Nationwide trends in molecular epidemiology of methicillin-resistant Staphylococcus aureus, Finland, 1997–2004

BACKGROUND: In Finland, the annual number of MRSA notifications to the National Infectious Disease Register (NIDR) has constantly increased since 1995, and molecular typing has revealed numerous outbreak isolates of MRSA. We analyzed the data on MRSA notifications of the NIDR, and MRSA isolates were identified mainly by pulsed-field gel electrophoresis (PFGE) at the National Reference Laboratory (NRL) in Finland during 1997–2004. One isolate representative of each major PFGE type was further characterized by multilocus sequence (MLST)-, staphylococcal cassette chromosome mec (SCCmec)-, and Panton-Valentine leukocidin (PVL)-typing. RESULTS: The annual number of MRSA notifications to the NIDR rose over ten-fold, from 120 in 1997 to 1458 in 2004, and the proportion of MRSA among S. aureus blood isolates tripled, from <1% during 1997–2003 to 2.8% in 2004. During the same period of time, 253 different strains among 4091 MRSA isolates were identified by PFGE: 215 were sporadic and 38 outbreak/epidemic strains, including 24 new strains. Two epidemic strains resembling internationally recognized MRSA clones accounted for most of the increase: FIN-16 (ST125:IA) from <1% in 1997 to 25% in 2004, and FIN-21 (ST228:I) from 6% in 2002 to 28% in 2004. Half of the ten most common strains carried SCCmec IV or V. CONCLUSION: The predominant MRSA strains seem to change over time, which encourages us to continue implementing active control measures with each new MRSA case.