Otitis Externa in Animals

Otitis externa can be infectious or noninfectious. It can be acute or chronic, and unilateral or bilateral. Otitis externa can also be classified according to causes and factors.

Causes are various disease conditions and infectious agents that induce otitis externa. Causes can be primary or secondary.

Primary causes of otitis externa create disease in a normal ear. They alter the environment in the ear, often allowing a secondary infection to develop. Primary causes include the following:

• allergy (adverse food reaction, atopic dermatitis, allergic contact dermatitis)

• parasitism, such as mite infestations (Otodectes cynotis in dogs and cats, Demodex spp in dogs and cats, Sarcoptes spp primarily in dogs, Psoroptes cuniculi [now reclassified as a variant of Psoroptes ovis] in rabbits, goats, sheep, horses, donkeys, and mules)

• autoimmune or immune-mediated conditions (pemphigus foliaceus, vasculitis, others)

• endocrine disease (hypothyroidism, hyperadrenocorticism)

• epithelialization disorders (sebaceous adenitis, zinc-responsive dermatitis)

• foreign bodies

• glandular disorders (sebaceous gland hyperplasia)

• fungal infection (Aspergillus spp; see aspergillosis)

• viral infection (distemper)

• miscellaneous conditions (proliferative and necrotizing otitis externa of cats, juvenile cellulitis of dogs)

Secondary causes of otitis externa create disease in an abnormal ear. These are often chronic or recurrent problems that develop when the primary cause is not addressed. Secondary causes include the following:

• bacterial infection (Staphylococcus spp, Streptococcus spp, Enterococcus spp, Pseudomonas spp, Proteus spp, and other organisms)

• yeast infection (Malassezia spp)

• medication reactions

• overcleaning

Factors may contribute to or promote otitis externa, often by altering the structure, function, or physiology of the ear canal. Predisposing factors are present before ear disease develops, whereas perpetuating factors occur as a result of disease.

Perpetuating factors result from otic inflammation and can be severe in chronic cases. They include the following:

• epithelial changes (failure or alteration of epithelial migration in the ear canal, which in normal ears provides a natural cleaning mechanism)

• ear canal changes (stenosis, edema)

• tympanum changes (rupture)

• glandular changes (sebaceous gland hyperplasia)

• calcification of pericartilaginous fibrous tissue

• middle ear disease

Predisposing factors increase the risk for developing otitis externa. These include the following:

• ear conformation factors (pendulous pinna, stenotic canals, hairy concave pinna, excessive hair in canals)

• excessive moisture in the ears (from the environment or swimming)

• obstructive ear disease (neoplasia, polyp, feline apocrine cystadenomatosis [ceruminous gland cystomatosis])

• primary otitis media

• systemic disease (immune suppression, catabolic states)

• treatment effects (changes in normal microbiota, trauma)

Although all causes and factors must ultimately be addressed, primary causes in particular must be managed to lessen continuation or recurrence of otitis externa.

Clinical signs of otitis externa in animals can include any combination of the following:

• head shaking

• aural pruritus

• pain

• malodor

• otic discharge or exudate

• erythema

• erosions

• ulcerations

• edema

There is no recognized sex distribution for otitis externa. Young animals may be more commonly affected. Breed dispositions for otitis externa reflect those for skin disease (eg, allergies in retrievers and terriers).

• Patient history

• Otoscopic examination

• Cytological evaluation

After a thorough physical and dermatological evaluation, the ears should be examined, starting with the least affected, least painful ear. Animals in extreme pain might require sedation or administration of systemic glucocorticoids for several days before an otoscopic evaluation can be performed.

Palpation of the ear canals and pinnae (see palpation image) helps to identify swelling, mineralization (due to chronicity), and pain.

Palpation, ear, dog

Image

Courtesy of Dr. Michelle Woodward.

Patients with severe chronic changes to the ear canals are more likely to need advanced ear imaging.

