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Microbiology, pathogenesis, and epidemiology of relapsing fever

Alan G Barbour, MD
Section Editor
Daniel J Sexton, MD
Deputy Editor
Jennifer Mitty, MD, MPH


Relapsing fever, caused by spirochetes of the Borrelia genus, is an arthropod-borne infection that occurs in two major forms: tick-borne (TBRF) and louse-borne (LBRF) [1]. As the name implies, relapsing fever is characterized by recurrent episodes of fever that accompanies spirochetemia.

The microbiology, pathogenesis, and epidemiology of relapsing fever will be reviewed here. The clinical manifestations, diagnosis, treatment, and prevention of this disorder are discussed separately. (See "Clinical features, diagnosis, and management of relapsing fever".)


Tick-borne relapsing fever is caused by eight or more Borrelia species, whereas louse-borne relapsing fever is caused only by B. recurrentis [2,3].

The agents of relapsing fever are spirochetes, a morphologically unique group of bacteria species, which cause syphilis, leptospirosis, and Lyme disease [3]. As their own phylum of bacteria, spirochetes differ substantially from both gram-negative and gram-positive organisms [4]. (See "Syphilis: Epidemiology, pathophysiology, and clinical manifestations in HIV-uninfected patients" and "Epidemiology, microbiology, clinical manifestations, and diagnosis of leptospirosis" and "Epidemiology of Lyme disease".)

The agents of relapsing fever and Lyme disease are in the genus Borrelia [1]. Every known species of Borrelia is host-associated; free-living forms have not been found. Borrelias are usually extracellular in location within the host, but they pass through endothelial cell layers to enter and leave the blood. Borrelia spp are susceptible to drying, hypotonic or hypertonic conditions, dilute detergents, and temperatures above 40ºC.


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Literature review current through: Sep 2016. | This topic last updated: Aug 15, 2012.
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