Tracing Scarlet Fever Transmission in English Classrooms

In recent years England has experienced an uptick in cases of scarlet fever leading to subsequent outbreaks.

Scarlet fever is caused by Group A Streptococcus (GAS), but little is known about transmission mechanisms. Investigators have hypothesized that transmission is driven by pediatric cases with symptomatic disease.

In a new study, funded by Action Medical Research and led by investigators from Imperial College London and Public Health England, investigators set out to learn more about transmission mechanisms. Their findings were slated to be presented at the European Congress of Clinical Microbiology and Infections Diseases 2020 (ECCMID). The in-person and virtual components of the meeting have been cancelled as a result of coronavirus disease 2019, but congress organizers have decided to publish abstracts.

For the investigation, the team conducted a prospective, observational study in nurseries that have experienced outbreaks of scarlet fever. The investigators assessed the impact of antibiotics on detection GAS, the prevalence of GAS in community contacts, and the presence of GAS in the classroom.

In order to evaluate transmissibility, the team used cough plates, hand and environmental swabs, and settle plates along with genome sequencing to confirm chains of transmission. The cases were tested for the first 3 days of antibiotic use, then weekly for 3-4 weeks. The contacts were also tested weekly over 3-4 weeks.

Over the course of the investigation the team evaluated 6 nursey classes and recruited 11 scarlet fever cases, 17 household contacts, and 142 classroom contacts.

Ten of the 11 cases were treated for their infection. All 10 participants had negative samples following the initiation of antibiotics; however, 4 of the 10 became GAS-positive by week 2 or 3. The case that was not treated with antibiotics remained positive.

Among household contacts, GAS was identified in 3 out of 17 individuals. The investigators found that GAS prevalence in classroom contacts was high and increased between weeks 1 and 2 (week 1, 0-19%; week 2, 9-56%; week 3, 18-50%). In total, 27 contacts (19%) were GAS-positive on 2 samples, and 4 on 3 samples.

The investigators reported that of the 60 surface swabs collected from 3 classrooms, only 1 yielded GAS. They also noted that genome sequencing showed clonality of isolates in the 3 classes, which confirmed that recent transmission accounted for high carriage; M1UK accounted for 2 of the 3 outbreaks. Settle plates were GAS-positive in 2 of the 3 classrooms tested.

Based on all of these findings the investigators indicated that GAS transmission within classrooms was extensive despite the short-term effectiveness of antibiotic treatment for infection. As such, transmission may be occurring prior to the use of antibiotics which supports the importance of rapid diagnostics and treatment.

With the lack of surface contamination, the investigators concluded that airborne transmission appears to play a key role in the transmission.

In a press release issued by ECCMID organizers, the abstract authors indicated that in the recent weeks, the notifications of scarlet fever cases have declined, which could be due to social distancing impacting transmission rates, or lower visits to clinicians due to stay at home messaging.

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

The abstract, Uncovering the role of airborne transmission and asymptomatic contact shedding in outbreaks of scarlet fever, was scheduled to be presented at ECCMID 2020.