Outbreak of methicillin-resistant Staphylococcus aureus (MRSA) USA300 skin and soft tissue infections among well newborns and post-partum women

Background: MRSA is a common cause of skin and soft tissue infections (SSTI) in the US and has been implicated in outbreaks among newborns and post-partum women (PPW).  In November 2008, Beth Israel Deaconess Medical Center (BIDMC) began receiving reports of PPW and well newborns with MRSA mastitis and diaper-area pustulosis.  CDC assistance was requested to help identify the source of infections and develop prevention strategies.

Objective: Assess community and healthcare risk factors for MRSA SSTI among PPW who delivered at BIDMC and their newborns.

Methods: PPW-newborn pairs with a delivery date at BIDMC from October 2008 to March 2009 were enrolled in a case-control study.  Cases were pairs in which one or both had a culture-confirmed MRSA SSTI within 60 days of delivery.  Controls were selected from pairs in whom neither developed an SSTI.  Data on community risk factors for MRSA and pregnancy, delivery, and newborn characteristics were collected by chart review and patient interviews.  Odds ratios and 95% confidence intervals were calculated using univariate logistic regression.  BIDMC staff (n=403) were screened for nasal and perirectal MRSA colonization. Clinical and colonization isolates were characterized using pulsed-field gel electrophoresis (PFGE).

Results: Forty MRSA infections were identified among 35 case-pairs during the study period.  Risk factors for newborns included peak total bilirubin >12mg/dL (OR=6.19, 1.78-21.67), male sex (OR=3.65, 1.16-11.51), and maternal parity (OR=0.36, 0.15-0.83).  The only risk factor for PPW was breast pump use while hospitalized (OR=3.57, 1.27-10.02), although no cultured breast pumps yielded MRSA.  No other healthcare exposures were associated with infection.  Community exposures were not associated with infection for newborns or PPW and no clustering by place of residence was observed.  All isolates with PFGE results available (n=28) were pattern USA300-0114.  Two employees were colonized with USA300-0114, but both had minimal patient contact on the affected units.  After implementation of universal gloving and dedicated equipment in the nursery, umbilical cord triple dye application, maternal chlorhexidine (CHX) showers, and bathing newborns with CHX wipes after delivery and before discharge, the frequency of infections fell sharply in April 2009.  The outbreak ended by October 2009.

Conclusions: Despite the lack of a single healthcare exposure that could account for the outbreak, the clustering of cases by delivery date, geographic distribution of cases, lack of community risk factors, and termination of the outbreak after hospital interventions suggest that most MRSA SSTI were healthcare-acquired.  Enhanced infection control efforts beyond Standard and Transmission-Based Precautions may be necessary in nursery and L&D settings to prevent MRSA transmission during outbreaks.