The Decline of Methicillin Resistance Among Danish Staphylococcus Aureus Strains

OBJECTIVE: To describe the occurrence and decline of methicillin-resistant Staphylococcus aureus in Denmark from 1966 to 1986, and to illustrate why it has been possible to retain a frequency of only 0.2% MRSA since 1984. DESIGN: A study of antibiotic susceptibility and phage-type of 522,978 S aureus strains isolated from hospitalized patients in Denmark during the years 1960 to 1988 combined with clinical information on patients with methicillin-resistant strains during the years 1986 through 1988. SETTING: All strains and information were collected at the centralized, national laboratory for S aureus phage-typing. PATIENTS: Hospitalized patients with S aureus isolates, and especially patients with methicillin-resistant strains. INTERVENTION: Antibiotic treatment. RESULTS: The frequency of MRSA rose to 15% in the years 1967 through 1971 but decreased to 0.2% in 1984, and has remained so ever since. The increase was due mainly to the spread of a single or a few clones of the phage-type complex 83A. Occurrence of strains of these phage-types declined from 18% in 1969 to 0.6% in 1989. In 1986 through 1988, at least 48% of the MRSA strains were imported by patients from abroad. Cross-infection occurred only in two cases. High awareness and special precautions were taken when MRSA was detected. CONCLUSIONS: MRSA of a single or a few clones spread in Danish hospitals in the years 1967 through 1971. Since 1984, only 0.2% of the Danish S aureus population has been MRSA, and imported MRSA strains have been prevented from spreading.