TAILIEUCHUNG - Chapter 165. Relapsing Fever

The term relapsing fever describes two distinct diseases. Tick-borne (endemic) relapsing fever (TBRF) is a zoonosis that is transmitted principally from rodents to humans by the bite of various soft ticks. Louseborne (epidemic) relapsing fever (LBRF) is a disease of humans that is transmitted from one person to another by the body louse. Both diseases are characterized by recurrent acute episodes of spirochetemia and fever alternating with variable periods of remission. | Chapter 165. Relapsing Fever The term relapsing fever describes two distinct diseases. Tick-borne endemic relapsing fever TBRF is a zoonosis that is transmitted principally from rodents to humans by the bite of various soft ticks. Louse-borne epidemic relapsing fever LBRF is a disease of humans that is transmitted from one person to another by the body louse. Both diseases are characterized by recurrent acute episodes of spirochetemia and fever alternating with variable periods of remission. Etiology Relapsing fever is caused by infection with spirochetal gram-negative bacteria of the genus Borrelia family Spirochaetaceae . The borreliae are helical in shape and average pm in width and 5-20 pm in length. They comprise an outer membrane an intermediate peptidoglycan layer and an inner cytoplasmic membrane which encloses the protoplasmic cylinder. A variable number of periplasmic flagella are situated beneath the outer membrane. Relapsing-fever borreliae are slow-growing and microaerophilic they grow best at 30 -35 C in Barbour-Stoenner-Kelly BSK II medium. B. recurrentis is the only species that causes LBRF. Most of the various species of Borrelia that cause TBRF are named after the species of Ornithodoros tick responsible for their transmission. In North America TBRF is caused mostly by B. hermsii and only occasionally by B. turicatae B. duttoni is the most common cause of TBRF in sub-Saharan Africa an area of high endemicity. Borreliae are unique among bacteria in having a genome composed of a linear chromosome and a series of linear and circular plasmids. The sequences of both the flagellin and the 16S ribosomal RNA genes are homogeneous among LBRF strains in contrast there is considerable heterogeneity of these genes between Old World and New World TBRF strains. A unique process of DNA rearrangement within vmp genes located on linear plasmids results in extensive variation in the expression of the surface antigens in relapsing fever borreliae. These vmp

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