Significant viral shedding of Streptococcus pyogenes among school-aged children exposed to scarlet fever highlights the need for rapid case management and effective interventions to decrease the risk for airborne transmission, according to results of a study published in The Lancet Microbe.
Researchers conducted a prospective, longitudinal, multicohort, contact-tracing study at 5 schools in the United Kingdom across 2 years between March 2018 and May 2019. Eligible schools included those that reported at least 2 cases of scarlet fever within 10 days among children aged between 2 and 8 years. Throat swab specimens were obtained for culture analysis at 4 timepoints across 4 weeks from children with scarlet fever and their classroom and household contacts. Specimens from hand and cough plate swabs also were cultured, as well as specimens from surface swabs of toys and other fomites in the classroom and settle plates. Of note, settle plates were only used in study year 2. Cultured specimens that grew S pyogenes underwent both emm genotyping and whole-genome sequencing to determine associations with the outbreak strain.
A total of 12 children with scarlet fever were included in the final analysis, of whom 6 (50%) had been diagnosed by a primary care physician, with subsequent antibiotic treatment initiated in 11 (92%). Genotypes of S pyogenes isolates obtained from these patients included emm6, emm1, emm4, and emm.393. Of the 11 children who received antibiotics, cultures from throat swab specimens grew the outbreak strain of S pyogenes in 4 (36%).
Among 17 household contacts included in the analysis, cultures from throat swab specimens grew S pyogenes in 3 (18%), of whom 2 were carrying the outbreak strain. Of children in whom throat swab specimens were obtained, asymptomatic throat carriage of the outbreak strains occurred in 10% (n=115) at week 1, 27% (n=126) at week 2, 24% (n=108) at week 3, and 14% (n=35) at week 4. Whole-genome sequencing showed that all S pyogene isolates of the same genotype were clonal, indicating a common source of transmission. Among 60 culture specimens obtained from swabbing the surfaces of toys and other fomites in the classroom, 1 (2%) grew the outbreak strain of S pyogenes. The researchers also noted that in 2 classrooms with 12 settle plates each, cultured specimens from 2 plates (17%) in the first classroom and 6 (50%) in the second grew the outbreak strain, indicating airborne transmission.
This study was limited as specimens from the surfaces of toys and other fomites were obtained at single timepoints, and surface plates were used only in year 2. In addition, findings from this study may not be generalizable to regions with different climates, as well as rural and low- and middle-income settings.
According to the researchers, “[these] findings might explain the periodic failure of interventions focused on hygiene alone to curtail outbreaks of S pyogenes, both in the classroom and other institutional settings.”
Cordery R, Purba A, Begum L, et al. Frequency of transmission, asymptomatic shedding, and airborne spread of Streptococcus pyogenes in schoolchildren exposed to scarlet fever: a prospective, longitudinal, multicohort, molecular epidemiological, contact-tracing study in England, UK. Lancet Microbe. Published online March 10, 2022. doi:10.1016/s2666-5247(21)00332-3