Host blood meal-dependent growth ensures transovarial transmission and transstadial passage of Rickettsia sp. phylotype G021 in the western black-legged tick (Ixodes pacificus)

In this study, we explored the growth dynamics of Rickettsia sp. phylotype G021 during transovarial transmission and transstadial passage by Ixodes pacificus using real-time quantitative PCR. Four parental engorged I. pacificus females were allowed to complete their developmental stages until the F2-generation eggs yielded unfed larvae. All eggs, larvae, nymphs, and adults tested through 2 generations were found to be infected with phylotype G021. Hence, we conclude that the efficiency of transovarial transmission and transstadial passage of this phylotype in I. pacificus was 100%. Acquisition of a blood meal by all 3 parasitic stages (larva, nymph, adult) significantly increased the rickettsial burden as fed larvae, nymphs, and adults had respective 19-, 12-, and 313-fold increases of rickettsiae compared with unfed ticks representing each developmental stage. I. pacificus eggs contained high rickettsial burdens at the time of oviposition. While I. pacificus egg cells underwent rapid proliferation during early embryonic development, the rickettsiae remained relatively quiescent, which resulted in depressed numbers of phylotype G021 per tick cell. However, the rickettsial burden remained constant over a period of 56 days, as the rate of I. pacificus cell division slowed during later embryonic development.