Correlation of Severity of Human Tick-Borne Encephalitis Virus Disease and Pathogenicity in Mice

Flaviviruses-Induced Neurological Sequelae

Flaviviruses, a group of single-stranded RNA viruses spread by mosquitoes or ticks, include several significant neurotropic viruses, such as West Nile virus (WNV), Japanese encephalitis virus (JEV), tick-borne encephalitis virus (TBEV), and Zika virus (ZIKV). These viruses can cause a range of neurological diseases during acute infection, from mild, flu-like symptoms to severe and fatal encephalitis. A total of 20-50% of patients who recovered from acute flavivirus infections experienced long-term cognitive issues. Here, we discuss these major neurotropic flaviviruses-induced clinical diseases in humans and the recent findings in animal models and provide insights into the underlying disease mechanisms.

Infection of wild-caught wood mice (Apodemus sylvaticus) and yellow-necked mice (A. flavicollis) with tick-borne encephalitis virus

The distribution of tick-borne encephalitis virus (TBEV) is expanding to Western European countries, including the Netherlands, but the contribution of different rodent species to the transmission of TBEV is poorly understood. We investigated whether two species of wild rodents native to the Netherlands, the wood mouse Apodemus sylvaticus and the yellow-necked mouse Apodemus flavicollis, differ in their relative susceptibility to experimental infection with TBEV. Wild-caught individuals were inoculated subcutaneously with the classical European subtype of TBEV (Neudoerfl) or with TBEV-NL, a genetically divergent TBEV strain from the Netherlands. Mice were euthanised and necropsied between 3 and 21 days post-inoculation. None of the mice showed clinical signs or died during the experimental period. Nevertheless, TBEV RNA was detected up to 21 days in the blood of both mouse species and TBEV was also isolated from the brain of some mice. Moreover, no differences in infection rates between virus strains and mouse species were found in blood, spleen, or liver samples. Our results suggest that the wood mouse and the yellow-necked mouse may equally contribute to the transmission cycle of TBEV in the Netherlands. Future experimental infection studies that include feeding ticks will help elucidate the relative importance of viraemic transmission in the epidemiology of TBEV.

Tick-borne encephalitis: Acute clinical manifestations and severity in 581 cases from Germany, 2018-2020

OBJECTIVES

Tick-borne encephalitis (TBE) is a growing public health problem with an average of 361 cases notified annually to Germany's passive surveillance system since 2001. We aimed to assess clinical manifestations and identify covariates associated with severity.

METHODS

We included cases notified 2018-2020 in a prospective cohort study and collected data with telephone interviews, questionnaires to general practitioners, and hospital discharge summaries. Covariates' causal associations with severity were evaluated with multivariable logistic regression, adjusted for variables identified via directed acyclic graphs.

RESULTS

Of 1220 eligible cases, 581 (48%) participated. Of these, 97.1% were not (fully) vaccinated. TBE was severe in 20.3% of cases (children: 9.1%, ≥70-year-olds: 48.6%). Routine surveillance data underreported the proportion of cases with central nervous system involvement (56% vs. 84%). Ninety percent required hospitalization, 13.8% intensive care, and 33.4% rehabilitation. Severity was most notably associated with age (odds ratio (OR): 1.04, 95% confidence interval (CI): 1.02-1.05), hypertension (OR: 2.27, 95%CI: 1.37-3.75), and monophasic disease course (OR: 1.67, 95%CI: 1.08-2.58).

CONCLUSIONS

We observed substantial TBE burden and health service utilization, suggesting that awareness of TBE severity and vaccine preventability should be increased. Knowledge of severity-associated factors may help inform patients' decision to get vaccinated.

