Prospective study of infection, colonization and carriage of methicillin-resistant Staphylococcus aureus in an outbreak affecting 990 patients

Methicillin-resistant Staphylococcus aureus outbreak: a consensus panel's definition and management guidelines

OBJECTIVE

To provide medical personnel with a definition of an outbreak of methicillin-resistant Staphylococcus aureus (MRSA) and guidelines for managing potential outbreaks.

PARTICIPANTS

Eighteen panel members were chosen from different specialties, types of institutions, and geographic regions. Representatives from the American Society of Consultant Pharmacists, the American Society of Health-Systems Pharmacists, the Society for Healthcare Epidemiology of America, and the National Association of Directors of Nursing Administration participated.

CONSENSUS PROCESS

In preparation for the conference, panel members reviewed the literature and wrote abstracts outlining their personal opinions on the core issues, which were circulated to all participants. During a weekend conference, the panel summarized the reviewed literature, defined an MRSA outbreak, and developed management guidelines.

EVIDENCE

Published literature, clinical experience, and expert opinion concerning the emergence and subsequent management of MRSA cases in health care institutions.

RESULTS

An outbreak of MRSA was defined as either an increase in the rate of MRSA cases or a clustering of new cases due to the transmission of a single microbial strain in the health care institution. An increased rate of cases can be defined statistically or experientially and includes both infected and colonized patients. A potential outbreak should trigger stepwise, multidisciplinary actions consisting of basic epidemiologic procedures (phase I) to form an initial epidemiologic hypothesis of an outbreak (phase II) followed by a standard epidemiologic workup (phase III) and microbiologic studies (phase IV) to confirm the hypothesis. Mupirocin calcium treatments should be considered to decolonize health care workers during the fourth phase, even before typing is completed.

CONCLUSIONS

Until studies can be conducted to delineate the effectiveness of different recommendations, the proposed guidelines may provide a useful starting point that can be adapted to meet an individual institution's specific needs.

Analysis of an outbreak of non-phage-typeable methicillin-resistant Staphylococcus aureus by using a randomly amplified polymorphic DNA assay

A cluster of methicillin-resistant Staphylococcus aureus (MRSA) infections among patients on an intensive care unit (ICU) was detected by routine infection control surveillance. In the period from 5 January to 22 June 1995, 10 patients on the ICU and a further 6 patients (5 on one ward that had received colonized patients transferred from the ICU) were affected by MRSA strains with the same antibiotic susceptibility patterns. Seven (44%) of these 16 colonized patients developed MRSA bacteremia. MRSA isolates with the same characteristics were also found on the hands of one member of the ICU staff. The isolates were untypeable by phage typing, but 15 of 17 outbreak strains analyzed genetically had identical randomly amplified polymorphic DNA (RAPD) and pulsed-field gel electrophoresis (PFGE) profiles. A single strain of MRSA that was nontypeable by phage typing and that was isolated on the ICU on 1 January and six nontypeable and epidemiologically unrelated MRSA isolates all had RAPD profiles distinct from that of the outbreak strain. Implementation of strict infection control measures stopped the further spread of MRSA on the ICU, the affected general ward, and seven other wards that received MRSA carriers from the ICU. Although nontypeable by phage typing and not previously recognized as an epidemic strain, this strain of MRSA was readily transmissible and highly virulent. RAPD typing was found to be a simple, rapid, and effective method for the epidemiological investigation of this outbreak, and performance of typing by this method was simpler and less time-consuming than that of typing by PFGE. RAPD typing may have more general application for the study of S. aureus infections in hospitals.

