Lyme Disease Transmission and Tick Saliva

Lyme disease is a spirochaetal infection which is initially observed as a localized cutaneous inflammation (erythema migrans). This Lyme disease rash only occurs in around 75% of cases however, but is sufficient in itself to diagnose the infection as it is so characteristic of Lyme disease transmission. The skin acts as a protective barrier for infection in most cases but there is evidence that tick saliva adversely affects the innate immunity of the host’s skin during feeding. Those substances modulating immunity include antimicrobial peptides cathelicidin and defensin which are key initiators of skin inflammation that usually occur following inoculation with bacteria but may be disrupted during Borrelia infection. Kern (et al, 2011) examined the different responses to Borrelia infection using either a syringe to administer the Lyme disease bacteria in mice or through direct tick bite and transmission. The syringe inoculation triggered skin inflammation and induction of CRAMP (mouse cathelicidin), and tumor necrosis factor-alpha. Direct tick transmission resulted in significant repression of the inflammatory reaction.

Tick Saliva Inhibits Immune Reaction

Further research from the Czech Republic looked at the specific components of tick saliva that may aid the replication of Borrelia and Lyme disease transmission. Rudolf (et al, 2010) observed a marked increase in growth of B. garinii, B. afzelii, and B. burgdorferi s.s. in culture medium containing tick salivary gland extract from I. ricinus compared to the usual bacterial culture medium BSK-H. In addition, an antialarmin effect of tick saliva was observed by Marchal (et al, 2011) during Lyme disease transmission involving Borrelia burgdorferia s.s. which inhibited the mobilization and activation of antigen-presenting cells. Thus, the invading bacteria were able to disseminate more easily without being recognized by the host’s innate immunity. The tick saliva protein Salp15 was found to be important in inhibiting the induction of an immune system response to Borrelia burgdorferi s.s. OspC. In this study the downregulation of cathelicidin, defensin, and both psoriasin and RNase7 were also observed and, although such antimicrobial peptides (AMPs) did inhibit the motility of the bacteria they did not kill the organisms in in vitro testing. Using elements of the ticks’ saliva to facilitate their initial invasion of the mammalian host then allows the spirochaetes to rapidly reproduce and disseminate to infect the heart, nervous system, joints, and other organs of the body.

Research at the CDC has isolated a specific gene that is essential for infection following bacterial transmission from a tick bite. Gilmore (et al, 2010) found that a strain of Borrelia deficient in the synthesis of the bba64 gene of Borrelia burgdorferi was incapable of infecting mice via tick bite. Those mice inoculated via injection with the same mutant strain were infected however and larval ticks feeding on infected mice could acquire the infection themselves. Further research into the relationship between the ticks which transmit Lyme disease, their saliva, and the genotype of infectious Borrelia bacteria may aid in detection, diagnosis, treatment, and prevention of Lyme disease transmission at some point in the future.

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