It has been reported that 3 percent to 10 percent of atopic dermatitis cases present with otitis externa and less so with
unilateral otitis externa. Generally speaking, if you examine closely the medial aspect of the pinnae of early cases of otitis
externa with atopic dermatitis, you will see subtle erythema on central and medial aspect (concave surface just where the
pinnae might "fold" in half) of the pinnae. This inflammation is the early stages of the disease and can lead to further increase
in inflammation, particularly in the canal. Prolonged inflammation can thus lead to an increase in glandular secretions and
eventually microbial overgrowth. It seems that pinnal inflammation is much easier to treat than canal inflammation. It also
has been reported that 20 percent of food-allergy cases present soley with bilateral or unilateral otitis. Furthmore, more
than 80 percent of confirmed food-allergy cases have at least otitis externa present. I have not found this to be true. I
uncommonly confirm food allergy in the dog.
Perpetuating causes include primarily microbial colonization or overgrowth (mainly bacteria and Malassezia). As mentioned
earlier, progressive changes on the pinnae and in the ear canal can also perpetuate the ear disease. Finally, extension into
the middle ear, termed otitis media can become a persistent nidus for inflammation and infection, and can also perpetuate
The ear canal is simply an extension of the skin, and can be similarly infected and affected by microbial infections, cutaneous
pathologic changes such as glandular (modified apocrine sweat glands called ceruminous glands) and epidermal hyperplasia (lichenification)
as well as edema, fibrosis and even calcification.
Chronic inflammation is the leading cause of these pathologic changes. It has been reported that breeds predisposed to otitis
have many more modified apocrine glands in the ear canal compared to normal dogs. Chronic inflammation that is progressive
leads to hypertrophy or hyperplasia of the epidermis, hyperkeratosis, follicular epithelium, glands, and results in microbial
overgrowth, erosions, ulcerations, pain, fibrosis and, in some cases, calcification. Not all cases progress this way, many
dogs never reach the calcification stage, but many dogs (at least histologically) are affected by fibrosis. Another important
fact is that most cases of otitis media have progressed from otitis externa. Fibrosis results in narrowing of the canal, which
is a common predisposing cause. The thickening or hyperplasia of the skin can also impede normal circulation, increase moisture
and interfere with the otoscopic examination and proper cleaning. The increased epithelial secretions and debris can favor
microbial overgrowth. Finally, a combination of microbial byproducts, glandular and epithelial debris and inflammatory mediators
released from the primary and predisposing causes can further contribute to the pathology.
Bacterial otitis externa/media
Apparently, bacteria are considered to be an uncommon cause of primary otitis externa. They generally are the result of chronic
allergy. Bacterial isolates from the ear canal include Staphyloccus, E. coli, Proteus, Enterococcus, Klebsiella and, uncommonly,
Pseudomonas. Obviously, identification of the primary cause is most important. Identification of bacteria is simply performed
with cytology. I prefer one that has a counter stain such as the Diff-Quick brand but Wright's stain will serve you well.
I have not found Gram stain to be of benefit. Generally, cocci organisms in the ear canal are gram positive and rod-form bacteria
are gram negative. Therefore, morphology of the bacterium is most important when examining cytology specimens. Gram-negative
organisms can be present with otitis externa, but generally they are associated with otitis media. Most cases of otitis media
have a ruptured tympanic membrane.
My criteria for performing a culture includes cases that are chronic or recurrent, patients that have a ruptured tympanic
membrane, and cases that demonstrate rod form bacteria visualized with cytology.
Some of the most frustrating cases that I must deal with include cases of chronic pseudomonal otitis media. I believe these
cases are the most frustrating cases to deal with due to a number of factors. The most difficult factor associated with pseudomonal
otitis media is the significant resistance to antibiotics that this organism demonstrates. This genus of bacteria is routinely
resistant to the penicillins (including cephalexin) and can be resistant to the fluoroquinolones and aminoglycosides. Another
factor that leads to frustration includes the pain and inflammation associated with severe cases of pseudomonal infections.
This pain can be a challenge to manage.