S. anginosus Infections Rise Sharply Across the UK

Serious infections caused by the Streptococcus anginosus group (SAG) are increasing among both adults and children in the UK, according to a large retrospective surveillance study.

Researchers found substantial increases in SAG infections involving normally sterile areas of the body, with rates rising at two major London hospitals over the study period. The findings suggest that the increase cannot be attributed solely to changes in diagnostic practices or increased testing and could represent a genuine shift in the epidemiology of these bacteria.

The SAG comprises three species: S. anginosus, S. intermedius, and S. constellatus. Although increasing rates of invasive intracranial SAG infections have previously been reported in children, the broader epidemiology of these organisms across different age groups and infection sites has remained poorly characterised.

Serious SAG Infections Increase Over Time

Researchers analysed microbiological laboratory data from Great Ormond Street Hospital (GOSH) between 2000 and 2023 and University College London Hospitals (UCLH) between 2012 and 2023. Nationally reported streptococcal bacteraemia data from 2015–2023 were also assessed.

At GOSH, the incidence of SAG sterile body site infections increased from 2.12 per 1,000 emergency admissions in 2012 to 11.51 per 1,000 in 2023. A similar trend occurred at UCLH, where incidence rose from 0.97 to 6.04 per 1,000 emergency admissions over the same period. Both increases were statistically significant (p<0.0001).

SAG diagnoses among external samples referred to GOSH for molecular testing also increased substantially relative to diagnoses involving other Streptococcus species.

Across the analysed groups, SAG had become the dominant cause of streptococcal sterile body site infection by the end of the surveillance period.

CNS Infections Particularly Common in Children

The anatomical distribution of infection varied considerably. Among 1,133 SAG sterile body site infections assessed by location, 24.9% involved intrathoracic sites, 19.6% involved the central nervous system (CNS), and 17.8% involved abdominal sites.

CNS involvement was particularly prominent among children, who accounted for 87 of the 222 CNS infections identified, representing 39.2% of these cases.

Different SAG species also demonstrated distinct infection patterns. S. intermedius was the predominant species associated with CNS and intrathoracic infections, whereas S. anginosus and S. constellatus predominated in abdominal infections. Importantly, however, all three species contributed to the overall increase.

Changing Epidemiology Requires Further Investigation

The researchers concluded that SAG infections, which commonly manifest as deep pyogenic collections in normally sterile sites, represent an increasingly important cause of serious streptococcal disease in the UK.

Although improvements in molecular diagnostics and changes in sampling practices could contribute to increased detection, the authors concluded that these factors alone were unlikely to explain the scale of the observed rise.

The mechanisms driving the trend remain uncertain. Potential explanations requiring further investigation include changes in SAG virulence, bacterial carriage rates or transmission dynamics, as well as other unidentified environmental or host factors.

The findings highlight the need for further epidemiological and microbiological research to determine why serious SAG infections are becoming increasingly common and whether additional surveillance or prevention strategies are warranted.

Reference

Gil E et al. Streptococcus anginosus group as a cause of serious streptococcal infection in the UK: a retrospective population-based surveillance study. The Lancet Microbe, 2026; 10.1016/j.lanmic.2026.101436 .