Background:
Accurately determining Streptococcus pyogenes (Strep A) carriage and infection burden in high-burden, low-resource settings is essential to inform public health interventions, policy and research in future. A sub-set of the SpyCATS household cohort in The Gambia underwent intensive weekly swabbing and PCR of clinical events to compare event detection with monthly visits and culture detection from the main study cohort.
Methods:
Sixteen households underwent six weeks of weekly oropharyngeal (OPS) and normal skin swabbing (NSS) swabbing for culture-based detection of Strep A carriage and infection. Clinical pharyngitis and pyoderma episodes were also identified and swabbed for culture and for quantitative PCR (qPCR) targeting speB. Household transmission was assessed using emm typing.
Results:
In children under 5, compared to monthly visits, weekly visits detected a substantially higher incidence of skin carriage (1197/1000 person-years [pyrs] vs. 239/1000pyrs) and pyoderma (997/1000pyrs vs. 520/1000pyrs). qPCR increased Strep A pyoderma detection from 48.2% to 75.1% and pharyngitis from 10.2% to 23.9% compared to culture. Emm-linked household secondary attack rate was higher in the weekly visit cohort (2.9 vs. 0.7). Skin carriage was the dominant source of household transmission, with an emm-linked attack rate of 4.1.
Conclusions:
Increasing visit frequency and using molecular diagnostics significantly increases Strep A event detection, especially in children, revealing a substantial undetected burden. Skin carriage plays a key role in household transmission. These findings will inform surveillance study and vaccine trial design, highlighting the need for frequent active case finding and PCR-based diagnostics to fully detect Strep A burden.