Ear: otitis externa

• Inflammation of the external ear canal .
• Affects 4-20% dogs.
• Causes: numerous, categorized into predisposing factors, primary and secondary causes and perpetuating factors.
• Signs: pruritus (usually earliest sign) shown by scratching or head shaking, aural discharge, malodor, and pain - shown by head tilt or dislike of handling of ear.
• Diagnosis:clinical signs, examination and cytology.
• Treatment: topical therapy (ear cleaner and ear drops), systemic therapy, surgery (where chronic irreversible change is present).
• Prognosis: Good if acute, in chronic disease primary causes must be identified and controlled to prevent recurrence. Where chronic change is present (perpetuating factors) this must be treated to prevent recurrence.
  Print off the owner factsheet on Chronic otitis Chronic otitis to give to your client.Follow the diagnostic tree for Evaluating and Managing Otitis Externa/Media Evaluating and Managing Otitis Externa/Media.

• Pruritus with headshaking or scratching affected ear.
• Otic discharge and malodor.
• Erythema, swelling, scaling, crusting, pain of affected ear.
• Localized dermatological signs often associated with otitis.

• Aural hematomas .
• Acute moist dermatitis of the face.
• There are also often other more generalized dermatological signs applicable to the primary cause of the otitis, eg in allergy - pruritus elsewhere on the skin (pedal saliva staining); and in epithelialization disorders - generalized seborrhea.

• Pendulous ear pinnae is a recognized predisposing factor but is of uncertain significance:
  • Spaniels.
  • Basset hounds.
• Predispositions are noted for Cocker English Cocker Spaniel and Brittany spaniels Brittany; Labrador and Golden retrievers Retriever: Labrador Retriever: Golden; West Highland White West Highland White Terrier; Scottish Scottish Terrier, Fox Fox Terrier and Maltese terriers Maltese; and German Shepherd dogs German Shepherd dog.
• Breed predispositions for primary factors is important, eg:
  • Allergy - atopic dermatitis, cutaneous adverse food reaction.
  • Idiopathic inflammatory/hyperplastic otitis of cocker spaniel.

• The PSPP (Primary, Secondary, Predisposing and Perpetuating) model categorizes causes and factors involved with otitis externa into:
  • Primary causes which are diseases that can affect a normal ear canal.
  • Secondary causes which are infections only affecting an ear canal due to inflammation caused by primary causes.
  • Perpetuating factors are usually chronic changes to the anatomy and physiology of the ear canal as a consequence of disease. Unless managed and where possible resolved, otitis will fail to respond to therapy.
  • Predisposing factors are usually breed associated conformational problems of the ear canal anatomy that pre-exist otitis externa and increase the chance of it occurring, eg pendulous ear pinna. They do not cause disease in their own right.
• Primary causes:
  • Allergy - atopic dermatitis Skin: atopy, cutaneous adverse food reaction Skin: food hypersensitivity, contact allergy Skin: allergic contact dermatitis, Otodectes cynotis Otodectes cynotis. Hypersensitivity reactions to other ectoparasites generally affect the ear pinna (Sarcoptes scabiei Sarcoptes scabiei, flea bite  Skin: external parasite bite reaction Skin: flea bite hypersensitivity.
  • Autoimmune diseases - pemphigus foliaceus Skin: pemphigus foliaceus, erythema multiforme Skin: erythema multiforme. Other immune-mediated diseases affect the ear pinna, eg vasculitis Cutaneous vasculitis.
  • Endocrine disorders - hyperadrenocorticism Hyperadrenocorticism, hypothyroidism Hypothyroidism, sex hormone abnormalities Skin: alopecia X.
  • Foreign bodies, especially grass awns .
  • Fungal infections - dermatophytosis Skin: dermatophytosis, aspergillosis Aspergillosis: disseminated.
  • Parasites Ear: parasitic otitis - Neotrombicula autumnalis Skin: neotrombicula autumnalis infestation, Demodex spp Skin: demodectic mange, Otodectes cynotis, Sarcoptes scabiei Skin: sarcoptic mange, tick spp.
  • Others - sebaceous gland hyperplasia Sebaceous gland; hyperplasia neoplasia, sebaceous gland hypoplasia, lipid-responsive seborrhea, primary idiopathic seborrhea Skin: primary seborrhea, granulomatous sebaceous adenitis Skin: idiopathic granulomatous sebaceous adenitis, vitamin-A responsive dermatosis Skin: vitamin A -responsive dermatosis, zinc-responsive dermatosis, idiopathic inflammatory/hyperplastic otitis of cocker spaniels, juvenile cellulitis Juvenile cellulitis and distemper Canine distemper disease.
• Secondary causes:
  • Bacteria.
  • Acute disease predominantly Gram positive organisms especially Staphylococcus spp Staphylococcus spp, Streptococcus spp Streptococcus spp and Enterococcus spp.
  • Chronic disease switch to a more Gram negative infection especially Pseudomonas spp Pseudomonas, E. coli Escherichia coli, Proteus spp Proteus spp.
  • Yeast - Malassezia Skin: malassezia disease Malassezia pachydermatis and Candida spp Candida albicans (uncommon cause of otitis).
• Perpetuating factors:
  • Chronic changes within the ear canal - inadequate epithelial migration (self-cleaning of ear canal), hyperplasia/fibrosis/calcification of ear canal wall, glandular hyperplasia (apocrine or sebaceous gland).
  • Tympanic abnormality - dilation or rupture, choleasteatoma.
  • Chronic change within the tympanic bulla - otitis media Otitis media.
• Predisposing factors:
  • Conformation - excessive hair, pendulous pinna, narrow canal.
  • Moisture in ear canal - swimming, bathing, humid climate.
  • Ear canal obstruction - polyp, neoplasia (can also be primary cause) .
  • Primary otitis media.
  • Systemic catabolic state.
  • Unnecessary overtreatment or reaction to topical treatment, eg irritation from prolonged use of topical medications, water based products, acids, proplyene glycol, alcohol.
  • Physical damage from cleaning - physical or chemical damage.