The pinnae should be evaluated for erythema, crusting, erosions, ulcerations, lichenification, hyperpigmentation, and the amount of ear canal debris or exudate.

Pinnae sampling procedures include the following: surface cytology (to detect yeast, bacteria, inflammatory cells), skin scrapings and mineral oil preparations (to detect Demodex spp, Sarcoptes spp), or dermatophyte culture.

Otoscopic evaluation should be performed on all patients with dermatological issues, if possible. A handheld otoscope is typically sufficient; however, video otoscopes can provide additional magnification, which improves visualization of the ear canal and tympanic membrane. A different, clean cone should be used for each ear.

During examination, the ear canal should be evaluated for stenosis, erythema, erosion, ulceration, glandular hyperplasia, exudate (amount, quality), and masses.

When possible, the tympanic membrane should be examined for bulging, rupture, or color changes. However, often, the tympanic membrane is not visible because exudate is present in the horizontal canal, and cleaning or flushing may be required. Samples (cytological samples, mineral oil preparations) should be taken before any cleaning is done.

Otoscopic examination of the vertical canal, horizontal canal, and tympanic membrane might not be possible if there is excessive tissue proliferation (see hyperplasia image) or swelling, if the ear is painful, or if the ear canal is filled with exudate.

Hyperplasia, ear canal, dog

Image

Courtesy of Dr. Michelle Woodward.

In cases of excessive pain, sedation might be required if otoscopic evaluation needs to be performed the same day. However, many patients with severe ear disease benefit from high anti-inflammatory doses of glucocorticoids plus topical therapy for 1–2 weeks before full examination. Otoscopic evaluation is often possible at that time without requiring sedation.

Cytological evaluation of exudate from the horizontal ear canal may provide immediate diagnostic information.

Exudate obtained with a cotton-tipped applicator should be rolled onto a glass slide, stained with a commercial three-step modified Giemsa stain kit or modified Wright stain, and examined under a microscope. Heat fixing is not necessary for ear swab cytology.

Stained smears should be evaluated microscopically using all available objective lenses, from low power (eg, 40X and 100X magnification) for scanning large areas to oil immersion (1000X magnification) for the highest resolution. The following should be examined for and noted:

• numbers and morphology of keratinocytes, bacteria, yeast, and WBCs

• evidence of phagocytosis of microorganisms

• presence of fungal hyphae

• presence of acantholytic or neoplastic cells

The external ear canals of most dogs and cats harbor small numbers of commensal gram-positive cocci and yeast. These organisms can become pathogenic if the microenvironment changes and encourages their overgrowth.

A smear stained using a commercial three-step modified Giemsa stain kit can quickly help identify microbial overgrowth. Coccal organisms are usually staphylococci or streptococci. Rod-shaped organisms are usually Pseudomonas aeruginosa, Escherichia coli, Proteus mirabilis, or Corynebacterium spp.

If a large number of rod-shaped organisms are observed with modified Giemsa staining, subsequent Gram staining may be beneficial (see Gram stain image), because many gram-negative rod-shaped organisms (Pseudomonas spp) respond better to certain antimicrobial classes (fluoroquinolones, aminoglycosides). Biofilm, which is also commonly produced by Pseudomonas spp, is an assemblage of microbial aggregates that is encased within a polysaccharide extracellular matrix. Biofilm often appears as veil-like material in aggregates on cytological evaluation; however, advanced techniques, such as confocal laser scanning microscopy or scanning electron microscopy, are needed for definitive diagnosis of biofilm (1).

Pseudomonas infection, Gram stain, ear, dog

Image

Courtesy of Dr. Sarah Hoff.

If a Gram stain is not available, ulceration of the ear canal, presence of a slimy green discharge, and cytological results showing only rod-shaped organisms are highly suggestive of Pseudomonas spp infection. Otitis externa with infection by primarily gram-negative rod-shaped organisms can also be associated with a particularly pungent odor.

