Methicillin-resistant Staphylococcus aureus Colonization inthe Long-term Care Setting: Characteristics of Nasal Carriage andTransmission

Methicillin-resistant Staphylococcus aureus Colonization in the Long-term Care Setting: Characteristics of Nasal Carriage and Transmission By Nimalie D. Stone, M.D. Methicillin-resistant Staphylococcus aureus (MRSA) is highly prevalent in long-term care facilities (LTCFs) but its transmission dynamics are not well defined in this setting. Over a 6-month period we classified MRSA nasal carriage into 3 groups: persistent (all cultures positive), intermittent (at least one, but not all cultures positive), and non-carrier (no cultures positive). We plated each nasal swab on MRSA selective media for culture and graded growth on a semi- quantitative scale from 0 (no growth) to 6 (heavy growth).We calculated a growth score for each subject by averaging the growth from all cultures obtained to estimate burden of carriage. We defined MRSA acquisition as an initial negative culture followed by >1 positive culture with no subsequent negative cultures. We collected epidemiologic data for risk factor analysis and typed MRSA isolates by pulsed-field gel electrophoresis (PFGE). Among 412 residents at 3 LTCFs, MRSA prevalence was 59% with distributions of carriage similar at all three facilities: 20% persistent, 39% intermittent, 41% non-carriers. Multivariate analysis showed MRSA carriage was associated with prior history of MRSA culture (OR 3.5, p<0.0001), device use (OR 1.8, p=0.03), and receipt of systemic antibiotics (OR 1.7, p=0.04). The mean growth score of MRSA differed significantly between the persistent and intermittent positive carriers (3.4 vs. 1.2, p<0.0001). Of 254 residents with an initial negative swab, 25 (10%) acquired MRSA over the 6 months; rates were similar at all three LTCFs. Multivariate analysis demonstrated receipt of systemic antibiotics during the study was the only significant risk factor for MRSA acquisition (OR 7.8, p=0.002). MRSA strains from acquisitions were related to those from a roommate by PFGE in 9/25 (36%) cases; 6 of these 9 roommates had persistent carriage. MRSA colonization prevalence was high across three separate VA LTCFs. MRSA acquisition was strongly associated with antibiotic exposure in the LTCF. Roommate sources were often persistent carriers but transmission from roommates accounted for less than half of all MRSA acquisitions.