Diagnosis and Medical Treatment of Otitis Externa in the Dog and Cat

Classification and causes of otitis externa Otitis externa is a syndrome, not a diagnosis 3,18. The term refers to inflammation of the external ear canal, rather than to a specific disease process 29. Clinically, otitis externa can be unilateral or bilateral, acute or chronic, mild to severe, non-recurrent or recurrent and amenable or resistant to routine therapy. It has been classified according to the type of exudate as erythematoceruminous or suppura-tive, with the former subgrouped as parasitic or nonparasitic 9. Otitis externa can be caused and perpetuated by many conditions and factors, frequently more than 1 at a time 29,53. These have been classified as predisposing factors (increase the risk of otitis), primary causes (directly induce otitis) and perpetuating factors (result from inflammation and pathology in ear that prevents resolution of otitis) 3 and this has become standard usage. The classification was recently adapted to include secondary causes, which were previously included as perpetuating factors 53. Secondary causes contribute to otitis only in an abnormal ear or in conjunction with predis-posing factors. Table 1 lists and defines causes of otitis, and indicates which are most common. As illustrated in the table, primary causes can be local or general-ised, while secondary causes and predis-posing or perpetuating factors are more likely to be local. Most microbial infections of the ear are secondary to another disease or factor and are usually opportu-nistic 17. Pathology and pathophysiology The pathophysiology of otitis externa is not complex – in fact perhaps the opposite. Fig. 1 shows the self-perpetuating nature of the condition if untreated or inadequately treated. The detailed pathology of otitis, particularly early on, differs to some extent according to the cause 51 , but in general, changes are rather stereotyped. Acute inflammation and oedema, if not resolved , progresses over time to chronic inflammation, characterised by glandular changes, fibrosis and scarring, and, eventually , progressive stenosis and occlusion of the ear canal 29,53. Permanent changes such as calcification and later ossification of cartilage can occur. Possible sequelae are otitis media and aural cholesteatoma (both also perpetuating factors) 29. Chronic changes favour proliferation of bacteria and yeasts, further perpetuating pathology 53. Ulceration of the ear canal can occur, usually in association with Pseudo-monas infection 36. The secondary lesions of chronic otitis are due to chronic irritation and microbial overgrowth 51. Diagnostic approach 'Diagnosis and clinical management of otitis externa is often frustrating because there are …


Classification and causes of otitis externa
Otitis externa is a syndrome, not a diagnosis 3,18 . The term refers to inflammation of the external ear canal, rather than to a specific disease process 29 . Clinically, otitis externa can be unilateral or bilateral, acute or chronic, mild to severe, nonrecurrent or recurrent and amenable or resistant to routine therapy. It has been classified according to the type of exudate as erythematoceruminous or suppurative, with the former subgrouped as parasitic or nonparasitic 9 .
Otitis externa can be caused and perpetuated by many conditions and factors, frequently more than 1 at a time 29,53 . These have been classified as predisposing factors (increase the risk of otitis), primary causes (directly induce otitis) and perpetuating factors (result from inflammation and pathology in ear that prevents resolution of otitis) 3 and this has become standard usage. The classification was recently adapted to include secondary causes, which were previously included as perpetuating factors 53 . Secondary causes contribute to otitis only in an abnormal ear or in conjunction with predisposing factors. Table 1 lists and defines causes of otitis, and indicates which are most common. As illustrated in the table, primary causes can be local or generalised, while secondary causes and predisposing or perpetuating factors are more likely to be local. Most microbial infections of the ear are secondary to another disease or factor and are usually opportunistic 17 .

