Otitis externa (OE) is extremely common and problematic in dogs and occasional in cats. OE is complex due to multiple causal factors, with underlying primary disease, secondary infections, and perpetuating inflammation all important. Otitis media (OM) is most often an extension of inadequately managed OE in dogs, but may occur as a primary problem occasionally in dogs (e.g. Pugs) and more frequently in cats.1-4
Some Key Points for Effective Management of OE are:
Key Point ONE: Ear cytology is the one basic test required to guide treatment of OE
Cytology indicates if there is infection, and if so, what organisms are present and their relative numbers. Culture and sensitivity testing is frequently misleading as true pathogens may not be cultured (a range of normal flora grow readily, even if in low numbers), results are not repeatable (multiple swabs from the same ear can give different results), and results won’t indicate which topical treatments will be effective.5-8
Consideration of “normal” numbers of yeast and bacteria on cytology is important: allergic ears may be irritated/inflammed without bacterial or yeast infections (see cytology tips). Wise choices from the myriad of products available is then required based on cytology (see treatment tips).5,6
Key Point TWO: Topical therapies are paramount
Topical therapies are the mainstay of effective treatment of OE, and systemic antibiotics are generally unnecessary, and also rarely effective if used alone.1-3,9 If owners are struggling to administer topical medications, time needs to be spent to find a solution. Wrestling with dogs is frequently unsustainable and ineffective. Slowly reconditioning them to the process, avoiding painful/uncomfortable administration (e.g. poking nozzle down canal; ear cleaners), and reward-based behaviour encouragement can all help with achieving effective administration (see administration tips).
Key Point THREE: Prescribe measured doses and give precise administration instructions
Topical antibiotics and antifungals should be accurately dosed, based on the size of the patient/ear canal volume.2,3,9 A "" into the canal gives imprecise, variable dosing and is to be strongly discouraged. Owners should be instructed to administer a measured dose via a syringe or metered pump, or by counting the drops.
Key Point FOUR: Adequate treatment duration is a vital concept
The importance of chronic inflammatory changes in perpetuating ear infections, and influencing effective treatment duration is often underestimated.1-3,12 Despite label instructions promoting short therapy, inadequate duration of treatment frequently results in poor response to treatment in OE, particularly if chronic.
Bacteria and yeast are normal flora in ear canals and readily over-grow when local conditions suit. Chronic inflammatory changes provide suitable conditions (hyperplastic epithelium narrows canals, inflammation produces heat, and hyperplastic glands increase moisture). Such changes take weeks to reverse, thus sustained anti-inflammatory treatment (weeks to months) is needed after infections are controlled to return ears to normal. Antiseptic treatment plans need to extend throughout this period to prevent quick recurrence of infections while conditions remain suitable.
Addressing the underlying disease is also important when managing OE and OM,1-4 although rarely needs to be an initial focus of treatment. Unless overt signs of a primary disease are present (e.g. obvious Cushing’s dog; more generalised skin disease) evaluation of the primary cause can often be delayed until infections are controlled (~4-8 weeks), or if infections recur despite initial complete response. Excluding easily managed diseases like food adverse reactions, and endocrinopathies is important if OE signs are persistent and non-seasonal despite adequately clearing infections. Many intermittently recurrent cases are related to underlying atopic dermatitis, which is a life-long disease requiring on-going preventative care; a prevention plan is VITAL for these patients. Many quickly recurrent OE cases have incompletely resolved infections or chronic inflammatory changes that have not been treated for sufficient duration to resolve rather than necessarily active underlying primary diseases.
Key Point FIVE: Effective treatment plans for chronic otitis require staged plans:
Step 1: Clear any current infections, checking response with cytology and canal examination every 2-3 weeks (minimum 3 weeks; may take longer, and may require change of drops and/or thorough ear flushing in some patients).
Step 2: Continue appropriate anti-inflammatory topical treatments until the ear canal looks grossly normal and cytology remains clear (effective treatment removes all micro-organisms on cytology; after treatment reduces, low numbers of normal flora should return). This step may takes weeks to months.
Step 3: Address the underlying cause, or start a preventative treatment plan if this isn’t readily possible. Investigation can begin when progressing well in Steps 1 or 2.
Cytology Tips: Is there bacterial and/or yeast infection?
