To My Quyen, Nguyen Khanh Thuan, Nguyen Phuc Khanh, Nguyen Thanh Lam*

1. Introduction

Otitis externa is inflammation of the external ear canal and is a common problem in dogs and cats. Otitis externa may be acute or chronic (persistent or recurrent otitis lasting for 3 months or longer). Changes that occur in the external ear canal in response to chronic inflammation may include glandular hyperplasia, glandular dilation, epithelial hyperplasia, and hyperkeratosis. These changes usually result in increased cerumen production along the external ear canal, which contributes to increase in local humidity and pH of the external ear canal, thus predisposing the ear to secondary infection. Diagnosis is based on otoscopic examination, cytology, and culture. Treatment depends on the specific diagnosis. The inciting cause must be addressed to prevent recurrence (Bajwa, 2019).

2. Aetiology

The bacteria most commonly isolated from ear canals of dogs affected by otitis are Staphylococcus spp.  Other bacteria commonly associated with otitis include Pseudomonas, Proteus, Enterococcus, Streptococcus, and Corynebacterium. Some bacteria such as Staphylococcus and Pseudomonas may produce biofilm, which can lead to persistence of infection despite adequate therapy, as the biofilm needs to be disrupted for any antimicrobial therapy to be effective in clearing the infection. Malassezia yeast is another common component of otitis externa in dogs. Some dogs appear to develop an allergic response to Malassezia spp., leading to significant discomfort and pruritus. Understanding the multifactorial nature of otitis and paying attention to the different causes and contributing factors, not just the infection, is critical because the infection is usually only part of the problem (Koch, 2017).

The pathophysiology of otitis externa is not complex – in fact perhaps the opposite. Figure. 1 shows the self-perpetuating nature of the condition if untreated or inadequately treated. The detailed pathology of otitis, particularly early on, differs to some extent according to the cause, but in general, changes are rather stereotyped. Acute inflammation and oedema, if not resolved, progresses over time to chronic inflammation, characterised by glandular changes, fibrosis and scarring, and, eventually, progressive stenosis and occlusion of the ear canal. Permanent changes such as calcification and later ossification of cartilage can occur. Possible sequelae are otitis media and aural cholesteatoma (both also perpetuating factors). Chronic changes favour proliferation of bacteria and yeasts, further perpetuating pathology. Ulceration of the ear canal can occur, usually in association with Pseudomonas infection. The secondary lesions of chronic otitis are due to chronic irritation and microbial overgrowth (Jacobson, 2002).

The most recently proposed classification for otitis includes primary and secondary causes and predisposing and perpetuating factors (Table 1).

Table 1: Causes of otitis externa in the dog and cat. Common conditions are in boldface, and the most common primary causes are indicated with an asterisk (Jacobson, 2002).

2.1 Primary factors

Primary factors are diseases that have a direct effect on the external ear canal and can cause otitis, including otic parasites such as Otodectes cyanotis, hypersensitivity disease [food allergy, atopic dermatitis, contact hypersensitivity], endocrine disease such as hypothyroidism, otic neoplasia and foreign bodies. Underlying hypersensitivity disease is the most common primary factor leading to otitis in dogs (Bajwa, 2019).

2.2 Predisposing factors 

Predisposing factors are factors that alter the local ear canal environment and create an increased risk for development of otitis externa. Ears with excessive hair, stenotic ears, increased cerumen production in the canals, otic masses, frequent ear cleaning, as well as changes in external environmental temperature and humidity can all act as predisposing factors (Bajwa, 2019).

2.3 Perpetuating factors 

Perpetuating factors are factors that do not initiate inflammation but lead to exacerbation of the inflammatory process and maintain ear disease even if the primary factor has been identified and corrected. Bacteria such has Staphylococcus and Pseudomonas, and Malassezia yeast are common perpetuating factors. If infection travels to the tympanic bulla, presence of this infection in the middle ear can also act as a perpetuating factor, leading to recurrent external ear infections. Perpetuating factors are often the main reason for treatment failure in dogs affected by recurrent otitis externa (Bajwa, 2019).

