Pseudomonas aeruginosa is a gram-negative bacterium that has been implicated as the causative agent in as many as 21% of cases of canine ulcerative keratitis. The organism can cause permanent structural ocular damage and decreased visual capacity. Thus, it is critical to quickly begin treatment with an appropriate antimicrobial. Empiric choice of an antimicrobial can be difficult, however, because resistance is common. This study was done to determine the susceptibility of a spectrum of commercially available fluoroquinolone ophthalmic preparations to treat P aeruginosa-associated ulcerative keratitis in dogs. P aeruginosa isolates from 27 dogs with ulcerative keratitis were collected over a 3-year period. They were tested in vitro for susceptibility to ciprofloxacin, ofloxacin, norfloxacin, lomefloxacin, levofloxacin, gatifloxacin, and moxifloxacin via disk-diffusion method. Isolates were defined as susceptible, intermediate, or resistant. Of the 27 isolates collected, 24 were susceptible to all fluoroquinolones tested. Susceptibility ranged from 88.9% to 100% for individual antimicrobials. The highest percentage of susceptible isolates (100%) was seen for ciprofloxacin and levofloxacin and the lowest percentage (88.9%) for moxifloxacin. These results suggest that administration of the fluoroquinolones evaluated in this study to treat confirmed or suspected P aeruginosa-associated ulcerative keratitis in dogs is likely to be successful.

COMMENTARY: The results of this study are good news for practitioners who empirically treat cases of canine ulcerative keratitis while waiting for culture and sensitivity results. In human medicine, resistance of P aeruginosa to ophthalmic fluoroquinolones, including some that are resistant to all currently available generations of these drugs, is increasing. However, according to this study performed at Cornell University, fluoroquinolone resistance in canine keratitis cases appears to be low. Granted, resistance patterns in humans can vary geographically and this may also be true for dogs. Regardless, judicious use of these drugs, including dosing at a high enough concentration only as long as needed to eliminate infection, is recommended to help counter resistance.

In vitro fluoroquinolone susceptibility of Pseudomonas aeruginosa isolates from dogs with ulcerative keratitis. Ledbetter EC, Hendricks LM, Riis RC, Scarlett JM. AM J VET RES 68:638-642, 2007.