Commonly there is a proliferation of commensal skin organisms: Gram-positive cocci (mainly Staphylococcus pseudintermedius) and/or the yeast Malassezia pachydermatis. Less often there is colonisation by opportunistic Gram-negative rods (such as Pseudomonas aeuriginosa, Proteus species and E. coli) (Saridomichelakis et al 2007; Lyskova et al 2007; Malayeri et al 2010). A mixed population of microorganisms may be present. The clinical presentation and nature of the discharge are not always indicative of the infectious organism involved. Recurrent or chronic otitis needs careful evaluation to identify and manage underlying predisposing factors and inflammation in the ear canals. (Nuttall 2016)
Cytology is helpful for broadly identifying microorganisms, parasites and inflammatory cells. It is key to determining the need for antimicrobial therapy and to monitoring the progress of treatment. (Nuttall & Bensignor 2014; Bond 2012; Nuttall 2016) There may be different microorganisms in each ear, so in bilateral otitis externa each ear should be sampled and treated separately. (Oliveira et al 2008)
Cytology is not always done in practice. (Bond 2012) It might sometimes be impractical during a consultation. In such cases, the pragmatic approach is to treat with a practice-designated first-line antimicrobial at first presentation, and employ culture and sensitivity testing if the treatment is ineffective. (Malayeri et al 2010)
There are nine topical preparations licensed in the UK for treating dogs with otitis externa. All are prescription-only medicines. All provide a broad spectrum of antibacterial activity; in some products, the broad spectrum is provided by a combination of antibacterial drugs (see the table). The antibacterial spectrum of all the products includes Staphylococci, but they differ in their coverage of Gram-negative organisms: polymyxin B is inactive against Proteus, and florfenicol has very limited activity against Pseudomonas. (Osurnia EPAR; Aurimic SPC) All the products contain an antifungal drug that is active against Malassezia, and a corticosteroid to reduce inflammation and pain. Some products (Aurimic, Canaural and Surolan) are authorised for the treatment of ear mite infestations; although they contain no acaricide, they are thought to work by drowning mites.
An antibacterial treatment course that is too short or not administered correctly may lead to bacterial resistance. Development of resistance in otic microorganisms is considered unlikely to be promoted through treatment of otitis externa because the antibacterial is administered in high concentrations in the ear. Whether otic medication can promote systemic antibacterial resistance because of systemic absorption of small amounts of antibacterial is not known. (Voget et al 2012) The fluoroquinolones are among the highest priority drugs on the World Health Organization’s list of antibacterials critically important to human medicine (WHO 2012). The British Small Animal Veterinary Association (BSAVA) and British Veterinary Association (BVA) advise that they should only be used when other antibacterials are inappropriate or ineffective, and culture and sensitivity testing indicates that they will be effective. (BSAVA 2012; BVA 2015)
Topical ear products licensed in the UK for dogs
There are currently only three published randomised comparative trials of topical antimicrobial products in the treatment of canine otitis externa. All are non-inferiority trials sponsored by pharmaceutical companies that used various criteria for evaluating the treatments.
In a non-blinded trial in Europe involving 176 dogs with bacterial and/or fungal otitis externa based on clinical signs and cytology, Easotic (gentamicin/miconazole/hydrocortisone, 1mL once daily for 5 days) was non-inferior to Otomax (gentamicin/clotrimazole/betamethasone, 4 or 8 drops twice daily for 7 days) in reducing clinical score. By day 14, 72% of dogs on Easotic vs. 70% of those on Otomax had clinically recovered (i.e. had a greater than 75% reduction in clinical score). (Rigaut et al 2011)
In a blinded trial in North America involving 176 dogs with otitis externa and confirmed bacterial and/or yeast infection, Surolan (polymyxin B/miconazole/prednisolone, 5 drops twice daily for 7 days) was non-inferior to Otomax (gentamicin/clotrimazole/betamethasone, 4 or 8 drops twice daily for 7 days) in reducing clinical score 2 to 4 days after the end of treatment. (Engelen et al 2010)
In a blinded trial in Europe involving 140 dogs with acute or subacute otitis externa (i.e. in progress for under 30 days), Aurizon (marbofloxacin/clotrimazole/dexamethasone, 10 drops once daily for 7 or 14 days) was non-inferior to Surolan (polymyxin B/miconazole/prednisolone, 5 drops twice daily for 7 or 14 days), with an overall clinical cure in 58% with Aurizon vs. 41% with Surolan (Rougier et al 2005). Relapse occurred in more dogs treated with Surolan, although the difference was not statistically significant (18.5% vs. 9.8% treated with Aurizon). In the relapsed cases in which a second sample was taken, the bacteria found were mainly Staphylococcus spp. and P. mirabilis, which were resistant to polymyxin B but susceptible to marbofloxacin.