Antimicrobial resistance in staphylococci. Epidemiology, molecular mechanisms, and clinical relevance

Staphylococcal infections continue to pose important clinical problems in children and adults. Antibiotic resistance among the staphylococci has rendered therapy of these infections a therapeutic challenge. Despite early, uniform susceptibility to penicillin, staphylococci acquired a gene elaborating beta-lactamase that rendered penicillin inactive and that is borne by nearly all clinical isolates. "Penicillinase-resistant beta-lactams," such as methicillin, were introduced in the early 1960s, but resistance to them has become an increasing concern. The mechanism of the so-called "methicillin resistance" is complex. Moreover, once confined to the ecology of hospitals and other institutions, a recent increase in community-acquired methicillin-resistant S. aureus infections has been observed. Glycopeptides, until now the only uniformly reliable therapeutic modality, have been increasingly used for therapy of staphylococcal infections. The recent recognition of clinical isolates with reduced susceptibility to glycopeptides is of concern.

Risk factors for developing clinical infection with methicillin-resistant Staphylococcus aureus (MRSA) amongst hospital patients initially only colonized with MRSA

In hospital outbreaks of methicillin-resistant Staphylococcus aureus (MRSA) many patients are initially colonized without infection. The reasons why some progress to infection while others do not are not known. A cohort of 479 hospital patients, initially only colonized with MRSA, was followed prospectively for the development of MRSA infection. Risk factors for progression to infection were assessed using Cox proportional hazards survival analysis. Fifty-three patients (11.1%) developed 68 MRSA infections. Intensive care setting, administration of three or more antibiotics, ulcers, surgical wounds, nasogastric or endotracheal tubes, drains, and urinary or intravenous catheterization were all associated with increased rates of MRSA infection. Multivariate analysis showed that intensive care patients, compared with medical patients, had a higher rate of developing MRSA infection within the first four days of admission, with a hazard ratio of 26.9 (95% CI 5.7-126). Surgical wounds, pressure ulcers and intravenous catheterization were also independent risk factors, with hazard ratios (and 95% CI) of 2.9 (1.3-6.3); 3.0 (1.6-5.7) and 4.7 (1.4-15.6), respectively. These findings suggest that, during an MRSA outbreak, clinical infection would be reduced if surgical and intensive care patients received priority for the prevention of initial colonization with MRSA. Prevention of pressure ulcers, and strict aseptic care of intravenous catheters and surgical wounds would also reduce the development of MRSA infection. Since early treatment with vancomycin is known to reduce the mortality, patients colonized with MRSA who also have one or more of these risk factors may warrant empirical vancomycin therapy at the earliest suggestion of infection.

Nasal carriage of Staphylococcus aureus: epidemiology, underlying mechanisms, and associated risks

Staphylococcus aureus has long been recognized as an important pathogen in human disease. Due to an increasing number of infections caused by methicillin-resistant S. aureus (MRSA) strains, therapy has become problematic. Therefore, prevention of staphylococcal infections has become more important. Carriage of S. aureus appears to play a key role in the epidemiology and pathogenesis of infection. The ecological niches of S. aureus are the anterior nares. In healthy subjects, over time, three patterns of carriage can be distinguished: about 20% of people are persistent carriers, 60% are intermittent carriers, and approximately 20% almost never carry S. aureus. The molecular basis of the carrier state remains to be elucidated. In patients who repeatedly puncture the skin (e.g., hemodialysis or continuous ambulatory peritoneal dialysis [CAPD] patients and intravenous drug addicts) and patients with human immunodeficiency virus (HIV) infection, increased carriage rates are found. Carriage has been identified as an important risk factor for infection in patients undergoing surgery, those on hemodialysis or CAPD, those with HIV infection and AIDS, those with intravascular devices, and those colonized with MRSA. Elimination of carriage has been found to reduce the infection rates in surgical patients and those on hemodialysis and CAPD. Elimination of carriage appears to be an attractive preventive strategy in patients at risk. Further studies are needed to optimize this strategy and to define the groups at risk.

