Ancient <i>Borrelia</i> genomes document the evolutionary history of louse-borne relapsing fever

Abstract

Several bacterial pathogens have transitioned from tick-borne to louse-borne transmission, often involving genome reduction and increasing virulence. However, the timing of such transitions remains unclear. We sequenced four ancient Borrelia recurrentis genomes, the agent of louse-borne relapsing fever, dating from 2,300 to 600 years ago. We estimate the divergence from its closest tick-borne relative to 6,000–4,000 years ago, suggesting an emergence coinciding with human lifestyle changes such as the advent of wool-based textiles. Pan-genome analysis indicates that much of B. recurrentis’ characteristic evolution had occurred by ~2,300 years ago, though further gene turnover, particularly in plasmid partitioning, persisted until ~1,000 years ago. Our findings provide a direct genomic chronology of the evolution of this specialized vector-borne pathogen.