3. Clinical signs and pathology

3.1 Clinical signs

Otitis externa is common in dogs and may be unilateral or bilateral. Evaluation for otitis and its diagnosis is based on ear canal palpation, visual inspection of ears, including otoscopic examination, and cytological analysis of otic contents.

Changes to the ear pinna may include alopecia, excoriation, crusting, erythema, and hyperpigmentation. The external ear canal may exhibit presence of hyperemia, ulceration, ceruminous or suppurative discharge, masses, stenosis, glandular changes, or foreign bodies. Usually more than one abnormal finding is noted within an affected ear. Evaluation of the tympanic membrane forms a key part of the otoscopic evaluation, though it may be difficult to assess the tympanum when otitis externa is present. It is reasonable to leave assessment of the tympanic membrane to a later date, after changes attributed to active otitis have been corrected (Bajwa, 2019).

3.2 Pathology

            Several biopsies were obtained and submitted for histopathologic examination and impression smears of the biopsied tissue were made and submitted for cytologic evaluation. Wright’s-stained smears of the mass consisted of many anucleate keratinized squamous epithelial cells, some of which contained melanin (Figure 6), low numbers of individual and small groups of spindle cells (presumptive fibroblasts, Figure 7), and low numbers of inflammatory cells in small bloody droplets. The inflammatory cells were mostly nondegenerate neutrophils with fewer vacuolated macrophages, some of which contained phagocytized melanin or bluegreen pigment (presumptive lipofuscin) in the cytoplasm. Numerous bacterial cocci were observed extracellularly. Based on the provided history, reported MRI findings, and predominance of keratinized squamous epithelial cells, the cytologic diagnosis of a cholesteatoma with concurrent inflammation and possible fibrosis was made. The bacteria were interpreted as likely superficial skin inhabitants. Histopathologic examination of hematoxylin–eosin-stained sections of formalin-fixed biopsy tissue revealed a core of fibrous connective tissue covered by a mildly hyperplastic keratinizing stratified squamous epithelium. A large nodular accumulation of cholesterol clefts mixed with numerous hemosiderin-laden macrophages, fewer lymphocytes and plasma cells, and rare multinucleate giant cells was present in the submucosa (Figure 8). The center of the lesion contained small areas of mineralization and fragments of woven bone. The histopathologic diagnosis was a cholesterol granuloma with osseous metaplasia (Newman et al., 2015). 

A clinical diagnosis of a cholesteatoma and meningoencephalitis was made. The meningoencephalitis was attributed to an extension of the preexisting otitis media, rather than the cholesteatoma. The dog was treated with enrofloxacin and ampicillin/sulbactam prior to a left ventral bulla osteotomy, which was performed 10 days later. Incision of the left bulla revealed abundant yellow-to-gray waxy material, which was completely removed and submitted for culture and histopathologic examination. This excisional biopsy revealed a mass lined by moderately to severely hyperplastic keratinizing stratified squamous epithelium devoid of adnexal structures. There was marked thickening of all layers of the epithelium, with a dense layer of surface orthokeratosis. There was vacuolation and dyskeratosis within the stratum spinosum (Figure 9). A histopathologic diagnosis of cholesteatoma was made. Decalcified sections from the bulla wall revealed mild, multifocal, chronic myelofibrosis with mild, locally extensive edema, which was interpreted as a degenerative change associated with chronic otitis media (Newman et al., 2015).

4. Diagnosis

4.1 Otic examination

The ear examination allows the clinician to evaluate the amount and type of exudate in the ear canals; estimate the amount of otic inflammation; identify hyperplasia (along with palpation of the horizontal and vertical ear canals), masses, and foreign bodies; and determine the status of the tympanic membrane (e.g., changes in structure or rupture). These findings help in determining whether medical management or surgery (total ear canal ablation with or without bulla osteotomy) is the best treatment. If the patient’s ears are painful, sedation or general anesthesia may be necessary before otoscopic examination.

