Acute otitis externa is defined as disease that has been present for up to one week (Bensignor and Forsythe 2012) and is a common presentation in small animal practice (Hill and others 2006). Otitis externa is a complex disease involving primary and secondary causes of inflammation and predisposing and perpetuating factors. It frequently recurs, despite apparently successful initial therapy. Repeated episodes of otitis externa along with failure to address the various causes and factors, may lead to progressively more severe disease and potentially irreversible chronic pathological changes within the ear canal. A careful, considered approach in a case of acute otitis externa is the first step to prevent such chronic disease. In this article, Peter Forsythe outlines an approach to acute otitis externa that should be practical within the confines of a 10 to 15 minute first-opinion consultation. Cases of chronic disease require more in depth investigation beyond the scope of the typical first-opinion consultation and additional time has to be set aside for these more complex cases or consideration given to referral to a dermatology specialist.
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Peter Forsythe graduated from the Royal (Dick) School of Veterinary Studies (R[D]SVS) in 1983 and worked in mixed practice for 14 years. He gained the RCVS Certificate in Small Animal Dermatology in 1992. He left general practice in 1998 and, following a two-year residency at the R(D)SVS, gained the RCVS Diploma in Veterinary Dermatology in 2001. He founded, and is currently a partner in, the Dermatology Referral Service with centres in Glasgow and Aberdeen. He is an RCVS Recognised Specialist in Veterinary Dermatology and an honorary lecturer in dermatology at University of Glasgow's School of Veterinary Medicine. He has particular interests in otitis and bacterial skin disease.
Ear canal physiology
An understanding of the anatomy and physiology of the ear canal is essential when investigating and treating otitis externa. The external ear canal is contained within, and supported by, the auricular and annular cartilages and bony portion of the horizontal ear canal. It is lined by skin containing hair follicles, sebaceous and ceruminous (modified sweat [apocrine]) glands. Cerumen coats the lining of the ear canal and is comprised of a complex mix of exfoliated cells and glandular secretions consisting of waxes, oils, fatty acids, esters and proteins that form a protective and antimicrobial layer. Epithelial cell migration, a lateral movement of the stratum corneum, is a mechanism that facilitates the removal of cerumen and debris from the external ear canal. This process has recently been demonstrated in the canine ear (Tabacca and others 2011). Normal bacterial flora in the ears consist of mainly Gram-positive bacteria and yeast, whereas Gram-negative bacteria are rarely identified (Tater 2003).
Otitis externa involves primary and secondary causes of inflammation, and predisposing and perpetuating factors. These causes and factors are discussed in detail in a separate article in this publication (see pages 7 to 11), but primary causes of inflammation are those that produce an initial inflammation within the ear canal and are often a manifestation of a generalised skin disease. The most common primary cause of inflammation is atopic dermatitis. Typically, the primary cause of inflammation results in a change in the microclimate within the ear canal that favours the growth of commensal organisms, such as Malassezia species and staphylococci and leads to infection. Such infections are the principal secondary causes of inflammation and are the usual reason dogs are presented for veterinary attention. Perpetuating factors are changes that occur within the ear canal in response to disease and include accumulation of discharge and swelling of the ear canal lining. Inflammation leads to increased production of cerumen and it is thought that it also impairs epithelial cell migration. It is important to identify and address perpetuating factors to avoid chronic disease.
Approach to acute otitis externa
As with any medicine case, the consultation should involve history taking, examination, construction of a list of differential diagnoses, discussion with the owner, and treatment. The focus should be on identifying and appropriately treating infection and inflammation, achieving a clean ear canal and noting possible primary causes of inflammation that may require further investigation in the future if disease recurs.
A brief history is important to determine the duration and severity of disease as well as possible primary causes of inflammation and predisposing factors. Key questions to ask are shown in Box 1. Knowledge of when the episode began and whether there have been previous episodes of otitis allow the clinician to categorise the disease as acute, sub-acute or chronic. The age at which disease first started is helpful in determining the primary cause of inflammation. Atopic dermatitis starts in young dogs, whereas an endocrinopathy or neoplasia would be more likely in a middle-aged or older dog presenting with disease for the first time. The owner should be questioned on whether the dog shows other signs of skin disease because otitis externa is frequently a manifestation of a generalised skin disease. If disease is of acute onset and obviously painful then this would raise the index of suspicion for a foreign body.