• Aural disease.
• Aural pain.

• Head shaking or scratching affected ear.
• Otic exudate.

• Pain on palpation.
• Malodor.
• Aural hematoma.
• Acute moist dermatitis of skin over vertical canal.
• Red, hot, swollen pinna.
• Pinnal scaling.
• Pinnal crusting.

• Bleeding from ears.
• Pain eating.
• Head tilt.

• Signs relating to the primary cause are important:
  • Allergy - chronic pruritus particularly of the front feet and ventral pinnae.
  • Hyperadrenocorticism - polydipsia/polyuria, truncal alopecia, thin skin and muscle weakness.
  • Hypothyroidism - non-pruritis alopecia and systemic signs (lethargy, weight gain, poor exercise tolerance).

Acute otitis externa

• Ear pinna - erythema, mild self-inflicted trauma due to scratching or head shaking.
• Changes within the ear canal:
  • Erythema and swelling .
  • Vasodilation .
  • Increased cerumen production.
  • Pain.

Later

• Ear pinna - erythema, swelling, scaling, crusting, alopecia.
• Changes within the ear canal:
  • Otic exudate, nature dependent on cause (however, these are not sensitive distinguishing signs):
    • Moist brown - cocci and yeast.
    • Purulent creamy - gram-negative (often green/yellow if Pseudomonas spp).
    • Ceruminous waxy yellow - primary or secondary keratinization disorders.
    • Dry coffee grounds - Otodectes cynotis.
• Pruritus or pain on palpation of ear.

Chronic otitis externa

• Ear pinna - hyperpigmentation, lichenification:
  • Aural hematomas.
• Changes within ear canal:
  • Yellowish/brownish ceruminous otic exudate .
  • Hyperplasia of ear canal, narrowing of lumen .
  • Thicker, firmer, less pliable vertical and horizontal canal.
  • Tympanic membrane damage.
• Acute moist dermatitis Skin: acute moist dermatitis face.

