Chapter 282

Relapsing fever is a vector-borne, remittent febrile illness, transmitted by lice and ticks, which is caused by several species of spirochetes of the genus Borrelia, including B recurrentis and B hermsii.

### Epidemiology

Louse-borne (epidemic) relapsing fever is caused by B recurrentis. The body louse, Pediculus humanus, becomes infected by ingesting blood from infected humans, and the disease is transmitted when the louse is crushed and the spirochetes (Borrelia) penetrate human skin of a new host.1 Epidemic Borrelia infection has disappeared from the United States, along with louse-borne typhus. The disease does occur in other areas of the world, particularly Africa, where epidemics occur, especially among the homeless and refugee populations. Endemic relapsing fever is transmitted by soft-bodied Ornithodoros ticks (especially O hermsii and O turicata). They are distributed worldwide and have painless bites. Patients are often unaware of a tick bite or exposure. Borrelia may be transmitted from the tick to the host in a matter of minutes.2,3 Most tick-borne relapsing fever in the United States occurs in the western states4 and is caused by B hermsii.1 In contrast to louse-borne relapsing fever, humans are incidental hosts for the Borrelia causing tick-borne disease. Exposure to rodent-infested cabins or caves is important to human infection with Borrelia associated with tick-borne disease.1,2,5-7

Borrelia do not infect blood cells directly, but the concentration of the spirochetes in the blood correlates directly with severity. Repeated episodes of spirochetemia, each involving a different predominate antigenic variant, account for the cyclic nature of the disease.3,7-9 IgM antibodies help clear the more common variant each time, but other variants proliferate between episodes.10 Organisms can be sequestered in the liver, spleen, central nervous system, and/or bone marrow.10,11 Periodic relapses continue to occur until the number of antigenic variations are eliminated or the patient receives effective treatment.

### Clinical Manifestations

After a variable incubation period (4–18 days, mean of 7 days), the illness starts abruptly with fever, chills, headache, myalgia, and arthralgia. Conjunctivitis, petechiae, and hepatosplenomegaly with tenderness may be present. This first phase of illness typically lasts 3 to 6 days and subsides spontaneously.10 During the next 5 to 7 days (5–7 days, typically), the infected patient experiences extreme fatigue and may have a diffuse maculopapular rash, but the patient is afebrile or has only a low-grade fever.

Return of fever and chills after 4 to 14 days signals the relapse phase of the disease.11 Several such relapses can occur (up to 10 in tick-related cases; a lesser number in louse-borne disease),9 although the duration of relapses typically becomes shorter and milder over time. Relapsing fever may resolve even among untreated patients with tick-borne disease. Myocarditis is prominent in fatal cases of tick-borne disease.

Vertical transmission of infection can occur, ...

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