This inflammation is the early stages of the disease and can lead to further increase in inflammation, particular in the canal.
Prolonged inflammation can thus lead to 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 and furthermore 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 primary microbial colonization or overgrowth (mainly bacteria and Malassezia). Progressive changes
on the pinnae and in the ear canal can also perpetuate the ear disease and will be discussed further. 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 overgrowth, cutaneous
pathologic changes such as glandular (modified apocrine sweat glands called cerminous 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, 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 that of course is a common predisposing cause. The thickening or hyperplasia of the skin
can also impede normal circulation, increase moisture, and interfere with 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
Therefore, morphology of the bacterium is most important when examining cytology specimens. Gram negative organism 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. The most difficult
factor associated with pseudomonal otitis media is the significant resistance to antibiotics that this organism demonstrates.
This genus of bacteria are routinely resistant to the penicillins (including cephalexin) and can be also 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.
Before we discuss treatment options for bacterial otitis, I will mention a few facts that can be helpful for the treatment
and management. When one obtains a culture and sensitivity, those values are based upon minimum inhibitory concentration in
the serum or plasma that is needed to achieve a kill.
Many of the antibiotics are available in topical form and can be instilled into the ear canal directly on on the bacteria,
thus achieving a higher concentration of the antibiotic. Many antibiotics are more effective at higher concentrations.
Also, when microbiologists and microbiology technicians select a few colonies (after culture is complete) for sensitivity
studies, the information may not reflect the resistance for the remaining colonies. That is why repeated cultures and sensitivities
on the same ear canal and organism can lead to different results.