• Initial symptomatic treatment based on history, examination and cytology is often required in order to allow adequate examination by otoscopy. Topical and systemic corticosteroid treatment for about a week (providing not contra-indicated for other health issues) is especially valuable to decrease pain, open up edematous ear canals and allow both more effective home cleaning and otoscopy and cleaning in the practice.
• Pain relief using opiates or acetaminophen (NSAID contra-indicated with corticosteroid use).
• If a dog will not allow visual examination of the ear canals or insertion of a swab they will often allow palpation and insertion of gloved fingers into the top of the vertical canal. Cytological preparations can be made from the material collected on the glove fingers.
• It is important that otoscopic examination is performed at some stage as part of identifying primary and perpetuating factors, eg presence of foreign bodies, neoplastic or hyperplastic lesions, canal narrowing and ear drum rupture.

Otoscopy/video otoscopy

• Sedation is often required for otoscopic examination and in dogs with painful and swollen ear canals it is often helpful, prior to scheduling the examination, to treat for 1-2 weeks with topical and/or systemic corticosteroids in combination with systemic analgesics (when the ears are too painful for the owner to treat at home).
• If the horizontal canal requires cleaning to visualize the eardrum, general anesthetic is often required Ear: cleaning. Dogs that are only sedated often continue to move when stimulated by cleaning this area.
• Video otoscopy provides a much enhanced view of the canal and the ear drum and enables recording of lesions.
• Evaluation of the ear drum is challenging:
  • Small tears may be difficult to see.
  • A plug of debris may be taken for an intact ear drum.
  • The medial wall of the middle ear cavity (cochlear promontory) may be mistaken for the ear drum.
  • When the ear drum is ruptured or absent then a stream of air bubbles is expected to rise from the air-filled middle ear when the ear canal is filled with fluid (saline should be the fluid used during these evaluations).
• Palpation of the ear drum with a soft feeding tube passed down an otoscope can be useful. When the ear drum is absent the end of the tube should fall out of sight in the middle ear cavity.
• Other changes that can be assessed include type and extent of exudate within the canal , proliferative changes , presence of foreign body , ulceration of the canal  , ruptured tympanic membrane , presence of ectoparasites - Otodectes cynotis mites .

Microscopy

• Samples of exudate should be obtained from the ear canal and rolled onto a microscope slide. These should be examined as stained (Diff Quik or Gram stain) to look at cellular detail and the presence of microrganisms unstained to check for ectoparasites (Demodex spp, Otodectes cynotis).
• Cocci:
  • Staphylococcus pseudintermedius Staphylococcus pseudintermedius.
  • Streptococcus spp.
  • Enterococcus spp.
• Rods:
  • Pseudomonas spp Pseudomonas aeruginosa.
  • Proteus spp Proteus spp.
  • Escherichia coli Escherichia coli.
  • Klebsiella spp Klebsiella pneumoniae.
• Yeast infection:
  • Malassezia pachydermatis: 5-10/high power field significant.
  • Candida spp.
• Other findings:
  • Inflammatory cell infiltrate - usually degenerative neutrophils with variable degrees of phagocytosis.
  • Acantholytic epidermal cells - suggestive of pemphigus complex.
• Otitis media can be diagnosed when discharge from the middle ear shows signs of an inflammatory infiltrate.
• Biofilm:
  • Where biofilm infection is present cytology will often reveal only small numbers of organisims but there will be an inflammatory cell infiltrate with fine lacy background appearance typical of extracellular polymeric substance produced by the biofilm organisms.

Bacteriology

• Cytology is more important than culture Bacteriology for cases of otitis externa. For otitis media bacteria and yeast rarely found on cytology and culture is more useful.
• Culture and susceptibility is indicated when cytology demonstrates presence of rods.
• Culture indicated where the infection has failed to respond to rational therapy on the basis of cytology and multi-resistant organisms are suspected, eg MRSP Meticillin-resistant Staphylococcus pseudintermedius.
• Cytology results should always be recorded so that if cytology and culture give conflicting results, cytology findings should be used to guide initial therapy.
• Culture should be taken from both the external (horizontal) canal and middle ear in cases of otitis media because different bacteria may be present in various parts of the canal. Guarded swabs should taken from middle ear or ear canal should be thoroughly cleaned before taking samples from middle ear.

