Scarlet fever may be widely spread through airborne transmission

Airborne transmission may be a key contributor to outbreaks of scarlet fever, according to a study led by Professor Shiranee Sriskandan at Imperial College London.

The researchers found strong evidence that the airborne spread of scarlet fever can be silently triggered by an individual with no symptoms of infection. 

Scarlet fever is a highly contagious disease caused by the same bacteria as strep throat, Group A Streptococcus (GAS), and mainly affects young children. The first signs of the illness are usually flu-like symptoms, including a fever and sore throat. 

While the transmission of scarlet fever is not entirely understood, it is widely accepted that the disease can be spread through airborne droplets. 

Over the last few years, England has been experiencing a sudden increase in scarlet fever infection, with outbreaks that run parallel with the school season.

The incidence of scarlet fever followed a typical seasonal pattern the first ten weeks of 2020, but rates of infection have recently declined. 

The authors that regularly update statistics for Public Health England (PHE) said the scarlet fever decline could be due to social distancing measures or to fewer visits to health care providers. 

“This is of concern given the importance of prompt treatment with antibiotics to limit further spread as well reducing risk of potential complications,” wrote the PHE authors. 

In the current study, the researchers set out to investigate the presence of Group A Streptococcus (GAS) in classrooms, the prevalence of GAS transmission, and the impact of antibiotic treatment on the detection of GAS. Transmission factors were assessed using cough plates, hand swabs, environmental swabs, and air settle plates. The experts used genome sequencing to confirm chains of transmission. 

The study was focused on six classes, involving 11 scarlet fever cases, 17 household contacts, and 142 classroom contacts. 

Overall, ten of the 11 cases tested negative after starting antibiotics. However, 40 percent of these individuals became GAS-positive by week two or three.

Furthermore, 28 classroom contacts who were reported to have no symptoms tested positive for GAS through throat swabs. Air settle plates that were placed in elevated locations turned out to be GAS-positive in two-thirds of the classrooms. 

“GAS transmission within classrooms was extensive despite short-term effectiveness of antibiotic treatment,” wrote the researchers. “Transmission may occur prior to receipt of antibiotics, underlining the importance of rapid diagnosis and treatment.”

“Despite exclusion of cases and guideline adherence, heavy shedding of GAS by classroom contacts, who may represent subclinical infection or carriage, are likely to perpetuate outbreaks. Airborne transmission appears to be a key factor in scarlet fever, in contrast to environmental contamination.”

The research will be presented at the European Congress of Clinical Microbiology and Infectious Diseases (ECCMID).

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By Chrissy Sexton, Earth.com Staff Writer