Community-associated meticillin-resistant Staphylococcus aureus: nosocomial transmission in a neonatal unit

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Summary

Community-associated meticillin-resistant Staphylococcus aureus (CA-MRSA) is an emerging pathogen, increasingly reported worldwide to cause infections in individuals without classical risk factors for acquiring healthcare-associated MRSA (HA-MRSA). This report describes the first documented transmission of CA-MRSA in a healthcare setting in the UK, involving four babies and a member of staff in a neonatal unit. Detailed microbiological characterization of the isolates revealed that they represented a single clone with the following characteristics: multi-locus sequence type (MLST) 1; staphylococcal cassette chromosome mec (SCCmec) type IVa; protein A (spa) type t127; agr group 3, and encoding enterotoxins A and H. The Panton-Valentine leukocidin genes were not detected. The CA-MRSA strain appeared to be circulating alongside several subtypes of epidemic MRSA-15, the most prevalent HA-MRSA in the UK. A combination of infection control measures contained the outbreak. This report highlights the changing epidemiology of MRSA in the UK, and emphasizes the need for healthcare personnel to be alert to the fact that CA-MRSA can occur not only in the community but also in the healthcare setting.

Introduction

Following the first reports of epidemic meticillin-resistant Staphylococcus aureus (EMRSA) in the UK, 17 strains (referred to as EMRSA-1 to -17) have been identified in UK healthcare settings.1 Currently, two major clones predominate in hospitals in the UK, namely EMRSA-15 and -16.2

New clones of MRSA have recently emerged in England and Wales and have been reported to occur in the community worldwide.3, 4, 5, 6, 7 These organisms have been found in patients without the classical risk factors for MRSA infection, such as recent hospitalization, care in an intensive therapy unit, surgery, nursing home residence or recent antimicrobial therapy.8 The emergence of such strains gained prominent profile following reports of four paediatric deaths in the USA in Minnesota and North Dakota in 1999,9 in addition to fatal cases of community-acquired necrotizing pneumonia and purpura fulminans occurring in previously healthy young adults without known hospital contact.10, 11 These isolates have been referred to variously as ‘community-acquired’, ‘community-onset’ and ‘community-adapted’ strains to highlight the fact that they represent clones with differing genetic backgrounds, pathogenicity and epidemiological characteristics compared with healthcare-associated MRSA (HA-MRSA). Whilst the terminology can be confusing, the term that is emerging as being increasingly favoured to describe these isolates is community-associated MRSA (CA-MRSA).

True CA-MRSA possess a number of features that differentiate them from HA-MRSA, including susceptibility to most antimicrobial agents other than β-lactams, carriage of staphylococcal cassette chromosome mec (SCCmec) type IV or V, presence of the Panton-Valentine leukocidin (PVL) genes in some lineages, and DNA fingerprinting profiles that are distinct from those of HA-MRSA.4, 12 CA-MRSA are therefore genetically distinct from HA-MRSA. Furthermore, multiple clonal lineages have been reported in the UK and worldwide, suggesting that CA-MRSA have arisen from diverse genetic backgrounds rather than the worldwide spread of a single clone.3, 4, 13, 14, 15, 16

Prior healthcare exposure may lead to the misidentification of HA-MRSA in individuals in the community as CA-MRSA, resulting in an overestimate of the prevalence of CA-MRSA.17, 18 It is often difficult to establish the origin of such isolates reliably, even when risk factors related to the healthcare environment are rigorously assessed. Various studies have shown that it is not possible to define CA-MRSA based solely on demographic and epidemiological information.17, 18 To identify true CA-MRSA and differentiate them reliably from HA-MRSA, detailed microbiological analyses must be performed.3, 4, 14, 16

Epidemiological data have shown that in addition to causing sporadic disease in the community, outbreaks of CA-MRSA have occurred among family members, prison inmates, homosexual men, military recruits, injecting drug users and ethnic groups.5, 6, 19, 20

Whilst HA-MRSA remains a serious threat to hospitalized patients, recent reports of CA-MRSA causing outbreaks in hospitals have raised concern over the apparent change in the pathogenesis and epidemiology of MRSA worldwide.7, 21, 22 This report describes the isolation of CA-MRSA from four patients and one staff member on a neonatal ward. These data highlight the fact that CA-MRSA are not restricted to the community setting, but have the potential to infiltrate and spread in healthcare establishments in the UK.

Section snippets

Setting

Birmingham Heartlands and Solihull Hospitals form a 1300-bed hospital trust in which approximately 6600 infants are delivered every year. The neonatal unit (NNU) admits 650 babies of all gestational ages annually, 90% of whom are born in the hospital. The NNU comprises one intensive therapy unit with five cots, one high-dependency unit that can admit up to six babies, and a special care unit that has a quota of 20 cots (two rooms with eight cots each, a room with three cots and one room with a

Characteristics of MRSA

From August to October 2004, 11 babies on the NNU and three staff members were identified as being colonized with MRSA. Antibiogram data together with phage typing and PFGE-based analyses identified five isolates (from four babies and one staff member) as being distinct from HA-MRSA currently prevalent in the UK. The remaining nine isolates, from seven babies and two staff, were shown by PFGE to be recognized subtypes of EMRSA-15 (data not shown).

Characterization of the five S. aureus isolates

Discussion

To the authors' knowledge, this is the first report of hospital transmission of CA-MRSA in the UK, and echoes similar experiences reported in Australia, the USA and Israel.7, 21, 22, 24 The authors were alerted to the appearance of an unusual MRSA strain as a result of enhanced surveillance of neonatal patients in a ‘high-risk’ environment. All five CA-MRSA were clonal (ST1-MRSA-SCCmecIVa). This strain proved to be indistinguishable from a CA-MRSA clone occurring in Australia, and is therefore

Acknowledgements

The authors thank the staff of the West Midlands Public Health Laboratory for their technical support and the NNU consultants for allowing the study of their patients. In addition, the authors thank Ilka McCormick Smith and Sayeh Sabersheikh for excellent laboratory support.

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    Copyright © 2006 The Hospital Infection Society. Published by Elsevier Ltd. All rights reserved.