Cytology is vital to answer this question, and thus an invaluable tool for effective management of otitis.1-3,7
A cotton-bud is rolled gently for 2-3 seconds on each ear canal wall (aiming for at least mid-way down the vertical canal), then rolled onto a glass slide for staining (e.g. Diff Quik®). Heat-fixation is not required. Each ear should be evaluated independently, as causal microbes will often vary.
- Change stains regularly (every 1-2 weeks): aged or contaminated stains may contain crystalline matter resembling bacteria, or clustered contaminate yeasts.
- Yeasts are readily identified as well-outlined budding, peanut or snowman-shaped, microbes, readily seen with 40X lens and notably larger than bacteria (Figure 1).
- Bacteria are small and can be more challenging; the oil immersion lens (100X lens; 1,000 X magnification) is essential. Reliable differentiation of bacteria from debris or melanin granules, and bacterial rods from cocci is important (Figures 2,3).
Tips for accurate categorisation of bacteria: Bacterial rods have blunt ends and parallel sides (Figure 2), but may have central pale zones mimicking paired cocci. Bacterial cocci will be clearly spherical, and many will be present in pairs, closely abutted together.
How many bacteria or yeast are normal? Normal numbers per 400X field (40X lens) are reported to range from 2-8 yeast and 1-30 bacterial cocci in dogs, and from 3-5 yeast and 4-10 bacterial cocci in cats. Bacterial rods and neutrophils are not normal findings.5,6 Even though the oil immersion lens (1000X magnification) is recognised as essential to accurately identify bacteria, no studies to date have evaluated bacteria in normal ears under oil immersion. Extrapolating from 40X data, up to 1-3 yeasts, and 1-10 bacterial cocci could be expected per oil immersion field (100X lens) in normal ears.
When present with neutrophils, any bacteria or yeast are more likely significant. However numerous bacteria and/or yeast may occur in OE without neutrophils. Generally, more than 1 yeast and more than 2-3 bacterial cocci per oil immersion field, in conjuction with clinical signs of inflammation and irritation, is considered to warrant antimicrobial treatment.
Treatment Tips One: Which treatments - antibiotics, antifungals, anti-inflammatories?
Many products contain combinations of antimicrobials and anti-inflammatories. Product choice can be guided by considering:
1. Are there secondary bacterial and/or yeast infections?
Empirical choice of topical antimicrobials based on cytology is frequently used to guide treatment of OE +/- OM. Culture and sensitivity testing, even assuming pathogens are accurately identified, often underestimates drug efficacies due to high local drug concentrations achievable with topical treatments. Table 1 outlines empirical selection guidelines for antimicrobial therapies based on cytology.
2. How severe are inflammatory changes?
Current advice on glucocorticoid (GC) use for OE is largely anecdotal.12 More potent GC (e.g. mometasone, hydrocortisone aceponate) may be indicated with severe gross inflammation (e.g. narrowed canal entrances, lichenification/excessive skin folds on inner pinnae), and/or cytological inflammation (numerous neutrophils in the absence of notable infection (i.e. few or no bacteria or yeast)). Persistence of neutrophils without microbes may indicate medication reactions, or maceration from excessive cleaning.
Products with less potent GC (e.g. prednisolone [Surolan®, Canaural®], or low GC concentrations (e.g. 0.1% betamethasone [Otomax®], 0.1% dexamethasone [Aurizon®], 0.025% fluocinolone [Topigen®]) have less effect than potent GC (e.g. 0.1% hydrocortisone aceponate [EasOtic®], 0.1% mometasone [Mometamax®]. Systemic absorption and local atrophy/erosion are more likely with more potent GC.
3. Is there likely underlying atopic dermatitis?
Although hypersensitivities, and in particular atopic dermatitis, are commonly linked to recurrent OE in dogs and occasionally in cats, there is very limited scientific evidence for optimal treatment.1-4 Ear cleaning is often advocated, however response may be poor, even with good compliance. GC-containing drops may be more useful for management of allergic OE. Commercial products are not available in Australia: compounded silver sulfadiazine 0.5%/dexamethasone 0.1% can be used 2-3 times weekly.
