Molecular epidemiology of methicillin-resistant Staphylococcus aureus in Finland

This study reports the recent trends in the occurrence of methicillin-resis tant Staphylococcus aureus in Finland, with special focus on characterizati on of the strains linked to interhospital epidemics and local outbreaks. Be tween 1981 and 1997, the annual number of methicillin-resistant Staphylococ cus aureus isolations ranged from 89 to 272. Of all blood isolates of Staph ylococcus aureus reported to the National Infectious Disease Register durin g the period 1995-97 (n = 2049), only six were resistant to methicillin. Be tween 1992 and 1997, typing analysis by various methods (i.e., antibiogram, phage typing, ribotyping, and pulsed-field gel electrophoresis) identified 18 different strains capable of causing intrahospital outbreaks or interho spital epidemics. These 18 strains were separated into 13 different ribotyp es and 14 major pulsed-field gel electrophoresis types. Multiresistance was investigated as a possible marker for epidemicity. Eight of the ten interh ospitally spread strains were multiresistant compared to only three of the eight intrahospitally spread outbreak strains. More than one-third of the e pidemic and local outbreak strains were suspected to be of foreign origin. The majority (6 of 10) of the epidemics were localized in southern and west ern Finland, and the largest epidemic, which occurred in the Helsinki metro politan area, involved over 200 persons. Thus far, the epidemics have remai ned primarily intracity problems, and only two strains have become endemic.