Infectious keratitis is more common in patients who live in suburban areas and during non-winter months, according to a four-year study of keratitis patients in eastern Pennsylvania. This research, published in the March 2015 Cornea, comes on the heels of several studies conducted outside the United States that suggest geographical and seasonal variations play a role in the spread of causative organisms.1 

Researchers from the Wills Eye Hospital in Philadelphia and several universities in Pennsylvania, California and Rhode Island examined electronic health records of 311 patients representing 323 instances of infectious keratitis to identify type of infection, patient location and treatment method. 

Incidence of keratitis was highest in patients who live in large fringe (i.e., suburban) metro areas, followed by those who live in large central metro (i.e., city) areas, according to the investigators. Generally, winter and fall were the seasons with the lowest average occurrence of infections across all classes of organisms; however, the study indicated overall seasonal variation in culture-positive cases was not statistically significant. Bacterial infections were most common in the spring.

The infections were classified as either bacterial, fungal, Acanthamoeba, a combination of bacterial and fungal, or undetermined, with respect to both clinical impression and culture- or pathology-positive testing. Pseudomonas aeruginosa was found to be the most common bacteria and Fusarium the most common fungus in contact lens-related cases, while Staphylococcus aureus was identified as the most common bacteria and Candida the most common fungus in non-contact lens-related cases. 

As part of the study, researchers also evaluated bacterial and fungal susceptibility to select antimicrobials. All three fluoroquinolones tested—moxifloxacin, ciprofloxacin and gatifloxacin—were effective against P. aeruginosa, but only ciprofloxacin was effective against methicillin-resistant S. aureus isolates. This bacterial resistance, the researchers say, is concerning, given that fluoroquinolones are increasingly used as first-line monotherapy. None of the organisms tested were resistant to tobramycin, gentamicin, vancomycin or voriconazole.  

1. Ni N, Nam EM, Hammersmith KM, et al. Seasonal, Geographic and Antimicrobial Resistance Patterns in Microbial Keratitis: 4-Year Experience in Eastern Pennsylvania. Cornea 2015;34(3):296-302.