Clinical and Laboratory Characteristics and Outcome of Illness Caused by Tick-Borne Encephalitis Virus without Central Nervous System Involvement

A novel rapid visual nucleic acid detection technique for tick-borne encephalitis virus by combining RT-recombinase-aided amplification and CRISPR/Cas13a coupled with a lateral flow dipstick

Tick-borne encephalitis virus (TBEV), a zoonotic pathogen, can cause severe neurological complications and fatal outcomes in humans. Early diagnosis of TBEV infection is crucial for clinical practice. Although serological assays are frequently employed for detection, the lack of antibodies in the early stages of infection and the cross-reactivity of antibodies limit their efficacy. Conventional molecular diagnostic methods such as RT-qPCR can achieve early and accurate identification but require specialized instrumentation and professionals, hindering their application in resource-limited areas. Our study developed a rapid and visual TBEV molecular detection method by combining RT-recombinase-aided amplification, the CRISPR/Cas13a system, and lateral flow dipsticks. The diagnostic sensitivity of this method is 50 CFU/ml, with no cross-reactivity with a variety of viruses. The detection can be carried out within 1 h at a temperature between 37 and 42 °C, and the results can be visually determined without the need for complex instruments and professionals. Subsequently, this assay was used to analyze clinical samples from 15 patients suspected of TBEV infection and 10 healthy volunteers, and its sensitivity and specificity reached 100 %, which was consistent with the results of RT-qPCR. These results indicate that this new method can be a promising point-of-care test for the diagnosis of tick-borne encephalitis.

Tick-borne encephalitis infections without CNS involvement: An observational study in Latvia, 2007-2022

BACKGROUND

Tick-borne encephalitis (TBE) is a human viral infectious disease involving the central nervous system (CNS). It is caused by the tick-borne encephalitis virus (TBEV). At present, there is very limited information regarding the clinical importance and health burden of TBE infections without signs of CNS inflammation. Moreover, such cases are omitted from official TBE surveillances and there are no reports of population-based studies.

METHODS AND FINDINGS

A nationwide population-based study was conducted in Latvia by intensively searching for symptomatic TBEV infections recorded in outpatient and hospital settings between 2007 and 2022. In total, 4,124 symptomatic TBEV infections were identified, of which 823 (20.0%) had no CNS involvement. Despite the lack of neurological symptoms, non-CNS TBE patients still experienced severe health conditions that required management in a hospital setting for a median duration of 7 days. Furthermore, lumbar puncture information was available for 708 of these patients, with 100 (14.1%) undergoing the procedure, suggesting a high suspicion of CNS involvement.

CONCLUSIONS

Clearly, non-CNS TBE has the potential to negatively impact the health of patients. The actual burden of non-CNS TBEV cases may be higher than we think as these cases are omitted from official TBE surveillances and are challenging to recognize.

Elevated levels of serum muscle enzymes in the initial phase of tick-borne encephalitis

PURPOSE

Since some patients with tick-borne encephalitis (TBE) have pronounced myalgias, and since myositis is reported in Flavivirus diseases such as dengue, we performed systematic search for abnormalities of muscle enzymes in a group of patients in whom the presence of tick-borne encephalitis virus (TBEV) RNA in the first phase of the disease was demonstrated and who developed second phase of TBE.

METHODS

Total leukocyte and platelet blood counts were determined routinely at the initial examination during the first and the second phase of TBE. Activity of aspartate aminotransferase (AST), alanine aminotransferase (ALT), creatine kinase (CK), myoglobin and troponin was determined from the available stored serum specimens; the first and second phase disease specimens were tested simultaneously.

RESULTS

Of 24 patients with biphasic course of TBE, 83% had leukopenia, 65% thrombocytopenia, 83% elevated AST and 4% elevated ALT level. Furthermore, 33% had elevated serum CK, 26% myoglobin and 22% troponin activity; at least one of the muscle enzymes was elevated in 42% of patients. Leukopenia, thrombocytopenia, elevated liver enzymes and elevations of CK and myoglobin were present in the initial phase but resolve later, while troponin abnormalities were also found in the second phase of TBE.

CONCLUSIONS

The present study exposes that in addition to previously known leukopenia, thrombocytopenia and increased liver enzymes activity, the initial phase of TBE is relatively often associated also with elevated muscle enzymes. Clinical relevance of these findings remains to be determined.

Ancestral allele of DNA polymerase gamma modifies antiviral tolerance

Mitochondria are critical modulators of antiviral tolerance through the release of mitochondrial RNA and DNA (mtDNA and mtRNA) fragments into the cytoplasm after infection, activating virus sensors and type-I interferon (IFN-I) response1-4. The relevance of these mechanisms for mitochondrial diseases remains understudied. Here we investigated mitochondrial recessive ataxia syndrome (MIRAS), which is caused by a common European founder mutation in DNA polymerase gamma (POLG1)5. Patients homozygous for the MIRAS variant p.W748S show exceptionally variable ages of onset and symptoms5, indicating that unknown modifying factors contribute to disease manifestation. We report that the mtDNA replicase POLG1 has a role in antiviral defence mechanisms to double-stranded DNA and positive-strand RNA virus infections (HSV-1, TBEV and SARS-CoV-2), and its p.W748S variant dampens innate immune responses. Our patient and knock-in mouse data show that p.W748S compromises mtDNA replisome stability, causing mtDNA depletion, aggravated by virus infection. Low mtDNA and mtRNA release into the cytoplasm and a slow IFN response in MIRAS offer viruses an early replicative advantage, leading to an augmented pro-inflammatory response, a subacute loss of GABAergic neurons and liver inflammation and necrosis. A population databank of around 300,000 Finnish individuals6 demonstrates enrichment of immunodeficient traits in carriers of the POLG1 p.W748S mutation. Our evidence suggests that POLG1 defects compromise antiviral tolerance, triggering epilepsy and liver disease. The finding has important implications for the mitochondrial disease spectrum, including epilepsy, ataxia and parkinsonism.

