Otitis Externa in Animals

Classification of otitis externa is now broken down into causes and factors. Causes are different diseases/infectious agents that induce otitis and may be primary or secondary. Factors may contribute or promote otitis externa, often by altering the structure, function, or physiology of the ear canal. Predisposing factors are present before the ear disease develops, whereas perpetuating factors occur as a result of disease. Although ultimately all factors and causes need to be addressed, primary causes in particular must be managed to reduce continuation or recurrence of otitis.

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:

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

• parasites (Otodectes, Demodex, Sarcoptes)

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

• endocrine disease (hypothyroidism, hyperadrenocorticism)

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

• foreign bodies

• glandular disorders (sebaceous gland hyperplasia)

• fungal (Aspergillus)

• viral (distemper)

• miscellaneous (proliferative necrotizing otitis of cats, juvenile cellulitis)

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

• bacteria (Staphylococcus, Streptococcus, Enterococcus, Pseudomonas, Proteus, etc)

• yeast (Malassezia)

• medication reactions

• overcleaning

Perpetuating factors occur due to otic inflammation and may be severe in chronic cases. They include:

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

• ear canal (stenosis, edema, proliferative changes)

• tympanum (rupture)

• glandular (sebaceous hyperplasia)

• pericartilaginous fibrosis or calcification

• middle ear disease

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

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

• excessive moisture (environment, swimming)

• obstructive ear disease (neoplasia, polyp, feline apocrine cystadenomatosis)

• primary otitis media

• systemic disease (immune suppression, catabolic states)

• treatment effects (changes in normal flora, trauma)

• Diagnosis is based on history, otoscopic examination, and cytology

There is no recognized sex distribution for otitis externa. Young animals may be more commonly affected. Breed dispositions for otitis reflect those for skin disease (eg, allergies in retrievers and terriers). Clinical signs can include any combination of head shaking, pain with ear manipulation, malodor, exudate, erythema, erosion, ulceration, swelling, or ceruminous gland hyperplasia.

Palpation, ear, dog

Image

Courtesy of Dr. Michelle Woodward.

After a complete physical and dermatologic evaluation, the ears should be examined (least affected/painful first). Extremely painful cases may require sedation or systemic glucocorticoids for several days before an otoscopic evaluation is performed. Palpation of the ear canals and pinna will help to identify the presence of swelling, mineralization (due to chronicity), and pain. Patients with ear canals with severe chronic changes may be more likely to need advanced imaging of the ear. The pinnae should be evaluated for erythema, crusting, erosion, ulceration, lichenification, hyperpigmentation and the presence of exudate. Sampling of the pinnae may include surface cytology (yeast, bacteria, inflammatory cells), skin scrapes/mineral oil preps (Demodex, Sarcoptes), or dermatophyte culture.

Hyperplasia, ear, dog

Image

Courtesy of Dr. Michelle Woodward.

Otoscopic evaluation should be performed on all dermatologic patients, if possible. A handheld otoscope is typically sufficient, but video otoscopes may provide additional magnification, which increases visualization of the canals and tympanic membrane. A different, clean cone should be used for each ear.

During examination, the 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 changes in color. However, often, the membrane is not visible due to the presence of exudate in the horizontal canal, and cleaning/flushing may be required. Sampling (cytology, mineral oil preps) should be taken before any cleaning is performed.

Otoscopic examination of the vertical canals, horizontal canals, and tympanic membranes may not be possible if there is excessive tissue proliferation or swelling, if the ear is painful, or if the canal is filled with exudate. In cases of excessive pain, sedation may be required if otoscopic evaluation needs to be performed that day. However, many patients with severe ear disease benefit from high anti-inflammatory doses of glucocorticoids and topical therapy to reduce inflammation and infection for a week or two before full examination. Otoscopic evaluation is often possible at that time without requiring sedation.

