Spotted Fever Group Rickettsia Infection and Transmission Dynamics in Amblyomma maculatum

Abstract

Tick vectors are capable of transmitting several rickettsial species to vertebrate hosts, resulting in various levels of disease. Studies have demonstrated the transmissibility of both rickettsial pathogens and novel Rickettsia species or strains with unknown pathogenicity to vertebrate hosts during tick blood meal acquisition; however, the quantitative nature of transmission remains unknown. We tested the hypothesis that if infection severity is a function of the rickettsial load delivered during tick transmission, then a more virulent spotted fever group (SFG) Rickettsia species is transmitted at higher levels during tick feeding. Using Amblyomma maculatum cohorts infected with Rickettsia parkeri or “Candidatus Rickettsia andeanae,” a quantitative PCR (qPCR) assay was employed to quantify rickettsiae in tick salivary glands and saliva, as well as in the vertebrate hosts at the tick attachment site over the duration of tick feeding. Significantly greater numbers of R. parkeri than of “Ca. Rickettsia andeanae” rickettsiae were present in tick saliva and salivary glands and in the vertebrate hosts at the feeding site during tick feeding. Microscopy demonstrated the presence of both rickettsial species in tick salivary glands, and immunohistochemical analysis of the attachment site identified localized R. parkeri, but not “Ca. Rickettsia andeanae,” in the vertebrate host. Lesions were also distinct and more severe in vertebrate hosts exposed to R. parkeri than in those exposed to “Ca. Rickettsia andeanae.” The specific factors that contribute to the generation of a sustained rickettsial infection and subsequent disease have yet to be elucidated, but the results of this study suggest that the rickettsial load in ticks and during transmission may be an important element.

Interferon receptor-deficient mice are susceptible to eschar-associated rickettsiosis

“…route revealed dose-dependent lethality in Ifnar -/- Ifngr -/- mice, with 10 7 R. parkeri eliciting 100% lethality and 10 5 R. parkeri eliciting no lethality 21 . R. parkeri are present in tick saliva at a concentration of approximately 10 4 per 1 μl, and approximately 10 7 R. parkeri are found in tick salivary glands 17 . However, the number of bacteria delivered from tick infestation likely varies depending on many factors, and we therefore sought to examine the effects of different doses of R. parkeri upon i.d.…”

“…delivery of 10 8 bacteria 16 . However, this dose is substantially higher than the number of R. parkeri found in tick saliva or tick salivary glands 17 , and considerable effort is required to generate and concentrate this number of bacteria. An improved mouse model to investigate R. parkeri would greatly increase the ability to investigate virulence mechanisms, the host response to infection, and human rickettsial disease.…”

Annexin A2 depletion exacerbates the intracerebral microhemorrhage induced by acute rickettsia and Ebola virus infections

“…Rickettsioses represent devastating human infections [1][2][3][4][5][6][7]. These arthropod-borne diseases are caused by obligatory intracellular bacteria of the genus Rickettsia (R.).…”

Recent advances in understanding tick and rickettsiae interactions

“…52 Furthermore, a rat model was successfully used for A maculatum for an R parkeri transmission study, and traditionally used guinea pigs have been similarly proposed to be useful models for R parkeri tick-transmission studies. 64,65 The consensus tick transmission model for R parkeri will depend on the ease of infesting ticks, a sufficient pathogen load in animal organs, and accurate representation of human disease. The standard animal transmission model of spotted fever rickettsiosis is needed to perform many tick-transmission-blocking experiments.…”

“…Tick‐transmission mouse models (C3H/HeN) for R parkeri were established and provided evidence that tick transmission significantly increases the bacterial load inside mouse organs compared with intravenous injection 52 . Furthermore, a rat model was successfully used for A maculatum for an R parkeri transmission study, and traditionally used guinea pigs have been similarly proposed to be useful models for R parkeri tick‐transmission studies 64,65 . The consensus tick transmission model for R parkeri will depend on the ease of infesting ticks, a sufficient pathogen load in animal organs, and accurate representation of human disease.…”