Borrelia burgdorferi taxonomy pathogenicity and expansion

Borrelia burgdorferi sensu late is a bacterial complex of at least 10 speci es, most of which are non pathogenic for humans. Non pathogenic species see m to be more vector-specific, their distribution being restricted to that o f this vector. In opposition, the three species pathogenic for humans, Borr elia burgdorferi sensu stricto (B.b.s.s.), Borrelia garinii (B.g.), and Bor relia afzelii (B.a.), are transmitted by several species of ticks more dive rsified in their host choice. Each of these three species is associated wit h a preferential organotropism: articulation for B.b. s.s., neurologic syst em for B.g, and skin for B.a. Strains belonging to these three species are much more frequently isolated from vectors and have a much larger area of e xpansion than the non pathogenic species; Indeed, their geographical distri bution corresponding to that of their vector comprises one or two continent s. B.g. and B.a., transmitted by Ixodes ricinus and Ixodes persulcatus, are spread throughout Eurasia, B.b. s.s. is the only species spread on two con tinents separated by an Ocean: North America (vectors: I. scapularis and I. pacificus) and Europe (I. ricinus). Combining of epidemiological data and molecular analyses shows that B.b. s.s. probably originated in America and later on, very recently (after 1492), migrated to the European continent.