Cytologic evaluation of exudate from the horizontal ear canal may provide immediate diagnostic information. The external ear canals of most dogs and cats harbor small numbers of commensal gram-positive cocci and yeast. These organisms may become pathogenic if the microenvironment is changed and encourages their overgrowth. Exudate obtained with a cotton-tipped applicator can be rolled onto a glass slide, stained with a 3-step quick stain or modified Wright’s stain, and examined under a microscope. (A study has shown that heat fixing is not necessary for ear swab cytology.) Smears should be examined microscopically under 4x, 10x, and oil immersion to look for: numbers and morphology of keratinocytes, bacteria, yeast, and WBCs; evidence of phagocytosis of microorganisms; fungal hyphae; and acantholytic or neoplastic cells.

A Diff-Quick stained smear can quickly determine whether microbial overgrowth is present. Coccal organisms are usually staphylococci or streptococci. Rod-shaped organisms are usually Pseudomonas aeruginosa, Escherichia coli, or Proteus mirabilis. If many rods are identified, a gram stain may be beneficial because many gram-negative rods (Pseudomonas) respond better to certain antibiotic classes (fluoroquinolones, aminoglycosides). If a gram stain is not available, ulceration of the ear canal, a slimy green discharge, and cytology that shows rods only is highly suggestive of Pseudomonas infection. Otitis externa with primarily gram-negative rod infections may also be associated with a particularly pungent odor. Culture from the horizontal canal can be performed in these cases as well, but even resistant bacteria will often respond to topical therapy because the antibiotics are being applied at much higher levels than those evaluated in susceptibility reports, making ear cultures unnecessary in most cases. The presence of many neutrophils phagocytosing bacteria confirms the pathogenic nature of the organisms.

Yeast (Malassezia pachydermatis) may be found in low numbers in healthy ears of dogs and cats but frequently multiply with otitis externa. They are sometimes found on the surface of exfoliated squamous epithelial cells in surface cytology samples from affected ears. Yeast otitis typically responds well to topical therapy that includes an antifungal and steroid to help reduce inflammation.

In addition to stained cytology, otic exudate should be examined for eggs, larvae, or adults of the ear mite Otodectes cynotis and Demodex spp mites in dogs and cats, and Psoroptes cuniculi in rabbits and goats. In cats, Otodectes cynotis should be strongly suspected with dark, 'coffee-ground' exudate. 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, with the smear 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 and may be the only area on the body affected.

Histopathologic changes associated with chronic otitis externa are often nonspecific. Histopathologic evidence of a hypersensitivity response may support a recommendation for intradermal allergy testing or for 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 there is a mass obstructing the ear canal. If the primary problem is an allergy or an endocrine or autoimmune disorder, there are likely to be other clinical signs in addition to those found in the ear.

Radiography of the osseous bullae is indicated when proliferative tissues prevent adequate visualization of the tympanic membrane, when otitis media is suspected as a cause of relapsing bacterial otitis externa, and when neurologic signs accompany otitis externa. Fluid densities and proliferative or lytic osseous changes often provide evidence of middle ear involvement. Unfortunately, radiographs are normal in many otitis media cases. CT or MRI, if available, should be performed in cases of severe, chronic otitis that do not respond to appropriate treatment. Some clinically normal dogs have evidence of fluid in the middle ear on CT or MRI scans, however.

• 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).

Many animals do not like to have the cleaner squeezed directly into their ear canals. Many respond better to using cotton balls soaked in cleaner. These can be placed at the opening of the ear canal (in such a way that they can still be removed), the ear canal is massaged, and the cotton ball removed. This 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 two to three times weekly with a ceruminolytic solution such as carbamide peroxide or dioctyl sodium sulfosuccinate (DSS). Infected ears with copious purulent discharge may require cleaning once to twice daily. If rods are seen, the ear cleaner should contain squalene, because one possible cause is Pseudomonas, which can produce a biofilm that protects bacteria from antibiotics. The ears should be thoroughly rinsed with warm water to remove residual ear cleaner. If the tympanic membrane is ruptured, detergents and DSS are contraindicated; milder cleansers (eg, saline, saline plus povidone iodine, 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. 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).

