Otitis Externa in Pets: Complete Guide from Signs to Cure

Otitis externa is a common condition in both dogs and cats, characterized by inflammation of the external ear canal. This can be caused by a variety of factors, including infections, allergies, and underlying health conditions. Proper diagnosis and treatment are crucial, as untreated otitis externa can lead to chronic ear infections and even hearing loss.

Causes of Otitis Externa

Infections in otitis externa are invariably secondary and most commonly involve commensal (e.g. staphylococci and Malassezia spp.) or environmental (e.g. Pseudomonas spp.) organisms. Every case of otitis externa will have a primary (underlying) cause.

The main causes of otitis externa are hypersensitivity reactions, ectoparasitic infestations, foreign objects, tumours, endocrine disorders, autoimmune conditions, and keratinization abnormalities. Hypersensitivity (atopic dermatitis and/or food allergies), ectoparasites (Otodectes spp. and, less commonly, Demodex or Otobius ticks) and foreign bodies are by far the most common primary causes of otitis externa. To effectively manage and prevent recurrent otitis externa, it is crucial to identify and address the underlying cause.

Many cases of otitis externa will have a perpetuating (secondary) cause that prevents resolution. Perpetuating causes include: maceration or irritation of canal epithelium from medication, altered epithelial migration, ceruminous hyperplasia with excessive cerumen and/or exudate production, oedema, hyperplasia and stenosis of the ear canals, calcification and otitis media. Not addressing all the perpetuating factors of otitis externa often leads to recurring chronic otitis. Perpetuating causes can change over the course of chronic otitis and will eventually lead to irreversible changes that require a total ear canal ablation.

Predisposing factors must also be considered when managing otitis externa. Predisposing factors alone do not cause otitis, but combine with primary causes to make the animal more susceptible to  it. Predisposing causes include: narrow ear canals, excessive hair, pendulous pinnae, high density of ceruminous glands and excessive cerumen production, hair plucking, and swimming and/or overcleaning that macerates the ear canal. While predisposing factors may be challenging to manage, they should alert clinicians to animals with a higher likelihood of developing recurrent otitis.

Biofilms inhibit cleaning, prevent penetration and activity of antimicrobials and provide a protected reservoir of bacteria. They may also enhance the development of antimicrobial resistance, especially in gram-negative bacteria that acquire stepwise resistance mutations to concentration-dependent antibiotics.

All the primary, predisposing and perpetuating causes must be identified and managed as well as the secondary infections for a successful long-term outcome. 

Clinical Signs of Otitis Externa

Clinical features can vary between individuals due to differences in the primary cause, predisposing conditions, perpetuating factors, secondary infections, and disease expression. A few key points include:

• Acute, unilateral otitis externa is common in dogs and typically reflects foreign body penetration. Acute, unilateral otitis externa is unusual in the cat.
• Chronic unilateral otitis externa in the cat is often associated with neoplasia or inflammatory polyp formation, whereas bilateral otitis externa in the cat is most commonly associated with otodectic mange.
• Recurrent bilateral otitis externa in dogs strongly suggests hypersensitivity, such as atopic dermatitis or adverse food reactions. Atopic dermatitis and adverse food reactions can present with unilateral infected otitis, but the other ear is almost always also inflamed even if it is not infected. This still represents bilateral otitis – the clinical otitis may also affect each ear sequentially. Atopic dermatitis and food allergies show a characteristic diffuse erythema of the ventral pinnae and ear canals.
• Chronic otitis externa results in a quantitative (more bacteria) and qualitative (initially more gram-positive and eventually more gram negative bacteria) shift in microbial flora.
• Erythroceruminous otitis, with erythema, ceruminous discharge and pruritus is typically associated with a Malassezia and/or staphylococcal overgrowth.
• Suppurative otitis, with erythema, ulceration, purulent discharge and pain, is typically associated with a gram-negative (most commonly Pseudomonas spp.) infection.
• Moist suppurative otitis with a whitish granular discharge can be seen with maceration and/or contact reactions to medication.
• Pustules are rare on the concave aspect of the pinna and are often associated with pemphigus foliaceus or a medication reaction rather than superficial pyoderma.
• Otitis media can cause depression, pain, head tilt, deafness and difficulty in eating, but most cases are difficult to distinguish from otitis externa alone. 
• Head tilt, ataxia, nystagmus and Horner’s syndrome are associated with severe otitis media and otitis interna, and should be treated as emergencies.
• Very firm, immobile ear canals are often irreversibly fibrosed and/or mineralised.
• Sedation of the animal and cleaning of the ear are often necessary to perform a thorough otoscopic evaluation in severe cases.
• The normal tympanic membrane should be translucent with radiating stria.
• The tympanic membrane is more likely to be ruptured in suppurative otitis and where there is stenosis of the horizontal ear canal.