Key questions to ask in a brief history taking
▪ Duration of current disease; when did it start?
▪ Has the patient had this before? If so, how many episodes and how frequently have they occurred?
▪ If it has had this before, what age was it at the first episode?
▪ Is there evidence of more generalised pruritus (eg, does it chew its paws/rub its face, and so on)?
▪ Is there a possibility of a foreign body?
▪ Is it a swimmer?
Both the external ear canals and pinnae should be examined for the presence of erythema, discharge, odour, scaling, crusting, swelling or stenosis, excoriations and erosions. The vertical ear canals should be soft and pliable on palpation. Firm, immovable ear canals suggest fibrosis or calcification, indicating longer-standing disease. Digital examination of the ear canals with a gloved finger (Fig 1) is well tolerated, even where there is pain and ulceration and may reveal the presence of stenosis, fibrosis or calcification and the nature of discharge deeper within the ear canals. Material thus collected on the gloved finger may be used for cytological evaluation. In addition to examining the ears, the remainder of the integument should be briefly examined to help identify a possible primary cause of inflammation. Particular attention should be paid to the dorsal and plantar interdigital skin, the ventral neck, axillae, groin and perineum.
Clinical presentations of acute otitis externa
Having examined the external ear canals it should be possible to categorise the otitis based on the appearance of the external ear canal and nature of the discharge.
This is where the external ear canal and pinnae are erythematous, but there is no significant discharge (Fig 2). This may be seen in early atopic otitis and can indicate the presence of inflammation without infection.
Erythematoceruminous otitis externa
This term describes an erythematous ear with a ceruminous ‘waxy’ aural discharge (Fig 3). The discharge may be minimal to severe and is typically associated with yeast or Gram-positive infections. Many cases of acute otitis externa are erythematoceruminous and tend to be presented because of head shaking or pruritus.
Purulent otitis externa
A purulent aural discharge (Fig 4) is more commonly associated with long-standing otitis externa but, less frequently, may be a feature of acute otitis and is typically associated with Gram-negative infections, especially Pseudomonas aeruginosa. Purulent otitis cases typically present with signs of head shaking, obvious aural discomfort and pain. A foul odour and ulceration are also frequent features.
Otoscopic examination is indicated to assess the ear canals at the outset of disease and at follow-up visits to evaluate the effectiveness of therapy and ear cleaning programmes. A good quality hand held otoscope with a bright, white light source should be used. Both ears should be examined even if unilateral disease is suspected. Examine the less severely affected ear first to avoid spreading infection to the contralateral ear.
The ear is a sensitive structure, even when healthy, and every attempt should be made to avoid causing further discomfort when performing otoscopic examination, otherwise the dog may become ear phobic and strongly resent any future examination and treatment.
The canine ear canal is not straight and from the external orifice, the vertical canal runs ventrorostrally before turning medially into the horizontal canal (Fig 5). A ridge of cartilage projects from the medial wall of the canal at the junction between the horizontal and vertical canals. This area is especially vulnerable to trauma from otoscopic examination.
The dog should be gently restrained holding the chin or muzzle to prevent lateral movement of the head. The pinna should be held up, which helps to straighten the ear canal, and the otoscope cone is inserted first in a rostroventral direction and then once around the medial ridge, it is directed medially to view the horizontal canal. Avoid using large-sized cones in smaller ear canals. Warming the otoscope cone first improves tolerance. Additionally, the author finds it useful to first touch the external ear canal with the index finger of the hand holding the otoscope then slide the cone past the finger into the ear canal. Dogs seem to react less when this approach is used.
