Daily hazard of acquisition of methicillin-resistant Staphylococcus aureus infection in the intensive care unit

Ward-level factors associated with methicillin-resistant Staphylococcus aureus acquisition-an electronic medical records study in Singapore

Background

Methicillin-Resistant Staphylococcus aureus (MRSA) is endemic in hospitals worldwide. Intrahospital transfers may impact MRSA acquisition risk experienced by patients. In this study, we investigated ward characteristics and connectivity that are associated with MRSA acquisition.

Methods

We analysed electronic medical records on patient transfers and MRSA screening of in-patients at an acute-care tertiary hospital in Singapore to investigate whether ward characteristics and connectivity within a network of in-patient wards were associated with MRSA acquisition rates over a period of four years.

Results

Most patient transfers concentrated in a stable core network of wards. Factors associated with increased rate of MRSA acquisition were MRSA prevalence among patients transferred from other wards (rate ratio (RR): 7.74 [95% confidence interval (CI): 3.88, 15.44], additional 5 percentage point), critical care ward (RR: 1.72 [95% CI: 1.09, 2.70]) and presence of MRSA cohorting beds (RR: 1.39 [95% CI: 1.03, 1.90]. Oncology ward (RR: 0.66 [95% CI: 0.46, 0.94]) (compared to medical ward), and median length of stay (RR: 0.70 [95% CI: 0.55, 0.90], additional 1.5 days) were associated with lower acquisition rates. In addition, we found evidence of interaction between MRSA prevalence among patients transferred from other wards and weighted in-degree although the latter was not associated with MRSA acquisition after controlling for confounders.

Conclusion

Wards with higher MRSA prevalence among patients transferred from other wards were more likely to have higher MRSA acquisition rate. Its effect further increased in wards receiving greater number of patients. In addition, critical care ward, presence of MRSA cohorting beds, ward specialty, and median length of stay were associated with MRSA acquisition.

Genetic Lineages of Methicillin-Resistant Staphylococcus aureus Acquired during Admission to an Intensive Care Unit of a General Hospital

OBJECTIVES

The objectives of this study were to determine the frequency of methicillin-resistant Staphylococcus aureus (MRSA) colonization or infection while on admission to the intensive care unit (ICU), and examine the genetic backgrounds of the MRSA isolates to establish transmission among the patients.

SUBJECTS AND METHODS

This study involved screening 2,429 patients admitted to the ICU of Farwania Hospital from January 2005 to October 2007 for MRSA colonization or infection. The MRSA isolates acquired after admission were investigated using a combination of molecular typing techniques to determine their genetic backgrounds.

RESULTS

Of 2,429 patients screened, 25 (1.0%) acquired MRSA after admission to the ICU. Of the 25 MRSA, 19 (76%) isolates belonged to health care-associated (HA-MRSA) clones: ST239-III (n = 17, 68%) and ST22-IV (n = 2, 8%). The remaining 6 MRSA isolates belonged to community-associated clones: ST80-IV (n = 3, 12%), ST97-IV (n = 2, 8%), and ST5-IV (n = 1, 4%). The ST239-III-MRSA clone was associated with infection as well as colonization, and was isolated from patients from 2005 to 2007.

CONCLUSIONS

The HA-MRSA clone ST239-III persistently colonized patients admitted to the ICU, indicating the possibility of its transmission among the patients over time.

Rapid, highly discriminatory binary genotyping to demonstrate methicillin-resistant Staphylococcus aureus transmission in a tertiary care intensive care unit

OBJECTIVES

No previous studies of methicillin-resistant Staphylococcus aureus (MRSA) epidemiology in adult intensive care units (ICUs) have assessed the utility of rapid, highly discriminatory strain typing in the investigation of transmission events.

DESIGN

Observational.

SETTING

A 22-bed medical-surgical adult ICU. Patients Those admissions MRSA-positive on initial screening and all admissions <48 hours in duration were excluded, leaving a cohort of 653 patients (median age, 61 years; APACHE-II, 19).

METHODS

We conducted this study of MRSA transmission over 1 year (August 1, 2011 to July 31, 2012) using a multiplex PCR-based reverse line blot (mPCR/RLB) assay to genotype isolates from surveillance swabs obtained at admission and twice weekly during ICU stays. MRSA prevalence and incidence rates were calculated and transmission events were identified using strain matching. Colonization pressure was calculated daily by summation of all MRSA cases.

