Impact of ethnicity and socio-economic status on Staphylococcus aureus bacteremia incidence and mortality: a heavy burden in Indigenous Australians

Background: Investigations of the impact of ethnicity and socio-economic status on incidence and outcomes of Staphylococcus aureus bacteraemia are limited. Methods: We prospectively identified all S. aureus bacteraemia episodes in the Australian New Zealand Cooperative on Outcomes in Staphylococcal Sepsis cohort study between 2007 and 2010. We calculated population level incidence rates using regional postcodes and stratified the analysis by ethnicity, age and socio-economic status indexes. Results: There were 7539 episodes of S. aureus bacteraemia with an annual incidence of 11.2 episodes per 100,000 population. The age-adjusted incidence in the Indigenous population was 62.5 per 100,000 population with an age standardized incidence rate ratio of 5.9 compared to the non-Indigenous population and an incidence rate ratio of 29.2 for community-associated methicillin-resistant S. aureus (MRSA). Populations in the lowest socio-economic status quintile had an increased S. aureus bacteraemia incidence compared to higher quintiles. However, there was a disparity between Indigenous and non-Indigenous populations across all socio-economic status quintiles. The lower 30-day mortality for Indigenous patients (7%) compared to non-Indigenous patients (17%) was explained by differences in age. Conclusions: Indigenous Australians suffer from a higher rate of S. aureus bacteraemia than non-Indigenous Australians, particularly for community-associated MRSA. Ethnicity and socio-economic status had little impact on subsequent mortality, with other host factors contributing more significantly.