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Relapsing fever is an illness characterized by recurring episodes of fever and nonspecific symptoms (e.g., headache, myalgia, arthralgia, shaking chills, and abdominal symptoms) after infection with one of several species of Borrelia.

In North America, relapsing fever (a zoonosis) is transmitted by the bite of an Ornithodoros tick. In many other parts of the world, including Africa and Asia, relapsing fever is endemic and occurs after the bite of a tick or the human body louse (Pediculus humanus). Tick-borne relapsing fever (TBRF) is also reported from countries in the Middle East, including Israel, Iran, and Jordan. Louse-borne relapsing fever (LBRF) is occasionally imported into the United States by a traveler. TBRF is rarely fatal in North America, where it is most often sporadic; in some African countries (e.g., Senegal and Tanzania), TBRF is a more significant bacterial infection, causing morbidity and death. Conditions that favor infestation with P. humanus, such as living in refugee camps or other stressful situations in which many people are crowded together without access to good hygiene and nutrition, have led to large outbreaks of LBRF with substantial rates of morbidity and death; thus, LBRF is also known as epidemic relapsing fever.

The borreliae are helical or wavy motile spirochetes whose length ranges from 3 to 25 μm and whose width is usually 0.2–0.3 μm. In fixed Wright-stained differential smears, the organisms appear as loose coils (Fig. 172-1). Borreliae are transmitted to humans by exposure to the bite of an infected Ornithodoros tick (TBRF) or to the hemolymph of an infected human body louse, which may be found on clothing (LBRF). For louse-borne disease, it is not the louse's bite that causes transmission; rather, spirochetes are introduced when the louse is crushed (e.g., by scratching) and the insect's infected hemolymph is released and contaminates abraded or normal skin and mucous membranes. Relapsing fever results when variation in borrelial surface antigens leads to repeated bacteremia and stimulation of the immune system by each new antigen. Each time the organism changes its surface antigens, thus evading the immune system, another febrile response occurs. LBRF is caused by Borrelia recurrentis, whereas TBRF is caused by a variety of Borrelia species whose names sometimes correspond to their tick vectors. For example, B. hermsii is transmitted by the tick O. hermsi, and B. turicatae is transmitted by O. turicata (Fig. 172-2).

Figure 172-1

Photomicrograph of tick-borne relapsing fever spirochete (Borrelia hermsii) in a Wright-Giemsa–stained peripheral blood film. [Reprinted with permission from Dennis DT: Relapsing fever, in Harrison's Principles of Internal Medicine, 17th ed, AF Fauci et al (eds). New York, McGraw-Hill, 2008, p 1054.]

Figure 172-2

Ornithodoros turicata and O. hermsi, two of the many species of blood-feeding ...

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