Samples for culture from the horizontal canal can be taken in these cases as well; however, even antimicrobial-resistant bacteria may respond to topical therapy, because antimicrobials are being applied at much higher concentrations than those evaluated in antimicrobial susceptibility reports.

The presence of many neutrophils phagocytosing bacteria confirms the pathogenic nature of the organisms, as bacteria may be present in low numbers in healthy ears, but neutrophils are not a normal finding.

Yeast, specifically Malassezia pachydermatis, may be present in low numbers in healthy ears of dogs and cats but frequently multiply with otitis externa (see Malassezia otitis image). They are sometimes observed on the surface of exfoliated squamous epithelial cells in surface cytological samples from affected ears.

Malassezia (yeast) otitis, dog

Image

Courtesy of Dr. Stephen White.

In addition to stained cytological testing, microscopic examination of otic exudate for ear mite eggs, larvae, and adults is recommended. In cats, O cynotis should be strongly suspected if the exudate has a dark, "coffee-ground" appearance. Smears are made by combining cerumen and otic discharge with a small quantity of mineral oil on a glass slide. A coverglass should be used, and the smear should be examined under low-power magnification.

Rarely, refractory ceruminous otitis externa may be associated with localized proliferation of Demodex spp in the external ear canals of dogs and cats; in such cases, the external ear canal may be the only area on the body affected.

Histopathological changes associated with chronic otitis externa are often nonspecific. Histopathological evidence of a hypersensitivity response may support a recommendation for intradermal allergy testing or a hypoallergenic diet trial.

In addition, biopsies from animals with chronic, obstructive, unilateral otitis externa may reveal whether neoplastic changes are present. Biopsies are mainly indicated when a mass is obstructing the ear canal.

If the primary problem is an allergy or an endocrine or autoimmune disorder, there are likely to be other suggestive clinical signs in addition to those found in the ear.

Radiographic imaging of the osseous bullae is indicated when proliferative tissues make it difficult to view the tympanic membrane, when otitis media is suspected as a cause of relapsing bacterial otitis externa, and when neurological signs accompany otitis externa. Fluid densities and proliferative or lytic osseous changes often indicate middle ear involvement. Radiographs are normal in many otitis media cases.

CT or MRI, if available, should be performed in cases of severe, chronic otitis that does not respond to appropriate treatment. Some clinically normal dogs have evidence of fluid in the middle ear on CT or MRI scans, however.

• Management of underlying causes

• Management of pain, inflammation, and pruritus

• Appropriate antimicrobial administration

Successful treatment of otitis externa requires animal-owner compliance, management of inflammation, directed antimicrobial therapy, and workup for 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 or pruritis. Glucocorticoid therapy to decrease inflammation, swelling, and pain can ultimately make it easier for an animal's owner to treat and clean the ears at home. Oral prednisone and triamcinolone are used most commonly; duration and dosage depend on severity and chronicity of disease. In some cases, owners may not be able to clean the ears at home until glucocorticoids have had a few days to take effect.

Ear hygiene is important. If excessive hair is contributing to chronic or recurrent otitis externa, hair from the pre- and periauricular areas, as well as hair from the medial surface and tips of the pinnae, should be clipped to facilitate ear cleaning and topical treatment. Plucking of hair from the ear canal is controversial but may be needed to adequately resolve ear infection in some cases.

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, it is important that the cleaner has an appealing odor (to increase owner compliance) and a neutral pH (to lessen pain in inflamed ears).

Many animals dislike having cleaner squeezed directly into their ear canals and respond better to the use of cotton balls soaked in cleaner. In this method, a soaked cotton ball is placed at the opening of the ear canal (in such a way that it can still be removed); the ear canal is then massaged, and the cotton ball removed. The process should be repeated until the cotton ball is clean or blood is noted (indicating irritation).

