Circulation Of Tick-borne Encephalitis Virus Research Articles

Analysis of the zoonotic tick-borne encephalitis virus (TBEV) in raw milk and dairy products in mountain pastures of the Lombardy region, Italy

Over the last few decades, tick-borne encephalitis (TBE) has become a growing public health problem in Europe. The tick-borne encephalitis virus (TBEV) is a zoonotic virus that affects the central nervous system (CNS). TBEV has been detected in 27 European countries, and the rise in TBE cases is mainly due to environmental and ecological factors, and factors that increase the risk of human exposure to infected ticks. The infection via the alimentary route is the second most common means of TBEV transmission to humans. Raw milk from infected goats, sheep, or cows has been identified as a source of human food-borne infections. This study aims to gather new information on the prevalence of tick-borne encephalitis virus (TBEV) in raw goat’s and cow’s milk and related raw products in the Lombard Alps (Italy). This is important due to the close proximity of Lombardy to the Triveneto region, where TBE is endemic, and southern Switzerland, where numerous TBEV-positive mammals have been found. Throughout 2023, a passive monitoring plan was implemented on samples delivered for TBEV analyses from the Alpine pastures. In total, 248 specimens including raw milk, raw milk cheese, and butter were tested. This is the first monitoring of food at risk of TBEV transmission in a non-endemic region with evidence of TBEV circulation. Despite testing a wide range of dairy products, no sample tested positive for RNA-TBEV by real-time RT-PCR. Preliminary results suggest that raw milk and raw dairy products do not pose a significant risk of TBEV transmission to humans in the territory of Lombardy.

Differential susceptibility of geographically distinct Ixodes ricinus populations to tick-borne encephalitis virus and louping ill virus.

Tick-borne encephalitis virus (TBEV) is an emerging pathogen in the Netherlands. Multiple divergent viral strains are circulating and the focal distribution of TBEV remains poorly understood. This may, however, be explained by differences in the susceptibility of tick populations for specific viruses and viral strains, and by viral strains having higher infection success in their local tick population. We investigated this hypothesis by exposing Dutch Ixodes ricinus ticks to two different TBEV strains: TBEV-NL from the Netherlands and TBEV-Neudoerfl from Austria. In addition, we exposed ticks to louping Ill virus (LIV), which is endemic to large parts of the United Kingdom and Ireland, but has not been reported in the Netherlands. Ticks were collected from two locations in the Netherlands: one location without evidence of TBEV circulation and one location endemic for the TBEV-NL strain. Ticks were infected in a biosafety level 3 laboratory using an artificial membrane feeding system. Ticks collected from the region without evidence of TBEV circulation had lower infection rates for TBEV-NL as compared to TBEV-Neudoerfl. Vice versa, ticks collected from the TBEV-NL endemic region had higher infection rates for TBEV-NL compared to TBEV-Neudoerfl. In addition, LIV infection rates were much lower in Dutch ticks compared to TBEV, which may explain why LIV is not present in the Netherlands. Our findings show that ticks from two distinct geographical populations differ in their susceptibility to TBEV strains, which could be the result of differences in the genetic background of the tick populations.

Hybrids of Ixodes ricinus and Ixodes persulcatus ticks effectively acquire and transmit tick-borne encephalitis virus.

Ixodes rici nus and Ixodes persulcatus ticks are the main vectors of tick-borne encephalitis virus (TBEV), which has three main subtypes connected with certain tick species: the European subtype, associated with I. ricinus, and the Siberian and Far-Eastern subtypes, associated with I. persulcatus. Distribution ranges of these species overlap and form large sympatric areas in the East European Plain and Baltic countries. It has previously been shown that crossing of I. ricinus and I. persulcatus is possible, with the appearance of sterile hybrids. Hybridization of ticks can affect not only the spread of ticks but also the properties of natural foci of arbovirus infections, in particular TBEV. In the present study, we analyzed the effectiveness of virus transmission from infected mice to larvae and nymphs and trans-stadial transmission (from larvae to nymph and adult) in I. ricinus, I. persulcatus, and hybrids. For this purpose, we bred a hybrid generation from the crossing of I. persulcatus females and I. ricinus males, and we used the Siberian and European subtypes of TBEV. We showed that after feeding on infected mice, virus prevalence in engorged ticks decreased over time, and after molting, the opposite was true. In hybrids we observed the highest acquisition effectiveness and RNA copy numbers during Siberian TBEV subtype transmission. The efficiency of trans-stadial transmission of both TBEV subtypes was similar in hybrids and parental species. After the second trans-stadial TBEV transmission, a significant increase in ticks' infection rates was observed only in specific subtype-tick combination. Our data demonstrate the possible features of TBEV circulation in the I. ricinus and I. persulcatus sympatry area.