RESULTS

Of 1,030 admissions to ICU during the study period, 349 patients were excluded. MRSA acquisition occurred during 31 of 681 (4.6%) remaining admissions; 19 of 31(61%) acquisitions were genotype-confirmed, including 7 (37%) due to the most commonly transmitted strain. Moving averages of MRSA patient numbers on the days prior to a documented event were used in a Poisson regression model. A significant association was found between transmission and colonization pressure when the average absolute colonization pressure on the previous day was ≥3 (χ2=7.41, P=0.01).

CONCLUSIONS

mPCR/RLB characterizes MRSA isolates within a clinically useful time frame for identification of single-source clusters within the ICU. High MRSA colonization pressure (≥3 MRSA-positive patients) on a given day is associated with an increased likelihood of a transmission event.

Whole-genome sequencing in hierarchy with pulsed-field gel electrophoresis: the utility of this approach to establish possible sources of MRSA cross-transmission

BACKGROUND

In order to study the micro-epidemiology of meticillin-resistant Staphylococcus aureus (MRSA) effectively, the molecular typing method used must be able to distinguish between different MRSA strains. Pulsed-field gel electrophoresis (PFGE) can detect small genetic differences but is limited in its potential to distinguish isolates within a major lineage. Whole-genome sequencing (WGS) provides sufficient resolution to support or exclude links between otherwise indistinguishable isolates, but lacks the practical utility of conventional typing methods.

AIM

To explore the utility of WGS in a hierarchical approach with PFGE to help establish possible sources of MRSA cross-transmission in the intensive care setting.

METHODS

Possible transmission routes from donor to recipient via the hands of staff, the air or environmental surfaces were identified. Focused molecular typing used PFGE to explore these transmission hypotheses. WGS was applied when an acquisition event involved a common PFGE pulsotype.

FINDINGS

Thirty-eight of the 78 acquisition events could not be explored as clinical isolates were not available. PFGE excluded all potential donors from 26 of the remaining 40 acquisition events, but did identify a probable source in 14 new colonizations. Within the hypotheses tested, PFGE supported links between patients occupying the same bay, the same bed space, adjacent isolation rooms and different wards. When a patient source was not identified, PFGE implicated the ward environment and the hands of staff. However, WGS disproved three of these transmission pathways.

CONCLUSION

WGS can complement conventional typing methods by confirming or refuting possible MRSA transmission hypotheses. Epidemiological data are crucial in this process.

Assessment of the Influence of Test Characteristics on the Clinical and Cost Impacts of Methicillin-ResistantStaphylococcus aureusScreening Programs in US Hospitals

Objective.

To compare the impacts of different methicillin-resistantStaphylococcus aureus(MRSA) screening test options (eg, polymerase chain reaction [PCR], rapid culture) and program characteristics on the clinical outcomes and budget of a typical US hospital.

Methods.

We developed an Excel-based decision-analytic model, using published literature to calculate and compare hospital costs and MRSA infection rates for PCR- or culture-based MRSA screening and then used multivariate sensitivity analysis to evaluate key variables. Same-day PCR testing for a representative 370-bed teaching hospital in the United States was assessed in different populations (high-risk patients, intensive care unit [ICU] patients, or all patients) and compared with other test options.

Results.

Different screening program populations (all patients, high-risk patients, ICU patients, or patients with previous MRSA colonization or infection only) represented a potential savings of $12,158-$76,624 per month over no program ($188,618). Analysis of multiple test options in high-risk population screening indicated that same-day PCR testing of high-risk patients resulted in fewer infections over 1,720 patient-days (2.9, compared with 3.5 for culture on selective media and 3.8 for culture on nonselective media) and the lowest total cost ($112,012). The costs of other testing approaches ranged from $113,742 to $123,065. Sensitivity analysis revealed that variations in transmission rate, conversion to infection, prevalence increases, and hospital size are important to determine program impact. Among test characteristics, turnaround time is highly influential.

Conclusion.

All screening options showed reductions in infection rates and cost impact improvement over no screening program. Among the options, same-day PCR testing for high-risk patients slightly edges out the others in terms of fewest infections and greatest potential cost savings.

