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Thirty-one rickettsial isolates from ticks or patients in North Asian tick typhus (NATT) foci from the Ural region to the Russian Far East were obtained at the Omsk Research Institute of Natural Foci Infections between 1954 and 2001. Using citrate synthase (gltA) and outermenbrane protein a (ompA) gene sequencing, we identified these isolates as Rickettsia sibirica sensu stricto (25 isolates), R. sibirica strain BJ-90 (2 isolates), R. slovaca (1 isolate), and R. heilongjiangensis (3 isolates). We demonstrate that Ixodes persulcatus ticks should be considered potential vectors of NATT. We also demonstrate the presence of R. slovaca in Ural and R. heilongjiangensis in Siberia and Russian Far East, where they may cause human infections misdiagnosed as cases of NATT. Clinicians should be aware that several spotted fever rickettsioses with different prognoses coexist in Russia in areas where NATT was the only previously recognized rickettsiosis.

Rickettsiae represent a wide range of pathogenicity from classic and new pathogens to endosymbionts of eukaryotic cells. Recent studies of rickettsiae have widened the number of representatives of genus Rickettsia, especially in the spotted fever group (SFG). Rickettsiae of SFG are tick-borne microorganisms with effective transovarial and transstadial transmission. The main hosts are ticks (Dermacentor, Rhipicephalus, Haemophysalis, Ixodes, and Amlyomma). Strategy of maintenance of tick microorganisms is vector-type transfer and tropism to endothelial cells or blood cells of animals. The main epidemiological characteristics of SFG rickettsioses are different kinds of anthropogenic activity and connection of morbidity with seasonal tick activity. Two other important characteristics are quantitative and qualitative heterogeneity of its populations (different genotypes of Rickettsia in the same territory and species of ticks, for example) and coexistence of different tick microorganisms (rickettsiae, borreliae, ehrlichiae, tick-borne encephalitis complex viruses, etc.). The role of new rickettsial genotypes in infectious diseases is poorly understood. Simultaneous study of ticks after bites, blood and skin biopsies of patients may be used for detection of spectrum of tick-borne pathogens in mixed natural foci. Interference between rickettsiae with different virulence may affect its populations and levels of morbidity.

Restriction fragment length polymorphism (RFLP) analysis of polymerase chain reaction-amplified gene fragments was used to characterize 24 isolates of spotted fever group rickettsiae previously identified as Rickettsia sibirica from their serologic properties. These strains were obtained in Russia between 1946 and 1991 from humans and different species of Ixodid ticks. The RFLP analysis was performed using amplified DNA products obtained with a genus-specific primer pair derived from the R. prowazekii citrate synthase gene and two group-specific primer pairs from the R. rickettsii 190-kD and 120-kD surface protein antigen genes followed by Alu I, Pst I, and Rsa I restriction endonuclease digestions. Although some differences were detected in biological characteristics among the examined strains, only a single R. sibirica genotype was found with these molecular tools of identification.

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