Continued Circulation of Tick-Borne Encephalitis Virus Variants and Detection of Novel Transmission Foci, the Netherlands.

Tick-borne encephalitis virus (TBEV) is an emerging pathogen that was first detected in ticks and humans in the Netherlands in 2015 (ticks) and 2016 (humans). To learn more about its distribution and prevalence in the Netherlands, we conducted large-scale surveillance in ticks and rodents during August 2018-September 2020. We tested 320 wild rodents and >46,000 ticks from 48 locations considered to be at high risk for TBEV circulation. We found TBEV RNA in 3 rodents (0.9%) and 7 tick pools (minimum infection rate 0.02%) from 5 geographically distinct foci. Phylogenetic analyses indicated that 3 different variants of the TBEV-Eu subtype circulate in the Netherlands, suggesting multiple independent introductions. Combined with recent human cases outside known TBEV hotspots, our data demonstrate that the distribution of TBEV in the Netherlands is more widespread than previously thought.

Use of Wild Ungulates as Sentinels of TBEV Circulation in a Naïve Area of the Northwestern Alps, Italy.

Wild and domestic animals can be usefully employed as sentinels for the surveillance of diseases with an impact on public health. In the case of tick-borne encephalitis virus (TBEV), the detection of antibodies in animals can be more effective than screening ticks for detecting TBEV foci, due to the patchy distribution of the virus. In the Piedmont region, northwestern Italy, TBEV is considered absent, but an increase in tick densities, of Ixodes ricinus in particular, has been observed, and TBEV is spreading in bordering countries, e.g., Switzerland. Therefore, we collected sera from wild ungulates during the hunting season (October-December) from 2017 to 2019 in the Susa Valley, Italian western Alps, and screened them for TBEV antibodies by a commercial competitive ELISA test. We collected 267 serum samples by endocranial venous sinuses puncture from red deer, roe deer and northern chamois carcasses. The animals were hunted in 13 different municipalities, at altitudes ranging between 750 and 2800 m a.s.l. The serological survey for TBEV yielded negative results. Borderline results for five serum samples were further confirmed as negative for TBEV by a plaque reduction neutralisation test. To date, our results indicate that TBEV is not circulating in western Piedmont. However, monitoring of TBEV should continue since TBEV and its vector are spreading in Europe. The wide-range distribution of wild ungulates and their role as feeding hosts, make them useful indicators of the health threats posed by Ixodid ticks.

Bulk Milk Tank Samples Are Suitable to Assess Circulation of Tick-Borne Encephalitis Virus in High Endemic Areas.

A reliable surveillance strategy of tick-borne encephalitis virus (TBEV) is necessary to ensure adequate disease control measures. However, current approaches assessing geographical TBEV circulation are ineffective or have significant limitations. In this study we investigated a total of 1363 goat and 312 sheep bulk tank milk samples for the presence of TBEV. Samples were collected from systematically selected farms in Lithuania every 4–5 days from April to November in 2018 and 2019. To validate results, we additionally tested 2685 questing ticks collected in the vicinity of milk collection sites. We found 4.25% (95% CI 3.25–5.47) and 4.48% (95% CI 2.47–7.41) goat and sheep milk samples to be positive for TBEV, respectively. Furthermore, geographical distribution of TBEV in milk samples coincided with the known TBE endemic zone and was correlated with incidence of TBE in humans in 2019. When sampling time coincides, TBEV detection in milk samples is as good a method as via flagged ticks, however bulk milk samples can be easier to obtain more frequently and regularly than tick samples. The minimal infectious rate (MIR) in ticks was 0.34% (CI 95% 0.15–0.64). Therefore, our results confirm that testing milk serves as a valuable tool to investigate the spatial distribution of TBEV at higher resolution and lower cost.

