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.

Natural centers of tick-borne encephalitis and tick-borne rickettsiosis diseases are distinguished not only by stability and in- creasing level of epidemiological manifestation but also by ability to enlargement of areas. It is found that a contribution of some regions of Russia to the sick rate of tick-borne encephalitis and tick-borne rickettsiosis is changing in time. For tick-borne encephalitis the contribution of East Siberia increases and becomes equal to one of Ural area. For tick-borne rickettsiosis the general contribution of four regions (East Siberia, West Siberia, Ural and Far East) amount to 92% of diseases in country. The greater part of tick-borne rickettsiosis diseases (62,1%) is the contribution of East Siberia because of activi- ty of disease centers of Altai area.

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