Infected ears with thick, dry, or waxy material may require cleaning 2–3 times weekly with a ceruminolytic solution such as carbamide peroxide or dioctyl sodium sulfosuccinate (DSS). Infected ears with copious purulent discharge may require cleaning 1–2 times daily. If rod-shaped organisms are observed on stained cytological testing, a cleaner containing tris-EDTA should be used, because one possible organism is Pseudomonas spp, which can produce a biofilm that protects bacteria from antimicrobials. If the tympanic membrane is ruptured, detergents and DSS are contraindicated; milder cleansers (eg, saline solution [0.9% NaCl] or tris-EDTA) should be used to flush the ear.

In addition to cleaning, effective treatment may require both topical and systemic antimicrobial and anti-inflammatory therapy. Frequency of application, volume of product to be instilled, and length of treatment depend on the specific product being used and the severity of infection. The duration of treatment can 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 decrease exudation, pain, swelling, and glandular secretions. The least potent corticosteroid that will decrease the inflammation should be used. Most commercial prescription topical products contain a combination of an antimicrobial, an antifungal, and a glucocorticoid. Individual products should be chosen based on cytological test results (eg, infections involving gram-negative rod-shaped organisms may require an aminoglycoside or fluoroquinolone).

It is important to verify that the tympanic membrane is intact before applying topical treatments, as many agents (such as aminoglycosides) can cause ototoxic effects, and the risk for these effects is greater if the tympanic membrane is not intact. If the tympanic membrane cannot be visualized, pet owners must be cautioned to monitor for clinical signs of ototoxic effects (eg, decreased hearing, head tilt, vertigo). If such signs occur, the medication should be discontinued until the pet can be evaluated.

Irritatingtreatments (eg, home remedies and vinegar dilutions) should be avoided. They cause swelling of the lining of the ear canal and an increase in glandular secretions, which predispose to opportunistic infections. Substances that are usually not irritating in normal ear canals may irritate an ear that is already inflamed. This is particularly true of propylene glycol. Powders, such as those used after plucking hair from the canal, can form irritating concretions within the ear canal and should not be used.

Systemic glucocorticoids are often needed with both acute and chronic otitis externa to help manage pain, inflammation, and swelling. Glucocorticoids also decrease the amount of purulent discharge, which then improves the effectiveness of aminoglycosides (the most common antimicrobials in ear medications).

In dogs with chronic otitis externa with severely stenotic ears, triamcinolone dosages equivalent to 0.8 mg/kg every 24 hours should be administered until remission is achieved, and then the medication should be tapered. This high dose maximizes the chances that the ear can be medically managed rather than treated surgically (eg, with total ear canal ablation) (2). However, this dosage cannot be maintained over the long term because of adverse effects, and proper management of underlying disease and secondary infections is required.

Systemic antimicrobials are not required in cases of otitis externa but should be used when otitis media is suspected. However, the systemic antimicrobial should be selected based on cytological test results or culture, because many gram-negative rods are not responsive to typical first-tier dermatological antimicrobials, such as cephalexin.

Yeast otitis typically responds well to topical therapy that includes an antifungal and a steroid to help decrease inflammation. Most cases of otitis externa with yeast infection respond well to topical therapy; however, systemic antifungals such as ketoconazole (dogs) or terbinafine (dogs and cats) may be helpful.

If otitis media is suspected in addition to otitis externa, CT or MRI can help determine whether a myringotomy and middle ear flush are needed. Samples from the middle ear can be collected after myringotomy for cytological evaluation and culture. After the myringotomy establishes an opening into the middle ear, topical ear medication should be administered in large enough quantities to reach the middle ear. For both bacterial and fungal middle ear infections, middle ear flushing followed by large volumes of topical ear medication is more effective than systemic antimicrobials. Antimicrobials given systemically only reach the lining of the middle ear, not the lumen, which increases the likelihood of antimicrobial resistance.

