Topical antimicrobials. Topical antibiotic administration exposes Pseudomonas bacteria to high concentrations of an antibiotic (often more than 1,000 times the minimum inhibitory concentration [MIC]
of the organism). In some strains of Pseudomonas with in vitro resistance, high antibiotic concentrations can overcome the bacteria's resistance mechanisms and kill them.
Therefore, topical antimicrobials must be used in all Pseudomonas otitis cases. As a general guideline, five to 10 drops (depending on the dog's size) of topical antimicrobial agent should
be applied to the ear twice daily.
In cases with ruptured tympanic membranes, all topically applied antibiotics are potentially ototoxic. When the tympanic membrane
is not intact or its patency is unknown, pet owners must be warned of potential adverse reactions. In my experience, these
cases are rare, and the benefits of topical antibiotics far outweigh the risk of ototoxicity.
Furthermore, many suggested antibiotic treatments for Pseudomonas otitis are formulated in clinics, and their stability and safety is typically unknown. Fortunately, numerous products are
licensed and available for topical use in dogs' ears.
In acute Pseudomonas otitis cases, first-line topical antibiotics include neomycin, polymyxin, and gentamicin. Cases that fail to respond to initial
therapy or cases of chronic Pseudomonas otitis call for second-line topical antibiotics, such as tobramycin, amikacin, enrofloxacin, ticarcillin, and silver sulfadiazine.
In these circumstances, presoaking the ear canal with Tris-EDTA (T8 Solution—DVM Pharmaceuticals or TrizEDTA—DermaPet) enhances
the activity of the subsequently applied topical antibiotic. Tris-EDTA disrupts the cell membranes of gram-negative bacteria,
such as Pseudomonas species, allowing the antibiotics to permeate the bacteria and work more effectively.7
Systemic antimicrobials. Adding systemic antibiotics to treatment plans for Pseudomonas otitis depends on one or more of the following indications:
- the presence of otitis media,
- marked swelling and hyperplasia of the ear canal epithelium (indicating tissue infection),
- ulceration of the ear canal epithelium (indicating a portal of entry for bacteria into deeper tissue),
- inability of the owner to treat topically,
- adverse reactions to topical medications.
Few antibiotics with activity against P. aeruginosa are available for systemic use. Many of these antibiotics (e.g., amikacin, ticarcillin, carbenicillin, and ceftazidime) must be administered subcutaneously or intravenously, which is impractical
for medium- to long-term treatment.
Orally administered antibiotics for Pseudomonas infection are limited to fluoroquinolones, which should be selected based on bacterial culture and susceptibility testing
results. In theory, two specific ratios help determine whether the antibiotic is efficacious and whether resistance is likely
to develop. These ratios are based on the susceptibility of the infective organism and the pharmacokinetics of the selected
- Maximum plasma concentration (Cmax)/MIC90: A ratio of at least 8 is preferable. If the ratio is less than 8, then it's more likely that resistant Pseudomonas strains will emerge.8
- Area under the curve (AUC)/MIC: A ratio of at least 125 will likely indicate clinical efficacy.9
Unfortunately, it is seldom—if ever—possible to select a specific fluoroquinolone that meets these target ratios. One reason
is that many veterinarians don't receive the MIC results reported by laboratories for the isolated bacteria. Most labs supply
Kirby-Bauer disk diffusion results, which merely indicate whether the organism is susceptible, intermediately susceptible,
or resistant to the antibiotic. Second, no published reports exist listing the tissue levels of any fluoroquinolone in the
ear epithelium of dogs with otitis externa. Thus, the Cmax and AUC for the target tissue are unknown. Finally, clinical studies have not determined if failure to achieve these target
ratios equates to treatment failure and resistance.
However, there are some published data, albeit limited, that practitioners can refer to for general guidance when selecting