Borreliae Part 2: Borrelia Relapsing Fever Group and Unclassified Borrelia

SIMPLE SUMMARY: Following Borreliae Part 1, this second review describes Borreliae of the relapsing fever group (RFG) and unclassified Borreliae. The RFG is further divided according to vector transmission and geographical distribution in other subgroups, namely the soft-tick-borne relapsing fever (STBRF) group, the hard-tick-borne relapsing fever (HTBRF) one, the louse-borne relapsing fever (LBRF) group, and the Avian relapsing fever group. Where possible, according to the literature description, each sub-group of the RFG Borreliae is organized here in sections explaining the geographical distribution, the vectors, the hosts, the epidemiology, and the microbiology. In case of human infectiveness, clinical aspects are also discussed. Isolation and sequencing of Borrelia species is ongoing, and in addition to the groups mentioned in these reviews, there are Borreliae that at present cannot be cultivated, but according to sequencing data they share some characteristics with one specific group. Nevertheless, they still cannot be classified in one of them. This is the case of Borreliae with not-yet-identified pathogenicity for humans or animals, which are here named “unclassified Borreliae” and described separately, recalling the similarities with Borreliae already classified. In the future, we expect that those Borreliae are going to be characterized, including them in one of the previous groups. ABSTRACT: Borreliae of the relapsing fever group (RFG) are heterogenous and can be divided mainly into three groups according to vectors, namely the soft-tick-borne relapsing fever (STBRF) Borreliae, the hard-tick-borne relapsing fever (HTBRF) Borreliae, the louse-borne relapsing fever (LBRF) Borreliae, and the avian relapsing fever ones. With respect to the geographical distribution, the STBRF Borreliae are further subdivided into Old World and New World strains. Except for the Avian relapsing fever group Borreliae, which cause avian spirochetosis, all the others share infectivity in humans. They are indeed the etiological agent of both endemic and epidemic forms of relapsing fever, causing high spirochaetemia and fever. Vectors are primarily soft ticks of Ornithodoros spp. in the STBRF group; hard ticks, notably Ixodes sp., Amblyomma sp., Dermacentor sp., and Rhipicephalus sp., in the HTBRF group; and the louse pediculus humanus humanus in the TBRF one. A recent hypothesis was supported for a common ancestor of RFG Borreliae, transmitted at the beginning by hard-body ticks. Accordingly, STBRF Borreliae switched to use soft-bodied ticks as a vector, which was followed by the use of lice by Borrelia recurrentis. There are also new candidate species of Borreliae, at present unclassified, which are also described in this review.