The fall and rise of group B Streptococcus in dairy cattle: reintroduction due to human- to- cattle host jumps?

Group B Streptococcus (GBS; Streptococcus agalactiae) is a major neonatal and opportunistic bacterial pathogen of humans and an important cause of mastitis in dairy cattle with significant impacts on food security. Following the introduction of mastitis control programmes in the 1950s, GBS was nearly eradicated from the dairy industry in northern Europe, followed by re- emergence in the 21st century. Here, we sought to explain this re- emergence based on short and long read sequencing of historical (1953-1978; n=44) and contemporary (1997-2012; n=76) bovine GBS isolates. Our data show that a globally distributed bovine- associated lineage of GBS was commonly detected among historical isolates but never among contemporary isolates. By contrast, tetracycline resistance, which is present in all major GBS clones adapted to humans, was commonly and uniquely detected in contemporary bovine isolates. These observations provide evidence for strain replacement and suggest a human origin of newly emerged strains. Three novel GBS plasmids were identified, including two showing >98 % sequence similarity with plasmids from Streptococcus pyogenes and Streptococcus dysgalactiae subsp. equisimilis, which co- exist with GBS in the human oropharynx. Our findings support introduction of GBS into the dairy population due to human- to- cattle jumps on multiple occasions and demonstrate that reverse zoonotic transmission can erase successes of animal disease control campaigns.