|Dominate microbe on cytology||Active Ingredient (Brand)||Comments|
|Yeast||Miconazole# (Surolan®, EasOtic®^)||Good efficacy. Surolan® has good ceruminolytic activity.|
|Clotrimazole# (Otomax®, Mometamax®^, Aurizon®^)||SID dosing^ less effective? Reduced efficacy suspected in USA. Contain more potent GC.|
|Nystatin# (Canaural®, Topigen®)||Useful for suspected imidazole-resistant yeast 10|
|Bacterial cocci||Framycetin/Fusidic Acid*~ (Canaural®)||Good gram positive effect, including some MRSP. Some gram negative effect (Framycetin).|
|Polymixin B*~ (Surolan®)||Synergistic with miconazole, enhancing antibacterial effect; effective for some MRSP, Pseudomonas, E.coli; poorly effective for some rods (e.g. Proteus).|
|Bacterial rods||Gentamicin*# ~ (Topigen®, Otomax®, Mometamax®^, EasOtic®^)||Resistant strains of E-coli and Pseudomonas recognised.|
|Enrofloxacin# (Baytril Otic®; compounded 1.5%)||Resistant strains of Pseudomonas increasingly reported (USA); high topical concentrations achievable often effective in Australia; Baytril Otic® contains no GC:poor response if inflammed/ulcerated|
|Marbofloxacin (Aurizon®^)||Potentially more effective against Pseudomonas than enrofloxacin.|
|Ticarcillin* (Compounded 6%)||Good activity against cocci and rods, including resistant strains of Pseudomonas.|
|Silver sulfadiazine (Compounded 0.5-1%)||Good activity against cocci (including MRSP) and rods (including some resistant Pseudomonas); lesser activity against yeast. May aid healing of ulcers.|
|Mixed (yeast & cocci)||Miconazole/Polymixin B~ (Surolan®)||Both effective against most cocci including many MRSP, and yeast.
Both only contain low potency GC.
|Nystatin/Framycetin/Fusidic Acid (Canaural)®|
|Mixed (rods & cocci)||Gentamicin*# ~ (Topigen®, Otomax®, Mometamax®^, Easotic®^)||Good activity against most cocci; may be less effective against Streptococci.|
|Enrofloxacin# (Baytril Otic®; compounded 1.5%)||Variable activity against Streptococci reported.|
|Mixed (yeast, rods +/- cocci)||Gentamicin*# ~ (Topigen®, Otomax®, Mometamax®^, Easotic®^)||Good activity for most cocci; may be less effective for Streptococci. SID ^ less effective for yeast?|
* Ototoxic potential documented NB with gentamicin, mainly with parenteral use.
# Middle ear safety confirmed in some canine studies
~ Antimicrobials potentially inactivated in purulent environment
^ Once daily dosing
MRSP – methicillin-resistant Staphylococcus pseudintermedius
Treatment Tips Two: What About the Tympanic Membrane: Is it ruptured? Can we treat?
Considerable emphasis is often placed on establishing if the tympanic membrane (TM) is intact before considering topical treatments. However initial medication selection may be better focused on antimicrobial and anti-inflammatory needs primarily as:
1. Accurately establishing whether the TM is intact is often difficult, especially at first presentation and when using a hand-held otoscope. Narrowed canals restrict full examination, wax/discharge often obscures the TM, and many patients aren’t amendable to full examination unless sedated.
2. Avoidance of topical medications with known ototoxic potential if the TM has not been visualised will preclude the use of most first-line products in many patients unless sedation/GA and full ear canal cleaning and examination is performed each time, which is impractical.
3. Use of topical therapies is vital for effective control of the majority of OE cases.
4. Studies to date rarely clarify ototoxic potential even with an intact TM, considering frequent adoption of higher dose and duration of therapies than label recommendations.
If there are signs of OM, or it is clear there is a large defect in the TM, avoidance of products with known ototoxic potential is advisable assuming there is another suitable topical choice. As discussed, systemic antimicrobials used alone in this scenario appear rarely effective.
Products with more potential for otoxicity include:
- Polymixin B (e.g. Surolan®, Dermotic®)
- Fusidic acid (e.g. Canaural®)
- Gentamicin (e.g. Otomax®, Mometamax®, EasOtic®, Topigen®) - although a small study in 10 dogs found no ototoxicity with middle ear penetration of gentamicin
- Ticarcillin (compounded)
Treatment Tips Three: To Clean Or Not To Clean?
There is controversy around the use of ear cleaners in OE.1-3,11 Some suggest regular cleaning is essential to remove bacterial toxins/inflammatory products/epithelial debris that promote more inflammation, and purulent exudate that may impair antimicrobial efficacy (e.g. gentamicin, polymixin B, fusidic acid). Although some studies show benefit of individual cleaners in OE, no studies compare to medicated drops. Frequent cleaning may cause irritation and maceration that impair response to treatment. Ear canals have normal cleaning mechanisms; ear wax [cerumen] containing sloughed epithelial cells and glandular secretions is continually moved up and out of canals, along with trapped material and debris. Cerumen acts as a protectant, and removal with regular cleaning may be detrimental to epithelial health.