Rescue and in vitro characterization of a divergent TBEV-Eu strain from the Netherlands

Tick-borne encephalitis virus (TBEV) may cause tick-borne encephalitis (TBE), a potential life-threatening infection of the central nervous system in humans. Phylogenetically, TBEVs can be subdivided into three main subtypes, which differ in endemic region and pathogenic potential. In 2016, TBEV was first detected in the Netherlands. One of two detected strains, referred to as Salland, belonged to the TBEV-Eu subtype, yet diverged ≥ 2% on amino acid level from other members of this subtype. Here, we report the successful rescue of this strain using infectious subgenomic amplicons and its subsequent in vitro characterization by comparison to two well-characterized TBEV-Eu strains; Neudoerfl and Hypr. In the human alveolar epithelial cell line A549, growth kinetics of Salland were comparable to the high pathogenicity TBEV-Eu strain Hypr, and both strains grew considerably faster than the mildly pathogenic strain Neudoerfl. In the human neuroblastoma cell line SK-N-SH, Salland replicated faster and to higher infectious titers than both reference strains. All three TBEV strains infected primary human monocyte-derived dendritic cells to a similar extent and interacted with the type I interferon system in a similar manner. The current study serves as the first in vitro characterization of the novel, divergent TBEV-Eu strain Salland.

Activation of Early Proinflammatory Responses by TBEV NS1 Varies between the Strains of Various Subtypes

Tick-borne encephalitis (TBE) is an emerging zoonosis that may cause long-term neurological sequelae or even death. Thus, there is a growing interest in understanding the factors of TBE pathogenesis. Viral genetic determinants may greatly affect the severity and consequences of TBE. In this study, nonstructural protein 1 (NS1) of the tick-borne encephalitis virus (TBEV) was tested as such a determinant. NS1s of three strains with similar neuroinvasiveness belonging to the European, Siberian and Far-Eastern subtypes of TBEV were studied. Transfection of mouse cells with plasmids encoding NS1 of the three TBEV subtypes led to different levels of NS1 protein accumulation in and secretion from the cells. NS1s of TBEV were able to trigger cytokine production either in isolated mouse splenocytes or in mice after delivery of NS1 encoding plasmids. The profile and dynamics of TNF-α, IL-6, IL-10 and IFN-γ differed between the strains. These results demonstrated the involvement of TBEV NS1 in triggering an immune response and indicated the diversity of NS1 as one of the genetic factors of TBEV pathogenicity.

IRE1-Mediated Unfolded Protein Response Promotes the Replication of Tick-Borne Flaviviruses in a Virus and Cell-Type Dependent Manner

Tick-borne flaviviruses (TBFV) can cause severe neurological complications in humans, but differences in tissue tropism and pathogenicity have been described for individual virus strains. Viral protein synthesis leads to the induction of the unfolded protein response (UPR) within infected cells. The IRE1 pathway has been hypothesized to support flavivirus replication by increasing protein and lipid biogenesis. Here, we investigated the role of the UPR in TBFV infection in human astrocytes, neuronal and intestinal cell lines that had been infected with tick-borne encephalitis virus (TBEV) strains Neudoerfl and MucAr-HB-171/11 as well as Langat virus (LGTV). Both TBEV strains replicated better than LGTV in central nervous system (CNS) cells. TBEV strain MucAr-HB-171/11, which is associated with gastrointestinal symptoms, replicated best in intestinal cells. All three viruses activated the inositol-requiring enzyme 1 (IRE1) pathway via the X-box binding protein 1 (XBP1). Interestingly, the neurotropic TBEV strain Neudoerfl induced a strong upregulation of XBP1 in all cell types, but with faster kinetics in CNS cells. In contrast, TBEV strain MucAr-HB-171/11 failed to activate the IRE1 pathway in astrocytes. The low pathogenic LGTV led to a mild induction of IRE1 signaling in astrocytes and intestinal cells. When cells were treated with IRE1 inhibitors prior to infection, TBFV replication in astrocytes was significantly reduced. This confirms a supporting role of the IRE1 pathway for TBFV infection in relevant viral target cells and suggests a correlation between viral tissue tropism and the cell-type dependent induction of the unfolded protein response.

