Impact of extrinsic environmental factors

The phylogenetic tree topology for B. burgdorferi s. l. differs depending upon which gene is used as the basis (Godfroid et al., 2003; Kurtenbach et al., 2002a; Lagal et al., 2002; Wang et al., 1999a). Until a consensus is reached, conclusions about the impact of environmental factors on the evolution of this complex must rest on general patterns rather than statistical analysis. The high force of transmission of these bacteria (Randolph et al., 1996), and consequently the wide window of opportunity for persistent enzootic cycles, allows B. burgdorferi s. l. to occur more or less throughout the ranges of competent tick species under a wide range of ecological (biotic and abiotic) conditions. Therefore, compared with TBE virus, environmental factors are not expected to impose such constraint.

The wide, non-patchy geographic distribution of many B. burgdorferi s. l. species appears to confirm this, although there are clear differential ranges that cannot be explained simply by the availability of competent vector or host species (Fig. 4). The more recently described species from eastern Asia are currently known only from the countries where they were originally isolated (B. japonica, Borrelia tanukii and Borrelia turdi from Japan, and Borrelia sinica from northwest China), where they are associated with local tick species. This almost certainly reflects an absence of knowledge rather than a knowledge of absence elsewhere. In contrast, where survey work has been much more intensive, B. afzelii, B. garinii (Eurasian and Asian types) and B. valaisiana are found throughout Europe and Asia (Hubalek & Halouzka, 1997; Wang et al., 1999a), but Borrelia lusitaniae is known mostly from Portugal and N. Africa and more rarely, but increasingly, from elsewhere in Europe (Jouda et al., 2003). B. burgdorferi s. s. and B. bissettii are the only species found on both sides of the Atlantic, but both are much more prevalent in N. America than in Europe. In the Old

Species

B. andersoni B. bissettii B. burgdorferi s. s. B. lusitaniae B. garinii (Eurasia) B. afzelii B. garinii (Asia) B. valaisiana B. japonica B. tanukii B. turdi B. sinica

Fig. 4. Broad geographic distributions and principal tick vectors (Ixodes species) of each species of the B. burgdorferis. l. complex. Expanded from Miyamoto & Masuzawa (2002), with permission.

World, B. burgdorferi s. s. is much more common in western than eastern Europe, where it is transmitted by I. ricinus but not in Eurasia by I. persulcatus. B. bissettii, associated with woodrats and Ixodes spinipalpis in the New World, has been recorded from human patients in Slovenia (Picken et al., 1996). B. andersoni, associated with cottontail rabbits and Ixodes dentatus, is confined to the USA.

The absence of B. afzelii and B. garinii in America, despite the ability of the widespread American tick species Ixodes scapularis to transmit both these species (Dolan et al., 1998), together with the greater diversity of B. burgdorferi s. l. in the Old World, suggests that this bacterial complex originated in the Old World (Wang et al., 1999a). The New World, however, may be a (secondary?) centre of endemism. B. burgdorferi s. s. isolates from both Europe and N. America are closely related to one another, indicating 'unambiguously ... that a constant supply from one continent to the other has occurred' (Lagal et al., 2002). Genetic diversity is evidently higher amongst B. burgdorferi s. s. isolates from N. America than amongst isolates of this species from Europe (Foretz et al., 1997; Marti Ras et al., 1997). Furthermore, interspecific lateral transfers have occurred from B. burgdorferi s. s. to B. garinii and B. afzelii, but not in the opposite direction. These facts, it is argued, indicate that no European

America Europe Far East Russia, China, Nepal, Thailand

Korea Japan Yangtze Taiwan River

America Europe Far East Russia, China, Nepal, Thailand

Korea Japan Yangtze Taiwan River

Fig. 4. Broad geographic distributions and principal tick vectors (Ixodes species) of each species of the B. burgdorferis. l. complex. Expanded from Miyamoto & Masuzawa (2002), with permission.

. granulatus

. granulatus

B. burgdorferi s. s. isolate has been imported into N. America, only the opposite has occurred (Lagal et al., 2002), i.e. B. burgdorferi s. s. originated in America and was introduced into Europe.

Despite the apparent general permissiveness of both biotic and abiotic conditions for circulation of the B. burgdorferi s. l. complex of spirochaetes, there are clearly differences which favour the endemic Eurasian species of B. burgdorferi s. l. but limit B. burgdorferi s. s. in Eurasia, and vice versa in America. Statistical models of the distribution of each species (or subspecies if possible and necessary), incorporating biotic and abiotic predictor variables, would allow some flesh to be put on this obvious statement. Predictive mapping of the risk to humans of Lyme borreliosis (i.e. B. burgdorferi s. s.) in America (Dister et al., 1997; Glass et al., 1995; Guerra et al., 2002) has identified conditions that favour tick survival and therefore healthy tick populations, but I suspect that the differences between Borrelia species lie in rather more subtle abiotic factors that can determine the precise, quantitative tick-host relationships (Randolph & Storey, 1999). Both the questing behaviour of ticks and the availability of various host species in tick micro-habitats are under environmental determinants. We have a very long way to go on both sides before we can start to test for matches between phylogenetic and eco-climatic trees for B. burgdorferi s. l.

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