The Challenge of Chronic Otitis in Dogs: From Diagnosis to Treatment

Canine chronic and recurrent otitis externa can be extremely challenging to treat and require multifactorial, step-by-step strategic plans. Understanding otitis and its associated causes and contributing factors is an important initial step toward successful diagnosis and treatment.

Understanding Otitis: Cause and Pathogenesis

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 (Figure 1). The most recently proposed classification for otitis includes primary and secondary causes and predisposing and perpetuating factors (Table 1).^1–3

FIGURE 1. Pathologic cycle for chronic otitis externa.

Primary Causes

Primary disorders initiate the inflammatory process within the ear canal and alter the aural environment, allowing secondary complicating factors, such as infections, to develop. The primary cause may be very subtle and often is unrecognized by the owner or even the veterinarian until a secondary complicating factor arises. Most cases have a primary cause. In a retrospective study³ of 100 dogs with acute (37%) and chronic-recurrent (63%) otitis externa, the most common primary cause of otitis was allergic dermatitis (43/100 dogs; Figure 2), followed by grass awns (12/100) and otoacariasis (7/100). No primary factor could be identified in 32 of 100 cases. Allergic and hormonal diseases can be associated with unilateral or bilateral otitis, but bilateral otitis is more common. Foreign bodies, neoplasia, and polyps are usually associated with unilateral otitis; however, bilateral problems have been reported.³

FIGURE 2. Otitis externa secondary to allergies.

Secondary Causes

Secondary causes occur in combination with primary causes or predisposing factors. The most common secondary causes are infections.^1–3 Generally, secondary causes of otitis externa are easy to eliminate once identified. When they are chronic and difficult to treat, it is usually because primary causes or perpetuating factors have not been adequately addressed.

Predisposing Factors

Predisposing factors alone do not cause otitis externa, but they facilitate inflammation by permitting the external ear canal microenvironment to be altered, thereby allowing pathogenic or opportunistic bacteria or yeast to become established.^1,2 In conjunction with primary or secondary causes, these factors become a significant problem. It is important to eliminate as many of these factors as possible while realizing that some, such as ear conformation, cannot be changed.

Perpetuating Factors

Perpetuating factors sustain and aggravate the inflammatory process and prevent resolution of, or worsen existing, otitis externa.^1,2 Once present, they accentuate or permit the development of secondary causes, such as infection, by providing favorable environments and microscopic niches. In many cases, perpetuating factors prevent the resolution of otitis externa when treatments are directed only at primary and secondary causes. These factors may be subtle at first but can develop into the most severe component of chronic ear disease. They are not disease specific and are most commonly seen in chronic cases. Perpetuating factors are the most common reasons for surgical intervention.

Important Diagnostic Steps

History

A detailed and complete history is essential to help investigate the underlying cause and associated factors.

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 (eg, 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 (ie, handheld) otoscopes should have a strong light and power source. If available, fiberoptic video-enhanced otoscopy (eg, video-otoscope [Figure 3]) 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 (eg, myringotomy, otitis media). However, because of the expense to purchase and maintain this equipment, referral to a dermatologist may be necessary

FIGURE 3. Video otoscopic examination.

Physical Examination

Performing a complete physical examination, including a detailed dermatologic examination, can help in identifying an underlying or primary cause. In patients with otitis media and/or otitis interna, concurrent neurologic abnormalities (eg, facial paralysis, nystagmus, ataxia, head tilt) may be present; therefore, a detailed neurologic examination is indicated.

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.^1–6 Malassezia species are also common organisms^1–5 (Figure 4).

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 were considered abnormal in the external ear canals of dogs.⁵ Leukocytes are always abnormal, and bacteria in the presence of leukocytes signal infection.

FIGURE 4. Cytologic appearance of Malassezia species (high-power oil immersion field 100× objective).

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.

Typical indications for bacterial C/S include the following^1–4:

• —Chronic otitis associated with bacteria (cocci [Figure 5] and/or rods) seen on cytology
• — Rods seen on cytology (Figure 6)
• — 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

FIGURE 5. Cytologic appearance of degenerate neutrophils and cocci (high-power oil immersion field 100× objective).

FIGURE 6. Cytologic appearance of degenerate neutrophils and rods (high-power oil immersion field 100× objective).

Cytology should always be performed before aerobic C/S to assist with interpretation of results and identification of concurrent problems. In one study,⁷ however, cytologic results agreed with culture results only 68% of the time. The same study⁷ showed that different bacterial organisms were isolated from the same ear in 20% of the cases; therefore, it is important to remember that ear cytology or a single swab submitted for C/S may not reveal the total population of organisms truly present in the ear canal. This might explain why, in some cases, sensitivity results (in vitro) and response to topical therapy (in vivo) do not always correlate. This study poses questions regarding the true benefit of C/S for selecting antimicrobials for otic infections; therefore, clinicians should be careful and critical when interpreting otic cytologic and culture results.

