Otitis externa is an inflammation lining of the external ear canal. The external ear canal begins at the outside opening of the ear and extends inwards to the eardrum. There are often many causes of otitis externa. We divide these into;

Predisposing Causes – Increase the risk.

Type of ear

  • long pendulous +/- heavy ears
  • very hairy ear canals
  • stenotic (narrow) ear canals

Climate

  • humidity +/- high temperature
  • Environmental factors
  • Swimming in dam/stagnant water (salt water is likely not a predisposing factor, and may be protective).

Primary Causes – Directly cause.
Atopic (allergic) dermatitis is the most common underlying cause of otitis in dogs in Australia.

  • Foreign bodies e.g. grass seeds
  • Ear canal masses e.g. nasopharyngeal polyps (cats), ceruminal gland carcinoma
  • Parasites – Otodectes cynotis, Demodex spp.
  • Hypersensitivities (allergies) – atopic dermatitis, food adverse reactions
  • Endocrine (hormone) diseases – hypothyroidism, hyperadrenocorticism

Secondary Causes
Secondary factors produce otitis when predisposing and/or primary factors are present.

  • Typical normal flora that normally live on the skin but grow in great numbers.
    Bacteria e.g. Staphylococcus intermedius, Pseudomonas spp., Proteus spp., E. coli, Klebsiella spp.
  • Yeast e.g. Malassezia pachydermatis, occasionally Candida spp.

Perpetuating Causes
Such factors prevent resolution of otitis despite addressing primary and secondary causes.
Chronic inflammation, which produces:
Thickening of the skin
Swelling and scarring of the skin lining the canal and excess activity of the glands of the ear that produce wax and oil. These changes lead to thickening of the ear canal lining, narrowing of the ear canals, and in severe cases calicification of the canals

  • Tympanic membrane (ear drum) thickening and loss of transparency
  • Otitis media, (Middle ear inflammation) which produces reservoirs of bacteria and/or yeasts to re-infect external ear canals.

Diagnosis of the Cause

We will often take a swab of the ear canal to look at under the microscope. We will roll this out on a microscope slide, stain it with special stains and then heat fix it before looking at it at 1000x magnification. We will usually see little dots (Staphylococcus), rods (Gram negative bacteria) or purple “snowmen” (Malazzesia – yeast). This tells us which ear medication we should be using. Some cases of ear infection with gram negative rods will require us to send a sample to an external laboratory so they can grow the bacteria and find out what antibiotics they are susceptible to.

Many cases of otitis require sedation or an anaesthetic in order to clean the ear and look down the bottom of the ear canal.

Important Points in Treatment

  • General Anaesthesia is often necessary to allow thorough cleansing of the ear canal and to obtain specimens for bacterial culture.
  • The longer the infection has been present, the more difficult it is to clear up. In severe longstanding infections, surgery may help to control the problem.

Cleaning the Ear
In some cases it is important to remove wax and debris and allow medications to penetrate the ear successfully.

Medicating the Ear
To be effective, the medication must contact the causal agents ie microorganisms or mites deep in the ear canal. To achieve this contact, the canal must be kept clear of debris, and the medication must be placed deep within the canal. Refer to diagram on the next page for further details. Please call us if you are having trouble treating your pet’s ear.

Ear Check Ups
Ear infections often need to be rechecked several times to make sure that they have completely resolved. It is important that the infection completely resolve otherwise it will often recur quickly. Most cases need to be treated for several weeks.

Most cases of otitis require checking one or more times until resolution. Some cases will require checks for several months.