A new strain of Strep A is causing scarlet fever and invasive infections in England and Wales, according to data published in The Lancet Infectious Diseases journal.

In 2014, England experienced the biggest surge in scarlet fever cases since the 1960s. Numbers continued to increase, with 15,000 cases in 2014, 17,000 in 2015 and over 19,000 in 2016.

In this new study, the authors provide an explanation for the association between increased incidence of scarlet fever and increased incidence of more serious invasive infections such as bloodstream infections. They uncovered a new strain of Streptococcus pyogenes with increased capacity to produce scarlet fever toxin.

“Given that this strain has an apparently enhanced ability to cause all types of Strep A infection, it is important to monitor the bacterium both here and globally,” says joint first author, Dr Nicola Lynskey from Imperial College London.

What is scarlet fever?

Scarlet fever is caused by toxins released by the bacterium Streptococcus pyogenes, also known as Strep A, and cases follow a seasonal pattern peaking between March and May. Scarlet fever is easily treated with antibiotics. Cases of invasive infections caused by the same bacterium also increased in 2016 compared to the previous five years.

Symptoms, which affect young children, include a high temperature, sore throat, and a pink-red rash that feels like sandpaper.

Scarlet fever lasts for around a week, but patients are infectious up to seven days before the symptoms start until 24 hours after they take the first antibiotic tablets. People who do not take antibiotics can be infectious for two to three weeks after symptoms start.

Need to install global surveillance systems for scarlet fever

The authors speculate that the recent increase in activity of the Streptococcus pyogenes bacterium, which coincided with upsurges of scarlet fever, might have provided the conditions required for it to adapt genetically and spread within the UK.

“The distinct bacterial clone we have discovered appears so far to be largely limited to the UK, but the fact that we have identified two examples of it elsewhere suggests it has the potential to spread internationally and may already be present in other countries. However, it’s also possible that the lineage will not last. In the past, some lineages have appeared and then disappeared quickly. Only further research on recent strains will provide more insights.” says senior author Professor Shiranee Sriskandan from Imperial College London, UK.

Writing in a linked Comment, Professor Mark Walker from the University of Queensland, Australia, says: “The continuing increase in scarlet fever and invasive disease notifications in the UK exemplifies the essential need to install global surveillance systems and address the increased GAS disease activity as a public-health priority. We believe that the report by Lynskey and colleagues sends out an important warning for the global public health community - recently emerging scarlet fever GAS strains have enhanced invasive potential which may have profound implications for the future global health burden.”