Nosocomial outbreak of ampicillin resistant Enterococcus faecium: Risk factors for infection and fatal outcome

Abstract

A nosocomial outbreak caused by ampicillin resistant Enterococcus faecium (ARE) was detected at a Norwegian university hospital in January 1995. Prior to this outbreak, ARE were not common in this hospital or other hospitals in Norway. During 1995 and 1996, a total of 149 cases with clinical ARE infection were detected prospectively. A case control study was performed by allocating controls matched for gender, age and ward of admission. Altogether, 123 case control pairs with mean age 70.1 years were included. Isolates from 89 (72.4%) of the cases were identical or related to the defined outbreak strain as determined by pulsed-field gel electrophoresis (PFGE). In 75 of the patients (60.9%), ARE caused urinary tract infection, five (4.1%) had bacteraemia, 33 (26.8%) had wound infection and 10 (8.1%) had other infections. In a logistic regression model for 1:1 matched samples, the following factors were identified as significant risk factors for ARE infection: underlying neurological disease (OR=33.5), prescription of antimicrobial agents for more than 10 days (OR=8.99), prescription of cephalosporins (OR=4.69), underlying gastrointestinal disease (OR=3.36) and length of hospital stay per day (OR=1.04). The intrahospital death rate for the cases was 18.7% compared with 8.9% for the controls, corresponding to an excess mortality attributable to ARE infection of 9.8%. A history of carbapenem prescription was the only independent factor contributing to death (OR=5.64) when comparing ARE patients dying in hospital to those surviving.