In one study, the MICs for various fluoroquinolones for the ATCC strain 27853 of P. aeruginosa were measured. The results (in order of increasing resistance on a µg-to-µg basis): ciprofloxacin (0.52 µg/ml), marbofloxacin
(2.1 µg/ml), enrofloxacin (4.4 µg/ml), and orbifloxacin (10 µg/ml).10 According to the package inserts, the MIC ranges for P. aeruginosa are 0.25 to 1.1 µg/ml for marbofloxaxin (seven isolates), 0.5 to 8 µg/ml for enrofloxacin (four isolates), and 0.39 to 25
µg/ml for orbifloxacin (14 isolates). The insert for difloxacin does not list the P. aeruginosa MIC range.
Studies of the susceptibility of clinical isolates of P. aeruginosa to various fluoroquinolones have been reported: 93.4% of isolates were susceptible to ciprofloxacin and marbofloxacin and
71% of isolates were susceptible to enrofloxacin (106 of the 183 isolates were from infected ears);2 89.9% of isolates were susceptible to marbofloxacin and 42.1% of isolates were susceptible to enrofloxacin (19 isolates from
ears with chronic otitis externa).3
In another study, there was susceptibility to enrofloxacin in 51% of 38 isolates from the middle ear of dogs with otitis media
(some isolates [6/10] resistant to enrofloxacin using the Kirby-Bauer method were susceptible with MIC testing);4 another report showed susceptibility to enrofloxacin in 12.5% of isolates from the external ear canal and 35% of isolates
from the middle ear (in dogs with chronic otitis externa).1
In yet another study, 54 dogs with suppurative otitis (43.8% were infected with Pseudomonas species) were treated only with marbofloxacin (5 mg/kg orally once daily) for 21 to 42 days. A topical otic treatment was
not used. In this study, 27.8% of the dogs were cured, 42.6% showed partial to marked improvement, and 29.6% failed to respond.11
Finally, the half-life of marbofloxacin is considerably longer than other veterinary fluoroquinolones, resulting in a higher
AUC and a potentially greater AUC/MIC ratio.
Based on these studies and my experience with Pseudomonas isolated from ear canals at The Ohio State University, I most commonly use marbofloxacin for systemic treatment of Pseudomonas otitis when susceptibility testing indicates that the organisms are susceptible or intermediately susceptible. I also prescribe
at the high end of the flexible dosing range—5.5 mg/kg once daily.
Although ciprofloxacin appears to have good activity against P. aeruginosa,10 the product is not labeled for veterinary use and has low bioavailability after oral dosing. If Pseudomonas is cultured from the ear and susceptible to other fluoroquinolones, then those fluoroquinolones may also be appropriate choices
for systemic use at the high end of the flexible dosing range.
It should be noted that some organisms are resistant to all fluoroquinolones, and even exceptionally high doses won't resolve
the infection with systemic treatment alone.
Treatment for hyperplastic and stenotic ear canals
In many patients with chronic Pseudomonas otitis, the ear canals are swollen, hyperplastic, and severely inflamed, leading to stenosis of the canal. These chronic
changes must be reduced and reversed from the outset so the ear canal and tympanic membrane can be evaluated, the ear canal
cleaned, and appropriate topical antimicrobial agents applied.
To achieve this, veterinarians should administer corticosteroids, even in cases with severe infection. In all these cases,
patients should receive 1 to 2 mg/kg of prednisone orally once daily for one to three weeks. If necessary, additional high-potency,
topical corticosteroids may be used (e.g., fluocinolone, betamethasone, dexamethasone, or triamcinolone).