The aim of this review was to characterize the epidemiological situation regarding tick-borne viral encephalitis (TBVE) in the Russian Federation between 2016 and 2025 and to provide a forecast of TBVE incidence for 2026 for federal districts and constituent entities of the highest epidemiological risk. During 2016–2025, the average annual number of individuals affected by tick bites in the RF was 498575.1. In 2020–2021, an upward trend in this indicator was noted in certain entities of the Volga and Northwestern Federal Districts, among others. Climate warming has shifted the onset of Ixodidae tick activity to the last week of March. A lengthening of the epidemic season is observed. In 2025, 1,807 TBVE cases were identified, corresponding to an incidence rate of 1.24 per 100,000 population (compared to the long-term average of 1.17 per 100,000). The five entities with the highest incidence are Kirov Region, Krasnoyarsk Territory, Tuva Republic, Khakassia Republic, and Perm Territory. A total of 44 fatal outcomes were recorded (case fatality rate – 2.4 %). Based on the data from 2016–2025, 17 entities at highest epidemiological risk have been identified, eight of which are located within the Siberian Federal District. The average tick infection rate in the Russian Federation for 2025 (tested via PCR and ELISA) does not differ from the long-term average. A comparison of the genetic structure of tick-borne encephalitis virus strain samples from 1937–1989 and 1990–2025 has showed an increase in the proportion of the Siberian subtype from 54.8 to 89.7 %, while the ratio of individual genetic lineages within the Siberian subtype remained invariable. The volumes of specific and non-specific TBVE prevention measures in 2025 increased compared to 2024 and corresponded to the target figures in all federal districts. According to the forecast, the TBVE incidence in the Russian Federation in 2026 is projected to be 1.14 per 100,000 population, with a confidence interval from 0.95 to 1.32 per 100,000. An increase in morbidity is possible in the Volga and Far Eastern Federal Districts, while the epidemiological situation in the remaining districts is not likely to change.

Trans-Baikal Territory is endemic for tick-borne encephalitis virus, borreliosis, and rickettsiosis. Cases of human granulocytic anaplasmosis and human monocytic ehrlichiosis are also reported. The aim of this study was to analyze the prevalence of tick-borne diseases and assess epidemiological risks in the Aginsky, Dul’durginsky, Kyra, Kalgansky, Nerchinsko-Zavodskoj districts of Trans-Baikal Territory. Materials and methods. During a period of three years (2022–2024), 2095 ticks of three species (Ixodes persulcatus, Haemaphysalis concinna, and H. japonica) were collected from five districts of the Territory and 60 small mammals from two districts. Results and discussion. In a PCR study of 2071 tick suspensions for four pathogen groups, markers for one or more pathogens were detected in half of them. Two or more pathogens were found simultaneously in 19.8 % of arthropod suspensions. Markers for Anaplasma, Ehrlichia, and TBEV were found two times more often in combinations than as mono-variants. The opposite pattern was observed when detecting DNA of Borrelia, which was more common in single variants. Borrelia markers were identified in average in 51.1 % of the ticks studied, Anaplasma markers – in 16.9 %, Ehrlichia markers – 9.7 %, and TBEV – 3.2 %. RNA isolates of the Baikal subtype from the Alkhanay National Park are genetically close to previously studied TBEV strains, confirming the continued circulation of the virus in this area over a long term. In a study of small mammals, markers of one or more pathogens were detected in six of them, using PCR for four pathogen groups. The Baikal subtype of the TBEV was detected in one Myodes rufocanus. The epidemiological situation on tick-borne infections in the Trans-Baikal Territory remains tense.