[Biological properties and phylogenetic relationships of tick-borne encephalitis virus (Flaviviridae, Flavivirus) isolates of siberian subtype isolated in the south of East siberia in modern period]

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.

Contact-dependent transmission of Langat and tick-borne encephalitis virus in type I interferon receptor-1 deficient mice

Tick-borne encephalitis virus (TBEV) is primarily transmitted to humans through tick bites or oral consumption of accordingly contaminated unpasteurized milk or milk products. The detection of TBEV RNA in various body fluids in immunosuppressed human patients is documented. However, the risk of direct contact exposure remains unclear. Interferon-alpha receptor-1 deficient (Ifnar1^-/- ) mice, which are lacking the interferon-α/β responses, develop neurologic manifestations after infection with TBEV and Langat virus (LGTV). We showed that subcutaneous, intranasal, and peroral infection of LGTV lead to disease, whereas mice with intragastric application of LGTV showed no disease signs. With LGTV infected mice exhibit seroconversion and significant viral RNA levels was detected in saliva, eye smear, feces and urine. As a result, TBEV and LGTV are transmitted between mice from infected to naïve co-caged sentinel animals. Although intranasal inoculation of LGTV is entirely sufficient to establish the disease in mice, the virus is not transmitted by aerosols. These pooled results from animal models highlight the risks of exposure to TBEV contaminants and the possibility for close contact transmission of TBEV in interferon-alpha receptor-1 deficient laboratory mice.Importance Tick-borne encephalitis is a severe disease of the central nervous system caused by the tick-borne encephalitis virus (TBEV). Every year between 10,000-12,000 people become infected with this flavivirus. The TBEV is usually transmitted to humans via the bite of a tick, but infections due to consumption of infectious milk products are increasingly being reported. Since there is no therapy for an TBEV infection and mechanisms of virus persistence in reservoir animals are unclear, it is important to highlight the risk of exposure to TBEV contaminants and know possible routes of transmission of this virus. The significance of our research is in identifying other infection routes of TBEV and LGTV, and the possibility of close contact transmission.

Tick-borne diseases and co-infection: Current considerations

Over recent years, a multitude of pathogens have been reported to be tick-borne. Given this, it is unsurprising that these might co-exist within the same tick, however our understanding of the interactions of these agents both within the tick and vertebrate host remains poorly defined. Despite the rich diversity of ticks, relatively few regularly feed on humans, 12 belonging to argasid and 20 ixodid species, and literature on co-infection is only available for a few of these species. The interplay of various pathogen combinations upon the vertebrate host and tick vector represents a current knowledge gap. The impact of co-infection in humans further extends into diagnostic challenges arising when multiple pathogens are encountered and we have little current data upon which to make therapeutic recommendations for those with multiple infections. Despite these short-comings, there is now increasing recognition of co-infections and current research efforts are providing valuable insights into dynamics of pathogen interactions whether they facilitate or antagonise each other. Much of this existing data is focussed upon simultaneous infection, however the consequences of sequential infection also need to be addressed. To this end, it is timely to review current understanding and highlight those areas still to address.

The envelope protein of tick-borne encephalitis virus influences neuron entry, pathogenicity, and vaccine protection

Background

Tick-borne encephalitis virus (TBEV) is considered to be the medically most important arthropod-borne virus in Europe. The symptoms of an infection range from subclinical to mild flu-like disease to lethal encephalitis. The exact determinants of disease severity are not known; however, the virulence of the strain as well as the immune status of the host are thought to be important factors for the outcome of the infection. Here we investigated virulence determinants in TBEV infection.

Method

Mice were infected with different TBEV strains, and high virulent and low virulent TBEV strains were chosen. Sequence alignment identified differences that were cloned to generate chimera virus. The infection rate of the parental and chimeric virus were evaluated in primary mouse neurons, astrocytes, mouse embryonic fibroblasts, and in vivo. Neutralizing capacity of serum from individuals vaccinated with the FSME-IMMUN® and Encepur® or combined were evaluated.

