AUTHOR=Gerardi Monize , Ramírez-Hernández Alejandro , Binder Lina C. , Krawczak Felipe S. , Gregori Fábio , Labruna Marcelo B. 

TITLE=Comparative Susceptibility of Different Populations of Amblyomma sculptum to Rickettsia rickettsii

JOURNAL=Frontiers in Physiology

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2019.00653

DOI=10.3389/fphys.2019.00653

ISSN=1664-042X

ABSTRACT=<p>The bacterium <italic>Rickettsia rickettsii</italic> is the etiological agent of Brazilian spotted fever (BSF), which is transmitted in Brazil mainly by the tick <italic>Amblyomma sculptum.</italic> Herein, larvae and nymphs of six populations of <italic>A. sculptum</italic> were exposed to <italic>R. rickettsii</italic> by feeding on needle-inoculated guinea pigs, and thereafter reared on uninfected guinea pigs or rabbits. Two tick populations were exposed to autochthone <italic>R. rickettsii</italic> strains, whereas four tick populations were exposed to non-autochthone strains. The six geographically different populations of <italic>A. sculptum</italic> showed different susceptibilities to <italic>R. rickettsii</italic>, higher among the two tick populations that were exposed to their autochthone <italic>R. rickettsii</italic> strain. In addition, higher rates of transovarial transmission of <italic>R. rickettsii</italic> and vector competence success also included the two tick populations that were exposed to autochthone <italic>R. rickettsii</italic> strains. These results indicate that the susceptibility of <italic>A. sculptum</italic> to <italic>R. rickettsii</italic> varies among different tick populations, with a clear bias for higher susceptibility to an autochthone <italic>R. rickettsii</italic> strain that has already coevolved with a tick population for some time. Our results demonstrated that the <italic>R. rickettsii</italic> infection induces higher mortality of engorged larvae and nymphs, and tend to reduce the reproductive fitness of engorged females. All together, these results might explain the low <italic>R. rickettsii-</italic>infection rates of <italic>A. sculptum</italic> under natural conditions (usually &lt;1%), and indicate that an <italic>A. sculptum</italic> population should not be able to sustain a <italic>R. rickettsii</italic> infection for successive tick generations without the creation of new cohorts of infected ticks via horizontal transmission on vertebrate rickettsemic hosts (amplifying hosts). Finally, despite of the ubiquitous distribution of <italic>A. sculptum</italic> in southeastern and central-western Brazil, most of the populations of this tick species are devoid of <italic>R. rickettsii</italic> infection. This scenario might be related to two major factors: (i) insufficient numbers of susceptible amplifying hosts; and (ii) lower susceptibilities of many tick populations. While the first factor has been demonstrated by mathematical models in previous studies, the second is highlighted by the results observed in the present study.</p>