Pathology and pathophysiology
The pathophysiology of otitis externa is not complex -in fact perhaps the opposite. Fig. 1 shows the self-perpetuating nature of the condition if untreated or inadequately treated.
The detailed pathology of otitis, particularly early on, differs to some extent according to the cause 51 , but in general, changes are rather stereotyped. Acute inflammation and oedema, if not resolved, progresses over time to chronic inflammation, characterised by glandular changes, fibrosis and scarring, and, eventually, progressive stenosis and occlusion of the ear canal 29,53 . Permanent changes such as calcification and later ossification of cartilage can occur. Possible sequelae are otitis media and aural cholesteatoma (both also perpetuating factors) 29 . Chronic changes favour proliferation of bacteria and yeasts, further perpetuating pathology 53 . Ulceration of the ear canal can occur, usually in association with Pseudomonas infection 36 . The secondary lesions of chronic otitis are due to chronic irritation and microbial overgrowth 51 .

Diagnostic approach
'Diagnosis and clinical management of otitis externa is often frustrating because there are numerous factors and diseases that predispose to otitis and numerous secondary pathogens that perpetuate the process' 35 In the light of the widely divergent causes of otitis externa, a systematic diagnostic assessment is essential. The approach to the ear 2 decades ago was to examine and treat it in isolation 1 . The current approach differs substantially, as the ear canal has now been given its proper place as a specialised extension of the skin 8,29 and otitis externa is now recognised as a dermatological condition. Diagnosis of the syndrome is straightforward -it can be recognised by variable degrees of head-shaking, pruritus, pain, odour and exudation from the ear 50 . Othaemotoma may result from pruritus 53 . The diagnostic challenge in otitis is to determine the primary cause and identify secondary and perpetuating factors 59 . It is difficult to assess how often it is possible to make a specific primary diagnosis, as little data are available. A primary cause was identified in 8/12 cases of chronic, proliferative Pseudomonas otitis 40 and Griffin asserts that, 'In the majority of chronic ear cases I can find historical or physical evidence of the primary disease.' 18 It seems likely that the primary cause can be found in a reasonable number of cases and that predisposing, secondary and perpetuating causes can be identified and controlled in most. Identifying a primary cause is more important in chronic or recurrent otitis than acute otitis 49 . Table 2 shows recommended diagnostic procedures for otitis. The assessment in all cases should include a general and dermatological history, physical and dermatological examination, otoscopy, and cytology 8,17,18,26,29,48,50,53 . A standard dermatological questionnaire can be used to ensure that important details are obtained in all cases 48 .

Routine diagnostic procedures
Proper otoscopic examination is essential. Adequate visualisation depends on patient control (sedation or general anaesthesia are often required), a meticulously clean ear, and absence of severe inflammation and oedema. In some cases, local or systemic treatment might be required for a few days before otoscopy can be performed 17,53 . Otoscopy is used to assess the diameter of the ear canal, the amount and type of exudate, the presence of ulcers, foreign bodies, parasites, tumours and other space-occupying lesions as well as the integrity of the tympanic membrane 8 . In 1 study, otoscopic examination of the tympanic membrane was only considered adequate in 28 % of otitic ears (compared with 78 % of healthy ears) 28 . However, otoscopy was reasonably effective at diagnosing ruptured tympanic membranes, although the sensitivity and specificity were suboptimaltympanometry had 100 % sensitivity and specificity, compared with 83 % and 93 % for otoscopy 28 .
The odour and gross appearance of the exudate is somewhat helpful, but not very reliable 26,39,49 . Thus, although a particular kind of exudate can increase the index of suspicion for a particular kind of otitis (Table 3), gross examination alone is inadequate.
Cytology is the pre-eminent diagnostic tool in otitis externa 27,48,49,53 and is recommended for all cases where exudate or debris are present 36 . Sample collection and preparation has been covered in detail 12 , but is essentially straightforward.
A sample from the horizontal canal is collected onto a clean cotton-tipped swab, part of the sample is examined under oil and part is rolled onto a slide, dried, stained and examined for yeasts, bacteria, inflammatory and neoplastic cells. Cytology is more sensitive than culture 22 , and culture (where indicated; see below) should never be performed without simultaneous cytology 53 . Cytology can demonstrate the number and morphology of bacteria, number of yeasts, presence of fungal hyphae, presence of parasites, number and type of leukocytes and whether they are phagocytosing organisms, the presence of excessive cerumen, keratinaceous debris and neoplastic cells 12 .
Since microorganisms are present in normal ears, how does the clinician assess whether those seen on cytology are abnormal or not? The presence of inflam-   3,17,18,29,49,53 .  13 ), while healthy ears had <10 per high power field. This has since been used as the basis of recommendations. It has also been suggested that numbers of yeasts be evaluated in relation to numbers of bacteria 18 and the clinician should thereby assess which are the major organisms present. It is useful to grade numbers of organisms using a consistent scale, preferably on a pre-printed card, for follow-up purposes. As a general guideline, healthy ears contain some keratinaceous cells, may contain low numbers of Malassezia and cocci, and have no, or extremely few, inflammatory cells. Large numbers of Malassezia and/or cocci should be considered abnormal, while any rods, fungal hyphae, ectoparasites, neoplastic cells and inflammatory cells (unless extremely sparse) are abnormal. If present in suffi-cient numbers, Otodectes cynotis mites are easily identified under oil. Unfortunately, mites are not always detectable, particularly in dogs 8,17,53 . This is at least in part because very few mites (2-3) can initiate pathology. Table 2 lists additional diagnostic techniques recommended for chronic and recurrent otitis. In these cases, a record should be kept, for assessment and follow-up purposes, of the grade and severity of oedema, the degree of canal stenosis, oedema or occlusion from chronic hyperplasia, the quality, character and colour of exudate, and cytological findings.

