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Otitis externa: avoiding chronic disease

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01 January 2015, at 12:00am

DAVID GRANT reports on a recent webinar presented by Peter Forsythe of the Dermatology Referral Service, Glasgow, on a condition which is very commonly seen in small animal practice

THE Webinar Vet recently hosted a webinar on otitis externa, featuring Peter Forsythe, an RCVS recognised specialist in veterinary dermatology. It was sponsored by Elanco Animal Health.

Mr Forsythe began by referring to an article by Hill (2006) which showed that otitis externa is very common in small animal practice, accounting for 7.5% of cases. These conditions can be difficult to manage as demonstrated by statistics from Peter’s referral practice. Over six years with 2,362 new cases, 1,171 (49%) had otitis externa. Of these, 285 were of sufficient severity to warrant video otoscopy.

It is unusual for dogs to have just one episode of otitis externa. Progression to chronic disease can occur in many cases and these are: difficult to treat; impact on the dog’s welfare; and lead to client (and vet) frustration.

Welfare is important. Otitis externa is frequently painful. A dog may not show much evidence of suffering. It is only when the disease is brought under control that the owner will sometimes notice how much happier the dog seems.

It is important to prevent the development of chronicity, and how this could be achieved constituted the rest of the webinar. A thorough review of the anatomy of the ear reminded us that the whole of the ear canal is lined by skin.

The canals contain hair follicles, sebaceous and ceruminous glands. The last produces earwax. There is a need for a mechanism to get rid of copious wax. The process of epithelial cell migration, likened to a slowly moving escalator moving the wax upwards and outwards, enables this. Failure of the mechanism leads to an accumulation of wax, a factor in many otitis externa cases.

To illustrate the anatomy of the ear canals, extensive use was made of video otoscopy and also later on when discussing disease. They were consistently of a very high standard and contributed to making the presentation the best I have seen on the topic.

Otitis externa is more than just an infection. Treatment of infection is important but may only be partially successful. This is because most cases involve generalised skin disease with secondary changes in the ear canal. A useful classification of otitis externa is helpful in addressing the complexity of the disease.

PSPP classification:

Primary causes of inflammation in otitis externa.

Secondary causes of inflammation.

Predisposing factors in otitis externa.

Perpetuating factors.

Primary causes

In a survey of 300 cases, two-thirds were found to have an underlying allergic cause. Of these, atopic dermatitis was the most common. Other causes included hypothyroidism, Demodex, sebaceous adenitis and foreign bodies. Perhaps surprisingly, only seven cases were attributable to the ear mite Otodectes cynotis.

Secondary causes

Infection is an important secondary cause and the presence of pain, head shaking and malodour will often prompt the owner to seek veterinary advice, having failed to notice the signs of primary causes.

Various organisms can cause infection. The commensals Malassezia, Staphylococci and Streptococci will only occur when the environment becomes favourable for their proliferation. In more severe cases, transient organisms, such as Pseudomonas, E. coli, and Proteus, which are not found in healthy ears, will appear.

Predisposing factors

These increase the likelihood of a dog developing otitis externa. They include increased humidity, conformation, swimming, hair in the canal and any other obstructive disease such as foreign bodies.

Perpetuating factors

Perpetuating factors lead to inflammation within the ear canal. As a result there may be:

  • increased desquamation, glandular hyperplasia, and a failure of epithelial cell migration;
  • a build-up of secretions, which block the access of ear preparations to the lining of the ear canal;
  • large lumps of wax called cerumenoliths;
  • further complications in order of severity include swelling with epithelial hyperplasia leading to stenosis, fibrosis and calcification, which is irreversible.

A video demonstrating the removal of a large cerumenolith from a Boxer’s ear was shown – quite an amazing sight. Another equally amazing sight was the appearance of air bubbles from the tympanic bulla popping up through the ruptured eardrum in a case of otitis media.

Ideally, attention to all the factors involved will be necessary to diagnose and control otitis externa.

An approach to achieve this was outlined. This can begin with:

  1. A detailed history and physical examination, with the intention of providing clues to the many possible underlying causes.
  2. A detailed otoscopic examination followed by ear cytology (cytology is considered to be mandatory in otitis externa cases).
  3. The ear sample – it is heat fixed (the use of alcohol fixative washes it away) and then stained using solutions 2 and 3 of Diff-Quik.
  4. Under oil immersion Malassezia, cocci and rods may be seen and this will give the best indication of infective causes.
  5. If culture and sensitivity is performed, which is indicated if systemic therapy is considered or there have been multiple previous treatments, it should not be a substitute for cytology but used in conjunction. The aims of therapy are to:
  • ensure adequate cleaning;
  • resolve inflammation;
  • eliminate microbial infection;
  • manage primary causes of inflammation;
  • prevent further episodes by addressing perpetuating factors.

Most important of all is cleaning the ear canals. Again, video otoscopy is invaluable and a 4-gauge nasogastric tube is used as this fits through the scope. Cleaning can be done at home but preferably (this often needs some persuasion) is best done in the clinic under sedation or general anaesthesia.

There are many cleaning fluids available including cerumenolytic, astringent, antiseptic products and others such as tris-EDTA, which is useful in Pseudomonas aeruginosa infections. This latter organism can prove to be challenging but cleaning and drying is a key component of the management followed by analgesia, glucocorticoids, and topical antimicrobials based on sensitivity testing. Rarely, systemic therapy may be needed: for example, severe stenosis precluding topical therapy or in cases of otitis media.

Following ear cleaning, a suitable topical ear product is prescribed. Due to the very high levels of antimicrobial agent attained resistance in vitro may be overcome. Cleaning at home two or three times weekly helps prevent recurrence. Many of the licensed topical ear products contain antifungal and anti-bacterial agents active against Gram-positive and/or Gramnegative organisms and most contain glucocorticoids. Which one to use is usefully dictated by the cytological findings.

In summary, Peter emphasised that otitis externa is a complex, multifactorial disease. Successful management requires:

  • Identification and correction of the primary and secondary causes.
  • Management of predisposing factors.
  • Cytology to identify microbes. Cytology is also essential as an aid to monitoring treatment progress.
  • Topical therapy – effective in most cases but a clean ear is essential before starting this.
  • Long-term ear cleaning to help achieve control and prevention of the disease progressing to the chronic state.
  • Cure is difficult in recurrent cases but with frequent monitoring and advice these dogs can be kept comfortable.