Fifteen-year study of the changing epidemiology of methicillin-resistant Staphylococcus aureus

PURPOSE: The study's purpose was to elucidate the evolutionary, microbiologic, and clinical characteristics of methicillin-resistant Staphylococcus aureus ( MRSA) infections. METHODS: MRSA cases from military medical facilities in San Diego, from 1990 to 2004, were evaluated and categorized as community- acquired or nosocomial. Sequence type, staphylococcal chromosomal cassette gene type, and Panton- Valentine leukocidin gene status were determined for a subset of isolates. RESULTS: Over the 15- year period, 1888 cases of MRSA were identified; 65% were community acquired. The incidence ( 155 infections/ 100 000 person- year in 2004) and household- associated cases rapidly increased since 2002. Among persons with community- acquired MRSA, 16% were hospitalized and only 17% were initially given an effective antibiotic. Community- acquired MRSA cases compared with nosocomial MRSA cases were more often soft- tissue and less often urinary, lung, or bloodstream infections ( P <. 001). Patients with community- acquired MRSA were younger ( 22 vs 64 years, P <. 001) and less likely to have concurrent medical conditions ( 9% vs 98%, P <. 001). Clindamycin resistance increased among community- acquired MRSA isolates during 2003 and 2004 compared with previous years ( 79% vs 13%, P <. 001). Genetically, nosocomial MRSA isolates were significantly different than those acquired in the community. Although community- acquired MRSA isolates were initially diverse by 2004, one strain ( staphylococcal chromosomal cassette type IV, sequence type 8, Panton- Valentine leukocidin gene positive) became the predominant isolate. CONCLUSIONS: Community- acquired and intrafamilial MRSA infections have increased rapidly since 2002. Our 15 years of surveillance revealed the emergence of distinct community- acquired MRSA strains that were genetically unrelated to nosocomial MRSA isolates from the same community. (c) 2006 Elsevier Inc. All rights reserved.