Analysis of laboratory methods for monitoring of tick-borne encephalitis natural foci

Introduction. Ixodid ticks simultaneously are hosts and vectors of tick-borne encephalitis virus (TBEV), presenting a high risk to humans. Monitoring of the vectors part of TBEV population is usually held by means of express analysis methods (ELISA and PCR), but only isolation and identification of infectious virus is reliable evidence of TBEV circulation in the natural foci. Objectives — to demonstrate the TBEV infection rates of Ixodid ticks from natural TBE foci of Baikal Region, based on comprehensive study, including ELISA, PCR and isolation of virus on laboratory mice (LM) model. Methods. Questing adult Ixodid ticks (n = 20 111, mainly — Ixodes persulcatus Schulze, 1930), were collected in TBE natural foci of Baikal Region during 2013–2020. The suspension on saline solution was prepared from the each tick and analyzed by ELISA first. The samples with positive ELISA results were verified in PCR-RT. Furthermore, randomly selected samples with negative ELISA results were analyzed by PCR. Suspensions with positive ELISA and PCR results have been inoculated to suckling LM intracerebrally. Results. The samples with positive PCR results have been divided into two groups: group 1 — all suspensions with positive ELISA results, group 2 — randomly selected samples with negative ELISA results. The positive PCR results in group 1 made up 70.5 % with average Ct rate 24.9. The positive PCR results in group 2 have been obtained in 2.2 % of cases with average Ct rate 30.7. The isolation on LM model was more successful in group 1 (25.8 vs 13.0 %; р < 0.01; df = 69). Conclusion. ELISA is more useful for study of large amounts of ticks during monitoring of natural TBE foci, offering insight into the epidemically important vectors rate. To get the more full assessment of the ticks’ infection rate one must use ELISA and PCR simultaneously, and sum the results into general rate. For high strains isolation results the LM should be inoculated with the suspensions, which had shown positive both ELISA and PCR results.

The European bison (Bison bonasus) as an indicatory species for the circulation of tick-borne encephalitis virus (TBEV) in natural foci in Poland

Tick-borne encephalitis (TBE) is one of the most common zoonotic diseases in Europe transmitted by Ixodidae vectors. While small mammals such as bank voles and ticks constitute the main reservoirs for virus transmission, large sylvatic species act as a food source for ticks. Cervids such as roe deer and red deer are considered sentinel species for TBE in natural foci. In addition, an increase of the population size and density of large wild mammals in an area corresponds to an increase in the tick burden and may potentially increase the prevalence of TBE virus (TBEV) in ticks and tick hosts and further exposure risk in humans. Humans are considered accidental hosts. The prevalence of TBE relies on interactions between host, vector and environment. The present study examines the exposure of the largest European herbivore, the European bison (Bison bonasus) to TBEV infection. Assessed using the IMMUNOZYM FSME ELISA (PROGEN), the overall TBEV seroprevalence was 62.7% in the 335 European bison that were studied. ELISA results were confirmed by the gold-standard virus neutralization test (VNT) with 98.7% sensitivity and thus giving a true prevalence of 63.5%. TBEV seroprevalence was significantly correlated to the origin, age group, sex, population type (free living/captive) and sanitary status (healthy/selectively eliminated/found dead/killed in accident) of the European bison in the univariable analysis. The highest seroprevalences were observed in the three largest north-eastern wild populations (Białowieska, Borecka and Knyszyńska forests), which corresponded with the highest incidence of human cases reported in the country. The risk of TBEV seropositivity increased with age and was higher in female and free-ranging European bison. Additionally, to the epidemiological investigation, the continuous detection of TBEV antibodies was studied by repetitive testing of animals over the course of 34 months. Two of six seropositive animals remained seropositive throughout the study. The presence of antibodies was followed throughout the study in seropositive European bison and for at least a year in animals that seroconverted during the observation period.

Increasing awareness for tick-borne encephalitis virus using small ruminants as suitable sentinels: Preliminary observations