Additional procedures for chronic and recurrent otitis
The overall usefulness of culture and sensitivity in otitis externa is limited. Many research groups have studied culture and sensitivity characteristics 4,13,20,25,33,34 , but the results (and, indeed, results from individual cases) are quite difficult to translate into definite treatment recommendations. Agreement between cytology and culture is not always good. The sensitivity of culture is inferior to that of cytology 53 , with the exception of Pseudomonas infections 19 . In vitro sensitivity is unlikely to be the same as in vivo, as drug concentrations are much higher in the ear than on sensitivity discs. In addition, most ears are treated in multiple ways. Cleaning agents, the mechanical act of flushing, antiseptics, multiple antibacterials in some preparations and vehicles will alter the microenvironment and affect bacteria in ways that cannot be predicted by testing a single drug in a laboratory. Many studies (and laboratories) include drugs such as unpotentiated penicillins and others which are rarely indicated for either topical or systemic use in otitis.
For the above reasons, culture and sensitivity are only recommended in the following circumstances, which usually occur in association with recurrent and/or chronic otitis 11,17,19,20,49,53 : • Rods seen on cytology.
• Failure to respond to initial treatment. • Otitis media diagnosed or suspected.
• Pseudomonas infection suspected (even if rods not visualised on cytology). If concurrent otitis media is present, the exudate in the middle ear should be cultured separately, as different organisms and/or sensitivity patterns often occur 13 .
Unlike otitis externa, otitis media is difficult to diagnose 53 . It may be secondary to chronic otitis externa and may in turn perpetuate otitis externa 53 . The proportion of dogs with otitis externa that also have otitis media appears to vary regionally 49 , but has been estimated to be as high as 50 % 36 . In chronic otitis, careful attention should be given to otoscopic examination of the tympanic membrane, but even under ideal circumstances the membrane cannot be adequately visualised in many cases 28 and it is intact in almost three-quarters of cases of otitis externa with concurrent otitis media 13 . Myringotomy is a useful diagnostic tool for otitis media 53 .
Radiography is likely to be diagnostic of otitis media if the condition is very chronic, neurological signs are present, and/or the tympanic membrane is perforated 17,59 . However, normal radiographs do not rule out pathology in the middle ear 59 . Computerised tomography or magnetic resonance imaging are good diagnostic tools for acute otitis media 49 .
In chronic cases, biopsy of the proximal vertical canal and/or proximal pinna can be performed 49 . This is an underused technique, and can provide useful diagnostic and prognostic information 48,49 . Any tumour or proliferative mass should be biopsied 36 .
Any additional diagnostic procedures  depend on the suspected or known primary problems, but might include haematology, serum chemistry profile, urinalysis, endocrine tests, allergy testing, and evaluation of the immune system 48 .