Gastrointestinal colonization by methicillin-resistant Staphylococcus aureus in immunosuppressed mice

ICR mice were inoculated intranasally with methicillin-resistant Staphylococcus aureus (MRSA) N133, and the inoculated MRSA was quantitatively recovered from the ceca and feces. The viable counts of the MRSA recovered from ceca correlated well with those from feces. Some mice eliminated MRSA from the cecum by 14 days after inoculation. Intraperitoneal administration of cyclophosphamide at a dose of 200 mg/kg 3 days before inoculation inhibited the elimination of the MRSA from both ceca and feces. All mice treated with cyclophosphamide excreted more than 10(4) CFU of the MRSA per g of feces for at least 70 days, indicating persistent colonization of the MRSA in the gastrointestinal tract. Some beta-lactam antibiotics decreased the colonization level, but others did not. The colonization level was suggested to depend on the antibacterial activity of the antibiotic against the MRSA and the degree of disturbance of intestinal flora by the antibiotic.

Nosocomial Staphylococcus aureus bacteremia among nasal carriers of methicillin-resistant and methicillin-susceptible strains

OBJECTIVES

To determine the relevance of nasal carriage of Staphylococcus aureus, either methicillin-sensitive (MSSA) or methicillin-resistant (MRSA), as a risk factor for the development of nosocomial S aureus bacteremia during an MRSA outbreak.

PATIENTS AND METHODS

In this prospective cohort study, 488 patients admitted to an intensive care unit (ICU) during a 1-year period were screened with nasal swabs within 48 hours of admission and weekly thereafter in order to identify nasal S aureus carriage. Nasal staphylococcal carriers were observed until development of S aureus bacteremia, ICU discharge, or death.

RESULTS

One hundred forty-seven (30.1%) of 488 patients were nasal S aureus carriers; 84 patients (17.2%) harbored methicillin-sensitive S aureus; and 63 patients (12.9%) methicillin-resistant S aureus. Nosocomial S aureus bacteremia was diagnosed in 38 (7.7%) of 488 patients. Rates of bacteremia were 24 (38%) of the MRSA carriers, eight (9.5%) of the MSSA carriers, and six (1.7%) of noncarriers. After adjusting for other predictors of bacteremia by means of a Cox proportional hazard regression model, the relative risk for S aureus bacteremia was 3.9 (95% confidence interval, 1.6-9.8; P = 0.002) for MRSA carriers compared with MSSA carriers.

CONCLUSIONS

Among ICU patients, nasal carriers of S aureus are at higher risk for S aureus bacteremia than are noncarriers; in the setting of an MRSA outbreak, colonization by methicillin-resistant strains represents a greater risk than does colonization by MSSA and strongly predicts the occurrence of MRSA bacteremia.

Clinical and economic effects of mupirocin calcium on preventing Staphylococcus aureus infection in hemodialysis patients: a decision analysis

This study was performed to determine the clinical and economic consequences of alternative strategies of preventing Staphylococcus aureus infection in chronic hemodialysis patients by use of intranasal mupirocin calcium to clear nasal carriage of S aureus. Decision analysis evaluated clinical outcomes and cost-effectiveness of three likely management strategies to address S aureus nasal carriage and prevent subsequent infection in chronic ambulatory hemodialysis patients: (1) screen for S aureus nasal carriage every 3 months and treat those with a positive test result with mupirocin calcium; (2) treat all patients weekly with mupirocin calcium; or (3) no prevention strategy, treat infection only. Rates of nasal carriage of S aureus, S aureus infection rates, proportion of infections attributable to nasal carriage, efficacy of mupirocin, natural history of infection, and patient management strategies were derived from the published literature and supplemented by a panel of experts. Actual payments for medical services were obtained from Medicare parts A and B. Incremental cost-effectiveness was calculated from the perspective of Medicare and subjected to sensitivity analyses. Assuming that 75% of S aureus infections are attributable to nasal carriage in hemodialysis patients, eliminating nasal carriage of S aureus with mupirocin calcium (with or without screening) markedly reduces the number of infections (45% to 55%) and also reduces health care expenditures relative to treating infections when they occur. Annual savings to Medicare are $784,000 to $1,117,000 per 1,000 hemodialysis patients, depending on the prevention strategy. Preventing S aureus infection by eradicating nasal carriage in chronic hemodialysis patients reduces morbidity while simultaneously reducing medical care costs. The decision to eliminate nasal carriage on a regular basis or use a screening test to guide antibiotic therapy is dependent on the tradeoff between improved short-term clinical and cost benefits and the potential for bacterial resistance that may arise from widespread use of mupirocin calcium.

