Predictors of Clinical Virulence in Community-Onset Methicillin-Resistant Staphylococcus aureus Infections: The Importance of USA300 and Pneumonia

Background. Though USA300 community-onset methicillin-resistant Staphylococcus aureus (CO-MRSA) has emerged as a major public health concern in the United States, its relative virulence is unknown. We sought to evaluate if the USA300 strain of CO-MRSA causes more severe infections than other MRSA (ie, USA100, -500, -800, and others) strains. Methods. An epidemiologic study was conducted from 2000 to 2007 to measure rates of severe infection. A matched case-control study was conducted from 2004 to 2006 to assess the relationship of strain type, syndrome, and severity of infection. Severe illness was defined as CO-MRSA infections with medical intensive care unit (MICU) admission or death within 1 week of admission. Controls were those with CO-MRSA infection without MICU admission. Results. We found an incidence of 75 cases per 100 000 people of CO-MRSA infection in 2000, which increased to a rate of 396 per 100 000 in 2007 (relative risk [RR], 5.3; 95% confidence interval [CI], 4.47-6.27). The incidence of severe infections increased from 5 cases per 100 000 in 2000 to 17 per 100 000 in 2007 (RR, 3.4; 95% CI; 1.67-6.43). USA300 strains were negatively associated with severe clinical courses or death as compared with other MRSA strain types. The highest risk of severe infection was found in those with pulmonary embolic infiltrates and bacteremia in the setting of USA300 infection (odds ratio, 31.41; 95% CI, 6.40-154.23). Conclusions. Our findings suggest that USA300 infections are negatively associated with severe clinical courses, suggesting less virulence than other MRSA strains, except in the setting of pneumonia with septic pulmonary emboli.