Seven species of pathogenic rickettsiae were compared in five assay systems for group, species, strain, and phase differences in infectivity. The species examined include Rickettsia prowazekii (Breinl and Cairo 3 strains), R. typhi, R. canada, R. rickettsii (Sheila Smith and R strains), R. conorii, R. sibirica, and Coxiella burnetii in phases I and II. Pathogenicity was characterized in terms of fever in guinea pigs. All comparisons of infectivity and pathogenicity were described in terms of numbers of rickettsiae in the inocula, as determined by direct rickettsial count. The data characterize the various species and strains of rickettsiae in quantitative terms, which are also estimates of the sensitivity of the assay systems used. Phase I C. burnetii was found to be the most, and R. canada the least, infective of the species examined. In general the primary chicken embryo cell culture system proved to be the most, and that of the mouse the least, sensitive assay system.

A rickettsia related to but distinct from the spotted fever agent, Rickettsia rickettsii, has been detected in 167 (18.9%) of 884 Rhipicephalus sanguineus taken off dogs in central and northern Mississippi. The organisms could readily be isolated in male meadow voles (Microtus pennsylvanicus), where it produced massive infections in the tissues of tunica vaginalis. It was practically nonpathogenic for male guinea pigs, although inoculation of these animals with infected tunica vaginalis of voles afforded in 30 of 38 instances solid immunity to challenge with virulent R. rickettsii. The Rhipicephalus rickettsia grew well in monolayers of chicken embryo fibroblast, Vero, mouse L, and HeLa cells. Cytopathogenic effects were minimal unless large concentrations of rickettsiae were used as inocula. It also could be established in embryonated hen eggs but only after injection of massive doses of L cell-propagated organisms. Serological tests (complement fixation, microagglutination and/or micro immunofluorescence) indicated that the newly described rickettsia belongs to the spotted fever group but differs from R. rickettsii, R. akari, and R. conorii. Antigenic differences were also demonstrated by direct fluorescence microscopy as well as by vaccine potency and mouse-toxin neutralization tests.

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