Relapsing fever is an illness with fever, headache, muscle pain, and weakness but no signs pointing to any organ system. It lasts about 1 week and returns a few days later. The relapses may continue for as many as four cycles. During each re-lapse, spirochetes are present in the bloodstream. The causative Borrelia species are transmitted to humans from ticks or body lice.
Relapsing fever occurs in two forms linked to the mode of transmission and the Borrelia species involved. The louse-borne form usually appears in epidemics, because of circum-stances connected with body lice, whereas the tick-borne form does not. For this reason, the two forms are sometimes called epidemic (louse-borne) and endemic (tick-borne) re-lapsing fever. Here they will be identified simply by the insect involved.
The occurrence and distribution of tick-borne relapsing fever are determined by the biology of the relevant tick species and its relationship to the primary Borrelia reservoir in rodents and other small animals (rabbits, birds, lizards). Ticks may remain infectious for several years even without feeding and transovarial passage to their progeny extends the infectious chain even further. Humans are infected when they ac-cidentally enter this cycle and are bitten by an infected tick. The bite is painless and the feeding period is brief ( < 20 minutes). Because the ticks usually feed at night, cases are most often associated with overnight recreational forays into wild, wooded areas. The largest outbreak in the United States involved National Park employees and tourists who slept in tick- and rodent-infested cabins on the Northern Rim of the Grand Canyon.
The epidemiologic conditions associated with louse-borne relapsing fever are much more exacting. The human body louse has no other host, infected lice live no more than 2 months, and there is no transovarial passage to progeny. B. recurrentis is the only species involved. Lice are infected from human blood but the spirochetes multiply in their hemolymph, not any of the feeding parts or excrement. This means they can infect another human only if the louse is crushed by scratching and the Borrelia reach a su-perficial wound or mucosal surface. Infected lice must be passed human to human for the disease to persist. These conditions are met by circumstances that combine over-crowding with extremely low levels of general hygiene. War, other kinds of social breakdown, and dire poverty are the prime associates. Currently, this variety of relaps-ing fever appears to be limited to east and central Africa and the Peruvian Andes.
The disease manifestations develop at times when thousands of spirochetes are circulating per milliliter of blood. The febrile illness has endotoxin-like features, but the exact mecha-nisms of disease are unknown. Between episodes the organisms disappear from the blood and are sequestered in internal organs only to reappear during relapses. The OMPs are anti-genically different with each relapse. The relapsing cycles correlate with antibody production to the new protein followed by clearing followed by emergence of a new antigenic type.
Immunity to the disease is largely humoral and appears to involve lysis of the organism in the presence of complement. The disease is controlled when variants from the antigenic repertoire are no longer able to escape the immune response.