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Otitis externa is an acute or chronic inflammation of the epithelium of the external ear canal. It may develop anywhere from the tympanic membrane to the pinna. It is variably characterized by erythema, edema, increased sebum or exudate, and desquamation of the epithelium. The ear canal may be painful or pruritic depending on the cause or duration of the condition. It is the most common disease of the ear canal in dogs and cats, is occasionally seen in rabbits (in which it is usually due to the mite Psoroptes cuniculi), and is uncommon in large animals. Internal and external factors may directly induce inflammation and pruritus in the ear canal. Identifying these factors is key to successful management.
Etiology
The causes of otitis externa have been grouped into 4 areas. Primary factors are disease conditions that directly cause the otitis. Secondary factors, such as yeast and bacterial infections, intensify and complicate primary and perpetuating conditions. Predisposing factors are conditions that place an individual at risk of developing otitis. Perpetuating factors tend to prevent the resolution of the otitis once it develops. Often all 4 factors are involved, but each category must be identified and addressed separately. In this way a more accurate prognosis can be provided, a specific and safe therapeutic plan formulated, and the best possible outcome from treatment assured.
Primary factors include parasites (Otodectes, Psoroptes, Sarcoptes, Demodex spp), foreign bodies (grass awn, concreted wax, medications), tumors (cerumin gland adenoma, inflammatory polyps), hypersensitivity (atopic dermatitis, food sensitivity, contact dermatitis), disorders of keratinization, hypothyroidism, autoimmune diseases, juvenile cellulitis, and irritants (cleaners, plucking fur, etc).
Predisposing factors are often congenital or environmental and include conformation (pinnal carriage, narrow ear canal, excessive hair or ceruminous glands), maceration of the ear canal from overtreatment or swimmer's ear, and systemic disease. Small changes in the otic microclimate may alter the delicate balance of normal secretions and microflora and result in opportunistic infections. Any disease that affects normal responses to pathogens can predispose the ear canal to opportunistic infections.
Perpetuating factors include otitis media and progressive pathologic changes. Once the environment of the ear canal has been altered by a combination of primary and predisposing factors, opportunistic infections (a secondary factor) and pathologic changes occur, which prevent resolution of the disease. Chronic pathologic changes in the ears may also reflect a generalized systemic or skin disease. Unless all the causes are identified and treated, recurrence may be expected.
Clinical Findings and Diagnosis
Signalment and a thorough dermatologic history provide information suggestive of primary problems (eg, genetic, hypersensitivity, keratinization disorders). The history will determine if the otitis is acute, chronic, or recurrent. Acute conditions tend to be parasitic or a foreign body. Chronic conditions point toward a hormonal, allergic, or neoplastic process, or a keratinization disorder. A thorough physical and dermatologic examination provide diagnostic clues related to hypersensitivity, endocrine, immune-mediated, and keratinization disorders that also affect the ear. The type of and response to previous otic therapy is also important. Inadequate therapy can lead to a chronic condition.
The outside of the ear should be examined, and any erythema, edema, crusts, scale, ulcers, lichenification, hyperpigmentation, or exudates should be noted. In addition to an otoscopic examination, skin scrapings, cytologic evaluation of exudate, a Wood's lamp examination, and dermatophyte culture should be done in every case.
The pinnae and periauricular regions should be inspected for evidence of self-trauma, erythema, and primary and secondary skin lesions. Pinnal deformities, hyperplastic tissue in the canal, and head-shaking suggest chronic otic discomfort.
For animals with unilateral signs, the unaffected ear should be examined first to prevent iatrogenic contamination of the unaffected ear with organisms (eg, Pseudomonas aeruginosa or Proteus mirabilis) that may be present in the diseased ear. The unaffected ear may, in fact, be diseased, requiring an adjustment of the differential diagnosis list to include causes of bilateral otitis.
Ear disease is both painful and pruritic, so heavy sedation or general anesthesia may be needed to perform a thorough examination. This is especially true if the ear canal is obstructed with exudate or proliferative inflammatory tissue, or if the animal is uncooperative. An otoscopic examination will allow identification of deep otic foreign bodies, tumors, impacted debris, low-grade infections with Otodectes cynotis, and ruptured or abnormal tympanic membranes.