The aim of the review was to conduct a comparative analysis of the epidemiological situation on tick-borne encephalitis (TBE) over 2014–2023 in the constituent entities of the Russian Federation located in the European and Asian parts of the country and to give a forecast of incidence for 2024. The incidence of TBE in 2023 only marginally decreased as compared to the peak values in 2022, which leveled down the ten-year trend towards reduction. Grouping of the entities of the Russian Federation according to the average incidence of TBE over a decade made it possible to classify 17 of them as regions of low, 15 – of medium, and 17 – of high epidemiological risk. In general, the epidemiological situation regarding TBE is more unfavorable in the Asian part of the country. A list of species of Ixodidae ticks, the main vectors of the tick-borne encephalitis virus, is provided, as well as subtypes and genetic variants of the latter, identified in the European and Asian parts of the country. The peculiarities of the epidemiological situation on TBE and the scope of preventive measures undertaken in Russia in 2023 are considered. It is shown that there are no clearly outlined trends in the nature of changes in the incidence of TBE in endemic federal districts and entities of the country, and the expected incidence rates in 2024 will remain at the long-term average level.

The results of long-term monitoring of the taiga ticke I. persulcatus abundance and its infection rate with tick-borne encephalitis virus and bacteria of genus Anaplasma, Borrelia, Ehrlichia and Rickettsia in forests along the Baikalian highway (road No 25К-011 Irkutsk region, Russia). The ticks’ abundance varied in space and time from complete lack on the registration route up to 339 ticks per flag-hour. The dynamics during study period (2005-2022) had three waves of ups and downs. The maximum of long-term average seasonal activity occurred at second and third decade of May. The average numbers of ticks increased as distance from the city grew. The analysis with the help of multiplex test system for four infections (tick-borne encephalitis virus, Anaplasma, Borrelia, Ehrlichia) showed marker at least of one pathogen in 57 % of specimens, different mixes of two and more co-infections – in 16.3 %. From 10 variants of co-infections found in this sample, the most prevalent were combinations of Borrelia, Ehrlichia and Anaplasma (74.7 % of double and 7.5 % triple «mixes»). Least often, the ticks were co-infected with tick-borne encephalitis virus in combination with Ehrlichia and Anaplasma (0.5 % each). In average the ticks most often contained the genetic material of Rickettsia (68.1±2.94 % of tested samples) and Borrelia, (48.3±1.30 %), then, in descending order, – Anaplasma, (13.5±0.88 %), Ehrlichia (9.5±0.76 %) and tick-borne encephalitis virus (3.0±0.47 %). Despite the low proportion of ticks with the virus in natural foci, 68.5 % of the persons, bit by infected ticks on the territories along Baikalian highway, became ill with tick-borne encephalitis. Conversely, with a wide prevalence of Rickettsia and Borrelia in ticks, the tick-borne rickettsiosis was diagnosed in 3.1 % of the victims and borrelioses – in 28.4 %. No significant differences have been revealed between the infection rate of adult ticks and nymphs, and between males and females, except for Borrelia, which more often have been found in females than in males (51.9 % versus 45.7 %). The tendencies of the pathogens prevalence in time and different plots of territory studied and risks for human infection have been revealed. It was shown than more than a half of human tick-borne encephalitis, tick-borne borrelioses and tick-borne rickettsiosis contracting occurs in the closest suburbs of Irkutsk – up to the 30th km of the highway, where the ticks’ abundance is lower, than in more distant areas, in other words are more to do with human behavioral factor than with the vector’s activity.

INTRODUCTION:Tick-borne encephalitis virus (TBEV) is medically most important representative of the same-name serogroup of genus Flavivirus (Flaviviridae). In the view of various researchers there are 3 to 5 TBEV subtypes, of them siberian being the most prevalent. The aim of the work is to compare the biological properties and to reveal phylogenetic relationships of large group of modern (2006-2019) TBEV isolates of siberian subtype from natural foci in southern East Siberia.MATERIAL AND METHODS:Ixodid ticks (Ixodidae) and small mammals (Mammalia) from tick-borne encephalitis (TBE) natural foci in Irkutsk Region, Republic of Buryatia and Republic of Tuva, as well as specimens from TBE patients, were examined for TBEV markers using enzyme-linked immunosorbent assay (ELISA) and reverse transcription polymerase chain reaction (RT-PCR). Virus was isolated from suspensions with positive result, and its pathogenicity for white mice (Mus) (WM) was studied by different inoculation ways. Analysis of the nucleotide sequences of E gene was performed for isolates at 1st passage. Phylogenetic tree was constructed using MEGA X program.RESULTS:The phylogenetic analysis has shown that TBEV of siberian subtype that circulates in natural foci of the studied territory belong to two genetic lines. These lines are «Vasilchenko» and «Zausaev» with a strong predominance of the first. The differences in biological properties between the two groups of strains have been demonstrated. Most of the strains from both groups showed high virulence for WM both after intracerebral and subcutaneous inoculation. Only four strains demonstrated the reduced ability to overcome the blood-brain barrier. However, the analysis of the E protein coding sequences revealed evident correlation between phylogenetic clustering and geographical origin of isolates, but not with TBE host or pathogenicity for WM.CONCLUSION:Further search for TBE genome regions associated with pathogenicity require the analysis of complete genome sequences of representative group of strains with different biological properties.

