Methicillin‐Resistant Staphylococcus aureus (MRSA) Carriage in 10 Nursing Homes in Orange County, California
Nursing home residents have multiple risk factors for acquiring methicillin‐resistant Staphylococcus aureus (MRSA), including diabetes, hemodialysis, frequent hospitalization, and sharing rooms and common areas.1,2 Previous studies showed MRSA prevalence ranged 5%–40%, but few studied entire regions or explained this variation.1,3‐6 We measured MRSA prevalence among nursing homes within 1 county and identified factors predicting carriage.
We measured MRSA carriage in residents of 10 nursing homes in Orange County, California, from October 2008 through November 2009. At each facility, we obtained nares swab samples from 100 residents during point prevalence screening and 50 consecutive residents on admission. The Institutional Review Board of the University of California Regents approved this study.
Bilateral nares swab samples (BD Culture Swabs; Fisher Scientific) were cultured for MRSA. Isolates were tested for susceptibility to oxacillin, vancomycin, linezolid, quinupristin‐dalfopristin, rifampin, tetracycline, trimethoprim‐sulfamethoxazole, gentamicin, clindamycin, erythromycin, levofloxacin, and mupirocin.
Nursing home characteristics were derived from public sources, including the proportion of residents who were male, less than 65 years old, of Hispanic ethnicity, or Medicaid‐insured.7 For each swabbed resident, we recorded the nursing home day of swab collection, whether residents shared a room, and whether residents had a history of MRSA.
We tested the correlation between MRSA admission and point prevalence using the Pearson coefficient. We performed paired t tests comparing mean MRSA admission in relation to point prevalence, length of stay for MRSA‐positive residents compared with length of stay for MRSA‐negative residents, and facility proportions of isolates nonsusceptible to specific antibiotics at admission and corresponding proportions at point prevalence. We tested associations of resident and facility‐level characteristics with individual MRSA carriage at point prevalence. We performed χ2 tests on several variables: admission prevalence, annual admissions, residence in shared rooms, MRSA history, nursing home day of swab collection, and the proportion of residents who were male, less than 65 years old, nonwhite, Hispanic, Medicaid‐insured, or resident for more than 3 months. Variables with
We obtained 500 admission and 1,000 point prevalence nares swab samples from 10 nursing homes. Facility characteristics, including MRSA admission and point prevalence, varied substantially (Table 1). Overall, MRSA admission prevalence correlated well with MRSA point prevalence (coefficient, 0.6). However, even when facilities had similar admission prevalence, MRSA point prevalence differed significantly (eg, nursing home 3 vs nursing home 4: 22% vs 42%;
|Variable||Nursing home, by identifier|
|No. of beds||145||24||198||255||80||138||99||182||99||143|
|No. of annual admissions||1803||392||1071||443||393||323||350||759||390||723|
|LOS, mean, days||387||29||554||429||292||678||689||277||331||362|
|Residents with characteristic, %|
|Age <65 y||41||4||2||62||0||39||0||1||27||40|
|History of MRSA on admission||12||9||12||15||18||3||10||16||26||16|
|LOS >3 mo||87||97||90||91||92||77||87||54||82||88|
|MRSA admission prevalence, %||8||11||12||12||13||22||21||25||29||31|
|MRSA point prevalence, %||30||7||22||42||25||30||16||39||44||52|
|Point prevalence − admission prevalence, %||22||−4||10||30||12||8||−5||14||15||21|
Across facilities, median proportions of isolates nonsusceptible to antibiotics were as follows: gentamicin, 11% (facility range, 0–35%); vancomycin, 0%; linezolid, 0% (0–1%); quinupristin‐dalfopristin, 0% (0–1%); mupirocin, 10% (0–17%); clindamycin, 74% (38%–83%); erythromycin, 95% (84%–100%); levofloxacin, 98% (94%–100%); rifampin, 0% (0–14%); tetracycline, 3% (0–17%); and trimethoprim‐sulfamethoxazole, 3% (0–17%). Paired t tests revealed no significant differences between mean facility proportions of nonsusceptible isolates at admission versus point prevalence.
In multivariate models, predictors of MRSA carriage at point prevalence included MRSA history (odds ratio [OR], 2.7;
MRSA carriage varied substantially across 10 nursing homes. Overall point prevalence was 31% (range, 7%–52%) versus 6% in hospitals and 9%–24% in intensive care units.8,9 Although variability arose partly from differences in MRSA admission prevalence, evidence of transmission remained. Often, nursing homes with similar admission prevalence differed in MRSA point prevalence, suggesting some facilities cannot contain MRSA from spreading.
Overall, MRSA point prevalence was 67% higher than admission prevalence, which was not attributable to differential length of stay for MRSA‐positive versus MRSA‐negative residents. MRSA acquisition may relate to congregating in common areas or having roommates. Additionally, nursing homes lack standard contact isolation policies, and much variability exists.10 More research is needed to understand whether specific infection control and/or cleaning policies affect transmission.
Our study provides insight into risk factors associated with MRSA carriage. Residence in nursing homes with high MRSA importation predicted MRSA carriage at point prevalence, even after accounting for an individual’s MRSA history. Plausibly, living in nursing homes with many MRSA carriers increases individuals’ chances of exposure. MRSA carriage was also associated with residence in facilities with high proportions of Medicaid‐insured residents. This predictor was interchangeable with proportions of nonwhite and Hispanic residents. Thus, race may be a proxy for economic disadvantage or residence in resource‐poor facilities. Limited resources may impact patient care and cleaning staff ratios, availability of single rooms, and cleaning and infection control practices. Nevertheless, we cannot exclude the possibility that cultural factors may affect transmission.
Limitations include small sample size and point prevalence design, reflecting MRSA carriage at one time. We did not culture samples from multiple sites for MRSA, but nares screening detects the majority of carriers. We primarily used facility‐level risk factors and did not measure several patient‐specific factors, including wounds, devices, and antibiotic use.
MRSA carriage in 10 Orange County nursing homes varied greatly, reaching 50% in some. MRSA burden was associated with importation, but transmission was evident and may relate to facility resources. More research is needed to evaluate the contribution of nursing homes to regional MRSA transmission and ways to adapt infection control to this unique setting.
We thank the nursing homes that participated in this study.
Financial support. Agency for Healthcare Research and Quality (grant HHSA29020050033I‐TO9 to S.S.H.).
Potential conflicts of interest. All authors report no conflicts of interest relevant to this article.
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