The results of these few trials do not help much with choice of treatment. There is a need for a standard scale for assessing the clinical features of otitis externa and defining thresholds for clinically important changes in clinical scores. (Nuttall & Bensignor 2014) We also need well-designed trials with clinically meaningful outcome measures, such as the proportion of dogs with clinical and microbial cure of infection, time to cure, and effectiveness according to type of infection.
For some products, the dose depends on the size of the dog, while for others there is one dose for all dogs. The recommended duration of use varies between the different products. (See the table)
In the randomised trial comparing Aurizon with Surolan in dogs with acute or sub-acute otitis externa, most needed 14 days’ treatment to achieve a clinical cure (Rougier et al 2005). For Osurnia, the maximum response may not be seen until 21 days after the second dose (Osurnia SPC; Osurnia EPAR)
A clinical and cytological assessment of response to treatment at the end of the specified treatment period is essential to determine if the infection has resolved or if a longer duration or change of treatment is needed. Some practitioners re-examine 1 week after treatment ends so that examination is not hindered by the presence of medication in the ear.
Most products need to be given once or twice daily (see the table); Osurnia is given as two doses one week apart and so may be particularly useful if an owner cannot or will not administer treatment, as it could be administered in the surgery. It can be difficult for owners to administer the correct number of drops in an ear. Some practitioners recommend applying a ‘squeeze.’ Some clients might find it easier to use a pump dispenser (Easotic) or gel formulation (Osurnia) that give metered doses. However, there is no evidence that use of Easotic and Osurnia lead to improved clinical outcomes. (Rigaut et al 2011; Osurnia EPAR) Unlike other products, Osurnia needs to be stored in a fridge.
The summaries of product characteristics (SPCs) for all the products recommend cleaning the ear before the treatment course, to remove ear wax, exudates and other debris. There is a large variety of rinses available, some of which have antimicrobial properties. (Swinney et al 2008) In the clinical trials of the antibacterial products, cleaning was either not permitted, or water or saline only used, to avoid the antimicrobial effect of ear rinses interfering with the results. (Rougier et al 2005; Rigaut et al 2011; Engelen et al 2010) Consequently, the compatibility of many of the products in conjunction with antiseptic rinses has not been studied and, in general, the SPCs do not offer useful advice. The SPC for Posatex states that “studies with a range of proprietary ear cleaners have shown no chemical incompatibilities”, while the SPC for Osurnia advises against ear cleaning during treatment and until 21 days after the second dose of the product.
A double-blind randomised placebo-controlled trial evaluated the efficacy of a rinse containing chlorhexidine and Tris-EDTA together with ear drops (containing marbofloxacin + clotrimazole + dexamethasone) in 64 dogs with otitis externa. (Bouassiba 2012) The abstract of the trial report (only available in full in German) states that the cleaner or a placebo was used twice daily followed by ear drops once daily. It reports that there was no clinical difference between the groups at the end of the trial. It also reports that the combination of rinse plus ear drops was more effective than placebo plus ear drops in reducing the number of bacteria and neutrophils seen on cytology, but did not prevent the development of bacteria resistant to marbofloxacin.
Given that there is no evidence of a better clinical outcome if ear rinses are used in combination with antimicrobial ear products, but there is a risk of incompatibility and reduced effectiveness, it would be better to avoid their concurrent use unless compatibility is known.