Tick-borne encephalitis virus (TBEV) is one of the most common zoonotic vector-borne infections in Europe. An appropriate awareness is crucial to react quickly and efficiently to protect humans from this pathogen. From winter 2017 until spring 2018 serum samples were collected from 71 small ruminant flocks (3174 animals) in five German federal states. The sera were examined for TBEV antibodies by ELISA and serum neutralization test. In the TBEV risk areas, there was a coincidence in 14 districts between seropositive small ruminants and the occurrence of human TBE cases in 2017. In eight districts, the TBEV infection could not be detected in small ruminants although human cases were reported. In contrast, in five districts, small ruminants tested TBEV seropositive without notified human TBE cases in 2017. A changing pattern of TBEV circulation in the environment was observed by the absence of antibodies in a defined high-risk area. In the non-TBE risk areas, seropositive small ruminants were found in five districts. In two districts with a low human incidence the infection was missed by the small ruminant sentinels. An intra-herd prevalence of 12.5% was determined in a goat flock in the non-TBE risk area in 2017, two years prior the first autochthone human case was reported. All sheep and goats in this flock were examined for TBEV antibodies for three years. Individual follow-up of twelve small ruminants was possible and revealed mostly a short lifespan of TBEV antibodies of less than one year. The probability to identify TBEV seropositive sheep flocks was enhanced in flocks kept for landscape conservation or which were shepherded (p < 0.05). Our preliminary observations clearly demonstrated the successful utilization of small ruminants as sentinel animals for TBEV.

Disappearance of TBEV Circulation among Rodents in a Natural Focus in Alsace, Eastern France.

Tick-borne encephalitis virus (TBEV) depends mainly on a fragile mode of transmission, the co-feeding between infected nymphs and larvae on rodents, and thus persists under a limited set of biotic and abiotic conditions. If these conditions change, natural TBEV foci might be unstable over time. We conducted a longitudinal study over seven years in a mountain forest in Alsace, Eastern France, located at the western border of known TBEV distribution. The objectives were (i) to monitor the persistence of TBEV circulation between small mammals and ticks and (ii) to discuss the presence of TBEV circulation in relation to the synchronous activity of larvae and nymphs, to the densities of questing nymphs and small mammals, and to potential changes in meteorological conditions and deer densities. Small mammals were trapped five times per year from 2012 to 2018 to collect blood samples and record the presence of feeding ticks, and were then released. Questing nymphs were collected twice a year. Overall, 1344 different small mammals (Myodes glareolus and Apodemus flavicollis) were captured and 2031 serum samples were tested for the presence of antibodies against TBEV using an in-house ELISA. Seropositive rodents (2.1%) were only found from 2012 to 2015, suggesting that the virus disappeared afterwards. In parallel, we observed unusual variations in inter-annual nymph abundance and intra-annual larval activity that could be related to exceptional meteorological conditions. Changes in the densities of questing nymphs and deer associated with the natural stochastic variations in the frequency of contacts between rodents and infected ticks may have contributed to the endemic fadeout of TBEV on the study site. Further studies are needed to assess whether such events occur relatively frequently in the area, which could explain the low human incidence of TBE in Alsace and even in other areas of France.

Tick-Borne Encephalitis Virus and Its European Distribution in Ticks and Endothermic Mammals.

Tick-borne encephalitis (TBE) is the most common viral tick-borne disease in Europe causing thousands of human infections every year. Available risk maps in Europe are solely based on human incidences, but often underestimate areas with TBE virus circulation as shown by several autochthonous cases detected outside known risk areas. A dataset of more than 1300 georeferenced TBE virus detections in ticks and mammals except for humans was compiled and used to estimate the probability of TBE virus presence in Europe. For this, a random forests model was implemented using temperature- and precipitation-dependent bioclimatic variables of the WorldClim dataset, altitude, as well as land cover of the ESA GlobCover dataset. The highest probabilities of TBE virus presence were identified in Central Europe, in the south of the Nordic countries, and in the Baltic countries. The model performance was evaluated by an out-of-bag error (OOB) of 0.174 and a high area under the curve value (AUC) of 0.905. The TBE virus presence maps may subsequently be used to estimate the risk of TBE virus infections in humans and can support decision-makers to identify TBE risk areas and to encourage people to take appropriate actions against tick bites and TBE virus infections.

The Seroprevalence of Tick-Borne Encephalitis in Rural Population of Mazandaran Province, Northern Iran (2018 - 2019)

Background: Tick-borne encephalitis virus (TBEV) is a neurotropic tick-borne virus circulating in vast geographical areas of the northern hemisphere. Although the presence of TBEV vector has been documented in north Iran, there is no information about the circulation of TBEV in the country. Objectives: This study was designed in order to determinate the TBE serologic profile in the Mazandaran Province of Iran. Methods: In this cross-sectional seroepidemiologic study, from September to November 2018, 448 serum samples collected (by random sampling method) from general population residing in rural areas of Behshar, Galugah, Miandorud and Neka cities were evaluated for presence of anti TBEV IgG antibodies using a commercial Anti-TBE Virus ELISA (IgG) kit. Results: Sixteen (3.6%) samples were detected as reactive, in which 43.8% had a history of tick bite or tick squish. The highest seropositivity was observed in farming/animal industry-related jobs (33.3%) or were housewives (33.3%). All reactive cases reported a history of consuming local unpasteurized dairy. Conclusions: This study provides first evidence on the circulation of TBEV in Northern Iran, where climatic conditions, presence of Ixodes ticks, and variability of mammalian hosts might contribute to TBEV establishment. Overarching investigations on tick vectors, animals, and human population would be recommended to shed light on the epidemiology of the virus in this region.

