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a Divisions of Pediatric Infectious Diseases
c Neonatology, Chang Gung Childrens Hospital, Kweishan, Taoyuan, Taiwan
b School of Medicine, Chang Gung University, Kweishan, Taoyuan, Taiwan
d Department of Clinical Pathology, Chang Gung Memorial Hospital, Kweishan, Taoyuan, Taiwan
| ABSTRACT |
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METHODS. Between March 2003 and February 2004, surveillance culture specimens from the nares, postauricular areas, axillae, and umbilicus of infants admitted to the NICUs at a childrens hospital in Taiwan were obtained weekly for the detection of methicillin-resistant S aureus. All colonized and clinical isolates from each study infant with methicillin-resistant S aureus infection were genotyped with pulsed-field gel electrophoresis, with Sma1 digestion, and compared.
RESULTS. A total of 783 infants were included in this study. Methicillin-resistant S aureus colonization was detected for 323 infants during their NICU stays, with detection with the first 2 samples for 89%. Nares and umbilicus were the 2 most common sites of initial colonization. Methicillin-resistant S aureus colonization was associated significantly with premature birth (
28 weeks) and low birth weight (
1500 g), and infants with colonization had a significantly higher rate of methicillin-resistant S aureus infection, compared with those without colonization (26% vs 2%). Methicillin-resistant S aureus colonization was noted for 84 of 92 infants with methicillin-resistant S aureus infections. Of the 68 episodes with previous colonization and isolates available for genotyping analysis, colonized and clinical isolates were indistinguishable in 63 episodes, highly related in 2 episodes, and distinct in 3 episodes.
CONCLUSIONS. More than 40% of the hospitalized infants were colonized with methicillin-resistant S aureus during their stay in methicillin-resistant S aureusendemic NICUs; this was associated significantly with methicillin-resistant S aureus infection. Most infants with methicillin-resistant S aureus infections had previous colonization with an indistinguishable strain.
Key Words: methicillin-resistant Staphylococcus aureus colonization infection genotyping analysis neonatal intensive care unit
Abbreviations: MRSAmethicillin-resistant Staphylococcus aureus HCWhealth care worker PFGEpulsed-field gel electrophoresis MLSTmultilocus sequence typing ORodds ratio CIconfidence interval
Methicillin-resistant Staphylococcus aureus (MRSA) is among the most important pathogens of bacteremia in the ICU. Colonized patients are the chief source of S aureus in hospitals. Colonizing strains may serve as endogenous reservoirs for overt clinical infections or may spread to other patients.18 One study demonstrated the link between S aureus isolated from blood and S aureus isolated from nasal specimens, taken before and after bacteremia was detected, with the use of molecular methods.9 Most studies were conducted among adults, however, and no study has investigated such a link among infants hospitalized in NICUs.
In Taiwan, 50% to 80% of the S aureus isolates causing nosocomial infections in 12 major hospitals in 2000 were methicillin resistant.10 In our NICUs, MRSA has been among the most common nosocomial pathogens and accounted for >90% of S aureus isolates since 1998.11 Standard infection control measures were implemented but were unable to control the spread of MRSA effectively. To identify infants with MRSA colonization and to implement cohort care for these infants, we screened routinely for MRSA carriage for each infant admitted or transferred to the NICUs. We thus had the opportunity to evaluate the MRSA colonization rate among these infants and its association with the MRSA infection rate, with molecular methods.
| METHODS |
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Infants admitted to NICUs 1 and 2 in March 2003 to February 2004 were included in this study, and specimens from the nares, postauricular areas, axillae, umbilicus, and perineum were obtained weekly (every Monday) and sent for detection of MRSA. Because of a low yield rate, perineum specimens were discontinued 1 month after commencement of the study. The infants with MRSA colonization were separated from noncolonized infants and placed in a segregated area of the NICUs, and cohort care by designated nurses was implemented.
