Community-associated meticillin-resistant Staphylococcus aureus carriage in hospitalized patients in tropical northern Australia

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Summary

Background

Community-associated meticillin-resistant Staphylococcus aureus (CA-MRSA) was first reported in remote Australian Aboriginal communities. It is a prominent clinical pathogen in northern Australia with potential for transmission within the local hospital setting.

Aim

To determine epidemiological characteristics of S. aureus carriage within the Royal Darwin Hospital.

Methods

We screened two patient groups: an ‘admission group’ recruited within 48 h of admission; and an ‘inpatient group’ recruited five or more days after admission. S. aureus isolates were characterized by antibiotic susceptibility testing and genotyped by a multi-locus sequence type-based high-resolution melting scheme.

Findings

S. aureus carriage on admission was 30.7% of 225 compared with 34.8% among 201 inpatients, with MRSA carriage of 2.2% and 18.9% respectively. We isolated CA-MRSA from 0.9% and 10.4%, and healthcare-associated (HCA)-MRSA from 1.3% and 9.0% of the admission and inpatient groups, respectively. Among the inpatient group, hospital-associated ST239 was the most common MRSA strain. CA-MRSA was represented by one clonal complex (CC) in the admission group (CC5) and seven CCs in the inpatient group (CC1, 93, 5, 6, 30, 75, 88).

Conclusion

Inpatient carriage of multiple CA-MRSA lineages suggests selection for and transmission within the hospital of not only typical HCA-MRSA, but also diverse CA-MRSA strains.

Introduction

The first reported cases of community-associated meticillin-resistant Staphylococcus aureus (CA-MRSA) infection were from remote Aboriginal communities in Western Australia in 1989.1 These clones had a distinct antibiotic susceptibility pattern and genotype from that of healthcare-associated (HCA)-MRSA circulating in Australian hospitals. In the tropical Top End of the Northern Territory, where 28% of the population are Aboriginal Australians, CA-MRSA and HCA-MRSA represent 15% and 6%, respectively, of clinical S. aureus isolates at the Royal Darwin Hospital (RDH).2 These CA-MRSA disease isolates are represented by multiple lineages, most notably clonal complexes (CC) 1, 5, 30, 75 and 93.3 By contrast, the major HCA-MRSA lineage in Australia is sequence type (ST) 239.4 Whereas 62% of HCA-MRSA at the RDH were found to be nosocomially acquired, the corresponding proportion for CA-MRSA was only 16%.2

Hospital-based CA-MRSA infection and transmission is of increasing concern. The USA300 CA-MRSA strain currently represents half of nosocomial-acquired MRSA infections in some US hospitals.5 Spread of CA-MRSA into hospitals has also been identified in Canada, the UK and Asia.6, 7, 8 By contrast, Australia's only reported CA-MRSA hospital outbreak was at a metropolitan Perth hospital in 1997.9 This is despite frequent admission of patients with CA-MRSA disease to local hospitals and significant rates of community CA-MRSA carriage in screening studies.4, 9, 10 This suggests that, unlike in the USA and elsewhere, Australian CA-MRSA clones may not be well adapted to the nosocomial environment.

In this study, the molecular epidemiology of S. aureus carriage was investigated among the RDH patient population. Given that 15% of clinical infections are caused by CA-MRSA, we expected a significant prevalence of asymptomatic CA-MRSA carriage at hospital admission. The study also aimed to determine whether inpatient hospitalization at RDH conferred an increased risk for CA-MRSA carriage, suggesting nosocomial CA-MRSA transmission.

Section snippets

Study population

The RDH, a tertiary referral centre for the city of Darwin, two regional hospitals and more than 70 remote communities, is a 350-bed hospital with 49,000 annual admissions. Aboriginal Australians represent 28% of the Top End population but account for more than 50% of RDH admissions and inpatients.

We conducted two cross-sectional surveys of patients to assess S. aureus carriage. An admission group was recruited from patients within 48 h of hospital admission. A separate inpatient group was

Recruitment and S. aureus carriage

We recruited 225 patients within 48 h of admission and 201 patients admitted for five or more days (Figure 1 and Table I). S. aureus was isolated from at least one swab site in 30.7% of admission patients (yielding 73 non-duplicate isolates) and 34.8% of inpatients (77 non-duplicate isolates). There was no significant difference between groups in the prevalence of S. aureus carriage [odds ratio (OR): 0.83; 95% confidence interval (CI): 0.55–1.24] nor in the sites of detection (Table II).

Discussion

In this study of S. aureus colonization in two distinct groups, there were two notable findings. First, in the setting of previously documented high rates of CA-MRSA infection in the Aboriginal population, carriage of CA-MRSA on admission to RDH was infrequent. Second, the significantly higher prevalence of MRSA carriage in the inpatient group was associated with multiple CA-MRSA strains in addition to expected HCA-MRSA.

Despite CA-MRSA comprising 15% of clinical S. aureus isolates at the RDH,

Conflict of interest statement

None declared.

Funding sources

S.T. is supported by an Australian National Health and Medical Research Council Postdoctoral Training Fellowship (508829). A.C. is supported by an NHMRC Career Development Fellowship.

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