Analysis reveals new evidence of global surge in group A streptococcus infections

Group A Streptococcus (Strep A) is a common type of bacteria that commonly causes throat infections and scarlet fever. Although most infections are mild, in rare cases Strep A can cause invasive infections that can be fatal.

Between 2022 and 2023, health services around the world recorded a surge in severe invasive Strep A infections after pandemic restrictions were lifted. A new study by scientists from Imperial College London and the UK Public Health Agency confirmed that a bacterial variant called M1UK played a key role in this increase.

M1 strains are known to cause more invasive infections compared to other types of Strep A. Although this variant was first identified and sequenced in the UK, M1UK has recently been associated with an increase in severe infections in Europe, Australia, North America and Japan following lifting pandemic restrictions. It has also been found in South America, New Zealand and Taiwan.

The low rate of Strep A infections during the pandemic has likely left the population, especially children, particularly vulnerable to these infections, contributing to the surge in cases in many countries.

In a new study on the genetics of M1UK, published in Nature Communications, scientists provide new data on when the variant first emerged and how it compares to other strains.

Global distribution and potential introductions of M1UK and intermediate populations. A Phylogenetic tree of 2364 M1UK and intermediate strains collected worldwide from March 2005 to July 2023. B Simplified transmission tree generated with PastML showing the location of the ancestral epidemic origin of M1UK lineages and intermediate populations. C Estimation of the effective population size (Ne) of M1UK in the UK over time. Nature Communications (2024). DOI: 10.1038/s41467-024-47929-7

Using genetic sequencing of patient samples combined with computer modeling, they found that M1UK likely emerged around 2008 and began to increase gradually from 2010.

Compared to earlier M1 strains, which have remained relatively unchanged over the past 40 years, M1UK produces more of the toxins that cause scarlet fever.

Professor Shiranee Sriskandan, from the Department of Infectious Diseases at Imperial College London, who led the study with UKHSA colleagues, said: “Bacteria evolve over time, and because Strep A is exclusively a human pathogen, it acquires genetic changes as it spreads through the population. Most variants come and go over time, however M1UK appears to have now taken root in the UK and other countries. Whether this will remain so in the long term is unknown.”

Genetic history

Using searches of global genomic databases, the team showed that when M1UK was first discovered in the UK in 2019, only two M1UK strains could be identified from genomic sequences outside the UK. But subsequent analysis from other countries examining changing trends in Strep A infections (including invasive infections) shows that M1UK is now present in more than 10 countries around the world and in some cases has become the dominant strain in the post-pandemic period.

The analysis also highlights a bottleneck effect in which Strep A infections have declined during the COVID-19 pandemic, coinciding with widespread social distancing measures and lockdowns. But after this period, M1UK spread rapidly in the UK, with a marked increase in cases of scarlet fever and invasive Strep A infections in 2022.

According to the researchers, although reduced transmission of Strep A during the pandemic may have led to weakened immunity in the population, their work highlights genetic features of M1UK that indicate a viability advantage over other strains, allowing it to spread more easily and potentially cause more severe illnesses. Diseases. It is possible that increased population immunity to the M1UK strain may allow other types of strains to emerge.

Professor Sriskandan, also clinical director of the Center for Bacterial Resistance Biology, added: “Although we now know that the M1UK variant was responsible for the significant increase in cases last winter, this occurred at an unusual time of year, coinciding with respiratory viruses and winter stress. Which all contributed to the severity. Ultimately, the spikes we are seeing at various times around the world are likely due to decreased immunity to certain respiratory pathogens, especially Strep A. Vaccines could help prevent this, and we really need a vaccine against Strep A. These latest findings Our research was made possible through close collaboration between infectious disease researchers at Imperial College and the UK Public Health Agency's world-leading molecular surveillance system."