Histopathology

• Biopsy of ear canal (rarely indicated):
  • Neoplasia, polyps.
  • Keratinization disorders.
  • Auto-immune diseases.

Other

• Many tests for primary causes may be indicated depending on the signalment and examination findings.
• Skin scrape Scraping: skin for parasites, eg Sarcoptes Sarcoptes scabiei, Demodex Demodex canis , dermatophytes Skin: dermatophytosis.
• Elimination dietary trial for food allergy Skin: food hypersensitivity.
• Intradermal skin test Skin: intradermal test  or serological allergy test to aid treatment of atopic dermatitis Skin: atopy.
• Biochemistry, hematology, urine analysis and endocrine tests.

Radiography

• Skull radiographs Radiography: skull (basic) will only pick up severe chronic change where there is calcification of the ear canal. Radiographs are rarely indicated as a diagnostic imaging modality for ear disease. Calcification can generally be diagnosed by palpation of the ear canal without the need for radiographs for diagnosis of otitis media .
• Advanced diagnostic imaging techniques are superior for the diagnosis of otitis media and otitis interna. CT Computed tomography: head is superior to MRI scanning for otitis media. MRI is the modality of choice for otitis interna. This type of scanning is important if chronic irreversible damage of the bulla is suspected and bulla osteotomy Bulla osteotomy is being considered.

Utrasonography

• For evaluation of the middle ear has been described.

Brainstem auditory evoked response (BAER) hearing testing

• It is helpful to establish the extent of hearing that remains in an ear Hearing tests.
• In otitis externa most of the hearing loss is conductive loss due to chronic change within the ear canal leading to narrowing or occlusion of the canal.
• In otitis media hearing loss is conductive due to chronic change in the bulla and also sensorineural hearing loss due to damage to the inner ear through ototoxic effects of infection of drugs.
• The only value in retaining an ear canal is to enable hearing so once this is lost total ear canal ablation Ear: total ear canal ablation with bulla osteotomy is indicated rather than continued medical treatment.

• Investigate and manage obvious primary causes with a treatment, eg Otodectes, foreign body.
• Treat secondary infectious causes.
• Control predisposing and perpetuating factors.

• Ear canal cleansing with ceruminolytics for waxy ears or an antiseptic cleanser for purulent discharges (most are contraindicated if the tympanic membrane is ruptured, the exceptions are saline, aqueous solutions of TrisEDTA and squalene) using ear bulb syringe or syringe and feeding tube. This may be performed at home or in the clinic under sedation or general anesthetic as appropriate.
• Cleaning removes debris and microbes. Many antiseptic cleaners have both antibacterial and anti-yeast effects.
• Many ear cleaners also contain a drying agent to prevent maceration of the ear canal, these types of product are useful after an aqueous based product. Some of these products may be contraindicated if the ear is severely inflamed or ulcerated.
• On-going cleaning is important to prevent further build-up of secretions and debris in dogs where epithelial migration within the ear is poor. This ongoing cleaning may be required for months, years or life in these patients. The ideal frequency will be case dependent and over-cleaning can also be a cause of otitis externa.
• Many owners need help in learning how to perform effective ear cleaning and to apply medications.

