Epidemiology of emerging methicillin-resistant Staphylococcus aureus (MRSA) in Denmark: a nationwide study in a country with low prevalence of MRSA infection

Abstract

Strict infection control measures introduced during the 1970s have kept the incidence of methicillin-resistant Staphylococcus aureus (MRSA) infections extremely low in Denmark. Nevertheless, similarly to other countries, MRSA infections began to appear in the community in the late 1990s. A nationwide surveillance program has collected and stored all MRSA isolates since 1988 and, since 1999, clinical information has been also recorded. We used this information and isolates in a detailed epidemiological and molecular analysis of the 81 MRSA infections identified in Denmark in 2001. MRSA isolates were characterized by pulsed-field gel electrophoresis (PFGE), spa typing, multilocus sequence typing, and SCCmec typing. Comparison of the 45 community-onset MRSA (CO-MRSA) infections with the 36 hospital-acquired MRSA (HA-MRSA) infections showed several striking contrasts. Most CO-MRSA were recovered from skin and soft tissue infections caused by isolates carrying the Panton-Valentine leucocidin toxin genes, and the majority (84%) of isolates belonged to a single clonal type, ST80-IV, which has been found in the community in other European countries. Clone ST80-IV could be traced in Denmark back to 1993. ST80-IV was rarely found in HA-MRSA infections, which belonged to a large number of clonal types, including some pandemic MRSA clones. The low number of HA-MRSA infections and the diversity of MRSA clones in Danish hospitals may be the result of successful infection control measures that prevent spread of clones in hospitals. The mechanism of spread of the ST80-IV clone in the Danish community is not known, and new control measures are needed to control further spread of this and other CA-MRSA clones.

FIG. 1.

Classification of 81 MRSA infection isolates from Denmark in 2001.

FIG. 2.

Geographic distribution of clone ST80-IV in Denmark in 2001. One case from the island of Bornholm is not marked. H, hospital-acquired MRSA.

FIG. 3.

PFGE of SmaI macrorestriction fragments of representative isolates from the retrospective detection of clone ST80-IV compared with ST80-IV isolates from other countries. Lanes 1, 12, and, 27, molecular size standards (bacteriophage lambda oligomers); lanes 2 and 26, NCTC 8325; lane 3, DEN24329/93; lane 4, DEN1451/94; lane 5, DEN1452/94; lane 6, DEN18851/95; lane 7, DEN22940/95; lane 8, DEN4250/96; lane 9, DEN19957/96; lane 10, DEN11819/97; lane 11, DEN15779/97; lane 13, DEN23612/98; lane 14, DEN16143/98; lane 15, DEN16915/99; lane 16, DEN17475/99; lane 17, DEN2949/01; lane 18, DEN4199/01; lanes 19 and 20, GRE14 and GRE143, respectively (1); lane 21, E31 (34); lanes 22 and 23, HT0401 and HT0490, respectively (37); lanes 24 and 25, 1418/02 and 27/03 (Heck, unpublished).