We describe the characterization of a novel Rickettsia species cultivated from Dermacentor ticks collected in Russia and France, for which we propose the name Rickettsia raoultii sp. nov. Using multigene sequencing, we demonstrated that five rickettsial isolates from Dermacentor silvarum, Dermacentor reticulatus, Dermacentor marginatus and Dermacentor nuttalli ticks were classified within this novel spotted fever rickettsia species. This rickettsia also exhibited a serotype distinct from previously described Rickettsia species. The type strain of Rickettsia raoultii sp. nov. is strain Khabarovsk T (5CSUR R3 T 5ATCC VR-1596 T ).

In 1999, three new spotted fever group rickettsial genotypes, i.e. RpA4, DnS12 and DnS28, were detected in ticks from Russia by PCR [1]. Rickettsiae with identical genotypes were later isolated in Russia and France, and classified within a new species named Rickettsia raoultii [2]. This rickettsial species is currently considered as an emerging pathogen and is an agent of tick-borne lymphadenitis [2]. R. raoultii has been found in several European and Asian countries, mostly in Dermacentor ticks (D. nuttallii, D. marginatus, D. reticulatus, D. silvarum), and also in Rhipicephalus pumilio, Haemaphysalis concinna and Ixodes persulcatus [1][2][3][4]. However, to date, the relationships between ticks and R. raoultii have not been studied.In the present study, we investigated the efficiency of transovarial transmission and intensity of multiplication of three R. raoultii genotypes in four naturally-infected Dermacentor species. We used the method of experimental modelling of a natural cycle of the metamorphosis of vectors (MEMNCMv) that allows the study of the role of tick species as vectors of infectious agents, including rickettsiae. M A T E R I A L S A N D M E T H O D SWe compared the efficiency of vertical transmission and concentration of R. raoultii (genotypes RpA4, DnS14 and DnS28) in pre-imaginal stages of four Dermacentor species: D. nuttalli, D. silvarum, D. marginatus and D. reticulatus. All ticks, collected in different regions of Siberia (Omsk region and Altai (Western Siberia), Buryatiya (Eastern Siberia) and Kazakhstan), were naturally infected with R. raoultii, as evaluated in all evolutionary stages by ompA PCR and sequencing.Genotypic identification of studied strains was performed using ompA and gltA gene amplification and sequencing. Tick cultivation in laboratory conditions, study of transovarial and transstadial transmission rates, and study of rickettsial concentration (number of rickettsiae per visual field) in tick preimaginal stages, were carried out using immunofluorescence as previously described [5].

The review presents an analysis of the epidemic situation on infections of rickettsial etiology, the causative agents of which are transmitted by Ixodidae ticks in the territory of the Russian Federation. The data obtained through molecular-biological verification allow to unite under the name of “tick-borne ricketsioses” a group of infections caused by R. sibirica subsp. sibirica, R. conorii, R. heilongjiangensis and other species of rickettsiae circulating in natural foci of various regions of Russia. Cases of tick-borne rickettsioses in Siberia and the Far East, caused by various species of rickettsiae, are registered under the name of “Siberian tick-borne typhus” due to the lack of available methods of differential laboratory diagnostics. The paper presents the assessment of the incidence of Siberian tick-borne typhus, indicating not only the varying degrees of epidemic hazard of endemic regions, but also changes in the distribution of risk areas, including the identification of new, epidemically significant foci. In accordance with the risk-oriented approach to prophylaxis, forecasting of epidemic situation on tick-borne rickettsioses was given and differentiation of the endemic territories of the Russian Federation as regards Siberian tick-borne typhus was carried out with distinguishing of epidemiological zones of low, medium, above average, high and very high risk of population infection.

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