Otoscopic examination can be conducted using a handheld otoscope or a video otoscope. A handheld otoscope must have enough light and magnification to clearly visualize the external canal to the level of the tympanic membrane. Otoscopic cones designed for use in dogs and cats are available in several sizes to accommodate anatomic differences. Two types of heads are available. A diagnostic head, which has a large magnifying lens through which to view the canal, is used to examine the ear. A surgical head has a much smaller magnifying lens, but there is space between the lens and the cone holder to insert a swab or instrument. The surgical head is used when biopsies, foreign body removal, or deep flush of the canal is anticipated.
A video otoscope allows tremendous magnification of the ear canal and tympanic membrane. Findings can be recorded via a video or digital recorder. Most have a working channel through which biopsy instruments, catheters for flushing debris from the canal, and even laser tips can be passed. Video otoscopes allow visualization through water and saline, allow visualization of the integrity of the tympanic membrane, and facilitate sampling and culture of the middle ear.
During an otoscopic examination, the ear canal should be inspected for changes in diameter, pathologic changes in the skin, quantity and type of exudate, parasites, foreign bodies, neoplasms, and changes in the tympanic membrane. The tympanic membrane should be examined for evidence of disease or rupture. However, in many cases of otitis, the character of the ear canal and tympanic membrane cannot be visualized at all until the exudate is gently flushed from the canal. Samples for cytologic evaluation and culture should be obtained before the ear is flushed. Examination is attempted again after the ear is dried. In chronic cases the canal is often too stenotic, either from hyperplasia or edema, to be examined. Systemic glucocorticoids given daily for a week may reduce swelling enough to allow examination.
Cytologic evaluation of exudate or cerumen taken 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. These organisms may become pathogenic if the microenvironment is changed to encourage their overgrowth. Exudate obtained with a cotton-tipped applicator can be rolled onto a glass slide, heat fixed, stained with a 3-step quick stain or modified Wright's stain, and examined under a microscope. Smears should be examined first under low-power magnification and then under high-power (preferably using immersion oil) for numbers and morphology of keratinocytes, bacteria, yeasts, and WBC; evidence of phagocytosis of microorganisms; fungal hyphae; and acantholytic or neoplastic cells.
A stained smear can quickly determine if microbial overgrowth is present. Coccal organisms are usually staphylococci or streptococci. Rod-shaped organisms are usually Pseudomonas aeruginosa, Escherichia coli, or Proteus mirabilis; their appearance in large numbers indicates that a bacterial culture with antibiotic sensitivity should be performed because of their known resistance to many antimicrobial agents. The presence of many neutrophils phagocytizing bacteria confirms the pathogenic nature of the organisms.
The yeast Malassezia pachydermatis is found in low numbers in the ear canals of many normal dogs and cats. Because yeasts colonize the surface of the ear canal, they are most easily found adhered to clumps of exfoliated squamous epithelial cells. M pachydermatis is identified readily on microscopic examination and its numbers easily assessed. No more than 2–3 organisms per high-power field should be present on any aggregate of cells from a healthy ear. When unidentified yeasts or hyphal organisms are seen in significant numbers in cytologic smears, the species should be identified through culture. Concurrent bacterial infections, especially with gram-positive cocci, are common.
A dark exudate in the canal usually signals the presence of either Malassezia spp or a parasite but may also be seen with a bacterial or mixed infection. In addition to stained cytology, otic exudate should be examined for eggs, larvae, or adults of the ear mite Otodectes cynotis in dogs and cats, and Psoroptes cuniculi in rabbits and goats. 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 examined under low-power magnification. Rarely, refractory ceruminous otitis externa may be associated with localized proliferation of Demodex sp in the external ear canals of dogs and cats and may be the only area on the body affected.
Microbial cultures are taken before otoscopy is completed and before any cleaning is done. Samples for culture should be taken with a sterile culturette from the horizontal canal (the region where most infections arise) or from the middle ear in cases of tympanic rupture. A bacterial culture and antibiotic sensitivity and an antibiotic mean inhibitory concentration (MIC) should be done.
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. Additionally, biopsies from animals with chronic, obstructive, unilateral otitis externa may reveal whether neoplastic changes are present.
