Trends in Parasitology
ReviewUnpacking the intricacies of Rickettsia–vector interactions
Section snippets
Rickettsial diversity and vector transmission
With historical significance and recognized resurgence and emergence, vector-borne rickettsial diseases affecting human health are of critical importance worldwide. In the USA, this growing trend is evident in a recent report of notifiable diseases with cases of tick-borne spotted fever rickettsiosis increasing by 23% from 2016–2018i and the recent re-emergence of flea-borne rickettsiosis within endemic areas in the USA, including California and Texas [1]. Likewise, the reappearance of
The biology of rickettsial infection in arthropods
Rickettsiae can be acquired through feeding on a bacteremic host. Alternative acquisition routes have also been described, including cofeeding with an infected arthropod on a nonrickettsemic host or transovarial transmission. Due to the host cell requirement for rickettsiae to replicate, a critical initial step in rickettsial pathogenesis is the bacterial recognition of, and attachment to, target cells. Nevertheless, the study of Rickettsiology has been hampered by the lack of genetic tools to
Tick-borne rickettsiae
Classification of the genus Rickettsia is based on genetic and biological characteristics [44]. Tick-borne, SFG rickettsiae consist of species ranging from those that cause severe and often fatal human diseases, such as Rocky Mountain spotted fever, to strict tick endosymbionts. To ensure persistence within tick populations, tick-borne rickettsiae rely on both transovarial and transstadial transmission. If imbibed in a blood meal, it is presumed that rickettsiae traverse through the midgut
Insect-borne rickettsiae
Another group of Rickettsia species, the typhus group (TG), includes pathogenic Rickettsia typhi and Rickettsia prowazekii, associated with fleas and lice, respectively. Possessing shared genetic characteristics between SFG and TG yet maintaining distinguishable biological features, the transitional group (TRG) has been more recently classified [44]. The TRG contains species with varying pathogenicity, such as R. felis, that is associated with diverse arthropod hosts (e.g., fleas, lice, mites,
Microbial interactions and vector biology
In addition to vectors' chitinous exoskeleton, resident microflora can also defend against invading microorganisms. The role of endosymbionts in the transmission of vector-borne pathogens has gained increased attention due to their ability to alter an arthropod's vectorial capacity [40,43,80]. The influence of the microbiome in altering rickettsial vector immunity and metabolism is being assessed [53]. For example, tick endosymbionts in the genera Coxiella, Rickettsia, and Francisella, which
Concluding remarks
The diversity of Rickettsia genomes and transmitting vectors provide a rich substrate to interrogate host–pathogen interactions (see Outstanding questions). Untangling the complex events in which rickettsiae can adapt to the dynamic shift between vector and vertebrate hosts will enable a better understanding of the molecular drivers associated with transmission. Limiting rickettsial disease management is the scant knowledge of the molecular and biological factors conveying rickettsial virulence
Acknowledgments
We appreciate the reviewers' thorough effort in reviewing this manuscript and apologize for any references that were omitted due to space constraints. We would also like to thank members of the Macaluso laboratory for their valuable comments. This work was supported by the National Institutes of Health (NIH) to K.R.M (AI122672 and AI077784). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.
Declaration of interests
The authors declare no competing interests.
Glossary
- Antimicrobial peptides (AMPs)
- molecules that are secreted as an innate immune response to an infection via IMD or Toll pathways. AMPs directly act against microbial agents by causing membrane disruption, inhibition of membrane protein synthesis, metabolism interference, or direct lysis. These molecules can include defensins, varisins, lysozymes, lectins, and protease inhibitors.
- Antioxidant enzymes
- enzymes produced to decrease the effects of ROS, including catalase, superoxide dismutases, and
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