Whole genome sequencing in the investigation of recurrent invasive Group A streptococcus outbreaks in a maternity unit

Background: The clinical manifestations of Group A streptococcus (GAS) – (Streptococcus pyogenes) are diverse, ranging from asymptomatic colonisation to devastating invasive disease. Maternity related clusters of invasive Group A streptococcus (iGAS) infection are complex to investigate and control, especially if recurrent. Aim: We report on the investigation into three episodes of emm 75 GAS/iGAS infection in maternity patients at one hospital site over a 4 year period, two with monophyletic ancestry. Methods: The episodes are described, together with whole genome sequence isolate analyses. Single nucleotide polymorphism differences were compared with contemporaneous emm 75 genomes. Findings: Seven mothers had GAS/iGAS in over a 4 year period, emm 75, S.pyogenes and one had iGAS (in year 4) emm 3, S.pyogenes (subsequently discounted as linked). Three (clinical/screening samples) of the seven babies of emm 75 positive mothers and 3 screened healthcare workers were positive for GAS emm 75. Whole genome sequence similarity suggests a shared ancestral lineage and suggested a common source transmission but directionality of transmission cannot be inferred. However the findings indicate that persistence of a particular clone in a given setting may be long-term. Conclusions: Occupational health procedures were enhanced, staff were screened and antibiotic therapy provided to GAS positive staff and patients. The definitive source of infection could not be identified, although staff/patient transmission is the most likely route. The pattern of clonal GAS transmission over 4 years suggests long-term persistence of GAS may have occurred