Impact of the abundance of the Lyme disease bacterium in host tissues and transmission to feeding ticks

In North America, Borrelia burgdorferi is transmitted to vertebrate reservoir hosts by the blacklegged tick (Ixodes scapularis). This tick-borne spirochete establishes a chronic infection in the host tissues. Experimental infection studies have shown that transmission of B. burgdorferi from infected hosts to feeding ticks (host-to-tick transmission) often decreases over the course of the infection, which suggests involvement of the acquired immune response. Mice with severe combined immunodeficiency (SCID) lack an acquired immune system and often have a higher pathogen abundance in their tissues compared to immunocompetent (IC) mice. Despite the amount of knowledge on Borrelia-host interactions, the effect of the acquired immune system on the lifetime host-to-tick transmission of the spirochete is not well characterized. Immunocompetent and SCID mice (C57BL/6) were experimentally infected with B. burgdorferi via tick bite and were subsequently infested with I. scapularis larvae at weeks 4, 8, and 12 post-infection (PI) to measure host-to-tick transmission. Mice were euthanized and organs were dissected at week 16 PI. The infection status of the mouse organs and ticks were tested using qPCR. We found a high infection prevalence and spirochete abundance in organs of SCID mice compared to IC mice, with abundance being different between male and female mice. In host-to-tick transmission, we found differences in the spirochete abundance in ticks that fed on SCID mice compared to IC mice. This research will enhance our understanding of how the acquired immune system influences the host tissue spirochete abundance and host-to-tick transmission of this tick-borne pathogen.