Regular (i.e., handheld) otoscopes should have a strong light and power source. If available, fiberoptic video-enhanced otoscopy (e.g., video-otoscope is extremely helpful in improving diagnosis and therapy because it not only allows visualization of fine details that may not be seen with regular otoscopes but also facilitates proper flushing of the ears, determination of disease extent, and discovery of indications for additional diagnostics and treatment (e.g., myringotomy, otitis media). However, because of the expense to purchase and maintain this equipment, referral to a dermatologist may be necessary (Koch, 2017). 

Table 2: Recommended diagnostic procedure for otitis externa.

4.2 Otic cytology

Otic cytology establishes whether an infection is present in the ears and assists with the selection of topical therapy. Cytologic samples should be collected gently from the horizontal canal. Exudate samples can be smeared onto a slide with mineral oil to look for mites. The most common type of coccoid bacteria found in the ears of dogs with otitis externa is Staphylococcus pseudintermedius, and the most common type of rod bacteria is Pseudomonas aeruginosa. Malassezia species are also common organisms.

It is important to describe the presence of any inflammatory or neoplastic cells as well as quantify each type of bacteria and yeast per oil immersion field (100×) to establish severity and allow monitoring at future visits. In one study, mean bacterial counts per high-power dry field of ≥ 25 and mean Malassezia counts per high-power dry field of ≥5 was considered abnormal in the external ear canals of dogs. Leukocytes are always abnormal, and bacteria in the presence of leukocytes signal infection (Koch, 2017).

4.3 Bacterial culture and sensitivity

Culture and sensitivity (C/S) may be useful in identifying specific otic pathogens and assisting with treatment decisions; however, a limitation is that antibiotic sensitivity data reflect the serum level needed systemically and may not predict true susceptibility of otic topical antibiotics (Koch, 2017).

Typical indications for bacterial C/S include the following:

• Chronic otitis associated with bacteria (cocci and/or rods) seen on cytology
• Rods seen on cytology 
• Suspected or confirmed cases of otitis media (systemic therapy may be indicated)
• History of multidrug-resistant bacteria
• History of long-term oral or topical antibiotic therapy
• Bacteria persisting on cytology despite apparently appropriate therapy

4.4 Diagnostic imaging techniques

Dogs with chronic, recurrent, and severe otitis and those with neurologic signs (e.g., vestibular signs or facial nerve paralysis), para-aural swelling, or pain on opening the mouth usually require diagnostic imaging to help identify contributing problems, such as middle ear disease (e.g., otitis media, neoplasia) and otitis interna, that cannot be identified with regular otoscopy. Patients with an apparently normal tympanic membrane may also have otitis media. Although otitis interna is uncommon in dogs with chronic otitis externa, otitis media is common, with a reported incidence of 50% to 88.9%. In dogs with recurrent ear infections of 6 months or longer, up to 89% may have concurrent otitis media; about 70% have an intact but abnormal tympanic membrane (Koch, 2017). 

5. Treatment

                        CP = chronic proliferative

• Manage underlying causes of otitis externa
• Select antimicrobials based on history and cytology

Successful treatment of otitis externa requires owner compliance, management of inflammation, directed antimicrobial therapy, and workup of the underlying cause. Owners need to have reasonable expectations and understand that it may take time to resolve or improve otitis externa.

Initially, many patients require management of pain and/or itch. Glucocorticoids reduce inflammation, swelling, and pain, which ultimately increase the owner's ability to successfully treat and clean the ears at home. Prednisone and triamcinolone are used most commonly, with duration and dose depending on severity and chronicity of disease. In some cases, owners may not be able to clean the ears at home until the glucocorticoids have had a few days to take effect. Ear hygiene is important; in particular, the hair from the pre- and periauricular area should be clipped, as well as hair from the medial surface of the pinnae and tips of the pinnae. This facilitates cleaning and treatment of the ears. Plucking of hair from the ear canal is controversial but may be needed to adequately resolve the ear infection.

