The Prevalence of Tick-Borne Encephalitis Virus in Wild Rodents Captured in Tick-Borne Encephalitis Foci in Highly Endemic Lithuania

Wild rodents are considered to be one of the most important TBEV-amplifying reservoir hosts; therefore, they may be suitable for foci detection studies. To investigate the effectiveness of viral RNA detection in wild rodents for suspected TBEV foci confirmation, we trapped small rodents (n = 139) in various locations in Lithuania where TBEV was previously detected in questing ticks. Murine neuroblastoma Neuro-2a cells were inoculated with each rodent sample to maximize the chances of detecting viral RNA in rodent samples. TBEV RNA was detected in 74.8% (CI 95% 66.7-81.1) of the brain and/or internal organ mix suspensions, and the prevalence rate increased significantly following sample cultivation in Neuro-2a cells. Moreover, a strong correlation (r = 0.88; p < 0.05) was found between the average monthly air temperature of rodent trapping and the TBEV RNA prevalence rate in cell culture isolates of rodent suspensions, which were PCR-negative before cultivation in cell culture. This study shows that wild rodents are suitable sentinel animals to confirm TBEV foci. In addition, the study results demonstrate that sample cultivation in cell culture is a highly efficient method for increasing TBEV viral load to detectable quantities.

Analysis of effectivity of the tickborne encephalitis virus detection methods in ixodid ticks

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.

Seroprevalence of TBEV in bank voles from Poland-a long-term approach

Rodents are known to play a significant role as reservoir hosts for TBEV. During three sequential expeditions at 4-year intervals to three ecologically similar study sites in NE Poland, we trapped bank voles (Myodes glareolus) and then tested their blood for the presence of specific antiviral antibodies to TBEV. The strongest effects on seroprevalence were the extrinsic factors, site of capture of voles and year of sampling. Seroprevalence increased markedly with increasing host age, and our analysis revealed significant interactions among these three factors. Seroprevalence did not differ between the sexes. Therefore, based on the seroprevalence results, the dynamics of TBEV infection differ significantly in time, between local sub-populations of bank voles and with increasing host age. To fully understand the circulation of the virus among these reservoir hosts and in the environment, long-term monitoring is required and should employ a multi-site approach, such as the one adopted in the current study.

Deep sequencing analysis of tick-borne encephalitis virus from questing ticks at natural foci reveals similarities between quasispecies pools of the virus

Every year, tick-borne encephalitis virus (TBEV) causes severe central nervous system infection in 10 000 to 15 000 people in Europe and Asia. TBEV is maintained in the environment by an enzootic cycle that requires a tick vector and a vertebrate host, and the adaptation of TBEV to vertebrate and invertebrate environments is essential for TBEV persistence in nature. This adaptation is facilitated by the error-prone nature of the virus’s RNA-dependent RNA polymerase, which generates genetically distinct virus variants called quasispecies. TBEV shows a focal geographical distribution pattern where each focus represents a TBEV hotspot. Here, we sequenced and characterized two TBEV genomes, JP-296 and JP-554, from questing Ixodes ricinus ticks at a TBEV focus in central Sweden. Phylogenetic analysis showed geographical clustering among the newly sequenced strains and three previously sequenced Scandinavian strains, Toro-2003, Saringe-2009 and Mandal-2009, which originated from the same ancestor. Among these five Scandinavian TBEV strains, only Mandal-2009 showed a large deletion within the 3′ non-coding region (NCR), similar to the highly virulent TBEV strain Hypr. Deep sequencing of JP-296, JP-554 and Mandal-2009 revealed significantly high quasispecies diversity for JP-296 and JP-554, with intact 3′NCRs, compared to the low diversity in Mandal-2009, with a truncated 3′NCR. Single-nucleotide polymorphism analysis showed that 40 % of the single-nucleotide polymorphisms were common between quasispecies populations of JP-296 and JP-554, indicating a putative mechanism for how TBEV persists and is maintained within its natural foci.

Cyclic patterns in the central European tick-borne encephalitis incidence series

Tick-borne encephalitis (TBE) is peculiar due to its unstable dynamics with profound inter-annual fluctuations in case numbers – a phenomenon not well understood to date. Possible reasons – apart from variable human contact with TBE foci – include external factors, e.g. climatic forcing, autonomous oscillations of the disease system itself, or a combined action of both. Spectral analysis of TBE data from six regions of central Europe (CE) revealed that the ostensibly chaotic dynamics can be explained in terms of four superposed (quasi-)periodical oscillations: a quasi-biennial, triennial, pentennial, and a decadal cycle. These oscillations exhibit a high degree of regularity and synchrony across CE. Nevertheless, some amplitude and phase variations are responsible for regional differences in incidence patterns. In addition, periodic changes occur in the degree of synchrony in the regions: marked in-phase periods alternate with rather off-phase periods. Such a feature in the disease dynamics implies that it arises as basically diverging self-oscillations of local disease systems which, at intervals, receive synchronizing impulses, such as periodic variations in food availability for key hosts driven by external factors. This makes the disease dynamics synchronized over a large area during peaks in the synchronization signal, shifting to asynchrony in the time in between.

Risk of exposure to ticks (Ixodidae) and the prevalence of tick-borne encephalitis virus (TBEV) in ticks in Southern Poland

The article presents the results of the first study on seasonal activity of ticks and prevalence of tick-borne encephalitis virus (TBEV) in nymphs from the Silesian Province (Southern Poland). Previous studies on the prevalence of TBEV in ticks in Poland have been conducted mostly in northern and eastern regions, but none in the Silesian Province itself. The aims of this study were to analyse the seasonal variation in tick populations and compare TBEV prevalence in nymphs from different geographical locations in the Silesia. A total of 5160 questing Ixodes ricinus ticks were collected by the flagging method from 23 localities in southern Poland in 2010. Micro-climatic parameters (air temperature and humidity) were measured in order to estimate their influence on tick population. The highest tick activity was recorded in spring and was positively correlated with relative air humidity (RH). TBEV in the Silesian Province was analysed in 1750 nymphs and an overall prevalence was 0.11% (2 pools out of 175 analysed). The results of this study show that TBEV pool prevalence in nymphs is low in accordance with the low number of TBE cases reported within the region.