Tick-borne encephalitis foci in northeast Italy revealed by combined virus detection in ticks, serosurvey on goats and human cases

ABSTRACT Tick-borne encephalitis (TBE) is a severe zoonotic neurological disease endemic in northeast Italy since 1992. In the Province of Trento, a sharp increase in TBE incidence has been recorded since 2012, despite the vaccination efforts. To assess current TBE infection hazard in this area, we applied an integrated approach combining the distribution of human cases, the seroprevalence of tick-borne encephalitis virus (TBEV) in sentinel hosts and the screening of questing ticks for TBEV. A total of 706 goat sera from 69 farms were screened for TBEV-specific antibodies resulting in 5 positive farms, while the location of human cases was provided by the local Public Health Agency. Tick sampling was concentrated in areas where TBEV circulation was suggested by either seroprevalence in goats or human cases, resulting in 2,410 Ixodes ricinus collected and analyzed by real-time RT–PCR. Four tick samples from 2 areas with record of human cases were positive to TBEV corresponding to a 0.17% prevalence in the region, while risk areas suggested by serology on goats were not confirmed by tick screening. Our results revealed an increase in TBEV prevalence in ticks and the emergence of new active TBE foci, compared to previous surveys, and demonstrated the importance of an integrated approach for TBE risk assessment. A phylogenetic analysis of the partial E gene confirmed that the European TBEV subtype is circulating in northeast Italy and suggested that the different Italian TBEV strains originated independently as a result of different introductions from neighbouring countries, presumably through migratory birds.

Seroprevalence of Tick-Borne Encephalitis Virus in Three Species of Voles (Microtus spp.) in Poland.

Rodents play a role as reservoir hosts for tick-borne encephalitis virus (TBEV). We trapped 77 voles of the genus Microtus and tested their blood using an enzyme-linked immunosorbent assay for the presence of antibodies against TBEV. We detected TBEV antibodies in three adult individuals (4%). Voles might play a role in the circulation of TBEV.

Flaviviruses at the territory of Serbia – present situation and challenges

The aim of this study is to summarize the data on the occurrence, presence and prevalence of some zoonotic flaviviruses, which have been actively circulating in the territory of the Republic of Serbia during past decade. The genus Flavivirus, family Flaviviridae,encompasses vector-transmitted viruses with zoonotic potential. The genus is comprised of more than 70 viruses transmitted to humans by mosquitoes and ticks. Some of those are associated only with human diseases (such as yellow fever and dengue fever), whereas others can cause diseases in both humans and animals. Animals are major reservoirs and primary hosts for the latter group. The virus transmission to other animal species or humans occurs via diverse vectors – mostly mosquitoes and ticks. A range of these virus species is widely distributed worldwide, especially in tropical and sub-tropical climatic zones. Some representatives of these species have only recently been identified at the territory of Republic of Serbia. This paper gives an evidence on the presence and distribution of West-Nile virus, tick-borne encephalitis (TBE) virus and Usutu virus that were conducted within the study in Serbia. The research pointed out the presence of recently detected West-Nile virus and Usutu virus as well as the current circulation of tick-borne encephalitis virus, for which only limited serological evidence on the presence of virus-specific antibodies in humans was available so far. In the aspect of public health, the full understanding of the importance of identification and prevalence of different flaviviruses still require further comprehensive entomologicalacarological, seroepidemiological, clinical and virological research.

