Serologic Typing of Rickettsiae of the Spotted Fever Group by Microimmunofluorescence

Abstract The micro-immunofluorescence (micro-IF) method was used to type rickettsiae belonging to the spotted fever or typhus groups according to their surface antigens. Seventy-two strains of rickettsiae from diverse sources, with varying histories of laboratory manipulation and immunologic characterization by other methods, were cross-tested against their mouse antisera. Fifteen serologic patterns (serotypes) were observed, 12 associated with spotted fever-group (SFG) rickettsiae and three with typhus group (TG) organisms. The reaction patterns of strains within each serotype were homogeneous. Nine of the SFG serotypes and the three TG serotypes were characteristic for rickettsiae that have been classified by other biologic procedures as to species (classified serotypes). The classified SFG serotypes included Rickettsia rickettsii (R-like), R. rickettsii (Hlp-like), R. sibirica, R. parkeri, R. conorii, R. rhipicephali, R. montana, R. australis, and R. akari. The three TG serotypes included R. prowazekii, R. typhi, and R. canada. Three SFG serotypes included rickettsiae that have not been identified as to species. Each consisted of two or more strains isolated in different years from widely-spaced localities in the United States and were considered to be distinct serotypes, which for the time being are unclassified. They included two strains of rickettsiae isolated from Dermacentor occidentalis ticks in California, three strains recovered from D. parumapertus ticks in southwestern United States, and three strains obtained from Ixodes pacificus ticks in Oregon. Each of three other strains had patterns of reaction that differed from those of all other rickettsiae. However, they were single representatives and evidence derived from this study is not sufficient to consider these as comprising additional serotypes. The results obtained by micro-IF are in general agreement with other procedures for antigenic differentiation of rickettsiae belonging to the SFG and TG. Therefore, an immunologic basis for serologic classification of these rickettsiae is likely. Micro-IF should prove to be particularly useful for determining taxonomic and epidemiologic relationships among SFG rickettsiae because of its simplicity and general applicability.