Natural maintenance cycle and associated biological parameters for B. burgdorferi. (a) The spirochete is maintained in nature by alternating infections in white-footed mice and immature Ixodes ticks. Larvae (L) acquire spirochetes by feeding on infected mice and maintain the infection through the transstadial (TST) molt. Infected nymphs (N) transmit spirochetes during the next bloodmeal. (b) Spirochetes are exposed to alternating temperatures during tick and mammalian infections, and other parameters associated with tick attachment and feeding. (c) OspA and OspC are differentially expressed by spirochetes during the transmission cycle with alternate tick and mammalian hosts. (d) General trend in antibody titer to OspA and OspC in mice infected by tick bite, supporting the evidence for the lack of OspA expression during infection and the early presence of OspC following the tick bite.