Review
Community-associated meticillin-resistant Staphylococcus aureus strains as a cause of healthcare-associated infection

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Summary

Community-associated meticillin-resistant Staphylococcus aureus (CA-MRSA) was first noticed as a cause of infection in community-based individuals without healthcare contact. As the global epidemic of CA-MRSA has continued, CA-MRSA strain types have begun to emerge as a cause of healthcare-associated infections (HAIs) and hospital outbreaks have occurred worldwide. In areas where CA-MRSA clones have become established with high prevalence, for example USA300 (ST8-IV) in the USA, CA-MRSA are beginning to supplant or overtake traditional healthcare-associated MRSA strains as causes of HAI. The emergence of CA-MRSA as a cause of HAI puts a wider group of hospitalised patients, healthcare workers and their community contacts potentially at risk of MRSA infection. It also exposes CA-MRSA strains to the selective pressure of antibiotic use in hospitals, potentially resulting in increased antibiotic resistance, challenges traditional definitions of CA-MRSA and hampers control efforts due to the constant re-introduction of MRSA from an emerging community reservoir. There is thus an urgent need to clarify the definitions, prevalence and epidemiology of CA-MRSA and to develop systems for the identification and control of these organisms in the community, in hospitals and other healthcare facilities, and at the community–hospital interface.

Introduction

Community-associated meticillin-resistant Staphylococcus aureus (CA-MRSA) infections were first noticed in patients without prior healthcare contact in Western Australia and New Zealand in the early 1990s and in American children in the late 1990s.1, 2 CA-MRSA appears to have arisen by the acquisition of mobile SCCmec cassettes by community strains of meticillin-susceptible S. aureus (MSSA).3, 4 These strains, like their parents, can affect younger, healthy people and spread readily in community settings. By contrast, healthcare-associated (HA)-MRSA strains usually cause infection in hospitalised, compromised, elderly patients, often those with a history of surgery, indwelling devices and/or antimicrobial therapy.5, 6, 7

CA-MRSA strains are generally susceptible to non-β-lactam antibiotics and usually have small (types IV or V) SCCmec cassettes; many of them also produce the Panton–Valentine leucocidin (PVL).4, 7 Since the 1990s, CA-MRSA strain types have appeared in many countries; and although they are now frequently found in the USA where the USA300 type is endemic, they are sporadic, heterogeneous and much less common in most other developed countries.7, 8

Here, we review reports of CA-MRSA strains as a cause of outbreaks in healthcare settings and evidence that CA-MRSA strains are beginning to supplant or overtake traditional HA-MRSA lineages as a common cause of hospital infection. To identify literature to review, a PubMed search of papers written in English was performed using the search terms ‘community MRSA healthcare’. Relevant articles from the bibliographies of articles identified by PubMed searches were also included.

Section snippets

CA-MRSA strains as a cause of hospital outbreaks

As the global epidemic has evolved, CA-MRSA strain types have begun to emerge as a cause of hospital outbreaks.9, 10, 11 Nosocomial outbreaks of CA-MRSA have been reported since 2003 from North America, Germany, Israel, Switzerland, Greece and the UK, often affecting specialties where the prevalence of HA-MRSA is low, such as paediatrics and obstetrics.12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26

Table I summarises 18 outbreaks of CA-MRSA strain types in healthcare settings

CA-MRSA strains as a cause of endemic healthcare-associated infection (HAI)

Recent reports suggest that CA-MRSA clones are beginning to supplant or overtake HA-MRSA clones as a cause of endemic HAI.9, 11 USA300 is an increasingly common community pathogen in the USA with a parallel tendency to cause nosocomial infections after entry into hospitals. Initial reports of USA300 in healthcare settings included outbreaks in newborns and as a cause of postoperative prosthetic joint infections.12, 36 Subsequently, USA300 has been reported as causing a significant proportion of

Implications of the emergence of CA-MRSA strains as hospital pathogens

Data regarding the emergence of CA-MRSA strains as a cause of HAI are mostly limited to developed countries with sophisticated surveillance networks and access to the latest strain typing methods. The current status of CA-MRSA in general and as a cause of HAI remains largely unknown in less developed countries. There is a suggestion from a few sporadic reports that CA-MRSA may already have emerged as a common cause of HAI in parts of Asia, Africa and South America.47, 48, 49, 50, 51

The reasons

Conflict of interest statement

J.A.O. is employed part-time by Bioquell (UK) Ltd. G.L.F. has no conflict of interest.

Funding sources

None.

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      Recently, however, CA-MRSA strains have emerged as a cause of HA infection in some parts of the world [13], challenging definitions of CA-MRSA based on clinical epidemiology and where disease manifests [14–16] in favour of genotype-based definitions [17–19]. Nonetheless, CA-MRSA strains retain a number of important characteristics, notably the association with infection in previously healthy individuals in the community [7,8,12,16,20]. Outbreaks in hospitals and nursing homes are today caused by both HA-MRSA and CA-MRSA clones [21–23], and it has been suggested that it is no longer useful to regard HA-MRSA and CA-MRSA as separate entities [24].

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