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Relapsing fever is a vector-borne, remittent febrile illness, transmitted by lice and ticks, which is caused by several species of spirochetes of the bacterial genus Borrelia. Two clinical forms are distinguished by their vectors: louse-borne and tick-borne relapsing fevers.
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PATHOGENESIS AND EPIDEMIOLOGY
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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 predominant antigenic variant, account for the cyclic nature of the disease. IgM antibodies help clear the more common variant each time, but other variants proliferate between episodes. Organisms can be sequestered in the liver, spleen, central nervous system, and/or bone marrow. Periodic relapses continue to occur until the antigenic variations are eliminated or the patient receives effective treatment.
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Louse-borne (epidemic) relapsing fever is caused by Borrelia recurrentis. The vector is the body louse, Pediculus humanus, which 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. Epidemic Borrelia infection has disappeared from the United States, along with louse-borne typhus. Louse-borne relapsing fever does occur in other areas of the world, mainly confined to the Horn of Africa (Ethiopia), Sudan, regions of Peru, and Bolivia. In afflicted regions, it is prevalent among the homeless, refugee populations, and confined populations exposed to famine and war.
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Tick-borne (endemic) relapsing fever is a zoonosis caused by Borrelia hermsii, and less commonly by Borrelia turicatae, Borrelia parkeri, and Borrelia mazzottii. It is transmitted to humans by soft-bodied ticks of the genus Ornithodoros (especially Ornithodoros hermsii and Ornithodoros turicata) and, less commonly, the genus Carios. In contrast to louse-borne relapsing fever, humans are incidental hosts for the Borrelia-causing tick-borne relapsing fever. Animal reservoirs include rodents, other mammals, reptiles, and birds. Ticks causing endemic relapsing fever are distributed worldwide with the exception of Antarctica and Australia. Borrelia may be transmitted from the Ornithodoros tick to the host in a matter of minutes, often with a painless nocturnal bite. Patients frequently are unaware of a tick bite or exposure. As reported by the Centers for Disease Control and Prevention in 2015, most cases of tick-borne relapsing fever in the United States occur in the Rocky Mountain regions of the western states. Three states account for approximately 70% of the 504 reported cases from 1990 to 2011: California, 33%; Washington, 25%; and Colorado, 11%. Exposure to rodent-infested cabins or caves is important to human infection with Borrelia-associated tick-borne disease. Most cases of autochthonous transmission in the United States are caused by B hermsii. Cases of B turicatae infection have been reported mainly in Texas with tick exposures while spelunking in caves infested with rodents; 1 case of infection has been reported with B parkeri.