The nature of the discharge can indicate the type of otitis and infection.

Dried material at the entrance of the vertical ear canal can be deceptive, so discharge should always be evaluated using otoscopy or by obtaining a sample from the ear canals. These findings are only suggestive, and cytology should be conducted whenever possible for confirmation.

Diagnosis of Otitis Externa

A basic evaluation of the ear involves a thorough examination and ear cytology.

Clinical examination

The clinician should examine the patient for evidence of Horner’s syndrome or vestibular neuritis. The submandibular lymph nodes should be evaluated for enlargement. Jaw pain should be assessed. The canals should be palpated externally for signs of hyperplasia and mineralisation – healthy ears should be non-pruritic, non-painful, pliable and mobile. The pinnal–pedal reflex should be evaluated. The pinna, vertical canal and horizontal canal should be examined.

Some patients may require sedation and cleaning to verify the presence or absence of a foreign body or tumour in the canal. The tympanic membrane may not be easily visible in diseased ears. This should not preclude treatment, although the possibility of a ruptured membrane should be considered. There are some clues to the position of the tympanic membrane. It sits just inside a short bony tube (the external auditory meatus), which may be palpable with the otoscope tip. There are also some hairs that arise from the ventral ear canal at the insertion of the tympanic membrane.

Cytology

Cytology is mandatory in all cases of otitis. It can effectively identify the most likely organisms in most cases. This is especially valuable in mixed infections, where culturing can detect multiple organisms, each with distinct susceptibility profiles.

Cytology should be collected from the canal with swabs or curettes and examined microscopically. Mites can be found in material collected in mineral oil under low power (×40). Material air dried or heat fixed and stained with a modified Wright–Giemsa stain can be examined under high magnification (×400 or ×1000 oil immersion) to observe cells and microorganisms. The quantities of yeast, cocci, rods, neutrophils, and epithelial cells need to be measured. Biofilms form variably thick veil-like material that may obscure bacteria and cells. Ear cytology is generally more sensitive and specific than culture to determine whether bacteria and/or yeast is present in the ear canal.

Malassezia spp. and staphylococci are relatively easy to identify, and their likely sensitivity can be reasonably predicted by considering local resistance trends and prior treatment history. Gram-negative bacteria are harder to differentiate on cytology alone, although Pseudomonas spp. are most common.

Treatment of Otitis Externa

Treatment varies depending on the cause and severity of the infection but generally includes the following:

Ear cleaning

Cleaning the ears helps eliminate debris and microbes from the ear canal. Some ear cleaners have broad-spectrum antimicrobial activity. Very waxy or exudative ears should be cleaned daily during treatment. Ears that are less exudative can be cleaned less frequently, but at least once a week during treatment. Many types of ear cleaners are available. In many uncomplicated cases of otitis externa, the type of cleaner used may be less important than the actual act of cleaning the ears. It is important to demonstrate effective ear cleaning techniques to owners.

Cerumenolytic and ceruminosolvent cleaners (i.e. propylene glycol, lanolin, glycerine, squalene, butylated hydroxytoluene, cocamidopropyl betaine and mineral oils) are useful for softening and removing dry waxy debris and/or wax plugs. Surfactant based ear cleaners (i.e. docusate sodium, calcium sulphosuccinate and similar detergents) are better in more seborrheic ears and purulent ears. Tromethamine ethylene diamine tetra acetic acid/edetate disodium dihydrate (Tris-EDTA) has very little cerumenolytic or detergent activity, but is soothing in ulcerated purulent ears and is safer if the tympanic membrane is ruptured.

Astringents, such as isopropyl alcohol, boric acid, benzoic acid, salicylic acid, sulfur, aluminum acetate, acetic acid, and silicon dioxide, can aid in preventing the maceration of the canal’s epithelial lining. Some astringents also have antimicrobial activity but acids and alcohols may irritate some animals. Antimicrobials (i.e. p-chlorometaxylenol [PCMX], chlorhexidine and ketoconazole) are useful for treating and preventing infections. Tris-EDTA has little to no antimicrobial activity by itself, although high concentrations can potentiate the effect of antibiotics and chlorhexidine.

Anti-inflammatory treatment of Otitis Externa

Steroids decrease inflammation in the ear, which speeds resolution of infection because infectious agents thrive in inflamed tissue. In addition, glucocorticoids (particularly dexamethasone) reverse the ototoxic effect of Pseudomonas infections. Steroids are not specifically analgesic, but reduction of the inflammation will reduce pain. However, specific analgesia should be considered in severe cases.