Treatment of secondary infections

Topical antibacterials

• Topical antibacterial products should be considered when bacteria and white blood cells are identified by cytology.
• Some ear cleaners have antiseptic activity and may be used before antibiotic therapy. Products containing antimicrobial peptides are available in some countries and have antipathogenic effects.
• Concentrations of topical ear antibiotics are much greater (c1000 fold) than the concentrations tested in vitro so laboratory sensitivities may not always be a good guide that a particular agent will be ineffective.
• When cocci are seen on cytology Staphylococcus spp should be suspected. Narrow spectrum drugs should be used. First line drugs that should be considered are fusidic acid Fusidic acid and florfenicol Florfenicol, after this aminoglycosides, eg neomycin Neomycin, gentamicin Gentamicin, framycetin or a miconazole Miconazole/polymyxin B Polymyxin B combination. Fluoroquinolones should not be used for infections with cocci unless culture and susceptibility suggests other drugs are inappropriate.
• When rods are seen Pseudomonas spp should be suspected and culture is recommended. Aminoglycosides (neomycin, framycetin, gentamicin) and polymycin B/miconazole combinations should be used on the basis of culture and susceptibility before using fluroquinolones. Topical Polymyxin B is highly effective against Pseudomonas spp, but is inactive if a purulent exudate is present. Thorough cleaning is required if this drug is used. A combination product containing enrofloxacin Enrofloxacin and silver sulfadiazine (licensed for use in dogs in US only) is useful in the therapy of Pseudomonas spp infections but should only be used as a second line drug on the basis of culture and susceptibility.
• There is little justification for the use of third generation drugs such as amikacin, ticarcillin or tobramycin. They should only be used as last resort drugs where culture and susceptibility has shown other drugs are of no benefit.
• Agents usually safe in the presence of a ruptured ear drum:
  • Aqueous solutions of fluroquinolones, gentamicin (note NOT veterinary licensed fluoroquinolones or gentamicin due to the presence of other components in the products), silver sulphadiazine.
• Agents often unsafe in the presence of a ruptured ear drum:
  • Neomycin, veterinary gentamicins, polymixin B (as components of licensed veterinary ear drops), ticarcillin Ticarcillin, amikacin Amikacin.
• Solutions of fluoroquinolones, aqueous gentamicin or amikacin designed for injection may be useful in cases of otitis media.
• Some dermatologists use higher volumes of antibacterial products than recommended on the licences.
• Triz EDTA solutions enhance antibiotic activity when used with aminoglycosides.
• Ear wicks are useful for clients that cannot apply medications frequently at home as they act as local reservoirs of the agents, but they do require general anesthetics for application and replacement every 3-10 days.
• See ear therapeutics Therapeutics: ear Therapeutics: antimicrobial drug.

Topical glucocorticoids/anti-inflammatories

• Helpful in all cases except where Demodex spp is identified on cytology.
• See ear therapeutics Therapeutics: parasiticide.
• Used for significant inflammatory benefits in primary causes, eg to reduce inflammation in allergic otitis or perpetuating factors to reduce proliferative changes.
• Glucocorticoids will also reduce glandular hyperplasia to reduce secretions in the canal.
• A licensed veterinary ear drop containing triamcinolone Triamcinolone may be useful where a topical glucocorticoid is needed without antibiotic. Other glucocortocoids are used off licence. This includes dexamethasone Dexamethasone used topically or added to ear cleaning solutions. Some dermatologists use topical hydrocortisone aceponate off-label in ear canals or 1% prednisolone Prednisolone drops clinically. For chronic use, use least potent formulation which will be effective as percutaneous absorption has been shown to occur leading to iatrogenic hyperadrenocorticism.
• Topical tacrolimus Tacrolimus has been used to otitis externa due to atopic dermatitis and immune-mediated disease.

Topical antifungals

• See ear therapeutics Therapeutics: non-bacterial infection.
• If yeast identified by cytology in significant numbers (5-10/high power field or >10 if numerous bacteria and <5 if only yeast).
• Or if yeast are present at lower numbers and are suspected to be significant.

Topical ectoparasiticides

• See ear therapeutics Therapeutics: parasiticide.
• If parasites (Otodectes cynotis/Demodex spp) seen on otoscopic or microscopic examination.
• Otodectes cynotis in-contact animals should also be treated - probably with a systemic agent unless they have otitis as well.
• Affected animals will often have some mites exterior to the ear canals so additional systemic treatment should be considered.

Systemic antibacterials Therapeutics: antimicrobial drug

• When otitis media is present, systemic drugs should be prescribed on the basis of culture and susceptibility. Systemic drugs do not always reach therapeutic levels within the tympanic bulla especially where organisms have moderate to high MICs.
• In severe gram negative otitis externa, eg Pseudomonas infection systemic medication may be considered but topical treatment alone will often be effective.
• In chronic otitis externa where proliferative changes inhibit the penetration of topical therapy, empirical therapy should be avoided where at all possible.
• In acute otitis externa where pain inhibits use of topical therapy, analgesics should be employed before resorting to systemic antibiotics.

