Pseudo-Outbreak of Vancomycin Intermediate Staphylococcus aureus

Background: On April 27, 2009, Infection Prevention (IP) at Barnes Jewish Hospital, an academic, tertiary care facility, was notified of a blood culture growing vancomycin-intermediate Staphylococcus aureus (VISA) from a patient in our Neurology/Neurosurgical Intensive Care Unit.  This was the first VISA isolated at this facility.  Six additional VISA isolates from various hospital locations were reported in the following 2 months.

Objective:  Investigate, determine source and prevent the spread of VISA in our facility. 

Methods: The pre-existing Vancomycin Intermediate/Resistant Staphylococcus aureus (VISA/VRSA) policy/procedure was implemented.  It included staff education regarding meticulous hand hygiene, strict contact isolation, limiting staff assigned to care for the patient, a log of all persons who enter the room, surveillance cultures on all patients currently on the unit and weekly surveillance cultures thereafter.  Infection Prevention met with laboratory personnel and learned that a new vancomycin-impregnated agar screen for vancomycin susceptibility had been recently implemented. 

Results: Between May and October 2009, 33 cases of VISA were identified.  24 (73%) were methicillin-resistant S. aureus (MRSA). During this time, hospital-wide monthly VISA rates ranged between 1.7 to 3.7 cases per 10,000 patient days (chi square for trend=1.21, p=0.27).  Four of the first 5 isolates were clonal by repetitive sequence PCR. Two of the clonal isolates were present on admission from outside hospitals. To verify that the testing change was the reason for the emergence of VISA, the lab tested 180 saved MRSA isolates from blood cultures obtained between 2005 and 2007.  13 (7.2%) of these MRSA blood isolates were VISA. Since vancomycin-resistance in S. aureus is mediated by an alternative pathway, and unlikely to arise from VISA isolates, the IP department decided to treat VISA in the same manner as MRSA.  This includes contact precautions on the current and future admissions until clearance of the organism.

Conclusions:  A low level of VISA, as determined by the new screening methods, has been present in the facility and referring hospitals since at least 2005.