Etiology

Relapsing fever is an infection transmitted by arthropods (lice or ticks) and caused by spirochetes of the genus Borrelia.

Louse-borne (epidemic) relapsing fever is caused by Borrelia recurrentis and is transmitted from person to person by Pediculus humanus, the human body louse. Human infection occurs as a result of crushing lice during scratching, facilitating entry of infected hemolymph through abraded or normal skin or mucous membranes.

Tick-borne (endemic) relapsing fever is caused by several species of Borrelia and is transmitted to humans by Ornithodoros ticks. Borrelia hermsii and Borrelia turicatae are the common species in the western USA, and Borrelia dugesi is the major cause of disease in Mexico and Central America. Human infection occurs when saliva, coxal fluid, or excrement is released by the tick during feeding, thereby permitting spirochetes to penetrate the skin and mucous membranes.

Epidemiology

Louse-borne relapsing fever tends to occur in epidemics associated with war, poverty, famine, and poor personal hygiene, often in association with typhus. This form of relapsing fever is no longer seen in the USA but is endemic in parts of East Africa.

Ornithodoros ticks, which transmit endemic relapsing fever, are distributed worldwide, including the western USA, prefer warm, humid environments and high altitudes and are found in rodent burrows, caves, and other nesting sites (Fig. 213-1). Rodents (e.g., squirrels, chipmunks) are the principal reservoirs. Infected ticks gain access to human dwellings on the rodent host. Human contact is often unnoticed because these soft ticks have a painless bite and detach immediately after a short blood meal.

Figure 213-1 Number of cases of tick-borne relapsing fever 1990-2000.

(From Centers for Disease Control and Prevention: Tickborne relapsing fever outbreak after a family gathering—New Mexico, August 2002, MMWR Morb Mortal Wkly Rep 52:809–812, 2003.)

Pathology and Pathogenesis

Relapsing fever is cyclical because the Borrelia