MEDICAL TREATMENT OF OTITIS EXTERNA
'One of the most significant advances in the management of chronic otitis over the past 20 years is that we no longer expect that taping the ears over the head and applying a topical ointment for 7 to 10 days will take care of the problem.' 59 .
Treatment of otitis is tailored to each individual case 50 . Therapeutic agents and products should be targeted at known causes and problems, the choice being based largely on a combination of diagnostic findings and personal experience. The number of commercially available products used in the ear, the array of extralabel treatments recommended, plus the combination of types of otitis and the variety of contributing factors have precluded the establishment of a solid, objective body of literature detailing which specific treatments are most appropriate in which specific circumstances. The general approach to treatment is as follows 11,17,18,36,49,53 : identify and address predisposing and primary factors; clean the ear canal; institute topical therapy; institute systemic therapy (where needed); client education; follow-up; preventive and maintenance therapy (as required). Aggressive surgical management might be indicated when intractable proliferation and stenosis of the ear canal are present 35,49,53 . One of the aims of medical therapy in dogs with known risk factors for chronic, severe, intractable otitis externa is to prevent the condition deteriorating to the point where surgery is the only option.

Treatment of predisposing and primary factors
Management of predisposing and primary factors varies widely according to the cause(s), and is beyond the scope of this review. Recent texts and reviews should be consulted for specific information 18,19,50,53,59 .

