B. burgdorferi s.s. B. burgdorferi s.s. is a highly generalist species: several vectors, such as I. scapularis , I. pacificus and I. ricinus, are able to transmit it, as are minor ones, such as I. trianguliceps and I. hexagonus [42] . Both vector cycles and seasonal fluctuations shape the transmission potential of Borrelia . As a result, the prevalence of the disease may be drastically different between places close to each other [43] . Similarly, the expansion zone of B. burgdorferi s.s. is quite large in the northern hemisphere. In North America, B. burgdorferi s.s. has spread over the West Coast and the eastern half of the USA (mainly in the northeast), but also some southern areas such as Florida and Texas. In the Mid-West, some contaminated spots have been recorded. In Canada, the threat exists in the southeast of the country. In Europe, B. burgdorferi s.s. is present, but its density is lower than those of the 2 other main pathogenic species: B. garinii and B. afzelii . In Africa, ticks harboring B. burgdorferi s.s. have been reported in Morocco [44] . Currently, it may be abundant, as is the case in the western part of France. Towards the east, B. burgdorferi s.s. is considered to be absent from Asia. Indeed, in the borderline area between Asia and Europe, where both I. ricinus and I. persulcatus coexist, B. burgdorferi s.s. was identified only in I. ricinus [45] . B. burgdorferi s.s. has been isolated in South Central China, but restricted to a hare, Caprolagus sinensis, whose associated tick is Haemaphysalis bispinosa [46] . B. burgdorferi s.s. is also present in Taiwan, but both ospC and ospA genes from several sequenced Taiwanese isolates are almost identical. It mirrors a strictly clonal population in spite of the different hosts harboring the isolates [47, 48] . Similarly, European B. burgdorferi s.s. also represent a subset of the North American population of B. burgdorferi s.s., which is largely more diverse intraspecifically. Such genetic bottlenecks are called a ‘founder’s event’, and suggest that some North American clones of B. burgdorferi s.s. have been subsequently imported into Europe and then into Taiwan [4, 19, 49] . B. burgdorferi s.s. hosts are still characterized by their diversity in the USA: Peromyscus leucopus, Tamias striatus, Blarina brevicauda, Sciurus carolinensis and Sciurus griseus, and also passerine birds, blackbirds, robins, pheasants and veeries [42, 50] . Each distinct host harbors different B. burgdorferi s.s. genotypes at different frequencies, shaping the Borrelia population into distinct enzootic niches [43, 50] . However, in Europe the range of B. burgdorferi s.s. hosts are less well known, and include red squirrels and hedgehogs [42] . Concerning its pathogenicity, B. burgdorferi s.s. – like any other pathogenic B. burgdorferi s.l. species – is able to provoke EM. It has been shown that lesions correspond to the intradermic inflammatory response fighting the centrifugal migration of bacteria from the inoculation point [51] . The physiology of multiple EM is quite different: it reflects the ability of some Borrelia to penetrate the blood vessels and migrate via this route into different parts of the body, including the skin. This necessarily supposes these bacteria to be invasive ones. Further, Lyme borreliosis may be inconstantly characterized by secondary lesions distant from the inoculation point, sometimes in deep organs. Septicemia is the way that the Borrelia invade the whole organism at this late stage. Each pathogenic Borrelia species exhibits a preferential organotropism [20] . B. burgdorferi s.s. have been associated with arthritis. For instance in the USA, where B. burgdorferi s.s. is the only pathogenic B. burgdorferi s.l. species present, arthritis is the most reported late clinical presentation (33%) [52] . However, this organotropism is elective since B. burgdorferi s.s., still in the USA, also causes neurological problems (5%) [52] . In western Europe, too, B. burgdorferi s.s. has been reported as the species prominently isolated from arthritic forms [53] , but in eastern areas where B. garinii is highly represented, such as Germany, the etiology of arthritis is more diverse [54] . Lipsek and Jaulhac (2009) “Lyme Borreliosis. Biological and clinical aspects”, Karger
Posted on: Thu, 13 Jun 2013 14:49:18 +0000
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