The prevalence and significance of methicillin-resistant Staphylococcus aureus colonization at admission in the general ICU Setting: a meta-analysis of published studies

OBJECTIVE

To estimate the prevalence and significance of nasal methicillin-resistant Staphylococcus aureus colonization in the ICU and its predictive value for development of methicillin-resistant S. aureus infection.

DATA SOURCES

MEDLINE and EMBASE and reference lists of all eligible articles.

STUDY SELECTION

Studies providing raw data on nasal methicillin-resistant S. aureus colonization at ICU admission, published up to February 2013. Analyses were restricted in the general ICU setting. Medical, surgical, and interdisciplinary ICUs were eligible. ICU studies referring solely on highly specialized ICUs populations and reports on methicillin-resistant S. aureus outbreaks were excluded.

DATA EXTRACTION

Two authors independently assessed study eligibility and extrapolated data in a blinded fashion. The two outcomes of interest were the prevalence estimate of methicillin-resistant S. aureus nasal colonization at admission in the ICU and the sensitivity/specificity of colonization in predicting methicillin-resistant S. aureus-associated infections.

DATA SYNTHESIS

Meta-analysis, using a random-effect model, and meta-regression were performed. Pooled data extracted from 63,740 evaluable ICU patients provided an estimated prevalence of methicillin-resistant S. aureus nasal colonization at admission of 7.0% (95% CI, 5.8-8.3). Prevalence was higher for North American studies (8.9%; 95% CI, 7.1-10.7) and for patients screened using polymerase chain reaction (14.0%; 95% CI, 9.6-19). A significant per year increase in methicillin-resistant S. aureus colonization was also noted. In 17,738 evaluable patients, methicillin-resistant S. aureus infections (4.1%; 95% CI, 2.0-6.8) developed in 589 patients. The relative risk for colonized patients was 8.33 (95% CI, 3.61-19.20). Methicillin-resistant S. aureus nasal carriage had a high specificity (0.96; 95% CI, 0.90-0.98) but low sensitivity (0.32; 95% CI, 0.20-0.48) to predict methicillin-resistant S. aureus-associated infections, with corresponding positive and negative predictive values at 0.25 (95% CI, 0.11-0.39) and 0.97 (95% CI, 0.83-1.00), respectively.

CONCLUSIONS

Among ICU patients, 5.8-8.3% of patients are colonized by methicillin-resistant S. aureus at admission, with a significant upward trend. Methicillin-resistant S. aureus colonization is associated with a more than eight-fold increase in the risk of associated infections during ICU stay, and methicillin-resistant S. aureus infection develops in one fourth of patients who are colonized with methicillin-resistant S. aureus at admission to the ICU.

Do active surveillance and contact precautions reduce MRSA acquisition? A prospective interrupted time series

Background

Consensus for methicillin-resistant Staphylococcus aureus (MRSA) control has still not been reached. We hypothesised that use of rapid MRSA detection followed by contact precautions and single room isolation would reduce MRSA acquisition.

Methods

This study was a pre-planned prospective interrupted time series comparing rapid PCR detection and use of long sleeved gowns and gloves (contact precautions) plus single room isolation or cohorting of MRSA colonised patients with a control group. The study took place in a medical-surgical intensive care unit of a tertiary adult hospital between May 21^st 2007 and September 21^st 2009. The primary outcome was the rate of MRSA acquisition. A segmented regression analysis was performed to determine the trend in MRSA acquisition rates before and after the intervention.

Findings

The rate of MRSA acquisition was 18.5 per 1000 at risk patient days in the control phase and 7.9 per 1000 at-risk patient days in the intervention phase, with an adjusted hazard ratio 0.39 (95% CI 0.24 to 0.62). Segmented regression analysis showed a decline in MRSA acquisition of 7% per month in the intervention phase, (95%CI 1.9% to 12.8% reduction) which was a significant change in slope compared with the control phase. Secondary analysis found prior exposure to anaerobically active antibiotics and colonization pressure were associated with increased acquisition risk.

Conclusion

Contact precautions with single room isolation or cohorting were associated with a 60% reduction in MRSA acquisition. While this study was a quasi-experimental design, many measures were taken to strengthen the study, such as accounting for differences in colonisation pressure, hand hygiene compliance and individual risk factors across the groups, and confining the study to one centre to reduce variation in transmission. Use of two research nurses may limit its generalisability to units in which this level of support is available.