Cleaning the ear canal
Cleaning and drying the ear canal is an essential part of assessment and treatment 27,50,53 . Cleaning allows optimal visualisation; removes debris; reduces the microbial population; removes microbial by-products such as toxins and enzymes; allows topical drugs to reach their site of action; increases the effectiveness of topical medications (some of which can be inactivated by exudate) and has a soothing effect. Unremoved debris can function as small foreign bodies and act as the nidus for reinfection 35 .
In mild cases, home cleaning with a ceruminolytic is sufficient, but many cases require flushing under sedation or general anaesthesia 30 . In very severe otitis, systemic and/or topical medication must be administered for up to 2 weeks before the canal is sufficiently open to allow adequate cleaning 30 . Ear cleaning and drying products, and their uses, are listed in Table 4. Cleaning usually involves a ceruminolytic, a flushing agent and in some cases a drying agent. Ceruminolytics soften and emulsify waxy debris, and are usually detergents or surfactants 50 . Examples, in decreasing order of efficacy, are dioctyl sodium sulphosuccinate, propylene glycol, glycerine and mineral oil 50 . All ceruminolytics are potentially ototoxic and should not be used if the tympanum is known or suspected to be ruptured. Flushing solutions include saline, water, acetic acid, chlorhexidine and povidone-iodine 50 ( Table 4). Saline does not damage the middle ear even under extreme circumstances 32 and can thus be recommended for routine use.
Ear flushing is approached as follows 30,53,59 : the integrity of the tympanic membrane is assessed, using history, severity and clinical signs in addition to otoscopy. If the membrane is intact, the canal is filled with a ceruminolytic, massaged and left for 5-10 minutes. Omit this step if the tympanum is known or suspected to be ruptured. The canal is gently flushed with warm flushing solution, using a rubber bulb syringe or soft tube (urinary catheter or feeding tube) with a 10 m syringe. The latter apparatus is considered safest and is very effective. Use of a 3-way stopcock (attached to an infusion set leading to the saline bag, the flushing tube and an outlet tube) streamlines the process 27    be used but is not essential. The flushing tube must be sterile, narrow enough to ensure that there is a space between it and the canal to avoid pressure build-up, and atraumatic. The tube is inserted through an otoscope cone and the flushing process visualised through the otoscope. Debris is removed by gentle flushing and suction. Large particles and hairs can be removed using alligator forceps. After the first flush, excess liquid is removed from the ear by gentle suction, and the canal and eardrum reassessed. Any obstinate debris should be carefully removed using a curette or loop inserted through the otoscope head, and the canal flushed until it is clean. Cottonwool swabs should be avoided, as they are traumatic and can compact debris in the canal. If the tympanum is ruptured, flushing fluid may enter the mouth or nasal cavity, and swallowing or fluid leakage from the nose may be seen 53 . Intubation of anaesthetised dogs should be routine to prevent aspiration pneumonia. If the tympanum is only discovered to be ruptured after the initial flushing, or has is ruptured during the procedure, the middle ear is gently and thoroughly flushed with saline or water to remove any traces of ceruminolytic and/or debris. Once flushing is complete, the canal is dried using gentle suction. Topical medication and/or a drying agent are instilled if required.
The main danger of ear flushing is inadvertent rupture of the tympanic membrane; this is most likely if the membrane is already compromised 30 . Introduction of ototoxic substances into the middle ear through a ruptured membrane is a related hazard. Contact irritation or allergy can result from ear flushing with more caustic substances 3 0 . To minimise ototoxicity and irritation, the mildest possible products should be used and if more caustic products are needed, they should be rinsed out afterwards with warm saline or water 30 . Iatrogenic damage to the ear canal and tympanic membrane are further minimised by avoiding 'blind' introduction of catheters or instruments; these should always be introduced through an otoscope cone and the procedure visualised as it is being performed. Resistant pathogens can be transmitted from one animal's ears to another through inadequately sterilised equipment; this can be avoided by proper sterilisation and by discarding equipment that cannot be properly sterilised, such as rubber tubes 30 . Auditory or vestibular dysfunction may rarely follow ear flushing even if no ototoxic substances are used; this is more common in the cat than the dog 30 .
Owners can carry out maintenance or preventive cleaning at home, using products suited to the particular case (Table 4). A squeeze bottle or bulb syringe can be used; the latter should be cleaned with 50:50 vinegar:alcohol after each use, and should be changed at least every 2-5 weeks 30 . In very severe otitis, or with very fractious dogs, a temporary cleaning device (see reference 30 ) can be inserted in the ear and left in place for 5-10 days. Although frequent home cleaning might be required initially 30 , it is generally recommended that owners do not clean ears more often than once every 2 days 59 . Frequent home-cleaning can result in continual moisture in the ear with secondary infection 53 , and/or irritation of the ear 30 .

Topical therapy
Topical therapy is an important part of the treatment of otitis externa 17,18,27,49,53 . Combination or multipurpose products are frequently indicated, particularly initially, because of the mix of microorganisms, inflammation and sometimes, parasites that are present in most ears at the time of diagnosis 50 . Although symptomatic topical treatment is effective 27 and can be curative alone 49 , the short-term effectiveness of such treatment can lull practitioners and owners into a false sense of security and lead them to bypass attempts to identify factors contributing to the disease 3,59 . This is considered by some to be a perpetuating factor of otitis 11 .
Topical therapy should be selected on the basis of clinical findings, cytology, underlying causes and personal experience 8,17,27,36 . Treatment requirements may change as the case progresses 18,53 . Most routine topical otitis preparations contain a glucocorticoid, antibiotic, antifungal, and sometimes an antiparasitic agent, in an oily or aqueous vehicle 17 . Commercial products currently available in South Africa are listed in Table 5. Disinfectants, ophthalmic and self-formulated preparations are also effective in certain types of otitis 53,59 . The array of products highlights the fact that there is no 'magic bullet' for otitis externa. There is little scientific data to show that 1 combination treatment is better than another and personal preference plays an important role 11 .

