Relapsing Fever and Other Borrelia Infections

Definition

Relapsing fever is a spirochetal infection with bacteria of the genus Borrelia . The two modes of transmission are epidemic louse-borne and endemic tick-borne relapsing fever. Disease is characterized by recurrent bouts of fever and spirochetemia separated by short fever-free periods.

Epidemiology

Louse-borne epidemic relapsing fever, which is caused by B. recurrentis , is carried from person to person by the human body louse ( Pediculus humanus ). There is no animal reservoir. The spirochete lives in the louse hemolymph; infection is transmitted to humans when the louse is crushed on human skin and infective spirochetes penetrate the skin or mucous membranes. Epidemics have occurred during famines and wars, when breakdown in sanitation favors the transmission of body lice. Louse-borne disease remains endemic in Central and East Africa (Ethiopia, Somalia, Chad, and the Sudan) and in the South American Andes (Bolivia and Peru). It is also seen in Eastern Europe and in recent refugees to Western Europe.

Tick-borne endemic relapsing fever, which occurs throughout the world, is transmitted to humans by Ornithodoros soft ticks. The ticks become infected by feeding on wild rodents (including mice, rats, squirrels, and chipmunks), which serve as natural reservoirs for the organisms. In the United States, relapsing fever is limited to humid mountainous areas of the West at altitudes of 1500 to 8000 feet, where the tick vector Ornithodoros hermsii resides in forests of ponderosa pine and Douglas fir trees. A key diagnostic clue has been a history of sleeping in rodent-infested cabins in western U.S. national parks. In Tanzania, where house infestation with Ornithodoros tick vectors can be very high, relapsing fever can be identified in about 10% of children seen at a clinic with fever. By comparison, the prevalence of antibodies to Borrelia miyamotoi in the New England states is about 3%, which is about 25% as high as the prevalence for Borrelia burgdorferi ( Chapter 296 ).