Infant Colonization With Methicillin-Resistant Staphylococcus aureus or Vancomycin-Resistant Enterococci Preceding Neonatal Intensive Care Unit Discharge

Abstract

Rates of colonization with methicillin-resistant Staphylococcus aureus (MRSA) and/or vancomycin-resistant enterococci (VRE) were determined for 1320 infants within 7 days of neonatal intensive care unit discharge. Overall, 4% and 1% of the infants were colonized with MRSA or VRE, respectively. Predictors identified in fixed-effects models were surgery during hospitalization (for MRSA colonization) and prolonged antimicrobial treatment (for VRE colonization).

Infants hospitalized in the neonatal intensive care unit (NICU) who are colonized with methicillin-resistant Staphylococcus aureus (MRSA) or vancomycin-resistant enterococci (VRE) are at risk of developing infection with one of these pathogens and can serve as a reservoir for these multidrug-resistant organisms. MRSA and VRE colonization rates preceding NICU discharge have not been well studied and are likely to be underestimated by clinical culture results [1]. Furthermore, predictors of infant colonization with MRSA or VRE near the time of discharge and concordance of colonization with previous infection with one or both of these organisms are not known. The objectives of this study were to determine the rates of colonization with MRSA and/or VRE preceding NICU discharge, identify potential predictors of colonization, and determine concordance with previous infection.

METHODS

Study Design, Sites, and Subjects

This substudy was part of a larger prospective study, “Improving Antimicrobial Prescribing Practices in the Neonatal Intensive Care Unit” conducted between May 2009 and April 2012 in 4 NICUs with a total of 247 beds and approximately 4500 annual discharges [2]. The study sites were NewYork-Presbyterian Morgan Stanley Children’s Hospital at Columbia University Medical Center, NewYork-Presbyterian Komansky Center for Children’s Health at Weill Cornell Medical Center, Children’s Hospital of Philadelphia, and Christiana Care Health System. The institutional review board at each participating center approved this study.

To be included in this substudy, infants had to have been <7 days of age at admission to the NICU, hospitalized for ≥14 days, and swabbed for study-related surveillance cultures (discussed next) within 7 days of NICU discharge.

Surveillance Cultures

Study-related specimens obtained for surveillance cultures obtained within 7 days of NICU discharge included 2 swabs for MRSA and 1 for VRE (Liquid Amies swab transport system, bioMérieux, Inc., Durham, NC, and Becton Dickinson, Franklin Lakes, NJ). The first swab for MRSA sampled 3 skin sites (axilla, periumbilical, and groin), and the second swab was obtained from the anterior nares. For VRE, a single perirectal swab was obtained.

Swabs were shipped overnight to a core laboratory and subcultured on CNA blood agar (Becton Dickinson). Presumptive S aureus isolates were tested using the Staphaurex assay (Remel, Inc., Lenexa, KS), and presumptive enterococci were tested using the Identicult AE assay (PML Microbiologicals, Wilsonville, OR). The MicroScan WalkAway system (Siemens Healthcare Diagnostics, Inc., Tarrytown, NY) was used for species identification and susceptibility testing. Clinical Laboratory and Standards Institute 2012 breakpoints were used, including an oxacillin minimum inhibitory concentration (MIC) of >2 µg/ml for MRSA and a positive cefepime screening test by MicroScan and a vancomycin MIC of >4 µg/ml for VRE [3]. Mupirocin susceptibility was not assessed.

Colonization and Infections

Colonization with MRSA was defined as isolation of MRSA from the study-obtained skin or nares swabs. Colonization with VRE was defined as isolation of VRE from the study-obtained rectal swab.

The infants’ medical records were reviewed to identify MRSA or VRE clinical cultures associated with infection that was diagnosed by the attending neonatologist and treated with intravenous antibiotics. Each site’s clinical microbiology laboratory processed and performed susceptibility testing of the clinical cultures. Clinical culture results described by the treating clinician in the medical record as indicating colonization were not collected in this study.

Risk Factors and Statistical Analysis

Potential risk factors for colonization included demographic and clinical variables and antimicrobial treatment. Clinical variables included surgical procedures, central venous catheters, mechanical ventilation, and length of stay. Treatment was analyzed according to individual agents and antibiotic classes. The individual agents analyzed were cefazolin, meropenem, metronidazole, oxacillin, and vancomycin. The classes analyzed were penicillin agents (penicillin and ampicillin), aminoglycoside agents (gentamicin, tobramycin, and amikacin), β-lactam/β-lactamase combination agents (ampicillin-sulbactam, piperacillin-tazobactam, and ticarcillin-clavulanate), and third/fourth-generation cephalosporin agents (cefepime, cefotaxime, ceftazidime, and/or ceftriaxone). Agents administered to ≤5 infants (eg, rifampin and clindamycin) were not analyzed.