Results

We identified a highly pathogenic and neurovirulent TBEV strain, 93/783. Using sequence analysis, we identified the envelope (E) protein of 93/783 as a potential virulence determinant and cloned it into the less pathogenic TBEV strain Torö. We found that the chimeric virus specifically infected primary neurons more efficiently compared to wild-type (WT) Torö and this correlated with enhanced pathogenicity and higher levels of viral RNA in vivo. The E protein is also the major target of neutralizing antibodies; thus, genetic variation in the E protein could influence the efficiency of the two available vaccines, FSME-IMMUN® and Encepur®. As TBEV vaccine breakthroughs have occurred in Europe, we chose to compare neutralizing capacity from individuals vaccinated with the two different vaccines or a combination of them. Our data suggest that the different vaccines do not perform equally well against the two Swedish strains.

Conclusions

Our findings show that two amino acid substitutions of the E protein found in 93/783, A83T, and A463S enhanced Torö infection of neurons as well as pathogenesis and viral replication in vivo; furthermore, we found that genetic divergence from the vaccine strain resulted in lower neutralizing antibody titers in vaccinated individuals.

Geographical distribution and prevalence of tick-borne encephalitis virus in questing Ixodes ricinus ticks and phylogeographic structure of the Ixodes ricinus vector in Norway

The tick-borne encephalitis virus (TBEV), a zoonotic flaviviral infection, is endemic in large parts of Norway and Eurasia. Humans are mainly infected with TBEV via bites from infected ticks. In Norway, the main geographical distribution of ticks is along the Norwegian coastline from southeast (~59°N) and up to the southern parts of Nordland County (~65°N). In this study, we collected ticks by flagging along the coast from Østfold County to Nordland County. By whole-genome sequencing of the mitochondrial genome of Ixodes ricinus, the phylogenetic tree suggests that there is limited phylogeographic structure both in Norway and in Europe. The overall TBEV prevalence is 0.3% for nymphs and 4.3% for adults. The highest estimated TBEV prevalence in adult ticks was detected in Rogaland and Vestfold County, while for nymphs it is highest in Vestfold, Vest-Agder and Rogaland. The present work is one of the largest studies on distribution and prevalence of TBEV in ticks in Scandinavia, showing that the virus is wider distributed in Norway than previously anticipated.

Can Bats Serve as Reservoirs for Arboviruses?

Bats are known to harbor and transmit many emerging and re-emerging viruses, many of which are extremely pathogenic in humans but do not cause overt pathology in their bat reservoir hosts: henipaviruses (Nipah and Hendra), filoviruses (Ebola and Marburg), and coronaviruses (SARS-CoV and MERS-CoV). Direct transmission cycles are often implicated in these outbreaks, with virus shed in bat feces, urine, and saliva. An additional mode of virus transmission between bats and humans requiring further exploration is the spread of disease via arthropod vectors. Despite the shared ecological niches that bats fill with many hematophagous arthropods (e.g. mosquitoes, ticks, biting midges, etc.) known to play a role in the transmission of medically important arboviruses, knowledge surrounding the potential for bats to act as reservoirs for arboviruses is limited. To this end, a comprehensive literature review was undertaken examining the current understanding and potential for bats to act as reservoirs for viruses transmitted by blood-feeding arthropods. Serosurveillance and viral isolation from either free-ranging or captive bats are described in relation to four arboviral groups (Bunyavirales, Flaviviridae, Reoviridae, Togaviridae). Further, ecological associations between bats and hematophagous viral vectors are characterized (e.g. bat bloodmeals in mosquitoes, ingestion of mosquitoes by bats, etc). Lastly, knowledge gaps related to hematophagous ectoparasites (bat bugs and bed bugs (Cimicidae) and bat flies (Nycteribiidae and Streblidae)), in addition to future directions for characterization of bat-vector-virus relationships are described.