Systemic glucocorticoids 

• When marked inflammation or proliferative changes are present, eg for the first week or two of treatment when waiting for canal wall swelling to reduce to allow examination and perform cleaning.
• Chronic otitis externa where proliferative changes inhibit use of topical therapy.
• When controlling atopic dermatitis as a primary cause.
• Local injection of depot products into proliferative tissue in an ear canal may be helpful.

Systemic antifungals 

• Systemic antifungal drugs may not reach high enough concentrations in the tympanic bulla especially where there is chronic change in the bulla or there is a biofilm infection present.
• Useful for Malassezia otitis media.
• Owners are unable to administer topical agents.
• Severe recurrent Malassezia spp otitis externa is present.

Systemic antiparasiticide treatment

• Topically applied ectoparasiticides, eg selamectin Selamectin or moxidectin Moxidectin products can be used as treatments and also to rule out Otodectes cyanotis and Sarcoptes scabiei as primary causes.
• Orally administered isoxazolines (afoxolaner Afoxolaner, fluralaner Fluralaner, lotilaner, sarolaner Sarolaner) may be useful for Otodectes cyanotis, Sarcoptes scabiei and Demodex spp.

Surgery

• Lateral wall resection Ear: lateral wall resection:
  • This is rarely helpful as it only aids control of lesions confined to the vertical canal.
  • Indicated if chronic disease is confined to the lateral wall.
  • It may be most helpful when used early in the course of problems in animals with congenitally stenotic canals, eg Shar pei Shar Pei.
• Vertical canal ablation Ear: vertical canal ablation:
  • Rarely helpful as it only aids control of lesions confined to the vertical canal.
• Total ear canal ablation Ear: total ear canal ablation plus bulla osteotomy:
  • Chronic irreversible damage to the ear canal, eg stenosis, severe glandular hyperplasia.
  • Tumors or polyps in external ear canal or middle ear.
  • Chronic recurrent otitis externa where chronic irreversible damage is present to the canal and/or the tympanic bulla.
  • Where the ear is already deaf and surgery is a salvage procedure to make the dog comfortable.

• Worsening of symptoms: change medication (?irritant or contact hypersensitivity).
• Check for pruritus - shaking and scratching; discharge, odor and compliance to medication.
• Repeat cytology on each examination.
• Hematology, biochemistry, urine analysis and ACTH- stimulation tests ACTH stimulation test to monitor side-effects if long-term systemic or topical glucocorticoids are used.

Treatment

• Maintenance ear cleaning every 3-7 days.
• Topical low potency steroid if ceruminous glands continue to over-produce exudate, eg hydrocortisone-containing solution Hydrocortisone, +/- topical antibacterials and/or antifungals if secondary infection cannot be controlled by control of the primary cause and perpetuating and predisposing factors.

Monitoring

• Subsequent history, clinical and otoscopic findings and cytology.

• Good when signs are acute and primary, predisposing and perpetuating factors are eliminated or controlled.
• Poor if these not identified or controllable.

• Regression of clinical signs over several weeks.

• Predisposing, primary and perpetuating factors not identified or treated, eg presence of otitis media, ear canal narrowed due to stenosis.
• Inadequate cleaning.
• Resistance to antibiotics, especially when Pseudomonas spp is present.
• Poor compliance of either owner or dog. Where owner is unable or unwilling to medicate the dog or where the dog will not permit therapy. In such situations it is important to ensure adequate pain relief is used to make the dog comfortable and in addition desensitization therapy may be useful with a veterinary behaviorist.

