Experimental Infections of Lambs with Rickettsia Prowazeki

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  • 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, Hamilton, Montana 59840

Weanling lambs were inoculated by intraperitoneal, intravenous, and subcutaneous routes with 105.3 or 107.3 egg 50% infectious doses of the ZRS strain of Rickettsia prowazeki. Control lambs were inoculated with identical amounts of the same infectious material inactivated with formalin. Tissue and blood samples from animals inoculated with infectious material were tested for the presence of rickettsiae by inoculation into embryonated eggs, guinea pigs, and meadow voles (Microtus pennsylvanicus) and by fluorescence microscopy with specific antiserum. Generalized rickettsial infections in lambs were not produced despite the use of various routes of inoculation. R. prowazeki was isolated in some instances from brain, lung, spleen, liver, and subcutaneous tissue 6 or 7 days after inoculation, but not 29 days after inoculation. Rickettsemia was not demonstrated under any of the conditions tested. Serological and clinical responses in lambs receiving infectious inocula were not distinguishable from corresponding responses in lambs that had received comparable amounts of formalin-killed inoculum. The pattern of isolations of R. prowazeki from various lamb tissues was consistent with a picture of local self-limited infections unaccompanied by rickettsemia. These findings are compared with those obtained from infected guinea pigs and are discussed with regard to the hypothesis of extrahuman reservoirs of R. prowazeki.