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447 Not All Staphylococcus aureus Are Created Equal: Risk Factors for Staphylococcus aureus Surgical Site Infection during an Outbreak in Cardiovascular Surgery Patients

Sunday, April 3, 2011
Trinity Ballroom (Hilton Anatole)
Manal A. Tadros, MBBS, PhD , University of Toronto, Toronto, ON, Canada
Victoria R. Williams, BSc, BASc, CIC , Sunybrook Health Sciences Centre, Toronto, ON, Canada
Suzanne Plourde, RN, BScN, MPH , Sunybrook Health Sciences Centre, Toronto, ON, Canada
Lisa Louie, ART , Sunybrook Health Sciences Centre, Toronto, ON, Canada
Sandra Callery, RN, MHSc, CIC , Sunybrook Health Sciences Centre, Toronto, ON, Canada
Andrew Simor, MD, FRCPC , Sunybrook Health Sciences Centre, Toronto, ON, Canada
Mary Vearncombe, MD, FRCPC , Sunybrook Health Sciences Centre, Toronto, ON, Canada
Background:

Staphylococcus aureus is the most common cause of surgical site infections (SSIs) accounting for at least 20% of cases. An outbreak of S. aureus SSIs (MSSA and MRSA) occurred in patients undergoing cardiovascular surgery at our tertiary-care teaching hospital between January 2009 and March 2010.

Objective:

To determine risk factors associated with S. aureus SSIs in the setting of an outbreak.

Methods:

 SSIs caused by S. aureus were identified by prospective infection surveillance.  A case control study was done by retrospective chart review to analyze risk factors associated with S. aureus SSI by comparing  38 cases with age, sex and procedure matched controls who underwent surgery during the same time period and did not develop a SSI, or developed SSI due to an organism other than S. aureus. Two  controls were selected for each case.  Cases and controls were compared using univariate and multivariate logistic regression and p<0.05was considered significant.  Isolates from patients and  epidemiologically linked healthcare workers (HCWs) were characterized by pulsed field gel electrophoresis (PFGE).  

Results:

During the outbreak, we identified transmission of 3 clones of S. aureus by PFGE: 8 patients had SSI due to CVMSSA-1, 6 had CVMSSA-2 and 9 had CMRSA-2r. 13 patients had unrelated S. aureus SSI. Two patients with S. aureus SSI had cultures processed by outside laboratories, and their isolates were not available for typing. Univariate analysis of risk factors associated with S. aureus SSIs included prior cardiac surgery (Odds Ratio [OR]  5.61; 95% Confidence Interval [CI] 1.03-30.39), intraoperative (OR,1.25; 95% CI 1.02-1.53) and postoperative blood transfusion (OR,1.39; 95% CI 1.08-1.80), postoperative hemorrhage (OR, 4.17; 95% CI 1.38-12.55), re-sternotomy (OR, 14.32; 95% CI 1.66-123.96) and longer procedure time (OR, 1.45; 95% CI 1.07-1.96). In multivariate analysis the only risk factors that remained significant were prior cardiac surgery (OR, 7.41; 95% CI 1.05-52.16), procedure duration (OR, 1.49; 95% CI 1.00-2.21) and re-sternotomy (OR, 11.34; 95% CI 1.00-128.29). The mean ICU stay was 17.28 days for cases and 2.87 days for controls, p=0.118. The mean postoperative stay was 30.89 days for cases and 9.68 days for controls, p=0.015. The average deep sternal SSI rate during the outbreak increased to 2.036% compared to our baseline of 0.86%. 21 HCWs identified through nasal screening were carriers of outbreak strains of S .aureus: 17 were colonized with CVMSSA-1, 3 were colonized with CVMSSA -2, and 1 was colonized with CVMRSA-2r. 

Conclusions:

This outbreak occurred due to transmission of S. aureus from HCWs to patients.  Some operational and patient related factors may increase the risk of S. aureus SSI. The observed prevalence of CVMSSA-1 during this outbreak warrants further study.