'Supercharged clones' behind resurgence of deadly scarlet fever - study

A girl with scarlet fever.
A girl with scarlet fever. Photo credit: Getty

The resurgence of a disease as deadly as COVID-19 which primarily targets children is being blamed on 'supercharged clones' better at infecting people than their ancestors.

Scarlet fever, caused by the bacteria Streptococcus pyogenes, has made a comeback in recent years in the UK, China, South Korea and Vietnam. Until now it's been unclear why.

"The disease had mostly dissipated by the 1940s," said Stephan Brouwer of the University of Queensland. 

Case numbers in the UK quadrupled in just a few years, and in 2019 the outbreak had reached Australia. Isolated cases of scarlet fever have been detected in New Zealand too, but it's not clear if they're linked to the new version of the disease. So far we've avoided the kind of outbreak that's seen global scarlet fever infections hit their highest in more than half a century. 

Scarlet fever can cause chills, fever, sore throat, abdominal pain, a swollen tongue, red rash and peeling skin. Before the advent of modern medicine, it had a mortality rate up to 25 percent. Antibiotics reduced that markedly, but the new version of the disease has a case fatality rate of 15-20 percent, BBC News reported last year based on research published in medical journal The Lancet

"This global re-emergence of scarlet fever has caused a more than five-fold increase in disease rate and more than 600,000 cases around the world," said Dr Brouwer.

His team thinks they've figured out the cause - the bacteria have acquired "superantigen" toxins from viruses. Superantigens prompt the immune system to go into overdrive, causing inflammation (as has been seen in some severe cases of COVID-19 - particularly in children, where it can manifest as multisystem inflammatory syndrome in children, or MIS-C).

"The toxins would have been transferred into the bacterium when it was infected by viruses that carried the toxin genes," said co-author Mark Walker.

"We've shown that these acquired toxins allow Streptococcus pyogenes to better colonise its host, which likely allows it to out-compete other strains. These supercharged bacterial clones have been causing our modern scarlet fever outbreaks."

Removing the virus toxins reduced the bacteria's ability to infect new patients, they found.

"This year COVID-19 social distancing has kept scarlet fever outbreaks in check for now," said Prof Walker.

"And the disease's main target - children - have been at school less and also spending far less time in other large groups. But when social distancing eventually is relaxed, scarlet fever is likely to come back... Just like COVID-19, ultimately a vaccine will be critical for eradicating scarlet fever - one of history’s most pervasive and deadly childhood diseases."

Unlike COVID-19, scarlet fever primarily shows up in children. Outbreak Observatory, run by the Johns Hopkins Bloomberg School of Public Health, says it was "one of the most feared infectious diseases" of the 19th and early 20th centuries, and a "leading cause of child mortality".

"Most cases are not considered particularly serious, but complications can arise from the illness such as sepsis, acute rheumatic fever, or acute kidney diseases."

The new research was published in journal Nature Communications on Tuesday night (NZ time).