Concurrent outbreak and pseudooutbreak of Acinetobacter in multiple intensive care units

Background: Acinetobacter baumannii-calcoaceticus complex (ABC) is well known for causing both outbreaks and pseudooutbreaks in intensive care units (ICUs).  We describe the simultaneous occurrence of both in the same facility’s ICUs.

Objective: To inform an infection control response to an apparent outbreak of ABC involving multiple ICUs by determining relatedness of ABC isolates recovered, and evaluating possible seasonal variability in background rates of ABC recovery.

Methods: Clusters of drug-susceptible ABC were noted in a tertiary care military medical center’s neonatal, pediatric and surgical ICUs (NICU, PICU, SICU) from 7/2008-10/2008.  Active surveillance and enhanced infection control precautions were initiated in the ICUs and passive surveillance hospital-wide was undertaken.  40 environmental cultures were obtained from the NICU, SICU, and respiratory therapy.  Available ABC isolates were analyzed by pulsed field gel electrophoresis (PFGE).  ABC rates were determined by microbiology and admissions records from 4/2004 to 4/2009.  Cases were defined as any inpatient with a positive ABC culture.

Results: From 6/2008 to 12/2008, 20 cases (6 NICU, 3 PICU, 6 SICU, 1 MICU, 1 CCU, 3 on medical, surgical and pediatric wards) were identified, and all but one were susceptible to all standard drugs.  All environmental cultures were negative.  Clinical sources were predominantly respiratory (13).  3 patients, all in the NICU, had only surveillance and no clinical cultures positive.  PFGE was performed on 14 cases’ isolates and revealed a single clone for 3 of the 4 NICU isolates tested; all others were unrelated.  Review of the facility’s historical ABC rates revealed higher baseline rates from June-October than November-May (0.35 vs 0.11 /1000 occupied bed days [OBD]).  During the outbreak period the overall rate was higher (0.85- 1.17/1000 OBD) than the median baseline rates (seasonal and monthly) for 4 consecutive months.  However, excluding the clonal NICU cases, the adjusted ABC rate was no higher than baseline for any 2 consecutive months.

Conclusions: In evaluating a potential hospital outbreak of ABC, seasonal variability should be considered by the epidemiologist, and antimicrobial susceptibility patterns may be of little value in predicting clonality.