Treatment duration varies but should continue until infection is resolved, as determined by clinical reexamination and repeat cytological testing. Animals with bacterial and yeast infections should be physically examined, and cytological evaluations should performed weekly to every other week until there is no evidence of infection. For most acute cases, this takes 2–4 weeks. Chronic cases can take months to resolve; in some instances, maintenance treatment must be continued indefinitely. If otitis externa does not resolve—despite addressing underlying conditions, administering appropriate therapy, and confirming owner compliance—evaluation for otitis media should be considered.

Methicillin-resistant Staphylococcus pseudintermedius and Pseudomonas aeruginosa have emerged as frustrating and difficult perpetuating causes of otitis externa because of the development of resistance to most common antimicrobials. These infections are often chronic in course (> 2 months) and associated with marked suppurative exudation, severe epithelial ulceration, pain, and edema of the canal.

Successful treatment of these challenging forms of otitis is multifaceted and should include the following steps:

1. Identify the primary cause of the otitis externa and manage it.

2. Remove the exudate via irrigation of the ear canal.

3. Identify and treat concurrent otitis media.

4. Select an appropriate antimicrobial based on the results of culture and susceptibility testing, and use it at an effective dosage for an appropriate duration.

5. Treat the infection topically and systemically until it resolves (can take weeks to months).

For chronic, recurrent otitis externa, prevention is crucial. Once the cause of acute otitis externa has been identified, topical and, in rare cases, systemic medications should be chosen based on patient history and cytological evaluation. The medications should have a narrow spectrum and be specific for the current condition, taking into account which medications have already been used to treat the current infection.

Because many topical products contain a combination of glucocorticoid, antimicrobial, and antifungal medications, it is imperative to educate animal owners in their proper use (frequency and duration). Many owners discontinue treatment when the ear looks better but before the infection is resolved.

Polymyxin B and fluoroquinolones have most successfully controlled Pseudomonas otitis externa in cases in which antimicrobial resistance has been noticed by failure to clinically respond.

To avoid contributing to antimicrobial resistance, responsible antimicrobial stewardship principles should be followed. When an antimicrobial or antifungal agent is needed, selection of an otic product should be based on results of cytological evaluation or culture and antimicrobial susceptibility testing. 

For maintenance care of chronic, recurrent otitis externa in animals, owners should be shown how to properly clean the ears. Frequency of cleaning usually decreases over time, from daily to once or twice weekly as a preventive maintenance procedure.

Ear canals should be kept dry and well ventilated. Ear-canal maceration can be minimized by using topical astringents in dogs that swim frequently and by preventing water from entering the ear canals during bathing. Chronic maceration impairs the barrier function of the skin, which predisposes the skin to opportunistic infection. Preventive otic astringents may decrease the frequency of bacterial or fungal infections in moist ear canals.

Clipping hair from the concave side of the pinna and around the external auditory opening, and plucking it from overly hairy ear canals, improves ventilation and decreases humidity in the ears. However, hair should not routinely be removed from the ear canal if it is not causing a problem, because doing so can induce an acute inflammatory reaction.

• Canine Otitis Externa Part 1: Anatomy, Physiology, Pathophysiology and Diagnosis [webinar]. World Association of Veterinary Dermatology.

• Canine Otitis Externa Part 2: Management [webinar]. World Association of Veterinary Dermatology.

• Also see pet owner content regarding otitis externa in dogs, cats, and horses.

1. Parnell-Turner H, Griffin CE, Rosenkrantz WS, Kelly Keating M, and Bidot WA. Evaluation of the use of paired modified Wright’s and periodic acid Schiff stains to identify microbial aggregates on cytological smears of dogs with microbial otitis externa and suspected biofilm. Vet Dermatol. 2021;32(5):448-e122. doi:10.1111/vde.13009

2. Nuttall T. Managing recurrent otitis externa in dogs: what have we learned and what can we do better?J Am Vet Med Assoc. 2023;261(S1):S10-S22. doi:10.2460/javma.23.01.0002