Tick-borne encephalitis (TBE) is transmissible viral disease widely common in temperate zone of Eurasia. ELISA and PCR are used for express identification of the vector’s infection, but the results of the two methods often do not agree. Aim of the work is comparative analysis for TBE virus of Ixodid ticks from nature using complex of methods, including ELISA, PCR, and isolation of the virus in laboratory mice. 18608 Ixodid ticks were collected during 2013-2019 in TBE natural foci of the Baikal Region. The ticks suspensions were examined individually, using ELISA (n=17610) and PCR (n=2999). Suckling mice were inoculated with the suspensions positive in the both tests. The TBEV antigen was found in 1.2 % of ticks in average. All ticks positive in ELISA were examined in PCR (Group 1). Randomly selected part of negative-ELISA samples were examined in PCR too (Group 2). The PCR results were positive in 68.9±3.13 % of the Group 1, with average Ct index 24.6±0.38. Positive results of PCR in Group 2 accounted for just 2.7±0.31 % with average Ct index 31.0±0.70. The average Ct margin of the Groups 1 and 2 is statistically significant (p < 0.001; df = 118). Isolation of strains was significantly more successful in Group 1 (21.7±2.77 %), than in Group 2 (8.2±5.26 %; p < 0.05; df = 50). ELISA is more useful for examining large amounts of ticks. To get a more complex picture about epidemically dangerous part of the vectors in TBE natural foci, the results of the two express-methods is better to sum. The isolation of the virus is useful to carry out of the samples positive in ELISA and PCR concurrently.

Ixodid ticks (n=3714) from natural foci of Baikal Region (Irkutsk Region and Republic of Buryatia) have been examined for pathogenic Borrelia DNA during 2013-2010. On average 40.9 % of the samples were positive for Borrelia markers during the survey period; the range of variantion was – 32–55 %. The increasing of infection rate in ticks is traced in multi-year trends. During the ticks’ activity season, maximal infection rates have been noted at the end of the season (60 %), minimal – at the very beginning of the season (28,6 %) and on the peak of vectors’ abundance (36-39 %). The significant geographical, species and age differences have been detected in Borrelia infection rate of Ixodid ticks. Borrelia DNA have been detected considerably more often in taiga tick (the main vector of Ixodid tick borrelioses in Siberia) and in its nymphs more often, than in adult ticks. There was no difference in the infection rate of male and female ticks, and also between ticks, collected from the vegetation and from human and animals. The ratio og genotyped Borrelia species had been as follows: 64.2 % – B. garinii, 21,7 % – B. afzelii, 14,2 % – B. miyamotoi. Key words: Ixodid ticks, pathogenic Borrelia, PCR, genotyping, Baikal Region.

