Acquisition Dynamics of Borrelia burgdorferi and the Agent of Human Granulocytic Ehrlichiosis at the Host-Vector Interface

The population dynamics of two cotransmitted tick-borne pathogens, Borrelia burgdorferi and the agent of human granulocytic ehrlichiosis (aoHGE), were assessed at the skin-vector interface at intervals after tick attachment on infected mice. Quantitative real-time polymerase chain reaction targeting the B. burgdorferi flagellin gene revealed consistent decreases in spirochete numbers in skin at the sites of tick attachment compared with non-tick attachment sites. This phenomenon was found during early (2 weeks) and late (8 weeks) infection and at 24, 48, and 72 h after tick attachment. A nonspecific inflammatory stimulus, implantation of suture material, did not have this effect. In contrast to B. burgdorferi, copy numbers of an aoHGE p44 target gene target were significantly increased at the sites of tick attachment, compared with non-tick sites. The non-specific stimulus of suture material had the same effect on aoHGE recruitment as tick attachment in aoHGE infected mice. These results reinforce the concept that B. burgdorferi interfaces with its vector by virtue of its non-systemic dermatotropism, and not via systemic hematogenous acquisition. In contrast, results indicate that the aoHGE relies upon hematogenous acquisition. Thus, these two cotransmitted tick-borne pathogens utilize distinctly different means of vector acquisition.