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    Study explores methicillin-resistant Staph colonization

    Chicago—Results from a retrospective study provide insight on the incidence of colonization with methicillin-resistant Staphylococcus spp among patients seeking refractive surgery and the potentially limited efficacy of standard antibiotic treatment regimens for eradicating these organisms, said Edward W. Trudo Jr., MD.

    The study included data collected as part of a U.S. Army performance improvement process in which patients seeking refractive surgery who were considered at high risk for colonization with methicillin-resistant Staphylococcus (methicillin-resistant S aureus or S epidermidis, MRSA or MRSE) had cultures taken from the nares and eyes. The features that defined “high risk” included living in crowded conditions (military barracks, dormitories), participation in contact sports, frequent use of gym/sports facilities, being a health-care worker or having a health-care worker in the family, and previous MRSA soft-tissue infection.

    An initial analysis of the first 36 patients, reported in 2011, showed positive cultures in 6 (16.7%) individuals. In an updated series including 99 patients, 7 (7.1%) were found to have positive cultures. Of the latter patients, four had positive cultures for MRSA and three for MRSE. All seven patients had positive nasal cultures, while only one patient had a positive ocular culture.

    “MRSA infection is a risk after all ocular surgery, and in the 2007 ASCRS survey, MRSA had emerged as the number one pathogen in infections after laser vision correction,” said Dr. Trudo, Director, U.S. Army Refractive Surgery Research Center, Fort Belvoir, VA.

    “The rate of positive cultures for methicillin-resistant Staph in our high-risk population exceeds the initial reported rates of 1.5% to 2.5% among healthy individuals in the literature, but we know that the percentage is rising,” Dr. Trudo said. “We believe additional study is needed to investigate MRSA/MRSE risk and preoperative treatment prospectively. However, routinely [performing cultures in] all patients with risk factors may not be warranted, and any screening that is undertaken might be done by culturing the nares only as ocular culture [results] may underestimate risk.”

    All seven of the patients with positive cultures had been contact lens wearers, although in further analyses investigating risk factors for colonization, there was no association between having a positive culture and the interval from discontinuing contact lens wear to culture time. Age, race, gender, use of oral contraceptives, gymnasium attendance (measured in hours), previous history of MRSA treatment, or health-care worker occupation/exposure to a health-care worker (spouse) also did not predict a positive culture. Aside from contact lens wear, features that were associated with an increased risk of methicillin-resistant Staph colonization included living in a dormitory, participation in athletics, and culture testing during the spring.

    “In our series, the incidence of colonization was higher among the cadets living in the dormitory than in the active duty military living in barracks, and all six patients with positive cultures in our initial series of 36 patients were tested in the spring,” said Dr. Trudo.

    “While athletic participation was also a risk factor in our series, we found that the incidence of positive cultures was lower among individuals on the NCAA Division 1 teams compared with those playing club sports. We believe the difference between these groups may be explained by more education about and adherence to preventive hygiene measures among the NCAA athletes through the initiative in coordination with the CDC,” noted Dr. Trudo.

    Guided by data from a National Surveillance Efficacy study, all patients with positive culture results were treated with intranasal mupirocin twice daily for 5 days and topical polymixin B sulfate/trimethoprim ophthalmic solution 5 times a day for 14 days. Cultures were repeated 1 to 2 weeks after the end of treatment, and then only one patient colonized with MRSE remained culture positive. On further work-up, chronic sinusitis was diagnosed in the latter individual and cultures became negative after treatment with an oral antibiotic for the sinus infection.

    Findings from antimicrobial susceptibility testing for the seven isolates from the culture-positive patients showed many cases of resistance to antibiotics commonly used in ophthalmology, including ciprofloxacin, levofloxacin, moxifloxacin, and trimethoprim/sulfamethoxazole. Notably, the MRSE isolate from the patient who remained culture-positive after initial treatment with intranasal mupirocin and polymixin B/trimethoprim drops was sensitive to trimethoprim/sulfamethoxazole.

    “This case raises the question of whether in vitro susceptibility results for trimethoprim/sulfamethoxazole are representative for polymixin B/sulfamethoxazole,” said Dr. Trudo.

    He added that findings from a literature review and his personal experience treating patients with bacterial keratitis after refractive surgery or trauma further highlighted the potential of the trimethoprim/sulfamethoxazole in vitro cultures to overestimate clinical efficacy of polymixin B/sulfamethoxazole. Therefore, routine cultures preoperatively is not a recommended practice, said Dr. Trudo, noting that his future goals are to clarify MRS spp carrier risk factors and better define a preoperative and postoperative regimen that has the highest efficacy.

    Dr. Trudo has no financial interest in the subject matter presented. The views presented are his own and not those of the Department of Defense, U.S. Army, or the U.S. Military Academy.

    Dr. Trudo acknowledges Susan Gromacki, OD, as a co-investigator in the performance improvement project and the retrospective review.

    For more articles in this issue of Ophthalmology Times eReport, click here.

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