We found some differences among the sites. Sites 1 and 2 used mupirocin applied to the anterior nares and chlorhexidine gluconate (CHG) baths (for infants ≥1500 g) to decolonize infants identified by clinical cultures to have MRSA infection/colonization [4], whereas sites 3 and 4 did not use a decolonization protocol. Site 4 did not perform any surgical procedures.

Preliminary modeling of the variables designed to measure antibiotic treatment showed the strongest association with colonization for categorical variables of ≥5 or ≥10 continuous antibiotic-days compared with continuous variables such as days of antibiotic treatment or other categorical variables (data not shown). Thus, these measures of prolonged treatment, ≥5 continuous antibiotic-days and ≥10 continuous antibiotic-days, were used in the analysis.

Because of site differences, a fixed-effects model was used with each site treated as a fixed effect [2]. Variables found to be significant (P < .2) in bivariate analysis were included in multivariable models. Statistical analysis was performed using SAS 9.3 for Windows (SAS Institute, Inc., Cary, NC).

RESULTS

Subjects

Among 3128 infants who met the first inclusion requirement for the substudy (<7 days of age at admission to the NICU and hospitalized for ≥14 days), the second inclusion requirement (study-related surveillance specimens for culture obtained) was met by 1320 (42%) infants. As described previously, when stratified according to site, those infants who met only the first inclusion requirement (but did not have study-related surveillance cultures obtained because of prohibitive conditions [eg, death, medical condition, parental refusal, timing]) had a shorter length of stay and higher mean birth weight than the infants who met both requirements and were included as subjects in this substudy [2].

For the 1320 included subjects, study-related surveillance swabs were obtained within a mean of 1 day (median, 2 days) of NICU discharge. NICU lengths of stay were similar among the 4 study sites (44–51 days; P = .50), but the infants’ gestational age (31–36 weeks), birth weight (1587–2575 g), race, and ethnicity varied (all P < .01).

Colonization Rates

MRSA

Overall, 58 (4%) of 1320 infants were colonized with MRSA within 7 days of NICU discharge (14 [3%] from site 1, 19 [6%] from site 2, 14 [7%] from site 3, and 11 [3%] from site 4). MRSA was isolated from both the nares and skin swabs from one-third (19 of 58 [33%]) of the colonized infants, from only the nares swab from 26 (45%) colonized infants, and from only the skin-site swabs from 13 (22%) colonized infants. None of the isolates was resistant to vancomycin. Most (83% [48 of 58]) MRSA-colonized infants were discharged home, 6 (10%) were transferred to another hospital unit, and 4 (7%) were discharged to a long-term care facility.

VRE

Seventeen (1%) infants were colonized with VRE within 7 days of discharge (6 [1%] from site 1, 8 [2%] from site 2, 2 [1%] from site 3, and 1 [0.3%] from site 4). Of these infants, 10 (59%) were colonized with Enterococcus faecalis, and 7 (41%) were colonized with Enterococcus faecium. The vancomycin MICs at which 50% and 90% of the tested isolates were inhibited (MIC₅₀ and MIC₉₀, respectively) were both >16 μg/ml. Most (88% [15 of 17]) of the VRE-colonized infants were discharged home, and 2 were transferred to another hospital unit.

Association of Previous Infection with Colonization

Nine (1%) of the 1320 infants had an MRSA infection during their NICU hospitalization. Previous MRSA infection was more common among infants colonized with MRSA within 7 days of discharge than among infants who were not colonized with MRSA (8.6% vs 0.3%, respectively; P < .01). One infant had a VRE infection during NICU hospitalization and was colonized with VRE within 7 days of discharge. No MRSA or VRE outbreak occurred at the sites during the study period.

Risk Factors for Colonization within 7 Days of Discharge

MRSA

Risk factors for MRSA colonization within 7 days of discharge are shown in Table 1. The final multivariable model revealed surgical procedures (odds ratio [OR], 2.83 [95% confidence interval (CI)], 1.23–4.62; P = .01) to be a significant predictor of MRSA colonization, although ≥5 days of treatment with cefazolin trended toward being protective (OR, 0.14 [95% CI, 0.02–1.06]; P = .06).

VRE

Risk factors for VRE colonization within 7 days of discharge are shown in Table 1. The final multivariable model revealed that ≥10 days of treatment with oxacillin (OR, 23.40 [95% CI, 4.00–137.01]; P < .01) or penicillin/ampicillin (OR, 6.26 [95% CI, 1.89–20.77]; P < .01) was a significant predictor of VRE colonization.

DISCUSSION

Among infants hospitalized for at least 14 days in the NICU, 4% and 1% of them were colonized with MRSA or VRE, respectively, within 7 days of NICU discharge. In contrast to many previously published results, our findings do not provide colonization rates during outbreaks but, rather, provide insight into rates of endemic MRSA and VRE colonization preceding NICU discharge.