The persistence of Staphylococcus aureus decolonization after mupirocin and topical chlorhexidine: implications for patients requiring multiple or delayed procedures

Preoperative screening and decolonization of methicillin-sensitive and methicillin-resistant Staphylococcus aureus (MSSA and MRSA, respectively) are advocated to reduce surgical site infections. We determined the rate and duration of decolonization in patients undergoing elective orthopedic surgery. Patients undergoing elective orthopedic surgery were seen in our preoperative testing program (PAT) and had their anterior nares cultured for MRSA and MSSA. All patients were treated with intranasal mupirocin and a topical chlorhexidine solution. A cohort of patients returned to PAT before a subsequent elective procedure and were recultured. All culture results and time between PAT visits were recorded, and the rates of successful initial and persistent decolonization were determined. Six hundred ten patients visited PAT 1290 times. Overall, 94 (70.1%) of 134 patients with initially MRSA- or MSSA-positive cultures remained decolonized at a mean time of 156 days (SD=140), whereas 40 patients (29.9%) were not decolonized by the time of repeat testing at a mean time of 213 days (SD=187). At repeat testing, there were 2 newly MRSA-positive and 35 newly MSSA-positive patients. Staphylococcus aureus decolonization with intranasal mupirocin and topical chlorhexidine was effective but not persistent in a significant proportion of patients. A small number of previously uncolonized patients became colonized. Staphylococcus aureus screening and decolonization protocols must be repeated before any readmission, regardless of prior colonization status.

The Clinical Significance of Occult Thoracic Injury in Blunt Trauma Patients

Increased use of thoracic CT (TCT) in diagnosis of blunt traumatic injury has identified many injuries previously undetected on screening chest x-ray (CXR), termed “occult injury.” The optimal management of occult rib fractures, pneumothoraces (PTX), hemothoraces (HTX), and pulmonary contusions is uncertain. Our objective was to determine the current management and clinical outcome of these occult blunt thoracic injuries. A retrospective review identified patients with blunt thoracic trauma who underwent both CXR and TCT over a 2-year period at a Level I urban trauma center. Patients with acute rib fractures, PTX, HTX, or pulmonary contusion on TCT were included. Patient groups analyzed included: 1) no injury (normal CXR, normal TCT, n = 1337); 2) occult injury (normal CXR, abnormal TCT, n = 205); and 3) overt injury (abnormal CXR, abnormal TCT, n = 227). Patients with overt injury required significantly more mechanical ventilation and had greater mortality than either occult or no injury patients. Occult and no injury patients had similar ventilator needs and mortality, but occult injury patients remained hospitalized longer. No patient with isolated occult thoracic injury required intubation or tube thoracostomy. Occult injuries, diagnosed by TCT only, have minimal clinical consequences but attract increased hospital resources.

A veterinary perspective on methicillin-resistant staphylococci

OBJECTIVE

To familiarize the reader with the epidemiology, diagnosis, and infectious and zoonotic potential of methicillin-resistant staphylococci.

DATA SOURCES

Original research publications, scientific reviews and abstracts, case reports, and conference proceedings.

HUMAN DATA SYNTHESIS

Staphylococcus aureus is a common human commensal organism; acquisition of genes encoding an altered penicillin-binding protein confers resistance to beta-lactam antimicrobial drugs. Methicillin-resistant S. aureus (MRSA) are often resistant to non-beta-lactam antimicrobial drugs as well. Originally described as an important cause of nosocomial infection, MRSA colonization and infection are now often identified in humans outside healthcare settings. Like other S. aureus, MRSA may be present without clinical illness. However, when they do cause infection the consequences can be extremely serious.

VETERINARY DATA SYNTHESIS

The major domestic animal species, including pets and livestock, may become contaminated, colonized, or infected with methicillin-resistant staphylococci, including MRSA. Dogs and cats are more likely to be colonized/infected with Staphylococcus pseudintermedius than S. aureus, but this pathogen can acquire genes encoding methicillin resistance (ie, MRSP). Diagnosis of MRSA or MRSP has implications not only for treatment of infected animals, but for potential zoonotic transmission.

CONCLUSIONS

MRSA infection is an important cause of morbidity and mortality in humans. Animals may be contaminated, colonized, or infected with MRSA, with implications for the animal's health and as a potential reservoir for human infection. Staphylococci other than S. aureus may also acquire genes for methicillin resistance, and these species can also result in animal and occasionally human morbidity or mortality.