Is MRSA admission bacteraemia community-acquired? A case control study

Abstract

To compare characteristics of methicillin resistant Staphylococcus aureus (MRSA) and methicillin susceptible S. aureus (MSSA) bacteraemia detected on admission to a UK hospital and to determine whether these organisms are community-acquired. Consecutive cases of MRSA bacteraemia admitted to general medicine between 2003 and 2006 were identified and compared to MSSA age-matched and unmatched controls (35, 35 and 34 patients, respectively). Demographics, MRSA risk factors, previous health-care contact and clinical presentation were compared using patient notes. Multi-locus sequence typing was performed. 34/35 strains of admission MRSA bacteraemia were the health-care associated Sequence Types (ST)-22 (77%) or ST-36 (21%), whereas 20 different MSSA strains were identified. No MRSA cases fitted the CDC definition of community-acquired MRSA. Compatible with health-care associated acquisition, after matching for age MRSA cases had significantly higher levels of previous hospital exposure than MSSA controls, and more co-morbidities. Notably, 63% of MRSA cases were admitted from their own home, as opposed to secondary care facilities. Clinical presentation of MRSA and MSSA bacteraemias was similar. MRSA strains associated with health-care were responsible for almost all cases of MRSA bacteraemia on admission to hospital during the period studied. Despite this the majority of cases with MRSA admission bacteraemia were admitted from their own homes. Further research is needed into the determinants of MRSA bacteraemia among patients outside hospital.