A new strain of group A streptococcus, which is able to produce significantly more toxin, is spreading in England and Wales, scientists say.
Strep A causes a range of infections, from a sore throat to scarlet fever.
The new strain's emergence, reported in The Lancet Infectious Diseases, coincides with a rise in rare but potentially deadly invasive infections.
However, it does not explain the mysterious surge in scarlet fever cases during the past five years.
And there is no direct evidence the new strain causes more serious infection than other strains, experts say.
Scarlet fever is a very contagious infection that mostly affects young children.
The number of children with scarlet fever soared in 2014 and two years later there was an increase in cases of invasive strep A, where the bug penetrates deeper into the body and causes problems such as sepsis (blood poisoning).
The latest figures showed there were 1,500 invasive cases in England in 2018-19 – 8% higher than the average for the previous five years.
A team from Imperial College London analysed the DNA, or genetic code, of stored strep A samples from patients to see if a change in the bacterium could explain the rise.
Strep A is made up of several "families" known as emm-types. Emm1 has been linked to invasive cases in the past and the study showed it had mutated to form the new strain.
"We found no trace of it in the UK until about 2010," one of the researchers Prof Shiranee Sriskandan told BBC News.
"And it took off between 2011 and 2013, so that by 2016 it represented 80% of emm1 strains – it's taken over its own family."
The mutated strain is able to produce nine times more toxin (streptococcal pyrogenic exotoxin A) than the previous version.
This is the toxin that actually causes the symptoms of scarlet fever – flu-like symptoms, sore throat, swollen glands and a rash.
However, Prof Sriskandan told BBC News: "This study doesn't offer any answers to why scarlet fever came back in 2014."
There is also no evidence, she says, that the toxin is causing the invasive cases of the disease.
The new strain has largely been confined to the UK. There has been one case in Denmark and another in the US.
"It's important because although invasive disease is very rare due to strep A, it has quite a substantial impact on the people it affects," said Prof Sriskandan.
"Mortality from invasive disease is between 15% and 20% – these are very high mortality rates."
Scarlet fever – because it is easily noticed and cases have to be reported to Public Health England – is a good indicator of how much strep A is circulating.
In the past five years, England has seen the biggest surge in scarlet fever since the 1960s.
In 2013 there were about 4,000 cases, reaching 15,000 in 2014 and 19,000 in 2016. The number of cases is now coming down again but remains relatively high.
Prof Mark Walker, Read More – Source