Genome sequencing shows the clonality of outbreak isolates within each of 6 classes, which confirmed that recent transmissions accounted for high carriage.
The transmission of Streptococcus pyogenes in classrooms might occur before, or even despite, reported treatment of cases and calls for a need of rapid case management, according to the results of a study published in The Lancet.
In week 1, 11 of 115 children who were swabbed had asymptomatic throat carriage of the outbreak strains, which increased to 32 of 126 in week 2, 26 of 108 in week 3, and 5 of 25 in week 4.
By comparison, the carriage of outbreak S pyogenes strains, colonization with non-outbreak and non-genotype strains occurred in 2 of 115 of the children swabbed during week 1, 5 of 126 in week 2, 6 of 108 in week 3, and 0 of 35 in week 4.
Investigators used genome sequencing to show the clonality of outbreak isolates within each of 6 classes, which confirmed that recent transmissions accounted for high carriage.
When investigators tested for transmissibility, 1 of 11 asymptomatic carriers of emm4 and 5 of 14 asymptomatic carriers of emm3.93 had positive cough plate. The outbreak strain was identified in just 2 of 60 swabs taken from 3 classrooms. However, in 2 classrooms with settle plates placed in elevated locations, 2 of 12 and 6 of 12 settle plates had the outbreak strain.
Investigators gathered data from 6 classrooms in 5 schools in Greater London, United Kingdom, between March 1 and May 31, 2018 (year 1) and between March 1 and May 31 (year 2), which included 12 cases of scarlet fever, 17 household contacts, and 278 classroom contacts.
The nurseries and schools were eligible if they had reported 2 cases of scarlet fever within 10 days of each other among children aged 2 to 8 years from the same class. The most recent case had to be within the preceding 48 hours.
Investigators used throat swabs from the individuals with scarlet fever, classroom contacts, and household contacts at 4 time points. Additionally, they cultured the hand swabs and cough plates from all cases in both years of the study, but in year 2, they also included the samples from classroom contacts.
Surface swabs from toys and other objects likely to carry infection in classrooms were cultured in year 1, and in year 2, settle plates from classrooms were collected.
A sample with S pyogenes detected was recorded as a positive sample and underwent emm genotyping and genome sequencing to compare with the outbreak strain.
Investigators acknowledge the limitations of the study and included sampling infection-prone objects at a single time point, and settle plates being used in just years 2 restricted their ability to identify transmission routes.
They also said that the intensity of the transmission could be seasonal.
However, investigators attempted to sample outside the season, but schools were unwilling to participate.
Cordery R, Purba AK, 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. 2022. doi:https://doi.org/10.1016/S2666-5247(21)00332-3