Relapsing fever

Relapsing fever is a primary bacterial infectious disease resulting from infection with specific *Borrelia* species, including *Borrelia recurrentis* (louse-borne), *Borrelia hermsii*, *Borrelia duttoni*, and *Borrelia miyamotoi*. The disease is defined by its characteristic clinical course of intermittent febrile episodes separated by afebrile periods, reflecting antigenic variation of the spirochete. Transmission occurs via arthropod vectors: body lice for the epidemic form, and soft or hard ticks for the endemic forms. The etiologic agents are spirochetes that circulate in the bloodstream during febrile phases and are detectable in peripheral blood smears during active infection.

The illness typically manifests 5–15 days post-exposure with sudden onset of high fever, chills, severe headache, myalgia, and nausea. A maculopapular rash may be present but is not consistently reported. Each febrile episode lasts 2–9 days before resolving spontaneously, followed by an afebrile interval that may last several days to weeks, leading to recurrence. In louse-borne cases, symptoms tend to be more severe, with higher risk of complications such as hepatic dysfunction (jaundice), hemorrhage, altered mental status, and cardiac arrhythmias (e.g., prolonged QT interval). Without treatment, mortality ranges from 30% to 70%, whereas appropriate antimicrobial therapy reduces fatality to approximately 1%. Relapse frequency varies by species: *B. recurrentis* commonly causes three to four cycles, while *B. hermsii* often produces one or two.

Louse-borne relapsing fever is historically linked to epidemics in contexts of war, famine, and overcrowding, particularly in low-resource regions of sub-Saharan Africa; current endemic foci include Ethiopia and Sudan. Tick-borne forms are geographically diverse: *B. hermsii*-associated disease is most common in the western United States and parts of Canada; *B. duttoni* and related strains occur in East Africa; *B. miyamotoi* has been identified in North America and Europe, transmitted by *Ixodes* ticks. Rodents serve as reservoir hosts for many tick-associated *Borrelia* species. Surveillance data indicate limited global reporting, with case counts likely underrepresented due to diagnostic challenges and lack of routine testing in endemic areas.

Transmission occurs through the bite of infected arthropods: body lice (*Pediculus humanus humanus*) ingest *Borrelia* during feeding on an infected host, the bacteria multiply in the louse’s gut, and transmission to new hosts occurs when crushed lice release organisms into mucosal surfaces or abraded skin. Soft ticks (*Ornithodoros* spp.) transmit infection during brief, non-feeding bites, often at night, with no visible skin reaction. Hard ticks (*Ixodes* spp.) transmit *B. miyamotoi* in a manner similar to other tick-borne pathogens, though the exact mechanics remain less well characterized. Human-to-human transmission does not occur directly; all cases result from vector exposure.

High-risk groups include individuals in humanitarian emergencies (e.g., refugees, displaced persons, prisoners), military personnel in endemic zones, and travelers to regions with known louse- or tick-borne transmission. Populations with poor sanitation, overcrowded living conditions, or limited access to healthcare face elevated risk for louse-borne disease. In endemic areas, children and immunocompromised individuals may experience more severe manifestations. Occupational exposure among field workers, researchers, and veterinarians in rural or wilderness settings also contributes to risk for tick-associated forms.

Preventive measures focus on vector control and personal protection. For louse-borne disease, improved hygiene, clothing changes, and delousing are critical in outbreak settings. Environmental interventions targeting rodent reservoirs and tick habitat reduction are recommended in endemic areas. No vaccine is available. Chemoprophylaxis is not routinely indicated; however, early administration of antibiotics (e.g., doxycycline) following suspected exposure may prevent progression. Public health surveillance should prioritize identification of outbreaks in vulnerable populations, especially those experiencing displacement, poverty, or conflict.

Relapsing fever should be considered in patients presenting with recurrent fevers and relevant exposure history (e.g., travel to endemic regions, contact with lice or rodents, or recent natural disaster). Laboratory confirmation requires detection of spirochetes in blood smears or molecular methods (PCR); serology is not reliable due to cross-reactivity among *Borrelia* species. Surveillance systems should differentiate between louse-borne and tick-borne forms, as epidemiological patterns and intervention strategies differ significantly. Case reporting should include vector exposure details, geographic origin, and outcome data to inform risk stratification and resource allocation.

1. 1 Wikidata. relapsing fever borreliosis.
2. 2 Wikipedia. Relapsing fever - Wikipedia.

ICD-10

ICD-11

Total cases

85

Peak month

2022-10

Coverage

1 reporting countries · 2012-09-14 → 2026-05-02