If possible, an initial ear cleaning should be done at the veterinary clinic with the owner observing. Although selection of an ear cleaner depends on the type of infection or exudate present, it is important that the cleaner has an appealing odor (to increase owner compliance) and a neutral pH (to reduce pain in inflamed ears).

Table 3: Selected cleaning and drying agents for otitis externa (Jacobson, 2002).

In addition to cleaning, effective treatment may require both topical and systemic antimicrobial and anti-inflammatory therapy. The duration of treatment may vary from 7–10 days to several months, depending on the diagnosis. In treatment of acute bacterial otitis externa, topical antibacterial agents in combination with corticosteroids reduce exudation, pain, swelling, and glandular secretions. The least potent corticosteroid that will reduce the inflammation should be used. Most commercial topical products contain a combination of an antibiotic, an antifungal and a glucocorticoid. Individual products should be chosen based on cytology (eg, gram-negative rod infections may require an aminoglycoside or fluoroquinolone).

Malasone from Vemedim 

For the treatment of canine otitis externa associated with yeast (Malassezia pachydermatis) sensitive to clotrimazole and bacteria (Staphylococcus spp., including S. intermedius, Pseudomonas aeruginosa, Proteus spp. and Streptococcus spp.) sensitive to marbofloxacin.

Dosage: 

Apply ten drops into the ear once daily for 7 to 14 days. After 7 days of treatment, the veterinary surgeon should evaluate the necessity to extend the treatment another week.

One drop of the preparation contains 71 mcg marbofloxacin, 237 mcg clotrimazole and 23.7 mcg dexamethasone acetate.

After application, the base of the ear may be massaged briefly and gently to allow the preparation to penetrate to the lower part of the ear canal. When the product is intended for use in several dogs, use one cannula per dog.

6. Control and prevention

Few effective preventive measures exist for otitis externa. Thorough otic examination of all patients presented for a physical examination helps with early detection of mild and early cases of otitis. When dogs are presented with early ear disease, thorough client education and detailed diagnostic work-up, including frequent follow-up examinations, can help prevent development of complications that may lead to chronic otitis, hearing loss, otitis media, and end-stage ear disease (Bajwa, 2019).

Some type of maintenance otic therapy is usually required, such as a cleaning and drying agent (to keep the ear canal free of wax buildup), antimicrobial ear cleaners (e.g., for recurrent ear infections), and sometimes topical glucocorticoids (for severe hyperplasia or stenosis, when surgery is not an option) (Koch, 2017).

Surgical management

Surgical management may be recommended, particularly in cases of otic tumors and chronic end-stage otitis, when all medical therapeutic attempts are made, after detailed discussion of potential benefits, risks, and post-surgery complications. Histopathology and bacterial culture of removed tissue or masses should always be performed. Advanced imaging before surgery is idea (Koch, 2017).

Table 4: Surgical techniques for chronic otitis and otic masses/tumors (Koch, 2017). 

TECA-BO: total ear canal ablation + lateral bulla osteotomy

7. References

Bajwa, J., 2019. Canine otitis externa - Treatment and complications. Canadian Veterinary Journal 60, 97-99.

Ferreira, T.C., de Carvalho, V.M., Pinheiro, D.C.S.N., 2022. Canine atopic dermatitis: report of ten cases. Research, Society and Development 11, e12411427258-e12411427258.

Jacobson, L., 2002. Diagnosis and medical treatment of otitis externa in the dog and cat. Journal of the South African Veterinary Association 73, 162-170.

Koch, S., 2017. The challenge of chronic otitis in dogs: From diagnosis to treatment, Today's veterinary practice. Dermatology.

Newman, A.W., Estey, C.M., McDonough, S., Cerda-Gonzalez, S., Larsen, M., Stokol, T., 2015. Cholesteatoma and meningoencephalitis in a dog with chronic otitis externa. Veterinary Clinical Pathology 44, 157-163.

Woodward, M., 2022. Otitis Externa in Animals. MSD manual. Veterinary Manual.