Clinicians should sample the middle ear separately if otitis media is confirmed when the tympanic membrane is intact. The types and sensitivity pattern of bacteria isolated from the middle ear may differ from those of bacteria isolated from the external canal.⁸ In one study, different organisms were cultured from the middle and external ear, and even when Pseudomonas species were cultured twice from the same ear, different strains were suspected on the basis of the sensitivity pattern exhibited.⁸ Culture results should be interpreted with caution because mixed bacterial flora and light commensal and contaminant bacteria might be present and may not be relevant as pathogens.

Diagnostic Imaging Techniques

Dogs with chronic, recurrent, and severe otitis and those with neurologic signs (eg, 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 (eg, otitis media, neoplasia) and otitis interna, that cannot be identified with regular otoscopy (Table 2). 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%.⁸ 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.⁸

Deep Ear Flushing

This procedure is very helpful not only as a diagnostic tool but also as part of the treatment plan.⁹ A short course (2 to 3 weeks) of an anti-inflammatory dose of oral and/or topical glucocorticoids may be needed before deep ear flush in order to decrease inflammation and stenosis of the ear canals. This procedure should be performed under general anesthesia so that the ear can be completely cleaned and the ear canal and tympanic membrane examined. Anesthesia also allows the placement of an endotracheal tube, which precludes the aspiration of fluids that may pass through the middle ear into the auditory tube and then into the posterior pharynx. Ideally, computed tomography of the tympanic bulla should be performed before the flush to stage ear disease and help make the decision to perform myringotomy if otitis media is present.

Several techniques to clean and flush the ears exist.⁹ Follow-up visits after flushing are very important to monitor response to therapy and evaluate the status of the tympanic membrane. If myringotomy is performed, the tympanic membrane usually heals within 30 days after the procedure. Deep ear flush and myringotomy are best performed by experienced practitioners with a video-otoscope; therefore, referral to a dermatologist might be ideal. For a description of one deep ear flush technique, please see Deep Ear Flush: Step by Step.

Treatment

There are 5 general goals of otitis externa treatment:

1. Resolve discomfort and pain.
2. Remove debris and discharge.
3. Eliminate infection from the external and middle ears.
4. Reverse chronic pathologic changes, when possible.
5. Identify and treat the primary cause of the otitis.

Topical Therapy

In most cases of otitis externa, topical therapy alone is sufficient and is preferred when possible. In contrast, chronic, severe cases of otitis externa and otitis media often require additional systemic therapy. The amount of medication applied is important. Generally, the recommendation is to use about 0.5 to 1 mL (10 to 20 drops) per ear, depending on the size of the dog.

Ear Cleaners

Ear cleaners should be used at home as part of most treatment protocols initially (once daily to twice weekly depending on the severity of the otitis and amount of discharge present) and as maintenance therapy (usually once to twice weekly) to help prevent future infections once the otitis and infection are resolved.^10–13 Removal of debris and purulent material greatly improves the efficacy of topical antimicrobials, especially aminoglycosides and polymyxin B. However, overcleaning should be avoided because it can contribute to maceration and ear disease. Clients should be educated on the proper technique to clean the ears and to avoid using cotton balls and cotton swabs inside the ears. Available ear cleaners include drying agents, antiseptics, ceruminolytics, and combination products.

For a table listing these cleaners, please see Ear Cleaners for Use in Patients with Chronic Otitis.

Acaricidals

Many different acaricidal products may be used to treat infections caused by Otodectes cynotis (ear mites) and, less commonly, Demodex species.^10–13 Veterinary acaricidal products for label and extralabel use include ivermectin, milbemycin, selamectin, fipronil, monosulfiram, permethrin, piperonyl butoxide, pyrethrins, thiabendazole, and rotenone.¹

Antimicrobials

It is important to always use higher volumes or concentrations of topical antibiotics because they may prove efficacious, even when resistance has been suggested on a susceptibility panel. With topical drugs, concentrations 100 to 1000 times superior to the minimum inhibitory concentration may be reached.¹³ Antimicrobials should be used until 1 week past negative cytologic results for most bacterial and/or yeast otic infections.^10-13 Twice-daily applications are usually recommended. For Pseudomonas (Figure 7) and multidrug-resistant infections, I recommend treating the patient until 1 week past negative cytologic and culture results.

FIGURE 7. Chronic Pseudomonas otitis.

Antibacterial agents: These products are indicated when infection is present and cleansing solutions are insufficient. Most topical antibacterial products also contain glucocorticoids and antifungals.

