Volume 21, Issue 6
  • ISSN: 0002-9637
  • E-ISSN: 1476-1645



The possibility that ticks are involved as vectors of the typhus rickettsia, , in an extra-human cycle has been evaluated experimentally. Although adult and could readily be infected by intracelomic injection of infectious yolk-sac suspensions, all attempts failed to initiate generalized and prolonged infections by feeding larval and/or nymphal , and on rickettsemic guinea pigs and/or voles (). Testing of ticks by injection of tissue suspensions into normal guinea pigs and voles, immediately after infectious feeding and at varying intervals thereafter, indicated that was ingested by all of the designated species of ticks except ; in some instances, the rickettsia was maintained transstadially into the next developmental stage. However, fluorescent antibody microscopy of tick tissues revealed that rickettsial infections, wherever detectable (i.e. in and ), were atypical and limited to the tissues of the midgut. These results, as well as those of other workers, suggest that the minimum dose requirement for successful infection of ticks with is considerably higher than that provided by concentrations of rickettsiae circulating in the blood of infected laboratory animals used so far. Until animals are found in nature that meet this minimum dose requirement, incrimination of ticks as vectors of does not appear justified.


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