Otitis externa is an inflammatory condition with or without concurrent infection and is the most common dermatological disease
in a busy veterinary practice. Therefore, it is the most common claim forwarded to veterinary insurance companies.
For the most part, the majority of cases are simple, and treatment clears the disease in a few days. However, many cases are
recurrent and the management of those cases is frustrating and often veterinarians seek the aid of a specialist. This paper
will focus on the more complicated cases and will clarify and discuss the pathogenesis, treatment and preventive measures.
I will not discuss the uncommon causes of otitis such as parasites, foreign bodies and neoplasia within the context of this
article. The emphasis of this article will be on the dog with severe, complicated or recurrent bacterial otitis with a focus
on the pathology and treatment.
Anatomy-normal
The canine ear varies in the size, shape and conformation from breed to breed. The external ear is comprised of the pinnae
that can be large and pendulous or erect. It is speculated that a pendulous pinna predispose to otitis externa. However, even
erect pinnal breeds such as the German Shepherd are a common breed affected by otitis external The ear has some hair (some
breeds have excessive hair), modified apocrine sweat glands (cerumen glands) and sebaceous glands. A small amount of cerumen
should be present in the opening to canals as well as on the surface of the canals, leading down to the tympanum membrane.
Some dogs have small or stenotic canals (such as Shar Peis) as a breed-related problem. The canals are divided into vertical
leading into the horizontal canal which ends at the tympanum membrane. The tympanum membrane separates the external ear canal
from the middle ear and leads ventrally into the bulla. The tympanum membrane should be a bit opaque, not completely cloudy
and should be relatively smooth.
The microbiology of the ear canal is very similar to the hair and skin in dogs. In my recent article concerning pyoderma,
the normal bacterial flora was elucidated. Staphylococcus, E. coli, Corynebacteria, and even transient bacterial such as Proteus,
Enterococcus and Pseudomonas can be found. Malassezia, as a yeast organism, is also found normally on the pinna and canal
of dogs.
Predisposing causes
It is speculated that predisposing factors are not the primary cause of otitis externa/media but they either trigger or increase
the risk of its development. In my practice, these predisposing causes are not common, but in this paper they are definitely
worth a short discussion.
Conformation is the most common predisposing cause and includes the large and pendulous pinnal seen in certain breeds such
as the Cocker Spaniel or Springer Spaniel. These breeds are also predisposed to other causes of otitis, so careful history
and physical examination must be performed before one blames anatomy as the sole cause of otitis.
Excessive hair in the ear canal (or on the medial aspect of the pinnae) has also been implicated as a predisposing cause to
otitis. Breeds such as Poodles and certain Asian breeds are considered the bulk of the breeds and most definitely benefit
from routine plucking of hair from the vertical canals. I am very careful when I perform hair removal, because certain cases
seem to worsen after this procedure. Moreover, some specialists seem to believe removal of hair can worsen and not benefit
these patients. As I mentioned earlier, I uncommonly observe these cases, but when I do, I am very careful in selecting patients
for hair removal.
Lastly, the one predisposing factor that can complicate and trigger otitis is swimming. Swimming has definitely been linked
to otitis in human patients, and it seems that certain dogs are also affected with a similar problem. The real dilemma that
I have is the most common breeds that swim are also the most common breeds affected by allergy: the most common primary cause
of otitis.
There are numerous primary causes of otitis in the dog including parasites such as Otodectes, Demodex and pinnal parasites
such as Sarcoptes, seborrhea, autoimmune diseases, non-yeast fungal infections and glandular disorders. We will focus our
attention on the most common primary cause of otitis: atopic dermatitis. Because of its high incidence, atopic dermatitis
is more frequently associated with otitis externa than any other disease.
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