Glucocorticoids
Topical glucocorticoids are considered beneficial in most cases of otitis externa, regardless of the underlying cause of inflammation 8,29,36,53 and most otic prepa- rations contain a glucocorticoid 17  Systemic absorption of topical glucocorticoids may suppress the pituitary-adrenal axis. In a randomised study of 2 ear preparations, 1 containing triamcinolone, the other dexamethasone, 4 mg glucocorticoid daily in the ear caused significant laboratory suppression of the axis after 7 days in 7/8 dogs; and in 5/7 dogs, ACTH stimulation was still inadequate 14 days after cessation of treatment (the treatment period was 21 days) 38 . However, the clinical significance of these findings was uncertain 38 . Potentiation of ear infections by topical glucocorticoids is theoretically possible, but there is little evidence that this is a real problem 27 . In fact, human studies have shown that secondary infections such as those that occur in otitis externa are often better controlled by combined antibiotic/ corticosteroid preparations than by antibiotics or corticosteroids alone 27 . A syndrome of acquired folding of the pinna, apparently due to loss of cartilage, has been identified in adult cats 53 . All these cats had been treated daily for 8 months to 2 years with topical glucocorticoid-containing otic preparations.
Despite the above and other theoretical disadvantages, topical glucocorticoids are relatively safe in practice 29 . However, as with any glucocorticoids used for any condition, those used in the ear should be administered judiciously. The choice depends on the nature, severity and chronicity of the condition. The general rule is to use the least potent and shortest-acting preparation possible, for the shortest period possible 29 . Selection is particularly important if long-term treatment (>3 months) is required 17 . More potent glucocorticoids may be needed for acute or acutely exacerbated otitis, but once the inflammation is controlled, short-acting, low-potency drugs are preferred 53 . The potency of the glucocorticoids is expressed relative to hydrocortisone (cortisol). The exact numbers differ in different reports, but the following is reasonably representative: hydrocortisone 1, prednisolone and triamcinolone 5, betamethasone and dexamethasone 25, fluocinolone 100 49 . However, triamcinolone has also been considered twice as potent as prednisolone (10 vs 4) 27 .

Antibacterials
Bacterial infection is likely to be present in most cases of otitis when seen initially, and can easily be confirmed by cytology. Antibacterials are thus required in most cases initially, though they may be unnecessary in maintenance and preventive treatment. Neomycin, chloramphenicol, polymixin B and gentamicin are frequently included in topical otic medications, but a number of other drugs can be used to treat bacterial otitis 53 . Antibacterial drugs are not the only option for treating infection, and disinfectants such as povidone-iodine, chlorhexidine, dimethylsulfoxide and Tris-EDTA can be extremely effective 18,35 . These are especially recommended, usually in conjunction with antibacterial drugs, for the treatment of resistant Pseudomonas otitis 19,49 . Nonotic preparations are often used for chronic, resistant infections, and include ophthalmic antibacterials as well as self-formulated compounds 53 .
Empirical choice of antimicrobials, based on cytological findings, is recommended except in chronic, recurrent cases, and/or if otitis media is present. (In these cases culture and sensitivity testing are indicated -see above.) The major distinction that must be made on cytology is whether cocci or rods (or both) are present. Choice of treatment is made accordingly ( Table 6). Especially initially, topical drugs should be those unlikely to be needed systemically, so as not to limit the choices of systemic antibiotics for resistant cases of otitis externa, or subsequent otitis media 53 . Some authors suggest using 'first-line' drugs such as neomycin or polymyxin B initially, while 'second-line' choices would include drugs like gentamicin or chloramphenicol 49,53 . Resistant Gram-negative infections, particularly of Pseudomonas, can be a therapeutic challenge. Table 7 lists treatments that have been found to be valuable in these cases.
Many commonly used antibacterials are potentially ototoxic if used in the presence of a ruptured tympanum and/or otitis media, particularly if use is prolonged 53,60 . These include the aminoglycosides gentamicin, neomycin and amikacin, as well as chloramphenicol and polymyxin B 36 . In practice, ototoxicity is rare in small animals and the risk is probably somewhat overstated 17,19,27,35,55 . However, a non-otoxotic drug must be used if the tympanum is known to be ruptured. The impairment caused by aminoglycosides is likely to be auditory rather than vestibular and might remain undiagnosed in many cases, particularly if unilateral 35,42 . Inappropriate and/or long-term use of antibacterial agents can cause bacterial resistance; some authors therefore recommend that more potent and broad-spectrum antibiotics such as gentamicin and chloramphenicol should not be used as first-choice treatments 18 .  Chronic topical antibiotics can also predispose to yeast infection 17 . Follow-up examinations are important to assess efficacy of treatment and minimise the development of resistant organisms.