All infants were observed if they were infected with MRSA. If a study infant with MRSA colonization had MRSA clinical isolates, then the clinical isolates as well as the colonized isolates were genotyped and compared. MRSA isolates recovered from clinical diagnostic samples (beyond surveillance culture specimens) submitted to the clinical microbiology laboratory were regarded as clinical isolates. In accordance with the standard definition of nosocomial infection,12 any infant with clinical isolates of MRSA who was receiving antimicrobial therapy was categorized as experiencing an episode of infection. Episodes of MRSA infection involving a single infant were considered to be distinct if they were
2 weeks apart, a course of effective antibiotics had been administered, the clinical symptoms had resolved, and
1 negative culture from the infected site was documented.
Three surveillance cultures for health care workers (HCWs) were performed, at the beginning, middle, and end of the study period, and specimens were obtained from the nares of HCWs working in both units. Intranasal mupirocin treatment was administered to each HCW with MRSA colonization.
Surveillance specimens for culture were obtained with a cotton swab, placed in transport medium (Venturi Transystem; Copan Innovation Ltd, Limmerick, Ireland), and then processed in our microbiology laboratory within 4 hours. Identification of MRSA was confirmed according to National Committee for Clinical Laboratory Standards guidelines.13 Pulsed-field gel electrophoresis (PFGE) with SmaI digestion was used in this study to fingerprint the MRSA isolates, according to the procedures described previously.14 The criteria proposed by Tenover et al15 were used for analysis of the DNA fingerprints generated with PFGE. The genotypes were designated, as in our previous studies,14,1619 in alphabetical order; any new type, if identified, was designated consecutively. PFGE patterns with <4-band differences from an existing genotype were defined as subtypes of that genotype and were labeled with Arabic number suffixes. Two isolates were considered to be indistinguishable, highly related, or distinct if they had the same subtype, the same genotype, or a different type, respectively.
Multilocus sequence typing (MLST) was performed for some selected strains with representative PFGE patterns, as described elsewhere.20 The allelic profiles were assigned through comparison of the sequences at each locus with those of the known alleles in the S aureus MLST database and were defined as sequence types accordingly.
We compared the characteristics of the infants with and without MRSA colonization with
2 tests (continuity-adjusted) or Students t tests. Relative risks and/or odds ratios (ORs) were calculated with 95% confidence intervals (CIs). Statistical analyses of the data were performed with EpiInfo, version 6 (Centers for Disease Control and Prevention, Atlanta, GA) and SAS for Windows, version 6.11 (SAS Institute, Cary, NC).
| RESULTS |
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2 sites of colonization. Nares (229 infants, 71%) and umbilicus (195 infants, 60%) were the 2 most common sites of colonization. The sensitivities of nares and umbilicus for the detection of MRSA colonization were 71% and 60%, respectively. The sensitivity could reach 90% if both nares and umbilicus were sampled. Colonization was identified in the first sampling for 222 infants (69%) and in the first 2 samplings for 287 infants (89%). The distributions of the sites of initial colonization are shown in Table 2. Compared with no colonization, MRSA colonization was associated significantly with premature birth (gestational age of
28 weeks) and low birth weight (
1500 g) (Table 1).
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7 days, 26 infants; 8 to 14 days, 17 infants; 15 to 21 days, 9 infants; 22 to 29 days, 8 infants;
30 days, 4 infants. The infants with MRSA colonization had a significantly higher rate of MRSA infection than did those without colonization (26% vs 2%; P < .00001; relative risk: 2.64; 95% CI: 2.342.98). MRSA infection among these infants was associated significantly with MRSA colonization (P < .00000005; OR: 19.86; 95% CI: 9.1145.07). MRSA colonization with infection was associated significantly with low birth weight (
1000 g; P < .0005; OR: 3.79; 95% CI: 1.698.51) and premature birth (gestational age of
28 weeks; P < .0005; OR: 3.33; 95% CI: 1.666.70), compared with colonization but not infection.