Otoscopic examination should not be attempted in dogs that struggle excessively when held or where the ear canals are obviously intensely inflamed. In both situations the dog should be sedated before attempting otoscopic examination. Alternatively, unless there is a suspected foreign body reaction, systemic glucocorticoids may be administered for several days to resolve inflammation before otoscopic examination.
In a case of otitis externa the following should be assessed on examination:
▪ presence and nature of any discharge;
▪ evidence of ectoparasites;
▪ presence of foreign bodies;
▪ appearance of the ear canal lining and any ulceration;
▪ patency of the ear canal and the degree of stenosis;
▪ appearance and patency of the tympanic membrane; and
▪ the presence of neoplasms or polyps.
Otoscopic examination of a healthy ear should reveal a smooth, non-inflamed lining (Fig 6). Some breeds such as poodles and Bichon Frise have quite hirsute ear canals. There is often a small tuft of hair in the ventral pocket of the very proximal horizontal ear canal adjacent to the tympanic membrane, along with a small amount of cerumen. It should be possible to visualise the tympanic membrane and inability to do so denotes a problem and merits further investigation. This may be due to stenosis of, or the accumulation of discharge within, the horizontal canal. Discharge preventing visualisation of the remainder of the ear canal lining also denotes a problem because it prevents effective topical therapy and needs to be addressed by some form of ear cleaning.
Cytology should be performed in all cases of otitis externa. Its principal use is to identify microbial infection (Fig 7), enabling the use of targeted therapeutics and accurate monitoring of the response to treatment. If culture and sensitivity testing has been performed, the results should correlate with cytology findings. Both ears should be sampled as there may be different infections in either ear. Mixed infections are a common finding in longer-standing disease. Lack of time is often cited as the reason cytology is not performed in practice, but it may be worth considering having a trained member of staff take a sample for cytology before the dog enters the consult room (Angus 2004).
Samples for cytology may be obtained using cotton buds or, as previously mentioned, a gloved finger. When using a cotton bud, it should be gently inserted into the ear canal to the level of the junction between the vertical and horizontal canals. Collected material is gently rolled onto a glass slide. The same slide may be used for both ears. The slide should be stained using a modified Wright's stain such as DiffQuik or Rapi-Diff. Purulent discharges should be air dried and fixed with the alcohol fixative before being immersed in the two stains. Waxy deposits on the glass slide may be heat fixed by passing through a flame several times. A disposable cigarette lighter works well for this. Heat fixing helps prevent loss of material from the slide during the staining process. These samples are stained without using the alcohol fixative to avoid dissolving the material collected. The use of just the blue counterstain has been shown to be a rapid and effective way of staining otic preparations for Malassezia species (Toma and others 2006).
The interpretation of ear cytology is described on pages 13 to 15 of this publication, but samples should be examined using the x 100 oil immersion lens of a light microscope. Interpretation of cytology takes some experience, but this develops with practice. As Malassezia species, cocci and coryneform bacteria may all be found in healthy ear canals on cytological examination, the clinician has to make a judgment on what constitutes an excessive number of micro-organisms.
In addition to performing cytology, material should also be collected onto a glass slide, dispersed in liquid paraffin and examined on low power under a light microscope to check for Otodectes and Demodex canis infestation.
Should culture be performed?
Opinions are divided as to the value of bacterial culture and sensitivity testing in otitis externa. The high concentrations of antimicrobial agents achieved with topical therapy tend to overcome apparent in vitro bacterial resistance, meaning that culture and sensitivity testing is of lesser value. However, indications for culture and sensitivity testing would include finding rod-shaped bacteria on cytology, a previous poor response to empirical therapy or a history of exposure to a possible source of resistant bacteria, such as a known affected animal or human in close contact.
The aims of therapy in acute otitis externa are to:
▪ clean the ear;
▪ resolve pain and inflammation;
▪ eliminate microbial infection; and
▪ educate the owner and discuss further investigation if appropriate.
Ear cleaning is an important component of managing otitis externa. Ear cleaning is indicated if there is any discharge within the ear canal that prevents visualisation of the tympanic membrane or areas of the ear canal lining.
