relapsing fever, infectious disease caused by a spirochete bacteria of the genus Borrelia and characterized by a high fever that breaks and then recurs a one to two weeks later. Relapsing fever may be transmitted by body lice or soft ticks. Louse-borne relapsing fever (LBRF) is caused by B. recurrentis, and tick-borne relapsing fever (TBRF) is caused by more than a dozen Borrelia species and transmitted by several soft ticks of the genus Ornithodoros; hard ticks of the genus Ixodes can transmit B. miyamotoi, which causes infection in humans with similar symptoms but usually without a true relapsing fever. Symptoms of relapsing fever include fever and chills, severe headache, muscle and joint aches, and nausea and vomiting. The fever typically breaks after a crisis in which it spikes and is accompanied by rapid breathing and heart rate; the fever then drops precipitously, accompanied by profuse sweating. Recurring, generally less severe episodes of fever and other symptoms are much more numerous in TBRF, but untreated LBRF is more likely to lead to liver and spleen complications, myocarditis, and heart failure with a death rate that can reach 30–70%. Neurological complications may occur with both forms. Both forms of relapsing fever and B. miyamotoi infection may be treated with antibiotics. LBRF is now endemic only in the Horn of Africa, but outbreaks often occur in the overcrowded situations associated with war, famine, and refugee camps. TBRF is found on all continents except Australia; in the United States it occurs in the western half of the country, where patients are most typically infected when sleeping in rural cabins frequented by rodents.
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