Background. Invasive Group B streptococcus (GBS) is a major cause of serious neonatal infection. Current strategies to reduce GBS early onset disease (EOD) have limited impact on late onset disease (LOD). Following a cluster of linked LOD we undertook enhanced genomic surveillance of all GBS cases in one unit.
Methods. GBS isolates were serotyped and genome sequenced. Case reviews and enhanced surveillance for additional GBS LOD cases were undertaken, coupled with weekly rectal screening, supported by genome sequencing.
Results. Over 2 years, 12 cases of GBS LOD were detected. The first cluster comprised serotype V GBS isolates of multilocus sequence type (ST)1 carrying tetM and ermB resistance genes. Comparison with contemporaneous ST1 data showed neonatal and adult invasive GBS to be intermixed. Subsequent clusters of GBS LOD were identified and confirmed genomically, due to serotype Ib, serotype III, and serotype Ia. Overall, genomic analysis revealed that 11/12 (>90%) GBS LOD cases were linked to at least one other LOD case, highlighting that horizontal transmission in the neonatal setting was the most common mode of acquisition for GBS LOD.
Conclusion. Acquisition routes for GBS in the nosocomial setting are poorly understood. Large-scale genomic databases that include longitudinally collected data from both disease-associated and carriage isolates can provide context for outbreak investigation and allow more certainty regarding transmission events. Our experience suggests that a single case of LOD arising in hospital should be considered as a potential nosocomial transmission event warranting immediate investigation with heightened infection prevention vigilance and action where required.