A total of 121 clinical isolates from 64 infants with 84 episodes of infection were available for genotyping analysis. Twenty-six infants (41%) had
2 clinical isolates available (up to 8 isolates). The source of the 121 clinical isolates included bloodstream (19 isolates), sputum (most from endotracheal tubes aspirates) (64 isolates), central venous catheter (5 isolates), pus (24 isolates), urine (4 isolates), and others (5 isolates). From the 64 infants, 464 colonized isolates were collected and subjected to genotyping analysis together with the 121 clinical isolates. Of the 585 MRSA isolates, 3 genotypes (designated types A, C, and D), with 23 subtypes, were identified (Table 3). The largest population (521 isolates, 89%) belonged to genotype A (similar to Brazilian or Hungary clone), among which the most prevalent subtype, A10, accounted for 316 colonized isolates (68%) and 76 clinical isolates (63%) from 37 infants (58%). The clinical isolates from the same infant were indistinguishable for 23 of the 26 infants with multiple clinical isolates. For the remaining 3 infants, the clinical isolates were highly related, although not indistinguishable. All colonized isolates from the same infant were indistinguishable or highly related for all except 5 infants for whom 1 or a few distinct strains were identified.
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| DISCUSSION |
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In this study, MRSA colonization was detected for 41% of the infants admitted or transferred to the NICUs. The rate, although very high, was not beyond prediction, because most infants were premature, with birth weights of
1500 g, longer stays in the NICUs were usually required, and they might thus acquire MRSA colonization during their stays in the NICUs. As expected, MRSA colonization in this study was associated significantly with premature birth (gestational age of <28 weeks) and low birth weight (
1500 g). What was unexpected was that nearly 90% of the colonized infants were detected with the first 2 samples. Because most infants (60%) were admitted to our NICUs within the first 24 hours of life, the acquisition of MRSA by these infants would have occurred very soon after hospitalization or even before the infants were admitted to our NICUs. These findings suggest that the infants might have acquired MRSA from their mothers. However, maternal vaginal colonization of MRSA and its association with results for the neonates were not investigated in this study. A study is being conducted regarding this issue, because community-acquired MRSA infection (28%74%) and colonization (1.9%5.3%) among children without risk factors are seen not infrequently in Taiwan.1719,2528
Nares are considered to be the most common reservoirs of MRSA and are selected as the sampling sites in most studies regarding MRSA colonization. However, there have been scanty reports in the literature comparing the efficacy of different sampling sites in the neonatal population.29,30 In the current study, although nares and umbilicus were the 2 most common sites of colonization, the sensitivities of nares alone and umbilicus alone for the detection of MRSA colonization were 71% and 60%, respectively. Apparently the sensitivities were insufficient. However, the sensitivity could increase to 90% (negative predictive value: 93%) if both sites were sampled. We think that sampling of these 2 sites is necessary and might be adequate for surveillance cultures in this population.
Our previous study16 indicated that, in 1998 and 2000, genotypes A and C were the 2 major genotypes in these NICUs and accounted for 63% and 35%, respectively, of the MRSA clinical isolates. In the current study, we found that genotype A (sequence type 239, similar to Brazilian or Hungary clone31), particularly a specific subtype (A10), became the only dominant clone and accounted for nearly 90% of all 585 clinical and colonized isolates from the infants. It is intriguing that genotype C accounted for only
10% of all isolates from the infants, whereas 77% of all isolates from the HCWs belonged to genotype C. These findings suggest that, in addition to the nasal carriage of MRSA by HCWs, there were other factors associated with the spread of MRSA in these units. Cross-transmission among the infants through the transient carriage of MRSA on the hands of HCWs was a strong possibility that requires additional investigation.
From the genotyping results in this study, we also learn that, if an infant hospitalized in a NICU harbors a MRSA strain, then this strain can be found at multiple sites and can exist for a long period (>3 months), resulting in subsequent infection and even recurrent infections. MRSA colonization, acquired either before or after admission, was seen commonly among infants hospitalized in our NICUs, particularly premature infants with very low birth weights. MRSA infection among these infants was associated significantly with previous colonization with an indistinguishable strain.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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Address correspondence to Yhu-Chering Huang, MD, PhD, Chang Gung Childrens Hospital, 5 Fu-Shin St, Kweishan, Taoyuan, Taiwan. E-mail: ychuang{at}adm.cgmh.org.tw
The authors have indicated they have no financial relationships relevant to this article to disclose.
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