Refereed papers

• Recent references from PubMed and VetMedResource.
• Forster S L et al (2018) A randomized placebo-controlled trial of the effiacy and safety of a terbinafine, florfenicol and betamethasone topical ear formulation in dogs for the treatment of bacterial and/or fungal otitis. BMC Vet Res 14 (1), 1-11 PubMed.
• Paterson S et al (2018) A study to evaluate the primary causes associated with Pseudomonas otitis in 60 dogs. JSAP 59 (4), 238-242 PubMed.
• Paterson S (2018) Brainstem auditory evoked responses in 37 dogs with otitis media before and after topical therapy. JSAP 59 (1), 10-12 PubMed.
• Barnard N, Foster A (2017) Pseudomonas otitis in dogs: a general practitioner's guide to treatment. In Practice 39 (9), 386-398 VetMedResource.
• Noli C et al (2017) Impact of a terbinafine-florfenicol-betamethasone acetate gel on the quality of life of dogs with acute otitis externa and their owners. Vet Dermatol 28, 386-388 PubMed.
• Zur G, Lifshitz B & Bdolah-Abram T (2011) The association between the signalment, common causes of canine otitis externa and pathogens. JSAP 52 (5), 254-258 PubMed.
• Favrot C, Steffan J, Seewald W et al (2010) A prospective study on the clinical features of chronic canine atopic dermatitis and its diagnosis. Vet Dermatol 21, 23-31 PubMed.
• Caffier P P, Harth W, Mayelzadeh B et al (2007) Tacrolimus: a new option in therapy - resistant chronic external otitis. Laryngoscope 117 (6), 1046-1052 PubMed.
• Nuttall T & Cole L (2007) Evidence-based veterinary dermatology: a systematic review of interventions for treatment of Pseudomonas otitis in dogs. Vet Dermatol 18 (2), 69-77 PubMed.
• Paterson S (2003) A review of 200 cases of otitis externa in the dog. Vet Dermatol 14, 249.
• Crespo M, Abarca M & Cabanes F (2002) Occurrence of Malassezia spp in the external ear canals of dogs and cats with and without otitis externa. Medical Mycology 40 (2), 115-121 PubMed.
• Bensignor E & Legeay D (2000) A multicentric prospective study of otitis externa in France: 802 cases. Vet Dermatol 11 (supplement 1), 22.
• Martín B J L, Lupiola G P, Gonzalez L Z & Tejedor J M T (2000) Antibacterial susceptibility patterns of Pseudomonas strains isolated from chronic canine otitis externa. J Vet Med B Infect Dis Vet Public Health 47 (3), 191-196 PubMed.
• Cole L K, Kwochka K W, Kowalski J J & Hillier A (1998) Microbial flora and antimicrobial susceptibilty patterns of isolated pathogens from the horizontal ear canal and middle ear in dogs with otitis media. JAVMA 212 (4), 534-538 PubMed.
• Nuttall T J (1998) Use of ticarcillin in the management of canine otitis externa complicated by Pseudomonas aerginosa. JSAP 39 (4), 165-168 PubMed.
• Mansfield P D et al (1990) Infectivity of Malassezia pachydermatis in the external ear canal of dogs. JAAHA 26 (1), 97-100 VetMedResource.
• Moriello K A et al (1988) Adrenocortical suppression associated with topical otic administration of glucocorticoids in dogs. JAVMA 193 (3), 329-31 PubMed.
• August J R (1986) Evaluations of the patient with otitis externa. Dermatology Reports 5 (2), 1-8 VetMedResource.
• Woody B J & Fox S M (1986) Otitis externa - seeing past the signs to discover the underlying cause. Vet Med Small Anim Clin 81 (7), 616-24 VetMedResource.
• Wilson J F (1985) A practitioner's approach to complete ear care. Dermatology Reports 4 (2), 1-8 VetMedResource.
• Dickson D B & Love D N (1983) Bacteriology of the horizontal ear canal of dogs. JSAP 24 (7), 413-21 VetMedResource.

Other sources of information

• Harvey R G, Paterson S (2014) Otitis externa: an essential guide to diagnosis and treatment. CRC Press, Taylor and Francis Group.
• Miller W H, Griffin C E & Campbell K L (2013) Otitis externa. In: Muller & Kirk's Small Animal Dermatology. 7th edn. Elsevier Mosby, St Louis, pp 741.
• Paterson S, Tobias K (2012) Atlas of ear diseases of the dog and cat. 1st edn, Wiley Blackwell, Oxford.
• Griffin C E (2010) PSPP System. www.animaldermatology.com
• August J R (1988) Otitis externa in the dog and cat. Part II - Pathogenesis of the Disease. Western Veterinary Conference, Las Vegas, 162.