Otitis Externa (Inflammation of External Ear)
Basic Structure of Ear Canal
The external ear canal is subdivided into vertical ear canal and horizontal ear canal. An elliptical translucent membrane called tympanic membrane forms a separation between the external ear canal and the middle ear.
How Does Otitis Externa Develop?
In the early stage of otitis externa, we see changes to the skin lining of the ear canal similar to those seen in chronically inflammed skin. The skin lining thickens, hair follicles dilate, glands in the skin dilate and increase in number, and the surface of the skin becomes inflamed. These changes result in an overall thickening of the surface layer of the skin lining the canals. As the surface layer thickens it only has one way to go and that is inwards towards the lumen of the ear canal. The increased number of glands results in excessive secretion of 'ear wax' and this together with the skin changes results in a narrower ear canal filled with exudate (i.e. 'wax'). In this environment microorganisms multiply and produce toxins, which in turn contribute further to the development of otitis externa.
As these initiating factors continue the lining of ear canal thickens up to 5 or 6 times normal size. The normal skin is replaced with fibrous tissue which leads to scarring of the skin lining of the ear canal. This in turn causes further narrowing of the ear canal. Further progression results in rupture of the skin glands with release of their contents into the skin 'itself'. This causes an inflammatory response which ultimately turns into a 'bony type of skin'. At this stage the skin changes are irreversible and ears that have progressed to this stage often require surgical treatment as medical treatment fails to resolve them.
What are the Causes of Otitis Externa?
There are several predisposing factors, including actual ear conformation, environmental factors (e.g. temperature, moisture, relative humidity) and age of the dog, and specific causes and factors that promote the development of the disease in spite of the removal of initiating factors.
(i) Climate and seasonal factors - A seasonal increase in environmental temperature has been noted to be followed two months later with increased cases of otitis externa.
(ii) Age - There is a peak incidence of ear infection between 5 to 8 years of age. This often corresponds with the peak incidence of dogs suffering from hypothyroidism (a hormonal condition that is associated with skin changes) and a few years after atopies (i.e. skin allergies that cause inflamed skin) have commenced in dogs.
(iii) Primary factors - Factors that cause otitis externa on their own in an otherwise healthy dog.
Parasites - Mites, ticks and other insects can cause otitis externa. 'Otodectes' mites have been cited to cause otitis externa in 5 - 10% cases in dogs. As 'Otodectes' mites are obligatory parasite that inhabits storage places (such as ears), treatment of ears alone will not permanently solve the infestation. The whole dog, its environment and any contact animals also require treatment to prevent recurrence. Other parasites such as ticks and fleas can lodge in the external ear canal and create an inflammatory response which results in otitis externa.
Foreign bodies - Grass seeds are a common cause of otitis externa cases especially during summer months, and especially in dogs with an excessively hairy ear flap or canal. Other foreign bodies such as sand or dirt can also plug the external ear canal.
Tumours - They obstruct or even block ear canals and present as otitis externa, but the tumours can be identified after careful examination with an otoscope.
(iii) Other factors - They include the following:
Hypersensitivities - Such as atopy, food allergies and contact hypersensitivity. 55% of atopy and 50% of food allergy cases develop otitis externa and in some cases otitis externa is the only clinical sign seen.
Hormonal disorders - Such conditions can result in otitis externa being the only obvious sign though other skin changes such as oily crusty skin and thinning of hair coat are often seen together with systemic symptoms associated with the specific hormonal condition.
Primary keratinisation disorders - These conditions are rare and quite often produce seborrhoea symptoms elsewhere on the body.
Nevertheless, veterinary examination and early treatment are required if you notice discharge and/or bad odour from the ear canal of your dog and your dog always scratchs its ears.