Otitis Externa - Cat and Dog

Introduction

Otitis externa is an acute or chronic inflammation of the external ear canal, developing anywhere from the tympanic membrane to the pinna.

It is the most common disease of the ear canal in dogs and cats, and is much more common in dogs than in cats.

It can range from a mild to a severe disease and it sometimes extends to the middle ear.

Aetiology

There are three areas which regroup the causes of otitis externa.

Predisposing factors increase the risk of development of the disease.

Primary factors directly cause the otitis.

Perpetuating factors prevent the resolution of otitis externa once it develops.

Predisposing factors

Anatomy: pendulous pinnae, hairy, stenotic canals, long, steeply-sloping canals

Maceration: wetting due to swimming, high humidity

Trauma: usually from inappropriate cleaning

Concurrent systemic disease: pyrexia

Tumours, polyps, cysts: more common in cats

Primary factors

Parasites: Otodectes cynotis, Sarcoptes, Demodex, Neotrombicula, ticks

Fungi: Dermatophytosis

Allergies: atopic dermatitis, diet, contact

Foreign bodies: grass seeds, grass awns,

Abnormal epidermal turnover or sebum production: endocrinopathy, nutritional problems, sebaceous adenitis

Primary keratinisation defects: idiopathic seborrhoea

Autoimmune disease: pemphigus foliaceus

Miscellaneous: juvenile cellulitis

Perpetuating factors

Bacteria: S. intermedius, Pseudomonas aeruginosa, Proteus

Fungi: Malassezia pachydermatis

Topical treatments: innapropriate use of topical treatments

Progressive ear pathology: ulceration, glandular hypertrophy and atrophy, thickened, folded canals

Otitis media

Clinical signs

The animal will present with a history of head-shaking and ear scratching, either acute-onset and severe or chronic and intermittent.

The ears may show evidence of self-trauma, erythema, and primary and secondary lesions. There may be pinnal deformities and the ear canal may feel thickened on palpation.

The animal may present with an aural haematoma from head-shaking if it is severe.

There may be discharge coming from the ear which may be brown and waxy or purulent and green.

The animal may show signs of pain on clinical examination and may resent otoscopic examination unless it is sedated.

Diagnosis

Signalment and dermatological history will provide some clues as to the cause of the otitis.

Examination of the ears will help determine if this is a chronic problem or not.

Otoscopic examination should be carried out to search for any primary factors which may have caused the otitis, such as a mass, foreign bodies or parasites.

The tympanic membrane should be examined for evidence of disease or rupture. The ear canal may have to be flushed and cleaned for this to be possible.

Cytological examination of an ear swab should be performed in all cases to determine the flora present in the ear. It is normal to find low numbers of cocci and yeasts such as M. pachydermatis in the ear canal, but increased numbers or the presence of rods indicating Pseudomonas infection indicate that culture and sensitivity should be performed on the sample.

Biopsies should be taken from any mass observed.

Radiography of the bullae is indicated if the otitis is chronic and middle or inner ear infection is suspected.

If an allergy is suspected, various tests such as intra-dermal allergy testing could be performed in order to provide a long-term means of control.

Treatment and control

Underlying predisposing, primary and perpetuating factors should be corrected.

Hair can be removed from the ear canal to improve ventilation and access for treatment.

Efficient cleaning of the ear is important, and the initial clean may have to be done under general anaesthesia in the clinic. Further treatments should be done at home before any medicated topical treatments are given.

Medical therapy may be useful:

Oral corticosteroids can be given for a short period at a high dose to open constricted canals and improve topical treatment efficacy.

If there is a bacterial otitis, oral antibiotics are useful.

If Otodectes cynotis is detected, a general topical or systemic antiparasitic is indicated as this parasite can invade sites other than the ear.

Many topical treatments are available for otitis externa cases. These usually provide a combination of anti-bacterial, anti-fungal, anti-parasitic and anti-inflammatory agents.

Duration of treatment should continue until the infection is resolved.

Animals should be examined and cytology swabs taken every week until there is no cytological evidence of infection.

This usually takes 2-4 weeks for acute cases.

Chronic cases may take months to resolve and therapy may have to continue for the rest of the animal's life.

Prevention:

Ears should be cleaned before every treatment, and then regularly cleaned twice a week indefinitely.

Astringents can be used for dogs who are used to swimming to minimise maceration. Hairs can be regularly plucked from the inside of the ear to improve ventilation.

References

Merck and Co (2008) Merck Veterinary Manual Merial

Harvey, R. (2001) Ear diseases of the dog and cat Manson Publishing