Adverse effects on ears
Some antibacterials, including aminoglycosides and polymyxin B, have a potential to cause ototoxicity; the SPCs of several of the topical preparations mention rare reports of deafness (usually transient) associated with their use, mainly in elderly dogs. However, while gentamicin is known to be associated with ototoxicity when given systemically in high doses, in one study, it did not have this effect after application in the middle ear of healthy dogs for 21 days. (Strain et al 1995) The BSAVA PROTECT poster states that enrofloxacin, marbofloxacin and aqueous gentamicin appear to be safe in the middle ear. (BSAVA 2012) All the topical products authorised in the UK are contraindicated if there is a ruptured tympanic membrane; owners should be made aware of the potential risks of using these products when the tympanic membrane cannot be assessed.
Other adverse effects
Erythematous papules or lesions have been reported with some products but usually regress after treatment is stopped (Easotic, Otomax, Posatex). In healthy dogs with no ear inflammation, adrenal gland function was suppressed after 2 to 3 weeks’ ototopical therapy with dexamethasone, and recovered a week or two after stopping therapy (Abraham et al 2005; Ghubash et al 2004). Long-term topical corticosteroid therapy can cause cutaneous atrophy, comedones, and immunosuppression, which can trigger demodicosis. Easotic and Osurnia are contraindicated in dogs with generalised demodicosis. Posatex (which contains orbifloxacin, a fluoroquinolone) is contraindicated in dogs aged less than 4 months because of the potential for fluoroquinolones to adversely affect cartilage.
The SPCs of all the products have warnings about use in pregnant and lactating dogs, but these vary. Aurizon, Marbodex, Osurnia and Otomax should not be used in pregnant or lactating dogs. Canaural is not recommended during pregnancy and lactation. Posatex should not be used in pregnancy and is not recommended for use during lactation. Surolan is not recommended during pregnancy. Aurimic and Easotic should only be used after a risk benefit assessment.
Overall, in terms of the list price, there seems no great difference between the cost of the different products (see table).
A vet or nurse should show the owner how to give the treatment. Written information may also be helpful.
- Shake the bottle before use (except Osurnia); Marbodex needs to be shaken for 1 minute; Surolan needs to be shaken vigorously to ensure the product is fully re-suspended before use
- Ensure that the correct volume of product is placed in the ears
- Give the full course of treatment.
- For Osurnia explain that there might be transient wetness of the ears between doses and that the full effect may not occur until up to 21 days after the second dose.
- Emphasise the need for follow up to review the response to treatment.
All the available antibacterial treatments for canine acute otitis externa contain an antibacterial in combination with an antifungal and a corticosteroid. The available evidence from the few published comparative trials does not help with choice of treatment. The choice therefore depends on other factors:
- the need to preserve the antibacterial effect of fluroquinolones, a key antibacterial group (i.e. use only when other antibacterials are inappropriate or ineffective, and indicated by culture and sensitivity testing)
- risk of ototoxicity and other adverse effects
- dosing schedule, ease of use, and likelihood of correct use by the client
Follow-up is essential at the end of the treatment to determine if the infection has resolved or if a longer duration or change of treatment is needed. There is very little information available on the compatibility of ear rinses with antibacterial products. Given that there is no evidence of a better outcome if ear rinses are used concurrently with antimicrobial ear products, and there is a risk of incompatibility, it would seem prudent to avoid their concurrent use unless compatibility is known.
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Goal of activity: Update knowledge; help clinical decision-making
Authors/disclosures: Veterinary Prescriber editorial team/no conflict of interest
Specific learning objectives: to improve knowledge and understanding of the antibacterial products available to treat otitis externa in dogs.
There are no multiple-choice questions on this module. Instead, we encourage you to reflect on your own and your practice's use of antimicrobial ear products. Click on the link below to go to the reflective exercises.
Complete the feedback form so you can receive your CPD certificate as a record of your activity.
How we produced this module
Our modules start with a detailed outline and electronic literature search. We commission a collaborating author, who is a specialist in the module topic, to write a draft module. The collaborating author on this module was Andrea Tarr. The draft is circulated unsigned to a wide range of commentators, include practising first-opinion vets, other topic specialists, the companies that market any mentioned drugs and other organisations and individuals, as appropriate. They can raise points about the interpretation of evidence, ask questions that are important to clinical practice, and present alternative viewpoints. There is a rigorous editing and checking process and the result is a module that is evidence-based, impartial and relevant to clinical practice. The final module is unsigned because it is the result of collaboration.
For up-to-date summaries of product characteristics (SPCs) for the ear products go to the VMD product information database.
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