Oral Presentation Lancefield International Symposium for Streptococci and Streptococcal Diseases 2025

Unravelling the temporal and geographic strain dynamics of invasive group A streptococci in Australia (118508)

Ouli Xie 1 , Rebecca Chisholm 2 , Andrew Hayes 1 , Gerry Tonkin-Hill 1 , Ravindra Dotel 3 , Matthew O'Sullivan 4 , Katherine Bond 5 , Tony Korman 6 , Bart Currie 7 , Steven Tong 8 , Mark Davies 1
  1. The University of Melbourne, Parkville, VIC, Australia
  2. Department of Mathematical and Physical Sciences, La Trobe University, Melbourne, Australia
  3. Blacktown Hospital, Sydney, NSW, Australia
  4. Westmead Hospital, Sydney, NSW, Australia
  5. Royal Melbourne Hospital, Melbourne, Vic, Australia
  6. Monash Health, Melbourne, Vic, Australia
  7. Menzies School of Health Research, Charles Darwin University, Darwin, NT, Australia
  8. Department of Infectious Diseases, The University of Melbourne at the Peter Doherty Institute for Infection and Immunity, Melbourne, Vic, Australia

Background

Invasive group A streptococcal disease which is caused by the bacterial pathogen Streptococcus pyogenes disproportionately affects disadvantaged communities including Aboriginal and Torres Strait Islander Australians. To better define the temporal dynamics of invasive S. pyogenes, we examined the population genomics of clinical cases between the hyperendemic Top End of the Northern Territory and lower incidence regions in temperate south-east (SE) of Australia.

Methods

995 retrospective cases of invasive S. pyogenes isolated between January 2011 to February 2023 were retrieved and genome sequenced. A S. pyogenes-specific multi-strain simulated transmission model was used to examine the relationship between host population-specific parameters and observed pathogen lineage dynamics.

Findings

The crude incidence rate ratio (IRR) of bloodstream cases was 5·97 for the Top End compared to SE Australia across the entire study period and disproportionately affected First Nations compared to non-First Nations people in the Top End. Population genomics revealed that the Top End was characterised by waves of lineage replacement compared to endemic maintenance in the SE. Despite distinct circulating lineages, the frequency of genes in the bacterial population was maintained across geographical and temporal periods. A multi-strain transmission model was applied to this dataset which revealed that transmission rate, population size, and human movement, are key parameters in replicating the observed lineage dynamics.

Conclusion

These findings provide a baseline for defining the national genomic epidemiology of invasive S. pyogenes and suggest that interventions such as vaccines-in-development without broad cross-protection could also lead to lineage replacement.