Antifungals
Antifungal agents are indicated in most cases where yeast infection is present and probably in all fungal (as opposed to yeast) infections of the ear. In mild cases of yeast infection, glucocorticoids and flushing alone can clear the infection by normalising the environment 29 . By far the most common fungal infection in the ear is the yeast Malassezia pachydermatis, but many otic antifungals are also effective against dermatophytes, Candida and Aspergillus spp. Antifungals effective against Malassezia are ketoconazole, econazole, miconazole, nystatin, pimaricin, clotrimazole, cuprimixin and amphoterecin B 25,36,58 . Ketoconazole is considered the most effective of these. Nystatin may cause local hypersensitivity reactions. Griseofulvin, thiabendazole, tolcyclate and tolnaftate are ineffective in vitro 31 , but thiabendazole appears to be clinically effective 11,27,53 . Povidone-iodine, chlorhexidine and 2.5 % acetic acid are also effective 53 .

Antiparasitic agents
By far the most common parasite in the ear is the earmite, Otodectes cynotis. Earmites are the major single cause of feline otitis externa. To deal effectively with earmites, the entire animal should be treated with a standard acaricide, because the parasites can survive on other areas of the body. All in-contact animals should be treated 17,49,53 . The minimum duration of treatment is 3 weeks, to break the parasite's life-cycle 49,53 .
Lindane (the ( isomer of BHC or ( BHC) was traditionally the acaricide used to treat O. cynotis 27 . The use of lindane in cats is controversial, with some authors advocating it 37 but others maintaining that all chlorinated hydrocarbons are contra-indicated in this species 2,23,47 . Concentrations over 0.1 % may cause toxic reactions in cats 5 . One of the reasons cats are susceptible to poisoning by chlorinated hydrocarbons is their fastidious habit of licking products off their coats 6,54 . Care should thus be taken to wipe away any overflow medication. No side-effects were reported in studies of lindanecontaining otic preparations in cats 15,43 . Dogs and other mammals are quite resistant to the toxic effects of lindane 6,44 . In summary, lindane should be used with circumspection in cats, and should be used with care in any animal that is young, emaciated or systemically ill 21 .
Otic preparations containing thiabendazole, rotenone, pyrethrins and carbaryl are effective against O. cynotis 14,16,18,35,53 . All have low toxicity to mammals and are highly unlikely to cause detrimental effects at the doses used for otitis 5,60 . Interestingly, a number of products without a miticide performed very well against O. cynotis 15,41,43,52,57 , presumably due to unknown antiparasitic properties of the components or the effect of the oil base. In 1 of these studies, lindane performed substantially worse than a non-acaricide product 43 ; in another, an otherwise identical product performed equally well with or without a miticide 15 .
Systemic or topical ivermectin is effective against ear mites 16,46 . Systemic ivermectin can cause mydriasis, tremors and blindness in cats, is not recommended in dogs younger than 3 months and caused fatal toxicity in a 4-month-old kitten 16 . It can cause discomfort and pain after subcutaneous injection in cats 16 . Ivermectin is contra-indicated in Collies and Collie crosses 18,35 . Fipronil spray is effective against earmites and can be administered as a single treatment 53 ; it should also be considered for otic tick infestation. Thiabendazole can be used for Demodex otitis in cats 49 . Topical amitraz or systemic ivermectin or milbemycin are effective against otic Demodex and tick infestations in dogs 36 .

