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The blacklegged tick, Ixodes scapularis Say, transmits the
Lyme disease spirochete Borrelia burgdorferi, whereas the American dog tick, Dermacentor variabilis (Say), is unable to transmit the bacterium. We compared the innate immune response of these ticks against spirochetes directly inoculated into the hemocoel cavity of ticks. In I. scapularis, some Borrelia were found associated with hemocytes, while numerous other spiral-shaped, intact bacteria remained free in the hemolymph. In contrast, in D. variabilis only remnants of the bacteria were evident in the hemolymph, indicating lysis; intact spirochetes were rare. Spirochetes were observed bound to or within the organs of both tick species, although many more spirochetes were found associated with the I. scapularis organs. The few spirochetes observed with the D. variabilis organs appeared to be dead because D. variabilis tissues rarely contained culturable bacteria, unlike I. scapularis tissues. When spirochetes were incubated with I. scapularis hemolymph plasma in vitro, bacterial survival and motility were not reduced. In contrast, incubation of spirochetes with D. variabilis hemolymph plasma resulted in > 50% of the spirochetes becoming nonmotile by 45 min. The differences in the responses of the two different tick species indicate that I. scapularis is immunotolerant when challenged with B. burgdorferi and dependent on a slow phagocytic response to clear Borrelia from the hemolymph. In contrast, D. variabilis is highly immunocompetent (i.e., innate immunity), using plasma borreliacidal factors and a rapid increase in phagocytic cells to clear the infection and limit tissue invasion.