Otitis externa is a common presenting problem in clinical practice. In chronic cases, the infection frequently extends to
the middle ear. While multiple factors contribute to otitis development and subsequent secondary infections, the organisms
found most often in affected ears include Staphylococcus species, Malassezia pachydermatis, and Pseudomonas aeruginosa.1
P. aeruginosa, a gram-negative bacillus, is ubiquitous in the environment but an uncommon inhabitant of the normal external ear canal and
middle ear of dogs. As small animal veterinarians can attest, ears infected with P. aeruginosa can be some of their most challenging cases. Studies have reported moderate to high levels of resistance to some commonly
used antimicrobials of P. aeruginosa isolates from cases of otitis externa.1-4
Animals with P. aeruginosa ear infections typically exhibit one or more of the following clinical signs or conditions:
- head shaking or ear scratching
- purulent exudate (occasionally hemopurulent or mucoid) (Figure 1)
- swelling, inflammation, and pain
- otitis media
It has been reported that otitis media is present in as many as 83% of dogs with chronic otitis externa.1 The most common feature of otitis media is persistent and chronic otitis externa. Neurologic abnormalities (e.g., Horner's syndrome, facial nerve paralysis, peripheral vestibular disease, and deafness) are present in some cases of otitis
media. In addition, the tympanic membrane may be ruptured or intact, so don't rule out the possibility of otitis media if
you find an intact membrane. Intact tympanic membranes may be bulging, hemorrhagic, brown, gray, or opaque, and exudate may
be visible in the middle ear.
Figure 1: Chronic otitis with Pseudomonas aeruginosa infection. Note the purulent discharge and the erosive and ulcerative
lesions of the anthelix and tragus around the ear canal opening.
All otitis cases require swab cytology of the otic exudate. Evaluate prepared slides for bacteria (cocci and rods), yeast
(M. pachydermatis), and the type and number of leukocytes under high-dry (×400 to ×800) or oil-immersion (×1000) magnification. P. aeruginosa appear as rod-shaped organisms, typically in the presence of neutrophils (Figure 2). Practitioners should also repeat the cytology at each recheck to monitor the patient's response to treatment.
Figure 2: Cytology preparation of the exudate obtained from the dog shown in Figure 1. Note the rod-shaped Pseudomonas organisms
that are visible both extracellularly and intracellularly within neutrophils (Diff-Quik stain, 31000 magnification).
In some otitis cases, patients fail to respond to initial empirical therapy or the condition recurs rapidly after apparently
successful therapy. If this occurs, perform culture and susceptibility testing of the otic exudate. Samples are easily obtained
by inserting sterile swabs to the junction of the vertical and horizontal external ear canals.
If you suspect otitis media, obtain a sample from the middle ear (tympanic bulla cavity) using either a sterile calcium alginate
(Calgi) swab or a syringe attached to a 3-F tomcat catheter to instill, and subsequently suction, 0.5 ml of sterile saline.
If the tympanic membrane is intact—as is often the case—you must perform a myringotomy in the caudoventral quadrant of the
pars flaccida tensa to gain entry into the middle ear.
In otitis media cases, it's important to obtain separate cultures from the external and middle ears because the organisms
and their antibiotic susceptibility patterns often differ between these two locations.1