Methicillin-resistant Staphylococcus aureus at a Veterans Affairs Medical Center (1986-96)

Methicillin-resistant Staphylococcus aureus (MRSA) isolates are endemic at the Veterans Affairs Medical Center in Oklahoma City. Five hundred and four consecutively occurring pathogenic MRSA from 490 in-patients were characterized from 1986-96 in order to better understand their epidemiology. The percentage of S. aureus resistant to methicillin increased steadily for 8 years then unexpectedly began decreasing in 1994. During the study period there was a decrease in nosocomial MRSA from wounds and urines with an increase in community-acquired MRSA from sputa and wounds. However, 72% of all isolates from 1986-96 were nosocomial. MRSA from intensive care unit (ICU) patients were more likely to come from respiratory tract specimens than MRSA from non-ICU patients. During the study period, increases in the proportions of nosocomial MRSA from ICU patients and community acquired MRSA from ICU and non-ICU patients occurred. MRSA with many phenotypes (i.e., 69) were identified, suggesting a divergent (versus univergent) threat to patients. Phenotypic analyses, weekly tabulations, and other evidence suggested that MRSA from 3 of the 69 phenotypes were epidemic strains, occurring around 1989. This 11-year descriptive study revealed the epidemiology of MRSA to be a continuously evolving, complicated multifactoral process.