The Irkutsk region is the unique territory where all known subtypes of tick-borne encephalitis virus (TBEV) circulate. In the last years, the phenomenon of changes in TBEV subtypes (substitution of the Far-Eastern subtype by the Siberian one) was noted in some regions of the Russian Federation. The results of individual investigation of 11522 Ixodes persulcatus ticks and brain specimens from 81 small mammals collected in natural foci of the Irkutsk region during 2006-2014 are presented in the article. More than 60 TBEV strains have been isolated and studied by virological methods; E gene fragments (1193 b.p.) of 68 isolates have been typed. The majority of the strains (irrespective of subtype) were of high virulence for laboratory mice (LM) in case of both intracerebral and subcutaneous inoculation of virus. All isolates from warm-blooded small mammals and humans were of high virulence for LM, but placed in the same clusters of the phylogenetic tree with ticks collected in the same area. Tick-borne strains of different virulence also did not form separate clusters on the tree. Phylogenetic analysis showed that modern TBEV genotypic landscape of the studied territory is changing toward absolute predominance of the Siberian subtype (94.1%). This subtype is represented by two groups with prototype strains “Zausaev” and “Vasilchenko”. The “Vasilchenko” group of strains is spread on the whole territory under study; the strains of “Zausaev” group were isolated previously in the Irkutsk suburbs. The European subtype of TBEV circulates in natural foci of Pribaikalie permanently (at least 5% of the random sampling); the strains are of high virulence for LM. The Far-Eastern TBEV subtype was not found within the group of isolates collected in 20062014. The phylogenetic relationship of the strains under study had a higher correlation with the place of isolation than with the year or source.

Individually 470 Dermacentor nuttalli and 46 Haemaphysalis concinna ticks collected in Irkutsk Region were analyzed using RT-PCR test systems. Rickettsia contamination of D. nuttalli was 82,3% including 1,3% of the ticks with R. sibirica DNA. H. concinna ticks were infected by rickettsia to 8,7% and the detected DNA in 6,5% of cases belonged to R. heilongjiangensis. PCR results were confirmed by phylogenetic analysis of the nucleotide sequences of three gene fragments: citrate synthase (gltA) and ompA and ompB surface proteins. Thus, circulation of two Rickettsia species pathogenic to humans was determined at the examined area.

Objective of the work was to carry out complex assessment of the current state of epizootic activity and epidemiological significance of the infectious disease natural foci in the Aleksandrovsk-Sakhalin territory of the Sakhalin Region. Trapped were 56 samples of small mammals in July, 2010; collected were 180 specimens of taiga tick imago, caught were 1000 specimens of mosquitoes. 223 samples of blood sera were taken from residents of the region. All the field data were tested to detect specific antibodies, antigens and genetic material of agents. Based on the results of epizootiological investigations, serological and molecular-genetic assays, demonstrated was the occurence of natural foci of leptospirosis, tularemia, tick-borne encephalitis, borreliosis, rickettsiosis, human granulocytic anaplasmosis, human monocytic ehrlichiosis, West Nile fever, Inco fever, Batai and Geto fevers, as well as HFRS with varying degree of activity manifestation in the territory of the region. Isolated was tick-borne encephalitis virus from mosquitoes.

Complex analysis of the data on epidemiology and clinical picture of the tick-borne borrelioses in the territory of the Trans-Baikal Region over the last decade since 2003 to 2012 has demonstrated that there is a distinct upward tendency as concerns its morbidity rates. Spotted have been the potentially hazardous, as regards the infection, areas. Highest incidence rates are registered between May-July among adult men, and erythema form of the disease prevails. Based on the results of molecular-genetic investigation of Ixodidae ticks, for the first time ever in the territory of the Dul’durginsk Region identified has been circulation of Borrelia garinii , and B. afzelii , pathogenic for humans bacterial species of Borrelia genus.

Objective of the work was to carry out complex assessment of the current state of epizootic activity and epidemiological significance of the infectious disease natural foci in the Aleksandrovsk-Sakhalin territory of the Sakhalin Region. Trapped were 56 samples of small mammals in July, 2010; collected were 180 specimens of taiga tick imago, caught were 1000 specimens of mosquitoes. 223 samples of blood sera were taken from residents of the region. All the field data were tested to detect specific antibodies, antigens and genetic material of agents. Based on the results of epizootiological investigations, serological and molecular-genetic assays, demonstrated was the occurence of natural foci of leptospirosis, tularemia, tick-borne encephalitis, borreliosis, rickettsiosis, human granulocytic anaplasmosis, human monocytic ehrlichiosis, West Nile fever, Inco fever, Batai and Geto fevers, as well as HFRS with varying degree of activity manifestation in the territory of the region. Isolated was tick-borne encephalitis virus from mosquitoes.

