Emergence and spread of community-associated methicillin-resistant Staphylococcus aureus in rural Wisconsin, 1989 to 1999

We investigated the emergence and spread of community-associated strains of methicillin-resistant Staphylococcus aureus (CA-MRSA) in central and northern Wisconsin by determining the temporal and clonal relationships and geographic expansion among 581 of 956 clinical isolates of MRSA collected between 1989 and 1999. Based on EcoRI plasmid profiles (PP), two types, PP-11 and PP-13, were highly stable over time and were consistently associated with multidrug-sensitive strains recovered from outpatients treated at Native American community clinics. Pulsed-field gel electrophoresis (PFGE) yielded six major clonal groups (MCGs) and 19 minor clonal groups. The six MCGs represented 82% of the isolates. All strains with either PP-11 or -13 were present in MCG-2. Eighty-nine percent of the isolates in MCG-2 originated from Native American clinics, and 90% belonged to two PFGE types (19 and 20), the types associated with an outbreak of MRSA in a Native American community in 1992. MCG-2 isolates were multidrug sensitive, harbored type IVa staphylococcal cassette chromosome mec, and were very closely related by PFGE to the Midwestern CA-MRSA strain MW2. MCG-2 strains were mostly obtained from skin infections and affected patients with a mean age of 24 (+/-18.0) years. MCG-2 strains spread to four additional Native American communities and 20 other communities. Our findings suggest that CA-MRSA in Wisconsin likely originated in Native American communities in the early 1990s and since has become widespread throughout the state. Two early CA-MRSA strains (WI-33 and WI-34) in Wisconsin represent progenitors of the MW2 strain, based on their almost indistinguishable genotypic characteristics.