A new Borrelia species isolated from patients with relapsing fever in Spain

Background. Lyme disease and tick-borne relapsing fever are worldwide systemic borrelioses caused by several Borrelia species transmitted by hard ticks (family Ixodidae) and soft ticks (family Argasidae), respectively. A previous seroepidemiological study of Lyme borreliosis showed several serologically reactive patients with clinically atypical presentations, and this discovery led to the hypothesis that some of the cases of Lyme borreliosis had been caused by another borrelia organism. Methods. Blood from patients in southern Spain who had suspected Lyme disease or relapsing-fever borreliosis was cultured before treatment began. Isolates of Borrelia spp were inoculated into several strains of mice of different ages. The 16S rRNA and flagellin genes of Borrelia spp were sequenced by PCR and assessed by phylogenetic analyses. Findings. We isolated a species of Borrelia from three patients with relapsing fever and from Ornithodorus spp ticks in southern Spain. This organism (refractory to in-vitro cultivation) caused a relapsing spirochaetaemia with multiple organ involvement in laboratory mice that recreated the human disease. Phylogenetic analysis showed that this organism is a previously unrecognised species. Interpretation. We have discovered a new borrelia pathogen that is closely related to the other tick-borne agents of relapsing fever in Europe and Africa, and which causes a relapsing systemic disease with serological similarities to Lyme borreliosis.