Practical advice: Only the most capable and diligent of owners will be compliant with cleaning 5-15 minutes prior to medicating on a twice daily basis for many weeks, and I see some cases poorly responding with these regimes, so I rarely recommend this. Cleaning once to twice weekly seems important to resolution of some purulent infections, particularly in dogs with heavy occluding pinnae and copious neutrophils on cytology (e.g. cocker spaniels/springer spaniels). Thus, if owners can do it, a once to perhaps twice weekly clean may be beneficial. However, many mild to moderate waxy and less inflammatory OE cases resolve with medicated drops alone, without any use of ear cleaners. Focus on antimicrobial and anti-inflammatory treatments is usually more important than cleaning.
A range of ear cleaners are commercially available. Ceruminolytic cleaners (e.g. PAW Ear Cleaner®) dissolve cerumen more effectively, but have more potential to irritate. Antiseptic/drying cleansers may alter the local ear canal micro-environment (e.g. 2% acetic/boric acid may raise local pH), potentially reducing bacterial survival. Triz EDTA cleansers (e.g. Otoflush®) used preceding fluoroquinolones enhance bacterial cell wall damage, however twice daily administration away from medicated drops is challenging for many owners.
Thorough cleaning and inspection of the ear canal, including middle ear, may be vital to resolution of some chronic OE cases. If infections are not resolving within 2-4 weeks of appropriate medicated drops, or if owners prefer the most reliable response, an ear flush is warranted. A video auroscope gives optimal visualisation, and enables extraction of deep foreign bodies, biopsy, and myringotomy if indicated.
Administration Tips: Focus on correct dose and compliance
Precise dose and application instructions are important for each patient.
Despite label advice, metered dosage is considered a key factor to maximise successful treatment of OE. EasOticâ has a 1ml metered dose, which acknowledges this concept (although doses are excessive for small patients). Counting of drops (20 drops = 1ml), or syringe dosing is ideal for other products (0.2-0.3ml for cats and small dogs up to 10kg, 0.5-0.7ml for medium [10-25kg], 0.8-1.2ml for larger dogs [>25kg] or larger ear canals). Syringe adapters are available (e.g. Surolan®) to facilitate accurate dosing.
Instill medicated drops at the entrance to the external canal, which allows visualisation if counting drops, and gravity to distribute into the deeper canal, facilitated by gentle canal massaging for 3-4 seconds after application. This technique is better tolerated in contrast to plunging long nozzles down into canals, and improves compliance. It is important to shake suspensions prior to administration to mix ingredients (e.g. Canaural®, Otomax®).
3. Hair Plucking?
Although it can be beneficial to remove plugs of hairs and cerumen that have accumulated within canals during OE, routine plucking of hairy canals is not recommended as there is no apparent increased risk of OE, or changes to local temperature/humidity, in hairy versus non-hairy canals. Plucking also requires persistent effort to be effective, which is not easily achieved. Thorough ear cleaning early in treatment of OE, and prophylactic use of ear cleaners in such ears, if tolerated, may be beneficial.
Dr Linda Vogelnest BVSc (Hons) MACVSc (Feline Medicine) FACVSc (Dermatology)
Registered Specialist in Veterinary Dermatology
Small Animal Specialist Hospital
Linda graduated from the University of Sydney in 1984 and worked in private and university small animal practice in Australia and England for over ten years before following a long time interest in dermatology. Linda achieved Membership of the ANZCVSc in Feline Medicine in 1997 and Fellowship in Veterinary Dermatology in 2003. She ran a small and large animal dermatology service at the University of Sydney from 1999-2012, before moving to private referral practice at the Small Animal Specialist Hospital in 2013.
Linda has authored and co-authored numerous scientific publications, and lectured in both Australia and internationally. She continues to teach pre-clinical dermatology to veterinary students at the University of Sydney, and is passionate about promoting a greater understanding of dermatology for all vets. Linda loves everything dermatological, but her special interests include atopic dermatitis, otitis, skin surface cytology and biopsies, and also dermatology in exotics and large animals.
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- Cole LK. The rational use of glucocorticoids in canine otitis. Proceedings of the North American Veterinary Conference, Orlando, Florida, USA; 2011: 477-478.