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 spiro-chaetaemia - 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

SGM symposium 63: Microbe-vector interactions in vector-borne diseases.

Editors S. H. Gillespie, G. L. Smith & A. Osbourn. Cambridge University Press. ISBN 0 521 84312 X ©SGM 2004

Table 1. Borrelia species that cause relapsing fever (RF) or Lyme borreliosis (LB) and species with other disease associations

Species

Ecological region

Vector*

Diseaset

Relapsing fever group

B. anserina

?Afro-Tropical+

A

FS

B. coriaceae

Nearctic

A

?EBA

B. crocidurae

Palaearctic

A

RF

B. duttonii

Afro-Tropical

A

RF

B. hermsii

Nearctic

A

RF

B. hispanica

Palaearctic

A

RF

B. latyschewii

Palaearctic

A

RF

B. lonestari

Nearctic

M

?STARI

B. mazzottii

Nearctic

A

RF

B. miyamotoi

Palaearctic

P

B. miyamotoi s. l.

Palaearctic, Nearctic

P

B. parkeri

Nearctic

A

RF

B. persica

Palaearctic

A

RF

B. recurrentis

?Palaearctici

L

RF

B. theileri

?Afro-Tropical+

M

BS

B. turicatae

Nearctic

A

RF

B. venezuelensis

Neotropical

A

RF

Lyme borreliosis group

B. afzelii

Palaearctic

P

LB

B. andersoni

Nearctic

P

B. bissettii

Nearctic

P

B. burgdorferi

Nearctic

P

LB

B. garinii

Palaearctic, Antarctic

P

LB

B. japonica

Palaearctic

P

B. lusitaniae

Palaearctic

P

B. tanuki

Palaearctic

P

B. turdi

Palaearctic

P

B. valaisiana

Palaearctic

P

*A, argasid tick; L, human louse; M, metastriate hard tick; P, prostriate hard tick.

*A, argasid tick; L, human louse; M, metastriate hard tick; P, prostriate hard tick.

+ BS, bovine spirochaetosis; FS, fowl spirochaetosis; EBA, epizootic bovine abortion; STARI, Southern tick-associated rash illness.

^Suspected origins of B. anserina, B. recurrentis and B. theileri, which are global in distribution species that use humans or livestock as reservoirs are now cosmopolitan. There was a single report of a Borrelia-like spirochaete in rats in Queensland (Carley & Pope, 1962), but otherwise, indigenous Borrelia species have not been documented in the Australasia, Indo-Malayan or Oceania ecological regions. In general, Borrelia spp. are not enzootic in rain forests or in highly arid deserts, but most other landscapes with ticks probably support one or more Borrelia spp., most of which have yet to be identified.

The vertebrate host ranges of most Borrelia spp. are broad. The most common vertebrate reservoirs are rodents, but other host animals include lagomorphs, carnivores, ruminants, primates, bats, insectivores and birds (Barbour, 2004). In the laboratory, one Borrelia sp., Borrelia burgdorferi, for example, will infect mice, rats, hamsters, guinea pigs, rabbits, dogs, non-human primates and birds. Some Borrelia species may have a very limited range of hosts in nature because of the tick's adaptations (see below), but are able to experimentally infect a variety of animals. Some tick vectors of Borrelia spp. even feed on lizards, snakes and turtles, but the reservoir competence of these poikilothermic vertebrates has not, with a few exceptions (Lane & Quistad, 1998), been studied. The ability of an unidentified relapsing fever (RF) Borrelia isolate to infect the guinea pig was used to discriminate between species (Barbour & Hayes, 1986), but, for the most part, the Borrelia-vertebrate host associations were not particularly useful for taxonomic classification or species identification. The apparently specific associations between a Borrelia species and a particular group of animals do not appear to be due to the development of specific and restricted affinities between pathogens and their hosts. The lack of infection of some vertebrates, which might serve as reservoirs, more often appears to be due to toxicity of the vertebrate's innate immunity, such as the complement system, for the spirochaete (Kurtenbach et al., 2002; Lane & Quistad, 1998), rather than co-evolution between host and parasite.

There are two major groups of Borrelia spp. (Table 1 and Fig. 1). The first group, and the largest in number of identified species, includes several species that cause RF, such as Borrelia duttonii in the Afro-Tropical region and Borrelia hermsii in the Nearctic region. The second group includes three species documented to cause Lyme borreliosis (LB): B. burgdorferi, Borrelia afzelii and Borrelia garinii. Although the majority of the species of each group may not, after a final, future accounting, be associated with any human disease, the terms 'relapsing fever group' and 'Lyme borreliosis group' are used for ease of presentation, at the risk for some species of false guilt by their associations in a group. The RF group includes species, such as Borrelia miyamotoi and some related organisms (B. miyamotoi sensu lato), that have yet to be associated with a disease, as well as the species Borrelia anserina and Borrelia theileri, which are the respective agents of fowl spirochaetosis and bovine borreliosis, two disorders that may be equivalent to RF in their pathogenesis and clinical features but were given other names. This group also includes two species, Borrelia lonestari and Borrelia coriaceae, that are suspected but not proven to cause disorders of humans (Southern tick-associated rash illness) and domestic animals (epizootic bovine abortion), respectively (Barbour, 2001).

Whereas known LB group species, with the exception of B. garinii, which infects sea-birds (Olsen et al., 1995), are limited to the Holoarctic (Palaearctic and Nearctic), RF group species have been found in the Afro-Tropical and Neotropical regions as well as Palaearctic and Nearctic. It is possible that the difference in the distribution of the two groups reflects the time periods and geographic scopes of the collections. RF Borrelia spp. have been known for over a century, while the first known example of the LB group was not discovered until 1981 (Barbour & Hayes, 1986). There have only been sporadic, limited searches to date for hitherto unknown Borrelia spp. in the Afro-Tropical, Australasia, Oceania and Indo-Malayan ecological regions.

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