Ear cleaning has the following benefits:
▪ facilitates examination of the ear canal;
▪ removes microbes, material that harbours microbes, small foreign bodies;
▪ exposes the lining of the ear canal to topical therapy; and
▪ prevents inactivation of topical therapy.
Techniques for ear cleaning include manual ear cleaning on a conscious animal using a proprietary ear cleaner, and in-clinic, deep, retrograde ear flushing performed under general anaesthesia. As a rule of thumb, manual ear cleaning is more likely to be successful if the tympanic membrane can be visualised and there is just a moderate amount of cerumen coating the lining. Retrograde cleaning is likely to be required if there is material impacted within the horizontal canal obscuring the tympanum (Fig 8). Retrograde ear cleaning is less likely to be required in cases of acute otitis externa and is discussed elsewhere in this publication on pages 17 to 21.
Topical antimicrobial products have an oily excipient and applications of such products may be sufficient to break up mild ceruminous discharge. The addition of a proprietary ear cleaner would be indicated for more profuse discharges. Cerumenolytic ear cleaners are indicated if there is a waxy discharge and most contain organic oils and solvents that soften and dissolve cerumen and are useful in mildly waxy ears. Some contain surfactants or foaming agents that help the cleaning process by emulsifying and breaking up debris. Many ear cleaners also have antimicrobial and/or astringent effects. It is important to choose a product appropriate for each individual case. A cleaner with aqueous properties (such as tris-EDTA) or perhaps a surfactant/foaming agent would be indicated in a purulent otitis. Acidic cleaners should be avoided if the ear canal is eroded or ulcerated. Most cleaners (with the exception of tris-EDTA, acetic acid, boric acid and squalene) are contraindicated if there is rupture of the tympanic membrane.
It is imperative to actively demonstrate to the owner exactly how the preparation should be used and veterinary nurses can perform an invaluable role in this respect. The aim is to fill the ear canal with the cleaner and massage the vertical canal for 30 to 60 seconds (although probably considerably longer is likely to be required for an effective ceruminolytic effect). Material is then wiped away from the external canal. If a topical antimicrobial preparation is being used it should be applied 10 to 15 minutes after ear cleaning. Typically, an owner would be advised to use the ear cleaning product two or three times weekly in a case of acute otitis externa, reducing to once weekly when the ear canal is clean. Note that the ear canals should not be cleaned following application of one recently developed long acting product (Osurnia; Elanco).
The use of topical therapy, particularly ear cleaners, in acutely inflamed ears can cause marked discomfort and is another cause of dogs becoming ear phobic. Thus, ear cleaning should not be performed in a conscious animal if the ear canals are inflamed and painful and should be delayed until the inflammation is resolved. Consideration should be given to the use of systemic glucocorticoids in this situation.
Antimicrobial therapy is indicated if infection is identified on cytology, and topical therapy is the mainstay of treatment for acute otitis externa cases.
Most proprietary topical products contain antifungals, antibacterial agents and a glucocorticoid. Choice of product should be based on cytology findings and with regard to good antimicrobial stewardship. In addition, it may be desirable to use a more potent glucocorticoid if there is severe inflammation, and owner compliance should also be taken into consideration. The use of a long-acting product applied in the clinic may be desirable if there are doubts about an owner's ability to follow a treatment programme. Products containing nystatin, miconazole, clotrimazole and terbinafine are effective for the treatment of Malassezia infections. Fusidic acid, the combination of miconazole and polymyxin B, gentamicin, florfenicol and fluoroquinolones would all be effective against Gram-positive bacteria, but there is a strong argument for reserving the fluoroquinolones for Gram-negative infections (Bensignor and Forsythe 2012). Polymyxin B, the aminoglycosides and fluoroquinolones are generally effective for Gram-negative infections. The constituent ingredients of products available in the UK are shown in Table 1.
Polymyxin B and the aminoglycosides (with the exception of gentamicin) are variably ototoxic and should probably be avoided if the tympanic membrane is perforated. Many of these antibacterial agents will only work effectively in a clean ear and apparent treatment failure is often due to an accumulation of purulent material within the ear canal.
