5 - Specificity of Borrelia–tick vector relationships

Summary

BORRELIA

The genus Borrelia comprises diverse species of spirochaetes that are associated with haematophagous arthropods (Paster et al., 1991; Paster & Dewhirst, 2000). Some Borrelia species are pathogenic for humans or for livestock. Other spirochaete groups with human pathogens are the treponemes, which include the human-restricted agent of syphilis, and the leptospires, which are mostly free-living spirochaetes that infect a wide variety of animals. The spirochaete phylum also contains a number of species that are symbionts of invertebrates, such as molluscs and termites. Borrelia spirochaetes characteristically circulate in the blood of their vertebrate hosts and are transmitted between vertebrates by ticks, with the single, epidemiologically important exception of a louse-borne species. A common strategy of Borrelia spp. for prolonging spirochaetaemia – thus increasing the probability of vector transmission – is avoidance of the immune response through antigenic variation (Barbour & Restrepo, 2000; Barbour, 2002). Most types of Borrelia infections are zoonoses, but humans are the critical reservoirs for at least one species (Barbour & Hayes, 1986; Barbour, 2004).

The number of recognized Borrelia species has more than doubled over the last two decades, in part because cultivation methods improved (Barbour, 1988; Cutler et al., 1994) and technologies like PCR allowed identification and taxonomic classification without being able to culture the organism (Anda et al., 1996; Barbour et al., 1996; Kisinza et al., 2003; Scoles et al., 2001). Table 1 is a list of accepted and tentative species designations, as of early 2004. Borrelia species have been documented in the Palaearctic, Afro-Tropical, Nearctic, Neotropical and Antarctic ecological regions, and some species that use humans or livestock as reservoirs are now cosmopolitan.