Objective
Methicillin-resistant Staphylococcus aureus infection is associated with morbidity in the neonatal intensive care unit. The purpose of this study was to determine the relationship between preterm maternal methicillin-resistant S aureus colonization and subsequent colonization and infection in premature neonates.
Study Design
We conducted a prospective cohort study of 422 women admitted for preterm delivery. Methicillin-resistant S aureus cultures were collected from mothers and their neonates admitted to neonatal intensive care unit. We determined the proportion of women and neonates colonized with methicillin-resistant S aureus and examined possible factors associated with colonization and infection.
Results
Fifteen of 422 (3.6%) women were found to be colonized with methicillin-resistant S aureus . Thirteen of 212 (6.1%) neonates admitted to neonatal intensive care unit were methicillin-resistant S aureus colonized and 3 of 13 (23.1%) developed a methicillin-resistant S aureus infection. We identified 1 methicillin-resistant S aureus colonized maternal-neonatal pair. The infant became methicillin-resistant S aureus positive 30 days after admission and did not develop a methicillin-resistant S aureus infection.
Conclusion
These findings suggest that maternal methicillin-resistant S aureus colonization is not a significant risk factor for vertical transmission of neonatal methicillin-resistant S aureus colonization.
Methicillin-resistant Staphylococcus aureus (MRSA) is now the most common nosocomial pathogen worldwide, accounting for 19,000 deaths annually. In the past decade, MRSA has been recognized as an important infectious agent in obstetrics and gynecology (OB-GYN) as well as in neonatology. OB-GYN-related MRSA infections include but are not limited to skin and soft tissue infections (SSTIs), mastitis, wound infections, and vulvar abscesses. Newborn and neonatal morbidity, such as increased length of stay and cost of care, has been associated with MRSA SSTIs, pneumonia, osteomyelitis, sepsis, endocarditis, and lymphadenitis. The source of MRSA-colonization and subsequent risk of MRSA infection among neonates in the neonatal intensive care unit (NICU) has not been fully described. Suspected routes of transmission of the organism to the neonate include: contaminated hands and clothing of health care workers, parents and family members, equipment, and breast milk, or through vertical transmission. Reports of NICU MRSA outbreaks and infections raise the concern for containment and isolation of affected neonates.
In the past, our NICU has experienced outbreaks of MRSA colonization and infection among neonates. In response to this, an active surveillance program was initiated in 2001 to identify neonates colonized with MRSA. Neonates were screened with nares cultures on admission and weekly thereafter as well as more frequently during periods of high incidence of MRSA. MRSA colonized neonates were placed on contact precautions to limit horizontal spread.
On review of the results of our active surveillance program from 2004-2008, we found that 50 of 2757 (1.8%) neonates were MRSA colonized and 16 of 50 (32%) of these colonized neonates went on to have MRSA infection develop. Neonatal MRSA acquisition was thought to be secondary to transfer from contact with transiently colonized healthcare workers or the healthcare environment. However, we noted that 15 of 50 (30%) of neonates were positive for MRSA within 7 days of admission to the NICU, thus we considered the possibility of vertical transmission from mother to neonate. Using our retrospective data, we designed a prospective study to assess the possibility of maternal vertical transmission of MRSA as a possible mechanism for neonatal MRSA colonization. The objectives of this prospective study were (1) to determine the proportion of women admitted to labor and delivery at risk for preterm delivery (ie, with diagnoses of preterm labor, preterm premature rupture of membranes, and/or preterm iatrogenic delivery) who are colonized with MRSA, (2) identify risk factors for MRSA colonization among these women, and (3) determine whether vertical transmission is a plausible cause of preterm neonatal MRSA colonization.
Materials and Methods
Study site
The Medical University of South Carolina (MUSC) is a 700-bed tertiary care, academic medical center located in Charleston, SC. The Obstetrics and Gynecology ward of the hospital is a 25-bed unit that serves as the referral center for 7 surrounding counties. The NICU at the study hospital is a 38-bed level III facility that averages 2800 admissions per year and serves as the only level III NICU in the region. Patients are referred from the state of South Carolina as well as surrounding states in the region.
Study type
We conducted an institutional review board-approved prospective cohort study to determine the prevalence of MRSA colonization among women at risk for preterm delivery admitted from January 2009 through March 2010 to MUSC. These women were admitted for preterm labor, preterm premature rupture of membranes, or iatrogenic preterm delivery. Iatrogenic preterm delivery refers to an indicated delivery before 37 weeks’ gestation secondary to maternal or neonatal conditions, eg, severe preeclampsia, intrauterine growth restriction. We also determined the rates of MRSA colonization in these women’s neonates admitted to the NICU.
Screening protocol
Screening MRSA cultures were collected from the nares, vulva, and vagina-rectum of the study cohort of women during their initial evaluation before the initiation of antibiotic therapy. Specimens were collected on BD CultureSwabs with liquid Amies transport media (Becton, Dickinson Co, Sparks, MD). Specimen swabs were inoculated onto a chromID MRSA plate (bioMerieux Inc, Durham, NC) and the plates were incubated for 24 hours at 35°C. Green colonies consistent with MRSA were then identified by conventional microbiologic techniques. Neonates, delivered by women included in the study, who were admitted to the NICU were screened for MRSA colonization. Cultures were collected from the neonates’ nares, axilla, and diaper area on admission and then repeated twice weekly for as long as the neonate remained MRSA negative. The neonatal umbilicus was not screened for MRSA colonization to avoid disrupting an umbilical catheter if present.
When available, the MRSA isolates from both women and neonates were embedded in an argose gel. Pulse field gel electrophoresis (PFGE) was used to determine whether maternal and neonatal MRSA isolates were genetically similar. Before PFGE, the MRSA isolates underwent multiple washes. The MRSA bacterial DNA was then lysed using the restriction endonuclease, S ma 1. The argose gel was exposed to PFGE to produce a restriction banding pattern for each MRSA isolate. The restriction patterns were examined based on pairwise, fragment-for-fragment comparisons to determine relatedness of the MRSA strains. Four different categories were used to describe the genetic relatedness of MRSA strains depending on the number of fragment differences. These 4 categories include: indistinguishable (0 bands different), closely related (2-3 bands different), possibly related (4-6 bands different), and different (≥7 bands different).
Data collection and analysis
Descriptive data for the women screened for MRSA was collected from the electronic medical record (EMR). Descriptive variables included: age, ethnicity, parity, weight, insurer, group B Streptococcus (GBS) status, history of sexually transmitted infection (STI), history of previous MRSA colonization or infection, number of prior hospital admissions during the current pregnancy, admission diagnosis, and gestational age on admission. These women’s EMRs were monitored from admission up to 6 weeks’ postpartum for the development of MRSA infection. Diagnoses of chorioamnionitis, postpartum endometritis, wound infections, and other infectious morbidity were noted. Similarly, neonatal descriptive data were obtained from the EMR. Variables recorded included: sex, gestational age at delivery, mode of delivery, birthweight, 5-minute APGAR score, the day of life MRSA colonization was noted, and the development of sepsis or MRSA infection.
Characteristics among women found to be colonized with MRSA were compared with those among women without MRSA colonization as were those among neonates found to have MRSA compared with those without. Proportions were compared using χ 2 or Fisher exact tests. A Cochran-Armitage trend test was performed to determine a linear correlation between the number of admissions and maternal MRSA colonization. Multivariable logistic regression was used to determine potential risk factors among these populations for MRSA colonization. Variables with associations at the P = .2 level from univariable analysis were included in the final multivariable model. Statistical significance was set at a P value of ≤ .05.
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