ear leads to a thick, malodorous, ceruminous discharge (Fig. 5.12) often with secondary yeast infection. In chronic cases the ear canal can develop signs of glandular hyperplasia.

Figure 5.12 Primary idiopathic seborrhoea in the ear canal of a dog.

5.5 AUTOIMMUNE DISEASE

5.5.1 Pemphigus Foliaceus

Aetiology and Pathogenesis

See Chapter 4, section 4.4.2.

Clinical Signs

Can affect any breed but the Akita is predisposed and can develop severe otitis externa due to pemphigus with minimal involvement of the rest of the skin. In the early stages the ear is painful and the canal is hyperplastic, erythematous and ulcerated, primary pustules can be seen in some cases (Fig. 5.13). In more chronic disease secondary infection can occur with Gram-negative infection.

Figure 5.13 Pustules in the ear canal of a dog with pemphigus foliaceus.

5.6 IDIOPATHIC DISEASE

5.6.1 Juvenile Cellulitis

Aetiology and Pathogenesis

See Chapter 4, section 4.4.2.

Clinical Signs

Ear canal is swollen, erythematous, ulcerated and painful (Fig. 5.14). The discharge is often sero-purulent in nature but in the early stages of the disease is sterile. In more chronic disease Gram-negative infection can occur.

Figure 5.14 Ear canal in a dog with juvenile cellulitis.

5.6.2 Proliferative and Necrotizing Otitis

Aetiology and Pathogenesis

Rare disorder of unknown aetiology, which has only been reported in a small number of cats. All cats have been young (<5 years of age). Its response to immune-modulating drugs may suggest a possible immune-mediated cause.

Clinical Signs

Affected cats have large tan to dark brown-black coalescing plaques (Fig. 5.15) covering the concave surface of the ear pinnae and external ear canals. Friable material from the plaques and thick dark exudates occludes the ears. Secondary infection with bacteria and yeast is common.

Figure 5.15 Ear canal in a cat with proliferative and necrotizing otitis.

Key Diagnostic Tests

• History and clinical signs.
  
• Biopsy reveals acanthosis with pronounced hair follicle root sheath hyperplasia and neutrophilic luminal folliculitis, follicular hyperkeratosis. Necrotic keratinocytes are found in the outer root sheath.

5.7 FOREIGN BODIES

5.7.1 Naturally Occurring – Grass Awns, Dirt, Sand, Broken or Loose Hairs

Clinical Signs

Most commonly seen in young hunting type dogs who present with an acute onset, painful, unilateral otitis externa, often with no previous history of ear disease. Palpation of the vertical canal usually elicits pain. The foreign body can usually be seen on otoscopic examination (Fig. 5.16a). Some foreign bodies are completely asymptomatic (Fig. 5.16b). However, material can migrate into the deepest portions of the vertical canal (Fig. 5.16c), rupturing the tympanic membrane and causing otitis media.

Figure 5.16 Foreign bodies within the ear canals of dogs (A–C).

5.7.2 Iatrogenic – Fluid or Powder Medication

Clinical Signs

Ear disease is acute in onset and is often associated with a history of a visit to the groomer or vet. Medication can accumulate due to poor epithelial migration or due to excessive application without removal by an owner. Powder can become impacted at the base of the canal (Fig. 5.17) and may need mechanical removal, or the overuse of ear drops or flush solution can accumulate in the horizontal canal (Fig. 5.18). Where material accumulates in the horizontal canal the dog can present with head shaking or with a reduction in hearing.

Figure 5.17 Canker powder within the ear canal of a dog.

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