Tick-borne encephalitis virus sequenced directly from questing and blood-feeding ticks reveals quasispecies variance

The increased distribution of the tick-borne encephalitis virus (TBEV) in Scandinavia highlights the importance of characterizing novel sequences within the natural foci. In this study, two TBEV strains: the Norwegian Mandal 2009 (questing nymphs pool) and the Swedish Saringe 2009 (blood-fed nymph) were sequenced and phylogenetically characterized. Interestingly, the sequence of Mandal 2009 revealed the shorter form of the TBEV genome, similar to the highly virulent Hypr strain, within the 3′ non-coding region (3′NCR). A different genomic structure was found in the 3′NCR of Saringe 2009, as in-depth analysis demonstrated TBEV variants with different lengths within the poly(A) tract. This shows that TBEV quasispecies exists in nature and indicates a putative shift in the quasispecies pool when the virus switches between invertebrate and vertebrate environments. This prompted us to further sequence and analyze the 3′NCRs of additional Scandinavian TBEV strains and control strains, Hypr and Neudoerfl. Toro 2003 and Habo 2011 contained mainly a short (A)3C(A)6 poly(A) tract. A similar pattern was observed for the human TBEV isolates 1993/783 and 1991/4944; however, one clone of 1991/4944 contained an (A)3C(A)11 poly(A) sequence, demonstrating that quasispecies with longer poly(A) could be present in human isolates. Neudoerfl has previously been reported to contain a poly(A) region, but to our surprise the re-sequenced genome contained two major quasispecies variants, both lacking the poly(A) tract. We speculate that the observed differences are important factors for the understanding of virulence, spread, and control of the TBEV.

The three subtypes of tick-borne encephalitis virus induce encephalitis in a natural host, the bank vole (Myodes glareolus)

Tick-borne encephalitis virus (TBEV) infects bank voles (Myodes glareolus) in nature, but the relevance of rodents for TBEV transmission and maintenance is unclear. We infected colonized bank voles subcutaneously to study and compare the infection kinetics, acute infection, and potential viral persistence of the three known TBEV subtypes: European (TBEV-Eur), Siberian (TBEV-Sib) and Far Eastern (TBEV-FE). All strains representing the three subtypes were infective and highly neurotropic. They induced (meningo)encephalitis in some of the animals, however most of the cases did not present with apparent clinical symptoms. TBEV-RNA was cleared significantly slower from the brain as compared to other organs studied. Supporting our earlier findings in natural rodent populations, TBEV-RNA could be detected in the brain for up to 168 days post infection, but we could not demonstrate infectivity by cell culture isolation. Throughout all time points post infection, RNA of the TBEV-FE was detected significantly more often than RNA of the other two strains in all organs studied. TBEV-FE also induced prolonged viremia, indicating distinctive kinetics in rodents in comparison to the other two subtypes. This study shows that bank voles can develop a neuroinvasive TBEV infection with persistence of viral RNA in brain, and mount an anti-TBEV IgG response. The findings also provide further evidence that bank voles can serve as sentinels for TBEV endemicity.

Molecular phylogeography of tick-borne encephalitis virus in central Europe

In order to obtain a better understanding of tick-borne encephalitis virus (TBEV) strain movements in central Europe the E gene sequences of 102 TBEV strains collected from 1953 to 2011 at 38 sites in the Czech Republic, Slovakia, Austria and Germany were determined. Bayesian analysis suggests a 350-year history of evolution and spread in central Europe of two main lineages, A and B. In contrast to the east to west spread at the Eurasian continent level, local central European spreading patterns suggest historic west to east spread followed by more recent east to west spread. The phylogenetic and network analyses indicate TBEV ingressions from the Czech Republic and Slovakia into Germany via landscape features (Danube river system), biogenic factors (birds, red deer) and anthropogenic factors. The identification of endemic foci showing local genetic diversity is of paramount importance to the field as these will be a prerequisite for in-depth analysis of focal TBEV maintenance and long-distance TBEV spread.

Tick-borne encephalitis virus in wild rodents in winter, Finland, 2008-2009

Rodents might maintain tick-borne encephalitis virus (TBEV) in nature through latent persistent infections. During 2 subsequent winters, 2008 and 2009, in Finland, we detected RNA of European and Siberian subtypes of TBEV in Microtus agrestis and Myodes glareolus voles, respectively. Persistence in rodent reservoirs may contribute to virus overwintering.

Tick-borne encephalitis virus in dogs--is this an issue?

The last review on Tick-borne encephalitis (TBE) in dogs was published almost ten years ago. Since then, this zoonotic tick-borne arbovirus has been geographically spreading and emerging in many regions in Eurasia and continues to do so. Dogs become readily infected with TBE virus but they are accidental hosts not capable to further spread the virus. They seroconvert upon infection but they seem to be much more resistant to the clinical disease than humans. Apart from their use as sentinels in endemic areas, however, an increasing number of case reports appeared during the last decade thus mirroring the rising public health concerns. Owing to the increased mobility of people travelling to endemic areas with their companion dogs, this consequently leads to problems in recognizing and diagnosing this severe infection in a yet non-endemic area, simply because the veterinarians are not considering TBE. This situation warrants an update on the epidemiology, clinical presentation and possible preventions of TBE in the dog.