Two Overlapping Clusters of Group B Streptococcus Late-onset Disease in a Neonatal Intensive Care Unit

Objectives: Current predominant routes of group B Streptococcus (GBS) transmission in preterm neonates admitted to neonatal intensive care unit (NICU) are poorly defined. We report 2 overlapping clusters of GBS late-onset disease (LOD) from June to September 2015 in an Italian NICU. Methods: During the outbreak, possible sources of transmission (equipment, feeding bottles and breast pumps) were swabbed. Specimens from throat and rectum were collected on a weekly basis from all neonates admitted to NICU. Colonized or infected neonates had cohorting. Bacterial isolates were characterized by serologic and molecular typing methods. Results: GBS was isolated in 2 full-term and 7 preterm neonates. Strains belonged to serotype III, with 3 different sequence types (ST17, ST182 and ST19). Full-term neonates were colonized with GBS strains unrelated to the clusters (ST182 and ST19). Two distinct ST17 strains caused 2 clusters in preterm neonates: a first cluster with 1 case of LOD and a second, larger cluster with 6 LOD in 5 neonates (one of them had recurrence). ST17 strains were isolated from vaginorectal and milk samples of 2 mothers. Two preterm neonates had no evidence of colonization for weeks, until they presented with LOD. Conclusions: Molecular analyses identified the presence of multiclonal GBS strains and 2 clusters of 7 cases of GBS-LOD. The dynamics of transmission of GBS within the NICU were complex. Breast milk was suspected to be one of the possible sources. In a research setting, the screening of GBS carrier mothers who deliver very preterm could contribute to the tracking of GBS transmission.