U. S. Department of Health, Education and Welfare, Public Health Service, National Institutes of Health, National Institute of Allergy and Infectious Diseases, Rocky Mountain Laboratory, Medical Zoology Department, U. S. Naval Medical Research Unit Number Three (NAMRU-3), U. S. Interests Section, c/o Spanish Embassy, Hamilton, Montana 59840, Arab Republic of Egypt
The possibility that ticks are involved as vectors of the typhus rickettsia, Rickettsia prowazeki, in an extra-human cycle has been evaluated experimentally. Although adult Dermacentor andersoni and Amblyomma variegatum 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 D. andersoni, A. variegatum, A. cohaerens, Rhipicephalus simus, R. evertsi, and Hyalomma dromedarii on rickettsemic guinea pigs and/or voles (Microtus pennsylvanicus). 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 R. prowazeki was ingested by all of the designated species of ticks except A. cohaerens; 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 D. andersoni and H. dromedarii), 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 R. prowazeki 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 R. prowazeki does not appear justified.