Summit of Asian Pacific Economical Cooperation (APEC) is planned in September 2012 at the Russian (Primorye Territory) island. Numerous participants are expected. It is necessity to evaluate the danger of their possible contamination with infections, transmitted by Ixodes ticks. Therefore, employees of Primorye Antiplague Station and Irkutsk Antiplague Research Institute conducted a joint epidemiological inspection of the island territory in 2011. Six species of small rodents (Apodemus (Alsomys) peninsulae Thom., A. agrarius Pallas, Clethrionomys rufocanus Sundevall, Microtus (Alexandromys) fortis Buchn., Tamias (Eutamias) sibiricus Laxm., Rattus norvegicus Berkenhout) were caught. Five species of ticks (Ixodes persulcatus, I. pavlovskyi, Haemaphysalis concinna, H. japonica douglasi, Dermacentor silvarum) were collected from plants. Natural foci of tick-borne encephalitis, tick-borne borrelioses and rickettsioses, monocytic ehrlichiosis were revealed. Sylvan biotopes were evaluated as the most dangerous. The complex of the preventive measures to prevent tickborne infections in the Summit participants and the local population is proposed.

In our research are presented the results of tick-borne encephalitis (TBE) virus strains, isolated in 1999-2011 from Ixodes ticks and wild rodents in the National Park Alkhanay (Transbaikal region). Combined circulation of two TBE virus genotypes, Far Eastern and Siberian, was established. Moreover i886-84-likei strains differed from the Siberian and Far Eastern genotypes were isolated in the National Park and a mixed strain combining features of the Siberian and Far Eastern genotypes.

In spring, 2011 within the frames of epizootiological survey of the Isle Russian territory, identified were five species of Ixodic ticks: Ixodes persulcatus, I. pavlovskyi, Haemaphysalis concinna, H. japonica douglasi, Dermacentor silvarum. Those ticks were infected with the agents of tick-born encephalitis, borreliosis, rickettsiosis, and monocytic ehrlichiosis. Further preparations for the hosting of APEC Summit in the territory of the Isle in September 2012 involve large-scale construction of various facilities, transformation of natural biotopes, and significant increase in population numbers both permanently and temporally residing in the region. In this connection, a complex of preventive measures aimed at protection of local population and participants of the Summit from tick-borne infections is currently under consideration.

Ixodes ticks collected from plants were examined for tick-borne encephalitis virus infections during epizootological-epidemiological examination at the Russian island in connection with forthcoming APEC summit in 2012. The results of this investigation are presented in this work. Imago ticks of four species (Ixodes persulcatus, I. pavlovskyi, Haemaphysalis concinna, H. japonica douglasi) were individually examined by PCR (n = 455) and ELISA (n = 259). Infected ticks were 7,5% and 1,5%, respectively. Nine (4,7%) strains of tick-borne encephalitis virus were isolated from 190 individual tick suspensions in cell cultures. All of them were isolated from ticks collected in one natural biotope surrounding Rynda Bay. The strains belonged to the Siberian subtype have been defined by the results of gene Е 1330 bp sequencing.

Determination of the taxonomic position of microorganisms associated with diatoms was carried out using microscopic, microbiological, and phylogenetic methods. Examination of the cultures of the fresh-water diatom Synedra acus grown in Lake Baikal water by epifluorescence and electron microscopy revealed that bacterial cells colonized the mucilage surrounding the cells of living microalgae and their cell surfaces and penetrate into the frustules of dead diatoms. A total of 13 strains of heterotrophic bacteria were isolated in pure cultures and described. Based on the results of analysis of their morphological, physiological, and biochemical properties, as well as on data obtained by 16S rRNA gene analysis, the following strains were identified to the species level: Sphingomonas sp., Variovorax paradoxus, Pseudomonas fluorescens, Microbacterium trichothecenolyticum, Rhodococcus sp., Caulobacter vibrioides, and Brevundimonas vesicularis.