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Background: The introduction of USA 300, the most common traditional CA-MRSA strain, into a new geographic area is often associated with displacement of endemic CA-MRSA strains. USA 300 has recently emerged as an important cause of serious HCA infections. However, little is known about the clinical consequences of its introduction into the hospital environment.
Objective: To compare the incidence of and identify risk factors for development of MRSA infection in subjects colonized with traditional HCA-MRSA and CA-MRSA strains in a hospital setting.
Methods: A prospective observational cohort study was conducted. Patients with MRSA colonization were divided into three cohorts of strain types—HCA-MRSA, USA 300 CA-MRSA, and Others—based on SCCmec and spa typing, pulsed-field gel electrophoresis (PFGE), and identification of pvl and ACME/arcA genes. The risk of developing MRSA infection and risk factors for MRSA colonization and infection with CA-MRSA vs HCA-MRSA was determined using logistic regression. For all analyses only HCA-MRSA and CA-MRSA cases were included.
Results: A total of 861 subjects with HCA-MRSA and CA-MRSA nasal colonization were analyzed, 678 (78.7%) with HCA-MRSA and 183 (21.3%) with CA-MRSA. African American race (p < .001), male gender (p < .001), urgent admission (p=.03), and HIV/AIDS (p=.003) were independently associated with colonization with USA300 CA-MRSA. Older age (p < .001) and longer length of stay during hospitalization when MRSA nasal colonization detected (LOS) (p = .009) were independently associated with decreased risk.
A total of 86 subjects (10.0%) developed MRSA infection during 6 month follow up. The risk of development of MRSA infection in the CA- vs HCA-MRSA group [15 (8.2%) vs 71 (10.5%), p=.36] and the type of infection was not significantly different on univariate analysis. The time to develop an MRSA infection was likewise not significantly different between the 2 groups (18 vs 13 days, p=.50). However, among subjects who developed infection, a greater proportion of patients colonized with USA 300 had skin and soft tissue infection compared to patients colonized with HCA-MRSA [3 (20%) vs 3 (4.2%); OR 5.67 (95% CI, 1.02-31.5)].
Development of MRSA infection was independently associated with underlying immunosuppression from all causes (p=.02), cerebrovascular disease (p=.01), solid organ transplant surgery (p=.01), foley catheter use (p=.03), increased LOS (p < .001), and vancomycin use (p <.001). Use of trimethoprim-sulfamethoxazole (p <.001) and topical mupirocin (p=.02) was associated with decreased risk.
Conclusions: Colonization with USA 300 CA-MRSA was associated with African American race, male gender, urgent admission, HIV/AIDS, younger age, and shorter LOS, but not a higher risk of infection. Preventing colonization and infection with either HCA or CA-MRSA is important.