IMMUNE INTERLAYER TO TICK-BORNE ENCEPHALITIS VIRUS IN HUMAN POPULATION OF TRANSBAIKALIA AS AN INDICATOR OF NATURAL FOCI ACTIVITY

Studying of immune interlayer value to tick-borne encephalitis virus in human population of the Transbaikalia Territory resulted from natural immunization of the healthy population in 31 Municipal areas in 2011–2016 is presented. Human selections were formed proportionally to the population size in the concrete human settlement among persons of the various age and professional groups living at this territory not less than 10 years and unvaccinated against tick-borne encephalitis virus. Total 4367 blood sera were investigated. Laboratory testing for antibodies of G class to tick-borne encephalitis virus in blood sera of the human population was performed by immune-enzyme analysis using a set of reagents “VektoVKE-IgG” of Joint-Stock Company “Vektor-Best” (Novosibirsk city). The monitoring data indicated the presence of natural immunity to tick-borne encephalitis virus in the human population. Mean annual level of immune interlayer in Municipal areas varied from 3.1 to 52.7% (in Transbaikalian Territory — 13.1±0.51%). High level (from 20 to 52.7%) was characteristic for mountain-taiga-forest-steppe zone (Krasnochikoisky — 23.8±3.36%; Uletovsky — 52.4±4.48%; Gazimuro-Zavodsky — 29.4±4.94% districts) and mountain-tundra-taiga zone (Tungokochensky district — 20.0±3.58%). In steppe zone the level of immune interlayer was lower and observed in districts with elements of south-taiga larch and pine forests. It was established that levels of the immune interlayer in human population living in mountain-taiga-foreststeppe and mountain-tundra-taiga zones authentically higher than in steppe area — t = 3.8; Р 0.001 and t = 2.27; Р 0.05, respectively. Distinctions in the value of the immune interlayer between mountain-tundra-taiga and mountain-taigaforest-steppe zones were non-authentic: t = 0.1; P 0.05. Active circulation of tick-borne encephalitis virus was accompanied by authentic (Р 0.01) increase of the interlayer in persons with the virus antibodies in 2014–2016 (15.8±0.69%) in comparison with 2011–2013 (9.7±0.78%). Also it was noted in all landscape zones: in steppe zone the increase was to 42.8% (P 0.05); in mountain-taiga-forest-steppe — to 61.3% (P 0.05); in mountain-tundra-taiga — to 150.0% (Р 0.01). It was not possible to reveal correlation between the recourse for medical aid and the value of immune interlayer in the population formed as a result of latent immunization. Results of the population immunity studying essentially expand our knowledge about the condition of the natural foci and dynamics of development of the epidemic processes in it, and can be used at planning of the preventive actions.

Exposure to West Nile virus and tick-borne encephalitis virus in dogs in Spain.

In the past decade, the spread of emerging zoonotic flaviviruses (genus Flavivirus, family Flaviviridae) has been reported in many regions worldwide, representing a threat to both human and animal health. A serosurvey was carried out to assess exposure and risk factors associated with antigenically related flaviviruses, particularly West Nile virus (WNV), Usutu virus (USUV) and tick-borne encephalitis virus (TBEV), in dogs in Spain. Flavivirus antibodies were detected in 39 of 815 dogs (4.8%; 95% CI: 3.3-6.3) by bELISA. Significantly higher seropositivity was observed in hunting dogs compared to pet dogs. Virus neutralization tests confirmed WNV-specific and TBEV-specific antibodies in 11 and 14 bELISA-positive dogs, respectively. This is the first serosurvey of WNV and TBEV in dogs in Spain and the first report of TBEV circulation in this country. The seropositivity obtained indicates widespread, but not homogeneous, distribution of WNV and TBEV in dogs in Spain. In 2013 and 2015, WNV-seropositive dogs were detected in those areas of Andalusia where the highest number of WNV outbreaks were reported in both horses and humans. Antibodies against TBEV have been found in dogs sampled in two different periods and regions in Spain. Serosurveillance in dogs could be a complementary way of monitoring the activity of emerging flaviviruses in Spain.

ХАРАКТЕРИСТИКА ГЕНЕТИЧЕСКИХ И ФЕНОТИПИЧЕСКИХ СВОЙСТВ ШТАММОВ ВИРУСА КЛЕЩЕВОГО ЭНЦЕФАЛИТА, ИЗОЛИРОВАННЫХ ИЗ РАЗЛИЧНЫХ ИСТОЧНИКОВ НА ТЕРРИТОРИИ ВОСТОЧНОЙ СИБИРИ