Topical anaesthetics
Topical anaesthetics are used in some otic preparations to decrease pain and pruritus 8,60 . They cause superficial anaesthesia only 10 , and their efficacy in otitis is considered doubtful 27 .

Vehicle
The vehicle is a significant component of any topical preparation. Unfortunately, in many instances little is stated about the vehicle in the product information. The specific formulation of the vehicle is important, as is the question of whether it is oil-based or aqueous.
Water-miscible bases are often easier to apply and less messy than oil-based products. They are usually better solvents for the active agent 24 . Ointments, creams and gels soften, hydrate, facilitate removal of scales and crusts, lubricate, protect, and facilitate penetration of the skin by the active agent 24 . In dermatology in general, it is recommended that exudative conditions (usually acute) should be treated with a product formulated with a minimally occlusive vehicle, while chronic, usually thickened, lesions, need occlusive vehicles to rehydrate the dry, thickened surface 24 . Most authors therefore recommend an aqueous vehicle (solutions, lotions, tinctures) in ' wet' ears and an occlusive vehicle (ointment/oil-based /creams) in 'dry' ears 7,11,17,18,27,53,59 . However, choice of vehicle might be more dependent on factors such as active ingredients, experience and owner convenience 17,49,53 . The type of vehicle might need to be changed as the healing process proceeds 11 .
Many vehicles are potentially ototoxic if the tympanic membrane is ruptured. This applies particularly to oil-based preparations. Propylene glycol, which is quite commonly used in otic preparations, can be associated with hypersensitivity reactions 36 .

Systemic therapy
Systemic therapy is required if 50  Tris-EDTA ± gentamicin 1.2 g EDTA, 6.05 g Tris and 25 m white vinegar; make up to 1 in distilled water; 5-10 min soak before antibiotic; or 2-12 drops adjust pH to 8.0, autoclave. Can add gentamicin to 3 mg/m BID (with genta); for 14 days a OID = once daily, BID = twice daily, PG = propylene glycol.
• Adverse reactions to topical treatments are suspected. Systemic glucocorticoids are used for severe pain and inflammation 17 as well as chronic otitis with proliferative changes and allergic otitis 50 . Where systemic antibiotics are needed, appropriate empirical choices are trimethoprim-sulfas, clindamycin, cephalexin and enrofloxacin (for otitis media) 53 , but where possible, selection should be based on sensitivity testing. (See also section on topical antibacterials, and Table 6.)

Client education
The major areas of importance in client education are: • The nature of the syndrome -first, and crucially, the fact that what seems to be a local problem is often a manifestation of a generalised condition; second, that the underlying problem cannot always be cured; and third, that the local (secondary) consequences of otitis have to be addressed as well. The client must be informed about the possibility of chronic, proliferative otitis and the need to avoid this. Proper education will allow the client to understand the need for an in-depth assessment in some cases, and the need for follow-up examinations 36 . • Correct methods of applying topical medication and cleaners for use at home 59 .

Follow-up
Follow-up checks should include progress reports from the owner and otoscopic and cytological examination. Initially, visits should be scheduled every 2 weeks 49 , to monitor therapeutic response. Treatment often needs to change over time -initial response may not be adequate or initial therapeutic intervention may differ from long-term preventive or maintenance management. Owners and veterinarians should be aware that recurrence may be long delayed, and that a short-term improvement does not necessarily mean that the otitis is cured. In 1 study, the average time to recurrence was 3.6 months 56 .

Preventive and maintenance therapy
Ongoing management is critically dependent on identifying the underlying cause(s) and on proper owner education, as well as on repeated evaluation. Cleaning and drying agents are often part of maintenance/prevention therapy (see Table 4). Long-term interventions are dependent on underlying causes 17,30,53,59 .