The volume of medication needed for most dogs is 1 mL, although dogs with large ears may require 2–3 mL. Adequate treatment requires instillation of at least this volume twice daily. Irritating medications (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 cause irritation in 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 frequently 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 antibiotics in ear medications). Chronic, severely stenotic ears in dogs may require doses equivalent to 2 mg/kg/day prednisone for 2 weeks, followed by a taper. This high dose maximizes the chance the ear can be medically managed rather than require a surgical procedure such as a total ear canal ablation. However, this dose cannot be maintained longterm, and proper management of underlying disease and secondary infections is required. Systemic antibiotics are not required in cases of otitis externa but should be used when otitis media is suspected. However, the systemic antibiotic needs to be selected based on cytology, because many gram-negative rods are not responsive to typical first-tier dermatologic antibiotics such as cephalexin. Most cases of otitis externa with yeast respond well to topical therapy, but 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 be useful to decide whether a myringotomy and middle ear flush is needed. This is followed by topical ear medication in large enough quantities to reach the middle ear after the myringotomy establishes an opening into 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 just giving systemic antibiotics. Antibiotics given systemically will only reach the lining of the middle ear, not the lumen, which can lead to a greater chance of antibiotic resistance.

Duration of treatment will vary depending on the individual case but should continue until the infection is resolved based on re-examination and repeat cytology. Animals with bacterial and yeast infections should be physically examined, with cytologies evaluated weekly to every other week until there is no evidence of infection. For most acute cases, this takes 2–4 weeks. Chronic cases may take months to resolve, and in some instances, a maintenance treatment must be continued indefinitely. If cases of otitis externa do not resolve despite addressing underlying conditions, appropriate therapy, and owner compliance, evaluation for otitis media should be considered.

Methicillin-resistant Staphylococcus intermedius and Pseudomonas otitis (caused by Pseudomonas aeruginosa) have emerged as frustrating and difficult perpetuating causes of otitis because of the development of resistance to most common antibiotics. 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 is multifaceted and should include the following steps:

1. identify the primary cause of the otitis and manage it

2. remove the exudate via irrigation of the ear canal

3. identify and treat concurrent otitis media

4. select an appropriate antibiotic from the results of culture and mean inhibitory concentration on the organism and use it at an effective dosage for an appropriate duration

5. treat topically and systemically until the infection resolves (weeks to months)

The best treatment of chronic otitis is prevention. In addition to identifying the cause of acute otitis, topical and, in rare cases, systemic medications should be chosen based on history and cytology; they should have a narrow spectrum and be specific for the current condition, taking into account which medications have been used to treat the current infection. Neomycin should be a first-line treatment. Aminoglycosides and fluoroquinolone antibiotics should not be used unless absolutely required for successful treatment, but they are the most common ingredients in topical otic medications. Because many topical products contain a combination of glucocorticoid, antibiotic, and antifungal medications, it is imperative to educate the owner on proper use (frequency and duration). Many owners discontinue treatment when the ear “looks better” before the infection is resolved. Polymyxin B and fluoroquinolone antibiotics have most successfully controlled Pseudomonas infections in cases in which resistance has been noticed by failure to clinically respond. However, resistance is rapidly developing to fluoroquinolones because of unnecessary use, so responsible antibiotic stewardship principles should mean they're used as a last-resort treatment.

Owners should be shown how to properly clean the ears. The frequency of cleaning usually decreases over time from daily to once or twice weekly as a preventive maintenance procedure. The ear canals should be kept dry and well ventilated. Using topical astringents in dogs that swim frequently and preventing water from entering the ear canals during bathing should minimize maceration of the ear canal. Chronic maceration impairs the barrier function of the skin, which predisposes 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 meatus, 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.

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