Spirochaetes and Borrelia
Lyme disease is caused by the bacterium Borrelia burgdorferi. The genus Borrelia is a member of the larger family of bacteria called Spirochaetes which were originally thought to be spiral shaped. The bacterium that causes syphilis (Treponema pallidum) is also a member of this family.
See The EUCALB site for more details on the biology of spirochaetes.
In the genus Borrelia, approximately 20 species are associated with relapsing fevers and transmitted by soft ticks (Ornithodoros or Argas) or by lice in the case of Borrelia recurrentis. However, since the 1980s there has been much interest in Borrelia burgdorferi, the agent of Lyme borreliosis (LB), which is transmitted by hard ticks of the genus Ixodes. When B. burgdorferi was originally described it was believed to be the only species responsible for LB. However, in the last ten years reciprocal hybridization of a large number of strains clearly demonstrated that the genetic diversity of this species was greater than expected and it is now usually referred to as B. burgdorferi sensu lato (s.l.) and consists of several different genospecies.
B. burgdorferi can be divided into at least 15 genospecies which are: B. burgdorferi sensu stricto (s.s.), present in Europe and in the USA, but rare in Russia and apparently absent from Asia;, B. afzelii, B. bavariensis (formerly the B. garinii OspA serotype 4), B. garinii, B. valaisiana, B. spielmanii, B. lusitaniae and B. bissettii in Eurasia; B. japonica, B. tanukii and B. turdae restricted to Japan, B. sinica in China, and B. andersonii, B. bissettii, B. californiensi and B. carolinensis in the USA.
In addition, B. miyamotoi, one of the relapsing fever Borrelia species has been identified in Europe recently. See our news item Borrelia miyamotoi new or old?
B. burgdorferi s.s., B. garinii, B. bavariensis (a close relative of B. garinii), B. afzelii and B. spielmanii (a close relative of B. afzelii) are undoubtedly involved in clinical cases of LB. There is considerable evidence resulting from PCR and serological data and from isolation from patients, that the division of B. burgdorferi sensu lato (s.l.) into genospecies on genetic grounds has clinical relevance also. Thus, B. burgdorferi s.s. is most often associated with arthritis, particularly in North America where it is the only known cause of Lyme disease, B. garinii is associated with neurological symptoms and B. afzelii with the chronic skin condition, acrodermatitis chronica atrophicans (ACA). Overlap between species in relation to clinical manifestations occurs and all cause the pathognomonic symptom erythema migrans (EM). In Europe there is evidence that this early symptom occurs more frequently in B. afzelii infections, especially in women, than in those caused by B. garinii, which result in more frequent non-annular EMs and are more often associated with systemic symptoms (Bennet et al, Wien. Klin. Wochenshr. 2006. 118: 531-7). Neither B. valaisiana nor B. lusitaniae are currently regarded as pathogens, but B. valaisiana has been detected in cerebrospinal fluid by PCR (Diza et al. 2004. Emerg Infect Dis. 10:1692-3) and B. lusitaniae has been isolated from a case of vasculitis (Lopes de Carvalho et al. 2008. Clin Rheumatol. 27:1587-91). The least information is available for B. bissettii, a species mostly encountered in California. No strain belonging to this species has been isolated from a human patient in the USA, but rare and unconfirmed cases of human disease due to this species have been reported in Europe and recently B. bissettii was isolated from human cardiac valve tissue (Rudenko et al., 2008. J Clin Microbiol. 46:3540-3).
So far there has been little progress in identifying pathotypes within genospecies. However, small subsets of B. burgdorferi s.s. and B. afzelii, characterised by particular variants of Osp C, have been reported to be particularly invasive in humans (Wormser et al. 2008. J Infect Dis. 198:1358-64).
Using darkfield microscopy technique, this photomicrograph, magnified 400x, reveals the presence of spirochete, or “corkscrew-shaped” bacteria known as Borrelia burgdorferi, which is the pathogen responsible for causing Lyme disease.
Transmission dynamics of Lyme disease spirochetes Published by Oxford Tick Research Group.