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 provide evidence of middle ear involvement. Unfortunately, radiographs are normal in many otitis media cases. CT or MRI, if available, should be performed for cases of severe, chronic otitis.
Treatment
Underlying primary, predisposing, and perpetuating causes should be identified and corrected. The periauricular area should be clipped of fur, and hair removed from the ear canal to improve ventilation, to facilitate cleaning and drying of the canals, and to increase adherence to treatment recommendations.
Topical medications are inactivated by exudates, and excessive cerumen may prevent medications from reaching the epithelium. The ears should be gently cleaned and should be dry before treatment is started. In animals with painful ears, proper cleaning requires general anesthesia. There are many appropriate products available for use if the otitis is limited to the external canal. The ears may be flushed with an antibacterial cleansing solution (chlorhexidine or povidone iodine) or with saline if the material is of fluid consistency. Thick, dry, or waxy material requires a ceruminolytic solution such as carbamide peroxide or dioctyl sodium sulfosuccinate (DSS). Thorough rinsing of the latter with warmed saline to remove the cleaning agent must always follow after all debris has been removed. 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.
Medical therapy should be specific and simple. Contributing causes should be treated specifically and aggressively. In treatment of acute bacterial otitis externa, antibacterial agents in combination with corticosteroids may be used to reduce exudation, pain, and swelling, and to decrease glandular secretions. The least potent corticosteroid that will reduce the inflammation should be used (see Anti-Inflammatory Agents: Corticosteroids). Animals with recurring bacterial otitis externa and a history of infection with Otodectes cynotis should be treated with a topical product that contains antibacterial and antiparasitic agents to ensure that undetected low-grade parasitic infections are eliminated. Parasites may also affect extra-auricular sites. A general topical or systemic parasiticide will be most effective in suspected or confirmed recurring cases.
Topical therapy should be based on the character of the disease. Properly applied, the ideal medication coats the epithelium of the external ear canal as a thin film. Nonocclusive solutions or lotions should be used for acute or chronic exudative otitis externa and proliferative conditions. Occlusive oil-based ointments should be reserved for dry, scaly lesions within the ear canals. Changes in the skin of the ear canals during treatment may indicate an irritant contact reaction to a vehicle or base, and the medication should be changed.
Irritating medications 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 therapy should be incorporated into the treatment regimen in most cases of chronic otitis and in any case in which otitis media is suspected. In cases of severe atopic dermatitis or idiopathic seborrhea, systemic corticosteroids may be necessary to control the inflammation. Failure to use systemic antimicrobial therapy is an important perpetuating cause of chronic ear disease in dogs. Systemic antibiotics should be used when neutrophils or rod-type bacteria are found on cytology, in cases of therapeutic failure with topical antimicrobial agents, in chronic recurring ear infections, and in all cases of otitis media. (Also see Systemic Pharmacotherapeutics of the Integumentary System.)
Duration of treatment will vary depending on the individual case but should continue until the infection is resolved (often ≥12 wk). Animals with bacterial and yeast infections should be physically examined and cytologies evaluated weekly to every other week until there is no evidence of infection. For most acute cases, this takes 2–4 wk. Chronic cases may take months to resolve, and in some instances, a therapeutic regimen must be continued indefinitely. Animals with Otodectes cynotis or Psoroptes cuniculi should receive appropriate parasiticide treatment in the ears and on the whole body for at least 2–4 wk. Otobius megnini infestations are best treated by manual removal of the ticks, followed by an acaricide/corticosteroid otic preparation.
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 mo) 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 and dry the canal, 3) identify and treat concurrent otitis media, 4) select an appropriate antibiotic from the results of culture and MIC on the organism and use it at an effective dosage for an appropriate duration, and 5) treat both 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/or systemic medications should be chosen based on cytology or culture; they should have a narrow spectrum and be specific for the current condition. Aminoglycosides and fluoroquinolone antibiotics should not be used unless absolutely required for successful treatment, but 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 shown the best success in controlling Pseudomonas infections in cases in which resistance has been identified through culture. However, resistance is developing to fluoroquinolones.
Maintenance Care
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 mycotic infections in moist ear canals. Clipping hair from the inside of the pinna and around the external auditory meatus, and plucking it from hirsute 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.
Last full review/revision July 2011 by Patricia D. White, DVM, MS, DACVD
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