• —First-line antibiotics most commonly used include products containing neomycin alone or in combination with other agents (Tresaderm, us.merial.com) and gentamicin (Gentocin Otic; Otomax, merck-animal-health-usa.com; Mometamax, merck-animal-health-usa.com). Ototoxicity is reported with all gentamicin topicals. However, as with chlorhexidine, this concern may be overstated. One study showed no vestibulotoxic or ototoxic effects from 21 days of otic gentamicin applied q12h to ears with ruptured tympanic membranes.^13,14 Polymyxin B (Surolan, elanco.com) can also be a highly effective topical antibiotic and often is effective in many Pseudomonas infections; however, polymyxin can be inactivated by purulent exudates.
• —Second-line antibiotics include tobramycin (Tobrex ophthalmic solution, alcon.com), injectable amikacin mixed with saline at a final concentration of 25 mg/mL, and ticarcillin–clavulanate potassium, which may be ordered at compounding pharmacies. Care needs to be used with certain topical aminoglycosides because ototoxicity, based on brainstem auditory evoked response testing, was recently seen to occur more commonly in dogs treated with amikacin- and tobramycin-based topicals.^13,14
• —Third-line antibiotics include mupirocin and fluoroquinolones, ideally based on C/S. Mupirocin should be saved for multidrug- and methicillin-resistant Staphylococcus infections; the product is mixed as 1 tube of product (30 g) to 30 mL of sterile saline. Enrofloxacin and silver sulfadiazine (Baytril Otic, bayerdvm.com) is often ineffective in chronic severe cases. Injectable formulations of enrofloxacin (Baytril injectable) are preferable in many different extralabel recipes, such as a 25% mixture of injectable enrofloxacin (22.7 mg/mL diluted with water, saline, or other active agents with variable concentrations of dexamethasone, not exceeding 0.1% to 1%). Posatex (merck-animal-health-usa.com) contains orbifloxacin, posaconazole, and prednisone and may be used against multidrug-resistant infections, such as Staphylococcus and Pseudomonas infections.
• —Antifungal agents: These may be used in any otitis case associated with yeasts, such as Malassezia or Candida species.^10–13 Many available products contain glucocorticoids and antibiotics; however, products containing only antifungals can be found. Usually effective antifungals include clotrimazole (Otomax, Mometamax), miconazole (Conofite, merck-animal-health-usa.com), thiabendazole (Tresaderm), acetic acid (MalAcetic Otic, dechra-us.com), and TrizEDTA and ketoconazole flush (TrizUltra+Keto, dechra-us.com).

Glucocorticoids

Numerous topical preparations of variable potencies are available for use in the external ear canal.^10–13 The frequency of use varies from q6h to q24h, depending on the product and the severity of the otitis. Most cases of chronic otitis externa benefit from topical glucocorticoids. Glucocorticoids have antipruritic, anti-inflammatory effects and decrease exudation and swelling, thereby helping to reduce pain and discomfort. In addition, they cause sebaceous atrophy and decrease glandular secretions. Glucocorticoids may reduce scar tissue and proliferative changes, which helps to promote drainage and ventilation. Most ear products contain various combinations of glucocorticoids, antibiotics, antifungals, and parasiticides.

• Otic products containing betamethasone (Otomax) and dexamethasone (Tresaderm) are usually effective but can be absorbed systemically and cause adrenal suppression with long-term use, so they should be used cautiously. In one study, a more potent yet “soft” glucocorticoid, mometasone (Mometamax), showed no adrenal suppression after 1 week of therapy.¹² In cases of allergic otitis externa, long-term topical glucocorticoids may be required with careful monitoring for adrenal suppression or local side effects, such as pinnae hair loss. Products with weaker-strength glucocorticoids should be used in these situations, such as those containing 1.0% or 0.5% hydrocortisone (Zymox HC, zymox.com). I often recommend fluocinolone and dimethyl sulfoxide (Synotic, zoetisus.com), with great results for many chronic, hyperplastic, and stenotic otitis cases.

New Food and Drug Administration–Approved Single-Dose Antimicrobial and Steroid Otic Solutions

Two new veterinarian-administered products containing florfenicol, terbinafine, and mometasone furoate (Claro, bayerdvm.com) and florfenicol, terbinafine, and betamethasone acetate (Osurnia, osurnia.com) are indicated as single-dose treatments for canine otitis externa associated with susceptible strains of yeast (Malassezia pachydermatis) and bacteria (S pseudintermedius). The duration of effect is 30 days for Claro and 7 days for Osurnia. Ears should not be cleaned at home after application. The recommendation is for use in the clinic after ear cleaning, and only with intact tympanic membranes. These are great options for patients that do not allow topical therapy at home and to improve compliance, with potential benefit for acute or mild otitis cases. Their use in chronic severe otitis cases is limited because severe hyperplasia and stenosis preclude ear cleaning and evaluation of the tympanic membrane.

