Relapsing fever

Relapsing fever refers to a group of infectious diseases caused by Borrelia species of the relapsing fever group, which are distinct from the Borrelia burgdorferi sensu lato complex responsible for Lyme borreliosis ^[3][2]. The causative spirochetes are transmitted to humans chiefly by soft-bodied argasid ticks, with two notable exceptions: Borrelia recurrentis, transmitted by the human body louse, and Borrelia miyamotoi, transmitted by hard-bodied ixodid ticks ^[2]. Relapsing fever Borrelia exhibit multiphasic antigenic variation, which underlies the characteristic relapsing clinical course ^[2].

The hallmark clinical feature of relapsing fever is the eponymous relapsing fever pattern, characterized by recurrent episodes of high fever alternating with afebrile periods, present in essentially all cases ^[1]. Acute episodes typically present with abrupt onset of fever, rigors, headache, myalgia, and prostration, and may be accompanied by thrombocytopenia as the most suggestive laboratory finding ^[1]. Neurological complications are frequently reported, particularly in tick-borne relapsing fever, and severe manifestations can include Jarisch-Herxheimer reactions following treatment ^[1].

Tick-borne relapsing fever displays a patchy global distribution, with Borrelia-carrying ticks reported on all continents except Antarctica and Australia, and the epidemiogeographic pattern continues to evolve as molecular diagnostics expand knowledge of circulating species ^[1]. Because the disease is geographically focal, knowledge of several clinical aspects remains limited and large clinical studies are lacking ^[1]. The patchy occurrence and under-recognition of cases continue to constrain comprehensive epidemiologic characterization ^[1].

Most relapsing fever Borrelia species are transmitted to humans through the bite of soft-bodied argasid ticks, while Borrelia miyamotoi is transmitted by hard-bodied ixodid ticks ^[1][2]. Louse-borne relapsing fever is transmitted from human to human via the body louse Pediculus humanus corporis, through inoculation of louse coelomic fluid or feces during scratching, with possible nosocomial transmission from contaminated blood ^[4]. Tick vectors acquire the spirochetes during blood feeding from infected vertebrate reservoirs, maintaining the zoonotic transmission cycle ^[1][2].

Prevention of relapsing fever centers on reducing exposure to arthropod vectors, including avoidance of tick habitats and use of protective measures against tick bites, as well as control of body louse infestations in settings where louse-borne transmission occurs ^[5]. Early recognition and treatment of suspected tick-borne illness is emphasized to decrease morbidity and mortality, with published treatment guidelines available from public health authorities ^[5].

1. 1Jakab Á et al. Tick borne relapsing fever - a systematic review and analysis of the literature. PLoS Negl Trop Dis. 2022 Feb. PMID: 35171908. doi: 10.1371/journal.pntd.0010212.PubMed: https://pubmed.ncbi.nlm.nih.gov/35171908/
2. 2Lopez J et al. Pathogenesis of Relapsing Fever. Curr Issues Mol Biol. 2021. PMID: 33372163. doi: 10.21775/cimb.042.519.PubMed: https://pubmed.ncbi.nlm.nih.gov/33372163/
3. 3Cutler SJ et al. Diagnosing Borreliosis. Vector Borne Zoonotic Dis. 2017 Jan. PMID: 28055580. doi: 10.1089/vbz.2016.1962.PubMed: https://pubmed.ncbi.nlm.nih.gov/28055580/
4. 4Warrell DA et al. Louse-borne relapsing fever (Borrelia recurrentis infection). Epidemiol Infect. 2019 Jan. PMID: 30869050. doi: 10.1017/S0950268819000116.PubMed: https://pubmed.ncbi.nlm.nih.gov/30869050/
5. 5Choi E et al. Tick-Borne Illnesses. Curr Sports Med Rep. 2016 Mar-Apr. PMID: 26963018. doi: 10.1249/JSR.0000000000000238.PubMed: https://pubmed.ncbi.nlm.nih.gov/26963018/