Evaluation of the Possible Role of Ixodid Ticks in Natural Tularemia Foci in the Forest-Steppe Zone of European Russia

Adult ixodid ticks Dermacentor reticulatus (Fabricius, 1894), D. marginatus (Sulzer, 1776) and Ixodes ricinus (Linnaeus, 1758), collected from vegetation in April–May 2018 and 2019 in natural tularemia foci of the forest and floodplain-valley types in the forest-steppe zone of European Russia (Voronezh Province) were investigated. Ticks in pooled samples of 2 ind. each were tested by real-time PCR for the presence of a Francisella 16S rRNA gene fragment 218–226 bp long. All the positive samples were then checked using species-specific primers and probes complementary to the lpnA gene fragment and the ISFtu2 element. The occurrence of Francisella DNA in I. ricinus from the forest biotope was almost 20 times lower than that in D. reticulatus . The occurrence of Francisella DNA in D. reticulatus ticks collected in all the studied floodplain-valley foci was more than 5 times higher than that in D. marginatus . All the positive samples of D. marginatus ticks were identified as Francisella tularensis . By contrast, the positive samples of D. reticulatus contained not only DNA of F. tularensis but also DNA of unidentified bacteria from the genus Francisella ; 15% of such samples were obtained from a forest-type tularemia focus and 5%, from three floodplain-valley biotopes combined. This may indicate the presence of endosymbionts of the FLE group in D. reticulatus ticks. Thus, D. marginatus may be an important component of the tularemia parasitic system in the floodplain-valley foci in the European part of the forest-steppe zone, though a slightly less important one than D. reticulatus . Adult forest ticks I. ricinus are occasional hosts of F. tularensis .