Characterization of rickettsial infection dynamics within Dermacentor variabilis and Amblyomma maculatum

Abstract

Spotted fever group (SFG) Rickettsia are primarily associated with their reservoir host and vector, the tick. Rickettsial colonization and maintenance within the arthropod is a key component of vector competence and pathogen transmission to the mammalian host. Contemporary detection of novel tick hosts for rickettsial species, combined with an unprecedented rise in human cases of SFG rickettsiosis, necessitates a deeper understanding of tick/Rickettsia interactions. The hypothesis for this work is that if primary tick/Rickettsia pairings do not exist then rickettsial determinants account for primary vector/pathogen relationships. To this end, Dermacentor variabilis and Amblyomma maculatum ticks were exposed to R. rickettsii, R. parkeri, R. montanensis, R. amblyommii or R. felis. Rickettsial exposure negatively impacted the fitness of A. maculatum, but not D. variabilis. Transovarial and transtadial transmission of rickettsiae was most successful for R. amblyommii and R. parkeri in both, A. maculatum and D. variabilis eggs, larvae, and nymphs. Maintenance of rickettsiae in both tick species via transstadial transmission was diminished from unfed larvae to unfed F1 adults. To further investigate the maintenance of Rickettsia in the arthropod host, an in vitro and in vivo model of R. parkeri infection was utilized. Rickettsial proteins implicated in intracellular actin-based motility (Sca2 and RickA) were shown to function similarly in mammalian and tick cell culture, suggesting conserved functionality in both hosts. In vivo dissemination of a wild-type strain of R. parkeri was measured against two strains deficient in Sca2 and RickA expression. Wild-type, RickA, and Sca2 deficient strains R. parkeri persisted in all tick organs organs at 7 days post-exposure. The findings suggest transovarial transmission specificity to be tick species dependent and vertical transmission is not sustainable.