Methicillin-Resistant Staphylococcus aureus Carriage in a Long-Term Care Facility: Stability after One Year
Author Block: Nimalie D. Stone, MD1, Donna R. Lewis, MSN2, Zobair Nagamia, MD2, Ken Lowery, MS2, Judy Clairmont, BA2, Ted M. Johnson, II, MD,MPH2, John E. McGowan, Jr., MD1, Fred C. Tenover, PhD3, John A. Jernigan, MD, MS3, Chesley L. Richards, MD, MPH3, Robert P. Gaynes, MD3.
1Emory University, Atlanta, GA, USA, 2Atlanta VAMC, Atlanta, GA, USA, 3Centers for Disease Control and Prevention, Atlanta, GA, USA.
Abstract:
Background: Current surveillance and control strategies for Methicillin-resistant Staphylococcus aureus (MRSA) often rely on a single culture from the anterior nares to define carriage, even though the sensitivity of a single culture is well below 100%.
Objective: We describe the stability of MRSA nasal carriage in a cohort of 50 residents of a VA long-term care facility (LTCF) using longitudinal cultures collected during two distinct time periods, one year apart.
Methods: We defined MRSA carrier status based on weekly nares cultures. MRSA carriage was classified as stably positive (SP; every culture positive), intermittently positive (IP; at least one, but not every culture positive), and stably negative (SN; no positive cultures). We also graded the amount of growth in each culture on a semi-quantitative scale from 0 (no growth) to 6 (heavy growth). We compared the MRSA carrier status during two periods (T1 and T2) for the 50 subjects using 8 cultures obtained from November through December 2005 (T1) and 6 cultures obtained over a 4 month period one year later (T2). MRSA strains were typed using multiple-locus variable-number tandem repeat analysis and pulsed-field gel electrophoresis (PFGE).
Results: During T1, 15 patients showed SP MRSA carriage (mean growth 3.8, range 1.6-5), 11 patients were IP (mean growth 0.74, range 0.17-3.1), and 24 patients were SN (mean growth 0). After one year, the mean growth scores for isolates from SP and IP patients did not vary significantly between T1 and T2 (mean difference -0.8, [95%CI: -0.13-1.7], and -0.6 [95%CI: -1.7-0.43], p=0.09 and 0.21, respectively, [paired t-test]). Of 15 patients that were SP in T1, 2 became SN, 2 were IP, and the rest remained SP one year later. Among the 11 patients who were IP in T1, 4 patients shifted in T2 to SP, 5 became SN, and 2 remained IP. Of 24 patients who were SN in T1, 5 were IP, 2 were SP, and 17 remained SN in T2. The 7 subjects who originally were MRSA carriers and subsequently became SN had a significantly lower mean growth score than the 19 who remained MRSA carriers (1.1 vs. 3.0, p=0.02, [t-test]). Preliminary molecular typing demonstrated that a single strain, the majority of which were either USA100 or USA500 PFGE types, colonized 96% (25/26) of the MRSA carriers in T1.
Conclusions: Overall, MRSA carriage among these 50 patients remained 52% over one year. Carrier status did not change appreciably in the stable cohorts. Among those IP in T1, there were equal shifts to either stable colonization or loss of carriage. The value of a single swab for determining persistent MRSA carriage is likely inadequate, and a semi-quantitative scale of bacterial burden may better predict stability of MRSA colonization in LTCF patients.