Invasive Streptococcus dysgalactiae on the rise in Australia

By Sasha Ellery BM BCh

Invasive Streptococcus dys­galactiae subspecies equisimilis (SDSE) infections are emerging as a growing public health concern in Australia, revealing distinct transmission patterns, with First Nations Australians disproportionately affected, according to a large clinical and genomic epidemiology study published in The Lancet Microbe.

The 12-year retrospective study compared 693 cases of invasive SDSE with 995 invasive cases of Streptococcus pyogenes (Strep A), a close relative of SDSE, between 2011 and 2023, spanning both temperate southeast Australia and the tropical Top End of the Northern Territory.

Speaking to Medicine Today, Dr Ouli Xie and Associate Professor Mark Davies, first and co-lead author on the paper, respectively, said, ‘SDSE has long been considered a relatively benign pathogen or even spread from animals, but our work and that of other groups has shown that strains causing human disease are in fact human adapted.’ They noted that it was not a benign disease, and could cause the same serious illnesses as Strep A, such as bloodstream infection, toxic shock and necrotising fasciitis.

In southeast Australia, the researchers found that the overall incidence of SDSE infections increased over the study period and was comparable to that of Strep A. Strep A infections dropped significantly during the COVID-19 lockdowns of 2020 to 2021; in contrast, SDSE cases plateaued but did not decline, suggesting different transmission dynamics.

The pattern of infection was different in the Northern Territory.

‘The burden of Strep A disease [in the Top End] is among the highest in the world,’ explained Dr Xie and Associate Professor Davies. ‘Our data show that while [the incidence of] invasive SDSE disease is lower than that of Strep A in the Top End, there is still an elevated burden, particularly in First Nations individuals, reflecting the inequalities in the determinants of health.’

The researchers also identified a dominant genetic lineage, stG62647, responsible for more than a quarter of SDSE cases.

‘This finding is important for existing molecular surveillance systems... enabling pathways for tracking changes in the population and spread of this variant between health jurisdictions,’ Dr Xie and Associate Professor Davies noted. They explained that the targets of several vaccine candidates originally developed for S. pyogenes were also present in SDSE, meaning these vaccines could potentially target SDSE as well, especially its dominant and emerging strains.

Unlike Strep A, few SDSE isolates were genetically similar enough to suggest recent person-to-person transmission. Individuals with recurrent infections were often infected with nearly identical strains, implying long-term carriage rather than reinfection.

‘Our study suggests that SDSE may persist for longer in individuals before causing invasive disease. In contrast to Strep A, contact tracing may not be as successful in preventing secondary cases of invasive SDSE,’ Dr Xie and Associate Professor Davies said.

The researchers concluded that invasive SDSE represented a substantial but under-recognised burden in Australia and called for expanded surveillance, research into SDSE carriage and immunity and continued efforts to reduce health disparities.

‘Older individuals and those with underlying conditions such as diabetes or obesity are at higher risk of developing a serious SDSE infection. Additionally, individuals with bloodstream infections associated with cellulitis as the source are at greater risk of recurrent infections,’ said Dr Xie and Associate Professor Davies.

‘Overall, greater awareness is required of the significance of SDSE as a pathogen, with early antibiotic treatment and/or referral in those who present with signs of sepsis.’ 

Lancet Microbe 2025; 6: 101182; doi: 10.1016/j.lanmic.2025. 101182.