Systemic Therapy

Indications for systemic therapy include the following:

• —Otitis externa that is severe and unresponsive to topical therapy alone
• —Concurrent otitis media
• —Owner unable to medicate with topical therapy
• —Topical therapy precluded by adverse reactions
• —Marked proliferative chronic changes

Antibiotics

These drugs may be used in animals with otitis media, moderate or marked proliferative changes with suspected otitis media, or no response to appropriate topical therapy and cleansing. I usually recommend C/S before a systemic antibiotic is selected. Usually, higher doses are recommended to achieve good penetration in the middle ear. Treatment duration may vary; however, I usually treat for 1 month after resolution of clinical signs and healing of the tympanic membrane (from spontaneous perforation or myringotomy). Fluoroquinolones may be prescribed when Pseudomonas species, other relevant gram-negative organisms, or very resistant gram-positive bacteria are isolated and susceptibility is confirmed after culture. Higher doses than usually recommended may be needed.

Oral fluoroquinolones that may be used include enrofloxacin (Baytril) at 10 to 20 mg/kg q24h, marbofloxacin (Zeniquin, zoetisus.com) at 5 to 10 mg/kg q24h, or orbifloxacin (Orbax, merck-animal-health-usa.com) at 10 mg/kg q24h. Ciprofloxacin should be avoided in dogs because oral absorption is inconsistent and low (58.4%) with oral tablets,¹⁵ potentially leading to inefficacy and bacterial resistance. In rare cases, injectable antimicrobials, such as aminoglycosides, carbapenems, and ceftazidime sodium, may be required to treat multidrug-resistant otitis cases. Potential side effects with these therapies need to be considered. Referral of these cases to a dermatologist should be considered, particularly when treatment options are limited.

Antifungals

Antifungal agents can sometimes be used in severe cases of Malassezia otitis or cases with poor response to topical agents alone. Oral antifungals commonly used include ketoconazole (Nizoral, nizoral.com), fluconazole (Diflucan, pfizer.com), and itraconazole (Sporanox, janssen.com). All are dosed at 5 to 10 mg/kg q24h or q12h (divided). Terbinafine (Lamisil, lamisilat.com) may also be used at 30 mg/kg q24h.

Glucocorticoids

Glucocorticoids are usually indicated in cases of markedly inflamed and painful otitis with chronic pathologic changes, such as marked hyperplasia and stenosis of the canal. Oral anti-inflammatory dosages of prednisone or prednisolone (0.5 to 1 mg/kg q24h) can be used initially and then tapered to the minimum alternate-day dosage that controls the clinical signs. I typically recommend oral glucocorticoids for cases of Pseudomonas otitis and for dogs that have undergone deep ear flushing. Oral glucocorticoids can also be helpful to reduce pain and discomfort, particularly a few days before the owners clean and medicate the ears. I often combine opioids, such as oral tramadol at 2 to 4 mg/kg q84 to q12h, with oral glucocorticoids in severely painful cases. When longer-term treatment is expected, alternate-day glucocorticoid therapy may be indicated, with careful monitoring for adverse effects.

Cyclosporine

Oral cyclosporine (Atopica, us.atopica.com) may be considered a medical option for dogs with severe proliferative otitis externa when surgery is not an option. In a pilot study,¹⁶ 5 client-owned dogs were treated with cyclosporine at 5 mg/kg q12h for a minimum of 12 weeks. All dogs were evaluated clinically every 4 weeks to monitor progress; they all showed significant clinical improvement based on owner and clinical assessments. Individual owners also commented on improved disposition, hearing, and quality of life. I have seen limited benefits with oral cyclosporine in end-stage disease (Figure 8), but the drug may be considered in cases when surgery cannot be performed.

Figure 8. End-stage otitis.

Client Education and Follow-up Visits

It is very important to talk to clients about the lengthy process of managing otitis, including the need for proper home therapy and frequent follow-up visits, pain management, quality of life, long-term prognosis, and medical costs. Clients must also be taught how to clean the ears and place ear medications. Dogs with otitis should be reevaluated with otic examination and cytology every 2 to 4 weeks, depending on severity, to assess response to therapy. It is important to treat ear infections until 1 week past clinical improvement and negative ear cytologic results for most bacterial and yeast infections. For multidrug-resistant and Pseudomonas ear infections, I usually recommend treatment until 1 week past negative cytologic and culture results.

Maintenance/Preventive Ear Care at Home

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 (eg, for recurrent ear infections), and sometimes topical glucocorticoids (for severe hyperplasia or stenosis, when surgery is not an option).

Surgical Management

Surgical management may be recommended (Table 3), 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 postsurgery complications. Histopathology and bacterial culture of removed tissue or masses should always be performed. Advanced imaging before surgery is ideal.