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.
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.
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 ...