New threats to the control of methicillin-resistant Staphylococcus aureus

Several countries have achieved considerable success in the control of epidemic methicillin-resistant Staphylococcus aureus (MRSA). However, in several hospitals in the UK, MRSA strains of enhanced epidemicity, notably EMRSA-16, are becoming endemic. Our inability to eliminate the cause of a single-strain outbreak is unfamiliar and unnerving. Factors in 'market-led' health care delivery that hinder control of MRSA include a shortage of inpatient beds, patients moving from ward to ward, and more mixed-specialty wards. Increasing use of day treatments leaves an inpatient hospital population with more risk factors for infection. Early discharge of infected patients to convalescent homes, or to homes for the elderly, has created a new reservoir of infected and colonized patients. The emergence of high-level mupirocin resistance may soon also contribute to failure of control. The transfer of vancomycin resistance from Enterococcus faecium to a laboratory strain of S. aureus suggests that, especially in hospitals with both vancomycin-resistant enterococci and MRSA, there is the opportunity for the emergence of vancomycin-resistant MRSA for which there may be no effective antimicrobial prophylaxis or treatment. It is increasingly important to persuade hospital managers that even partial control of MRSA, whilst expensive, is still cost-effective and is a quality issue for individual hospitals. The control of EMRSA-16 in one hospital has recently been estimated to have saved more than 629,000 pounds extra costs. MRSA continues to be at the forefront of those organisms that seriously challenge modern technological medicine and surgery.

Antimicrobial resistance in staphylococci

Staphylococci have developed a variety of strategies for dealing with the presence of antibiotics encountered in clinical environments. Resistance to beta-lactams and other antimicrobial agents has been accomplished by a diverse array of molecular mechanisms. Options available to treat infections caused by staphylococci resistant to methicillin are limited, and the next generation of antibiotics to be introduced, should glycopeptide resistance become an important clinical problem, is not yet on the horizon.

Nasal carriage of Staphylococcus aureus and cross-contamination in a surgical intensive care unit: efficacy of mupirocin ointment

A six month prospective study was carried out in a surgical intensive care unit (SICU) of a university hospital to assess the incidence and routes of exogenous colonization by Staphylococcus aureus. A total of 157 patients were included in the study. One thousand one hundred and eleven specimens (nasal, surgical wound swabs, tracheal secretions obtained on admission and once a week thereafter, and all clinical specimens) were collected over a four month period from patients without nasal decontamination (A). They were compared with 729 specimens collected over a two month period from patients treated with nasal mupirocin ointment (B). All S. aureus strains were typed by restriction fragment length polymorphism (RFLP) pulsed-field gel electrophoresis after SmaI macrorestriction. The nasal colonization rates on admission were 25.5 and 32.7% in groups A and B, respectively. Thirty-one untreated patients (31.3%) and three patients (5.1%) treated with nasal ointment, acquired the nasal S. aureus in the SICU (P = 0.00027). Nasal carriers were more frequently colonized in the bronchopulmonary tract (Bp) and surgical wound (Sw) (62%) than patients who were not nasal carriers (14%) (P < 0.00001). The patterns were identical for nasal, Bp and Sw strains from the same patient. RFLP analysis characterized seven epidemic strains of methicillin-resistant S. aureus (MRSA) which colonized 60% of group A and 9% of group B patients (P < 0.00001). The bronchopulmonary tract infection rate was reduced in group B (P = 0.032). In conclusion, in an SICU, nasal carriage of S. aureus appeared to be the source of endogenous and cross-colonization. The use of nasal mupirocin ointment reduced the incidence of Bp and Sw colonization, as well as the MRSA infection rate.