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71 Impact of Methicillin Sensitive Staphylococcus aureus (MSSA) Carriage on Methicillin Resistant Staphylococcus aureus (MRSA) Acquisition

Friday, March 19, 2010: 10:45 AM
Centennial I-II (Hyatt Regency Atlanta)
Susan S. Huang, MD, MPH , Division of Infectious Diseases and Health Policy Research Institute, University of California Irvine School of Medicine, Orange, CA
Sheryl Rifas-Shiman, MPH , Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, Boston, MA
Hilary Placzek, MPH , Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, Boston, MA
Katherine Lave, MS , Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, Boston, MA
Ken Kleinman, PhD , Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, Boston, MA
Richard Platt, MD, MS , Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, Boston, MA
Background:

The protective value of carriage with sensitive pathogens in preventing acquisition of resistant pathogens is not well studied. As decolonization regimens become more commonplace, it is important to know whether collateral eradication of sensitive strains may inadvertently increase risk for acquiring resistant strains, particularly in high prevalence endemic settings.

 Objective:

We evaluated the impact of MSSA and vancomycin sensitive enterococcus (VSE) carriage on MRSA and vancomycin resistant enterococcus (VRE) acquisition, respectively, when controlling for other risk factors.

 Methods:

We conducted a retrospective cohort study in ten 10-bed ICUs in a tertiary academic medical center in Boston performing routine admission and weekly bilateral nares and rectal screens for MRSA and VRE, respectively. For the MRSA analysis, patients were identified who had a negative nares screen and no prior history of MRSA. We evaluated predictors for MRSA acquisition defined as a subsequent MRSA-positive clinical or screening culture compared to those with a subsequent MRSA-negative nares screen within the same hospitalization. Full text medical records were reviewed for a) the presence of MSSA on the initial MRSA-negative nares screen, b) age, c) gender, d) comorbidities, e) medical lines and devices, f) recent procedures, g) receipt of antibiotics, h) low albumin, and i) colonization pressure defined as the cumulative number of same-ward MRSA+ patient-days to which a patient is exposed between the initial MRSA-negative nares swab and subsequent negative screen or positive culture/screen.  Bivariate and multivariate models were used to assess predictors of MRSA acquisition. A similar assessment was performed for VRE.

 Results:

In multivariate models, we found that carriage of MSSA was significantly protective of subsequent MRSA acquisition (OR = 0.6, p=0.03), even when controlling for comorbidities and other risk factors. Other predictors of MRSA acquisition included mechanical ventilation (OR = 6.4, p<0.001) and receipt of fluoroquinolones (OR = 2.4, p<0.001). In contrast, carriage with VSE did not protect against VRE acquisition, which was instead predicted by hemodialysis (OR = 2.2, p = 0.03), wounds (OR = 2.1, p = 0.006), rash (OR=1.9, p=0.05), intubation (OR=2.3, p=0.04), low albumin (OR=1.9, p=0.02), receipt of fluoroquinolones (OR = 2.5, p<0.001), and receipt of 3rd gen cephalosporins (OR=2.6, p<0.001).

 Conclusions:

We found that MSSA nasal carriage resulted in a 50% reduced risk of MRSA acquisition in ICU patients.  Use of universal decolonization regimens in ICU or other hospital settings should studied to ensure that decolonization of MSSA carriers does not increase the risk of acquiring MRSA in either current or later healthcare settings (e.g. readmission, nursing homes) where endemic MRSA prevalence may be high.