New concepts for the old challenge of African relapsing fever borreliosis.

Relapsing fever, caused by spirochaetes belonging to the genus Borrelia, was once the cause of worldwide epidemic disease. This was largely through infection with the louse-borne form of the disease, caused by Borrelia recurrentis (louse-borne relapsing fever (LBRF)). During the last century, we have witnessed the demise of this infection, largely owing to improved standards of living and the introduction of the insecticide DDT, resulting in a reduction in the incidence of the body louse, the vector for relapsing fever. In areas of extreme poverty this disease persists, causing a significant burden of disease. It is now looking probable that this infection is caused by a louse-adapted variant of Borrelia duttonii, transmitted by Ornithodoros moubata 'soft' ticks in East Africa. Like LBRF, infection still causes impact, particularly affecting young children and pregnant women. Over recent years, the true burden of relapsing fever caused by infection with the closely related Borrelia crocidurae, transmitted by Ornithodoros sonrai ticks, has only just begun to emerge. Here, the current state of knowledge concerning relapsing fever in Africa is reviewed.