Relapsing fever is caused by several species of Borrelia, all but one of which are transmitted from reservoir animals to humans by soft (argasid) ticks. The exception is B. recurrentis, which is transmitted from one human to another by the body louse and, under certain conditions, may cause large epidemics. Relapsing fever Borrelia species have a number of properties that facilitate invasion and dissemination in the mammalian host; these include neurotropism and adhesiveness for erythrocytes and platelets. But their most characteristic pathogenetic feature is multiphasic antigenic variation. Surface-exposed lipoproteins largely determine the antigenic identities of the spirochetes. Switching between genes encoding these highly polymorphic proteins is accomplished by a recombination in which the active gene at a single expression site is replaced by a copy of an archived, silent gene. The order of appearance of serotypes during infection is partially the function of the location of the archived gene and the surrounding sequences. (Samuels and Radolf, 2010)

References
Samuels, D.S. and Radolf, J.D. (2010) Borrelia: Molecular Biology, Host Interaction and Pathogenesis . Caister Academic Press, Norfolk, UK.

Labels: Argasid ticks, B. recurrentis, Body louse