Identifying conditions for elimination and epidemic potential of methicillin-resistant Staphylococcus aureus in nursing homes.
BACKGROUND
Residents of nursing homes are commonly colonized with methicillin-resistant Staphylococcus aureus (MRSA) but there is a limited understanding of the dynamics and determinants of spread in this setting. To address this gap, we sought to use mathematical modeling to assess the epidemic potential of MRSA in nursing homes and to determine conditions under which non-USA300 and USA300 MRSA could be eliminated or reduced in the facilities.


METHODS
Model parameters were estimated from data generated during a longitudinal study of MRSA in 6 Wisconsin nursing homes. The data included subject colonization status with strain-specific MRSA collected every 3 months for up to 1 year. Deterministic and stochastic co-colonization and single-strain models were developed to describe strain-specific dynamics of MRSA in these facilities. Basic reproduction numbers of strain-independent MRSA, non-USA300 and USA300 MRSA were estimated numerically. The impact of antibiotic use in the past 3 months on the prevalence of strain-specific MRSA and associated basic reproduction numbers were evaluated.


RESULTS
Our models predicted that MRSA would persist in Wisconsin nursing homes, and non-USA300 would remain the dominant circulating strain. MRSA eradication was theoretically achievable by elimination of MRSA-positive admissions over the course of years. Substantial reductions in MRSA prevalence could be attained through marked increase in clearance rates or reduction in MRSA-positive admissions sustained over years. The basic reproduction number of strain-independent MRSA was 0.18 (95 % CI = 0.13-0.23). Recent antibiotic use increased the prevalence of strain-specific MRSA and associated basic reproduction numbers, but was unlikely to lead to an outbreak.


CONCLUSIONS
Based on our model, MRSA elimination from nursing homes, while theoretically possible, was unlikely to be achieved in practice. Decolonization therapy that can sustain higher clearance rates or lower MRSA-positive introductions over years may reduce strain-specific prevalence of MRSA in the facilities, and antibiotic stewardship may contribute to this effort. Large-scale MRSA outbreaks were unlikely in this setting.
Department of Industrial and Systems Engineering, University of Wisconsin-Madison, 3270 Mechanical Engineering Building, 1513 University Avenue, Madison, WI 53706 USA.
Nataliya G
NG
Batina
Department of Medicine, University of Wisconsin-Madison, Madison, WI USA ; William S. Middleton Veterans Affairs Hospital, 2500 Overlook Terrace, B5112E, Madison, WI 53705 USA.
Christopher J
CJ
Crnich
Department of Mathematics, University of Wisconsin-Madison, 617 E B Van Vleck Hall, 480 Lincoln Dr, Madison, WI 53706 USA.
David F
DF
Anderson
Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, 2027 Veterinary Medicine Building, 2015 Linden Dr, Madison, WI 53706 USA.
Dörte
D
Döpfer
Colonization; Conditions for MRSA reduction; Epidemic potential; Non-USA300 MRSA; Nursing homes; USA300 MRSA
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