Previously published MRSA colonization rates have ranged from 1% to 41%; the higher rates were found during outbreaks [1, 5–10]. Molecular typing and, more recently, whole-genome sequencing have revealed that outbreaks are generally clonal [8, 10–12], that epidemic clones, most notably USA 300 (also called t008 by spa typing), often reappear over years [10, 13], and that over the past decade, there has been a shift from healthcare- associated to community-associated strains [14]. A genomic epidemiology study also found that ~70% of MRSA colonizations are a result of transmission events within the NICU [10].

Colonization with MRSA is associated with subsequent infections, usually with the same clone [1, 5, 10, 15], and 22% to 35% of colonized infants become infected [1, 5, 10]. Strategies for preventing infection in MRSA-colonized infants have included decolonization with mupirocin applied to the anterior nares and/or CHG baths [16]. Performing surveillance cultures with or without subsequent decolonization has been shown to end outbreaks and reduce the rate of endemic MRSA infections [7, 14, 17–19]. However, the strategies for performing surveillance cultures and decolonization have varied, implementation challenges exist, and the cost-effectiveness of routine surveillance cultures and decolonization is uncertain [6, 18–21]. For example, the optimal site for obtaining swabs to assess MRSA colonization is under debate. In our study, we found that obtaining specimens for surveillance cultures from both the nares and skin substantially improved the detection of MRSA colonization. Other authors have suggested obtaining surveillance specimens for culture from the throat or rectum and the nares and skin sites [19]. Furthermore, neonatologists are rightfully concerned about the risks of CHG tolerability (eg, dermatitis) and toxicity (eg, neurologic reaction) and mupirocin resistance. Nonetheless, the neonatology community is eager to develop evidence-based strategies for preventing MRSA colonization and infection [22].

Less is known about VRE colonization rates and molecular epidemiology. Colonization rates have ranged from 12% (with interannual variability in a single NICU ranging from 2% to 30%) to as high as 40% during outbreaks [23, 24]. The progression from colonization to infection has been reported to occur in <1% to 3% of VRE-colonized infants [23, 24]. During outbreaks, both a single clone and multiple clones have been found [25–27]. There are no accepted decolonization strategies for VRE.

Infants colonized with MRSA (or, less likely, VRE) near the time of NICU discharge can be a source of MRSA transmission within the community or other healthcare settings, including long-term care facilities [28, 29]. Although the duration of colonization after NICU hospitalization is not fully known, during 1 MRSA outbreak, the median decolonization time was 36 days, although some infants remained colonized 30 months after discharge [30]. However, to our knowledge, the risk of MRSA infection in the community or in pediatric long-term care facilities related to transmission from colonized NICU graduates has not been described.

Previously identified risk factors for MRSA colonization include low birth weight, younger gestational age, and prolonged length of stay [1, 31]. Similar to 2 previous studies, including a case-control study of normal-birth-weight infants conducted at 1 of our study sites, we found that surgery during hospitalization was a risk factor for MRSA infection [4, 32], which suggests that MRSA acquisition can occur outside the NICU, potentially in the operating suite. Strategies for reducing this risk should be explored.

VRE colonization was rare within 7 days of discharge and, somewhat unexpectedly, was associated with treatment with oxacillin or a penicillin agent for ≥10 days, but it was not associated with treatment with vancomycin. However, results of this model should be interpreted with caution until they can be confirmed by additional studies with larger numbers of VRE-colonized infants, because the parameter estimates might be biased by the small number of VRE-colonized infants in our sample. To our knowledge, no other studies have evaluated risk factors for colonization with VRE except during an outbreak in which lower birth weight and longer duration of antimicrobial treatment were associated with colonization [33]. Our findings suggest that antimicrobial stewardship may potentially modify the risk of VRE colonization.

This study had limitations. Colonization was studied only within 7 days of NICU discharge, so it is unknown when the acquisition of MRSA or VRE occurred and if additional infants were colonized at other times during their NICU hospitalization. We did not study maternal colonization or collect clinical specimens for culture thought to represent colonization. We did not measure the impact of MRSA decolonization strategies. Furthermore, it is unknown if colonization preceded healthcare-associated infections. Molecular characterization of the isolates was not performed.

In conclusion, a minority of infants harbor MRSA within 7 days of NICU discharge, and even fewer of them harbor VRE. These findings have potential implications for dissemination of these potential pathogens, particularly MRSA, into long-term care facilities, other hospital units, and the community.

Notes

Acknowledgments. We thank Kristina Rivera and Dana O’Toole for their assistance.

Financial support. This work was funded by the National Institute of Nursing Research (grant R01 NR010821-05).

Potential conflicts of interest. All authors: No reported conflicts. All authors have submitted the ICMJE Form for Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

References

Author notes