Since ixodid ticks and their feeders (vertebrates) form the basis of the tick-borne encephalitis virus (TBEV) population, it was interesting to obtain a genetic characteristic of strains isolated from various sources in the territory of Eastern Siberia. In our study, it was found that the circulation of TBEV of genotypes 1, 2, 3, 5 and polytypic strains in the territory of Eastern Siberia is maintained both by the main vectors (I. persulcatus ticks) and by their feeders (small and large mammals, birds). The distribution of genotypes in strains groups from ixodid ticks and vertebrates varied. TBEV strains of genotypes 1 and 2 were significantly more often isolated from rodents, and genotype 3 from ixodid ticks (p < 0.05). Isolates of genotype 3 (85.5 %) prevailed in the group of strains isolated from ixodid ticks, of which 29.6 % belonged to Vasilchenko, and 25.6 % to Zausaevsubgenotypes. Genotypes 1 and 2 were represented by single isolates (3.3 % and 1.3 %, respectively). Among strains isolated from warm-blooded hosts, the proportion of genotype 1 was 35.3 %, genotype 2 -11.8 %, genotype 3 - 35.3 %. More than half of genotype 3 strains (58.3 %) were related to Vasilchenko subgenotype, and the strains of Zausaev subgenotype were not detected. The obtained data suggest that ixodid ticks and warm-blooded animals are amplifiers of TBEV of genotypes 3 and 1, respectively. The certain differences in the phenotypic characteristics of strains isolated from ticks and warm-blooded animals have been revealed. It has been found that strains isolated from ticks are more homogeneous in their antigenic characteristics, less heterogeneous in S-feature than strains isolated from warm-blooded sources. The strains isolated from warm-blooded animals demonstrate somewhat greater resistance to warming but reducing the reproduction of the virus at 42 °C, and have greater virulence compared to strains isolated from ticks.

A nationwide seroprevalence screening for West Nile virus and Tick-borne encephalitis virus in the population of Bulgaria.

To assess local circulation and risk for human infections with West Nile virus (WNV) and Tick-borne encephalitis virus (TBEV) in Bulgaria, a nationwide seroprevalence study was conducted. In total, 1451 residents of all 28 districts in Bulgaria were tested for WNV-specific and TBEV-specific IgG antibodies. The survey found overall seroprevalence of 1.5% and 0.6%, respectively. The highest WNV seroprevalence was found in Sofia Province and districts near the river Danube. TBEV circulation was detected among residents of six districts. The results showed that the two virus infections seem to be more wide-spread in the country as has been described.

Comparative analysis of genomes of tick-borne encephalitis virus strains isolated from mosquitoes and ticks

The tick-borne encephalitis virus (TBEV) strain Lazo MP36 was isolated from the pool of mosquitoes Aedes vexans collected in Lazo region of Khabarovsk territory in August 2014. Phylogenetic analysis of the strain Lazo MP36 complete genome (GenBank accession number KT001073) revealed its correspondence to the TBEV Far Eastern subtype and differences from the following strains: 1) from ticks Ixodes persulcatus P. Schulze, 1930 [vaccine strain 205 (JX498939) and strains Khekhtzir 1230 (KF880805), Chichagovka (KP844724), Birobidzhan 1354 (KF880805) isolated in 2012-2013]; 2) from mosquitoes [strain Malyshevo (KJ744034) isolated in 1978 from Aedes vexans nipponii in Khabarovsk territory; strain Sakhalin 6-11 isolated from the pool of mosquitoes in 2011 (KF826916)]; 3) from human brain [vaccine strain Sofjin (JN229223), Glubinnoe/2004(DQ862460). Kavalerovo (DQ862460), Svetlogorie (DQ862460)]. The fusion peptide necessary for flavivirus entry to cells of the three TBEV strains isolated from mosquitoes (Lazo MP36, Malyshevo and Sakhalin 6-11) has the canonical structure 98-DRGWGNHCGLFGKGSI-113 for the tick-borne flaviviruses. Amino acid transition H104G typical for the mosquito-borne flaviviruses was not found. Structures of 5’- and 3’-untranslated (UTR) regions of the TBEV strains from mosquitoes were 85-98% homologous to the TBEV strains of all subtypes without recombination with mosquito-borne flaviviruses found in the Far East of Russia. Secondary structures of 5’- and 3'-UTR as well as cyclization sequences (CS) of types a and B are highly homologous for all TBEV isolates independently of the biological hosts and vectors. similarity of the genomes of the TBEV isolates from mosquitoes, ticks and patients as well as pathogenicity of the isolates for new-borne laboratory mice and tissue cultures might suggest a possible role of mosquitoes in the TBEV circulation in natural foci as an accidental or additional virus carrier.