Prevalence of USA300 strain type of methicillin-resistant Staphylococcus aureus among patients with nasal colonization identified with active surveillance.
BACKGROUND
USA300 is the most prevalent strain type of community-acquired methicillin-resistant Staphylococcus aureus (MRSA), but the proportion of patients colonized with USA300 strains and their risk for infection are largely unknown.


OBJECTIVE
To determine the prevalence of USA300 strains among MRSA nasal colonizing isolates in our hospital, to identify risks for USA300 nasal colonization, and to determine risks of developing infection for nasally colonized patients.


METHODS
Retrospective cohort study of patients found to be nasally colonized with MRSA through active surveillance at a tertiary care hospital from January 2005 through December 2007. MRSA isolates were identified as USA300 or non-USA300. Risks for colonization and infection were identified with logistic regression.


RESULTS
Among 1,306 MRSA nasal isolates, 307 (24%) were USA300 strains, and this proportion significantly increased over time (17.4% in 2005 and 26.7% in 2007; P = .003). African American race was an independent risk for USA300 nasal colonization (odds ratio [OR], 1.81 [95% confidence ratio {CI}, 1.38-2.38]). Older age (OR, 0.97 [95% CI, 0.96-0.98]) and female sex (OR, 0.74 [95% CI, 0.56-0.97]) decreased risk. Among nasally colonized patients, 238 (18.2%) developed infection. Increased length of stay (OR, 1.03 [95% CI, 1.01-1.06]) independently increased risk for infection among patients nasally colonized with USA300 MRSA, and female sex decreased risk (OR, 0.48 [95% CI, 0.24-0.95]). Increased length of stay (OR, 1.03 [95% CI, 1.02-1.04]) and treatment in an intensive care unit (OR, 1.64 [95% CI, 1.10-2.44]) independently increased risk for infection among patients nasally colonized with non-USA300 MRSA, and female sex decreased risk (OR, 0.67 [95% CI, 0.47-0.94]).


CONCLUSIONS
The proportion of MRSA nasally colonized patients with USA300 strains significantly increased during the study, and risks included African American race. Strain type had no significant effect on the proportion of patients who developed infection, and risk factors for infection were similar.
Department of Medicine, Medical University of South Carolina, Charleston, South Carolina, USA.
Eduardo A F
EA
Freitas
Rebecca M
RM
Harris
Ruth K
RK
Blake
Cassandra D
CD
Salgado
D002353:Carrier State / Q000453:epidemiology* / Q000382:microbiology; D005260:Female; D006785:Hospitals, University / Q000706:statistics & numerical data; D006801:Humans; D016015:Logistic Models; D008297:Male; D055624:Methicillin-Resistant Staphylococcus aureus / Q000145:classification / Q000235:genetics / Q000302:isolation & purification*; D008875:Middle Aged; D009296:Nasal Cavity / Q000382:microbiology*; D011159:Population Surveillance / Q000379:methods*; D015995:Prevalence; D012307:Risk Factors; D013022:South Carolina / Q000453:epidemiology; D013203:Staphylococcal Infections / Q000453:epidemiology* / Q000382:microbiology