It is imperative that the owner also understands how to apply these products and that the correct dosage and dosing intervals are adhered to. Most dosages are based on a ‘number of drops’ principle, according to the weight of the dog. It can be difficult to count the drops going into the ear canal and consideration may be given to administration of a known volume of product using a 1 ml syringe.
Glucocorticoids are very useful drugs for the treatment of acute otitis externa (Logas 2000) and are included in most topical aural preparations.
In acute otitis externa, the benefits of glucocorticoids include:
▪ relief of pruritus and pain;
▪ resolution of ear canal stenosis due to soft tissue swelling and epithelial hyperplasia; and
▪ reduced exudation and glandular secretion.
As discussed previously, consideration should be given to using systemic therapy in acutely inflamed ears where examination or topical therapy would result in discomfort to the animal. Systemic prednisolone or methylprednisolone may be used at 0.5 to 1 mg/kg. Three to five days of therapy is usually sufficient.
Follow up consultations
Revisits should be scheduled for a week to 10 days' time. Otoscopic examination and repeat cytology should be part of the re-examination process. Otoscopic examination should reveal a clean ear canal (Fig 9) and it should be possible to visualise the tympanic membrane. Client compliance and ear cleaning techniques should be appraised if there is persistence of discharge within the canal. Topical therapy is continued until no micro-organisms are seen on repeat cytology and there should be no evidence of inflammation. Inflammation within the ear canal is identified by not only the presence of intact inflammatory cells, such as neutrophils, but also by purple-staining strands of nuclear material.
Even following apparently successful treatment, the owner should be advised that otitis externa often recurs and that further investigation into causes of the otitis and addressing perpetuating factors will be required if disease recurs. Ongoing weekly or fortnightly ear cleaning is often beneficial to prevent further accumulation of cerumen and reduce the likelihood of reinfection. It is important to avoid repeatedly prescribing further topical therapy for subsequent episodes of otitis even though clients may be insistent on this course of action. To do so will encourage the formation of resistant organisms and is likely to result in severe perpetuating factors and the development of chronic disease.
Otitis externa is a complex disease involving primary and secondary causes of inflammation, as well as perpetuating factors. The approach to the case of acute otitis externa involves brief history taking and examination of both the ears and integument. Otoscopic examination should be performed when possible, but every attempt should be made to avoid causing the animal further discomfort. Cytology is mandatory in all cases of otitis externa and this will guide treatment selection and facilitate assessment of response to therapy. Topical therapy is the mainstay of treatment for acute otitis externa, but short-term systemic glucocorticoids are also very useful. The clinician should take the opportunity to discuss with the owner that acute otitis often recurs and that further investigation may be required to determine underlying causes and that failure to do so may, ultimately, result in chronic irreversible ear disease.
Quiz: the successful first-opinion ear consultation
This is a cytology sample from the ear of a cocker spaniel with acute otitis externa. Identify the cytological features in the photomicrograph.
What is the most common primary cause of inflammation in otitis externa:
Otodectes ear mites
Which of the following statements is incorrect?
Miconazole has activity against Gram positive bacteria
Polymixin B has activity against Gram positive bacteria
Fluoroquinolones are active against Gram negative bacteria
Florfenicol has activity against Gram negative bacteria
Cytology from a canine ear. What cytology features do you seen on this photomicrograph? What would be appropriate topical antibacterials to use?
Rod-shaped bacteria, neutrophils, neutrophilic stranding and squames
d Atopic dermatitis
b Polymixin B does not have activity against Gram positive bacteria
Predominantly coccoid bacteria, neutrophils and neutrophilic stranding. Appropriate topical antibacterials would include fusidic acid, miconazole, gentamicin and florfenicol. It is better to reserve fluoroquinolone antibiotics for Gram negative infections.
Competing interests In the past 10 years the author has, in the capacity of an independent veterinary dermatologist,consulted or lectured for the following companies: Animalcare, Zoetis, Dechra, Merial, Elanco and Vetruus.
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