Community-associated strains of methicillin-resistant Staphylococcus aureus and methicillin-susceptible S. aureus in indigenous Northern Australia: epidemiology and outcomes.

BACKGROUND: Some strains of non-multidrug-resistant, methicillin-resistant Staphylococcus aureus (nmMRSA) in Australia are likely to have emerged from strains of methicillin-susceptible S. aureus (MSSA) in remote Aboriginal communities.

OBJECTIVE: To describe the clinical epidemiology of infection due to community-associated MRSA strains in an Australian tropical hospital setting with a significant Aboriginal population and to compare infections caused by community-associated strains of MRSA, health-care-associated strains of MRSA, and MSSA strains with respect to demographic risk factors and clinical outcomes. Methods. We queried the microbiology database for the Top End of the Northern Territory, Australia, to determine population incidences for S. aureus infection and conducted a prospective matched case-control study to compare infection due to nmMRSA, MSSA, or multidrug-resistant MRSA at the Royal Darwin Hospital.

RESULTS: The annual incidence of S. aureus bacteremia was 65 cases per 100,000 population, but in the Aboriginal population the incidence was 172 cases per 100,000 population (odds ratio [OR] compared with non-Aboriginal population, 5.8 [95% confidence interval {CI}, 3.8-8.9). Female sex (adjusted OR [aOR], 1.5 [95% CI, 1.1-2.0) and remote residence (aOR, 1.8 [95% CI, 1.2-2.5]) were associated with the isolation of nmMRSA rather than MSSA, but disease spectrum and outcomes were similar. Among those from whom nmMRSA was isolated, Aboriginal patients were younger (aOR for each additional year, 0.94 [95% CI, 0.92-0.96]), more likely to be female (aOR, 3.8 [95% CI, 1.7-8.5]), and more likely to reside in a remote community (aOR, 29 [95% CI, 8.9-94]) than non-Aboriginal patients. The presence of Panton-Valentine leukocidin in nmMRSA was associated with double the odds of sepsis (aOR, 2.2 [95% CI, 1.1-4.6]).

CONCLUSIONS: The association of nmMRSA infection with female sex and remote residence supports the hypothesis that nmMRSA arose from MSSA strains in remote Aboriginal communities where staphylococcal disease is highly prevalent. The similar clinical spectrum and outcomes for nmMRSA infection and MSSA infection suggest that virulence is not correlated with resistance phenotype.