Steroids can be administered as individual topical drops, or as a component of either an ear medication or an ear cleaner. Topical steroids are generally well tolerated, although dermal atrophy of the ear canal epithelium or iatrogenic Cushing’s syndrome can occur. These side effects are uncommon and can be minimised by reducing the frequency or potency of the steroid. Hydrocortisone is the least potent steroid. Mometasone and hydrocortisone aceponate are potent anti-inflammatory agents but are also the least likely to cause dermal atrophy. Prednisolone is a moderately potent steroid. Dexamethasone, a powerful steroid, is highly effective, and the injectable form can also be used as a topical drop for the ear. Fluocinolone is also a highly potent steroid and when combined with dimethyl sulphoxide (DMSO) it effectively reduces hyperplastic epithelium. However, DMSO may be irritating to an ulcerated ear. Oral steroids are needed if the pet will not tolerate topical steroids, if the ear canal is stenosed or ulcerated, if otitis media is suspected and/or with generalised inflammatory skin disease.

Regular anti-inflammatory treatment is often required to prevent ongoing inflammation once the infection has been cleared (e.g. in atopic dermatitis). Otherwise this leads to a cycle of recurrent infection and chronic inflammation; the progressive pathological changes and end-stage otitis may require surgical intervention. Chronic inflammation makes each infection more difficult to treat, and repeated use of antimicrobials can contribute to the development of resistance.

Systemic antibiotic therapy

Systemic therapy may be less effective for otitis externa because the bacteria are confined to the external ear canal and cerumen, with no inflammatory discharge, leading to poor penetration into the canal lumen. Systemic treatment is recommended when topical treatment of the ear canal is not possible, such as in cases of stenosis, compliance issues, suspected adverse reactions to topical medications, or in the presence of otitis media. High doses of drugs with good tissue penetration (e.g. fluoroquinolones) should be considered.

Antifungal therapy

Clotrimazole, miconazole, Posaconazole, ketoconazole, nystatin and terbinafine are effective against Malassezia spp. Antifungals may be administered as individual topical drops, as a component of either an ear medication or an ear cleaner. Antifungal resistance seems to be on the rise, but switching to an antifungal from a different class often proves effective if resistance is suspected. Oral itraconazole (5  mg/kg  po  q24  h) or ketoconazole (5–10 mg/kg po q24 h) can be administered if systemic therapy is indicated.

Treatment of biofilms and mucus

Biofilms can be disrupted and removed through comprehensive flushing and aspiration techniques. Topical Tris-EDTA and N-acetylcysteine can break down biofilms, making them easier to remove and improving the penetration of antimicrobial agents. N-acetylcysteine, when administered systemically, has shown to be well-tolerated and effective in disrupting biofilms that form on mucous membranes, such as those in the middle ear. Systemic N-acetyl cysteine and bromhexine can also liquefy mucus, facilitating drainage in cases of primary secretory otitis media in dogs and feline otitis media due to inflammatory polyps.

Tris-EDTA

Tris-EDTA damages bacterial cell walls and increases antibiotic efficacy, which can overcome partial resistance.13 It is best given 20–30 minutes before the antibiotic but can be co-administered. 

It  is  well tolerated and non-ototoxic. At elevated concentrations, Tris-EDTA demonstrates synergistic or additive effects when combined with antimicrobial agents like chlorhexidine, gentamicin, and fluoroquinolones.

Length of treatment of Otitis Externa

Most cases of uncomplicated otitis externa show significant improvement and usually clear up within a 5- to 10-day period. However, more complex infections with chronic inflammatory changes, multi-drug-resistant bacteria and/or otitis media may require 2–3 weeks of treatment. Severe cases may require 6–8 weeks to fully resolve. Careful clinical examination and cytology are required to determine whether the ear infection has completely resolved.

Long-term treatment of Otitis Externa

Ear infections will recur unless the underlying (primary) cause is treated. In some cases, i.e. atopy, the primary, perpetuating or predisposing factors for otitis cannot be eliminated or completely controlled. In these cases, continuous ear treatments are often effective. Regular ear cleaning, combined with the topical application of antifungal and/or steroid treatments one to two times a week, can effectively manage inflammation and infection, reducing the likelihood of otitis recurrence. It is important to impress on owners the need for regular treatment even if the ears look normal.

Key points

• Ear cytology at every ear examination is essential to determine the status of the infection and to direct treatment. This is true even if the affected ear appears normal on exam.
• Most cases of otitis externa require a combination of cleaning, steroid and antimicrobial to achieve resolution of infection.
• Chronic and recurrent cases need thorough investigation and treatment to identify and manage primary conditions, predisposing factors, perpetuating changes and secondary infections.