Evidence of tick-borne encephalitis virus neutralizing antibodies in Serbian individuals exposed to tick bites

Introduction

Tick-borne encephalitis (TBE) is an emerging vector-borne and food-borne disease caused by the tick-borne encephalitis virus (TBEV; Orthoflavivirus encephalitidis), with a distribution spanning the Eurasian continent. Despite its significant public health impact in various European regions, TBE remains largely underdiagnosed in Serbia due to limited awareness and diagnostic challenges. In response to this, our study aimed to comprehensively assess TBEV exposure in individuals infested with ticks and to identify potential TBEV foci within Serbia.

Materials and methods

From 2019 to 2021, we conducted an observational study involving 450 patients who reported tick infestations.

Results

Our demographic analysis revealed a median age of 38 years, with a slight male predominance among the participants. We documented tick infestations in 38 municipalities across 14 districts of Serbia, with a notable concentration in proximity to Fruška Gora Mountain. The ticks most frequently removed were Ixodes ricinus, with nymphs and adult females being the predominant stages. On average, nymphs were removed after about 27.1 hours of feeding, while adult females remained attached for approximately 44.4 hours. Notably, we found age as a significant predictor of infestation time for both nymphs and adult females. Furthermore, we detected TBEV-neutralizing antibodies in 0.66% of the serum samples, shedding light on potential TBEV foci, particularly in Fruška Gora Mountain and other regions of Serbia.

Conclusion

Our study emphasizes the urgent need for active TBE surveillance programs, especially in areas suspected of hosting TBEV foci, in order to assess the true TBE burden, identify at-risk populations, and implement effective preventive measures.

Tick-Borne Encephalitis Vaccine: Recommendations of the Advisory Committee on Immunization Practices, United States, 2023

Tick-borne encephalitis (TBE) virus is focally endemic in parts of Europe and Asia. The virus is primarily transmitted to humans by the bites of infected

Ixodes species ticks but can also be acquired less frequently by alimentary transmission. Other rare modes of transmission include through breastfeeding, blood transfusion, solid organ transplantation, and slaughtering of viremic animals. TBE virus can cause acute neurologic disease, which usually results in hospitalization, often permanent neurologic or cognitive sequelae, and sometimes death. TBE virus infection is a risk for certain travelers and for laboratory workers who work with the virus. In August 2021, the Food and Drug Administration approved Ticovac TBE vaccine for use among persons aged ≥1 year. This report summarizes the epidemiology of and risks for infection with TBE virus, provides information on the immunogenicity and safety of TBE vaccine, and summarizes the recommendations of the Advisory Committee on Immunization Practices (ACIP) for use of TBE vaccine among U.S. travelers and laboratory workers.

The risk for TBE for most U.S. travelers to areas where the disease is endemic is very low. The risk for exposure to infected ticks is highest for persons who are in areas where TBE is endemic during the main TBE virus transmission season of April–November and who are planning to engage in recreational activities in woodland habitats or who might be occupationally exposed. All persons who travel to areas where TBE is endemic should be advised to take precautions to avoid tick bites and to avoid the consumption of unpasteurized dairy products because alimentary transmission of TBE virus can occur. TBE vaccine can further reduce infection risk and might be indicated for certain persons who are at higher risk for TBE. The key factors in the risk-benefit assessment for vaccination are likelihood of exposure to ticks based on activities and itinerary (e.g., location, rurality, season, and duration of travel or residence). Other risk-benefit considerations should include 1) the rare occurrence of TBE but its potentially high morbidity and mortality, 2) the higher risk for severe disease among certain persons (e.g., older persons aged ≥60 years), 3) the availability of an effective vaccine, 4) the possibility but low probability of serious adverse events after vaccination, 5) the likelihood of future travel to areas where TBE is endemic, and 6) personal perception and tolerance of risk

ACIP recommends TBE vaccine for U.S. persons who are moving or traveling to an area where the disease is endemic and will have extensive exposure to ticks based on their planned outdoor activities and itinerary. Extensive exposure can be considered based on the duration of travel and frequency of exposure and might include shorter-term (e.g., <1 month) travelers with daily or frequent exposure or longer-term travelers with regular (e.g., a few times a month) exposure to environments that might harbor infected ticks. In addition, TBE vaccine may be considered for persons who might engage in outdoor activities in areas where ticks are likely to be found, with a decision to vaccinate made on the basis of an assessment of their planned activities and itinerary, risk factors for a poor medical outcome, and personal perception and tolerance of risk. In the laboratory setting, ACIP recommends TBE vaccine for laboratory workers with a potential for exposure to TBE virus

Tick-Borne Encephalitis (TBE): From Tick to Pathology

Tick-borne encephalitis (TBE) is a viral arthropod infection, endemic to large parts of Europe and Asia, and is characterised by neurological involvement, which can range from mild to severe, and in 33-60% of cases, it leads to a post-encephalitis syndrome and long-term morbidity. While TBE virus, now identified as Orthoflavivirus encephalitidis, was originally isolated in 1937, the pathogenesis of TBE is not fully appreciated with the mode of transmission (blood, tick, alimentary), viral strain, host immune response, and age, likely helping to shape the disease phenotype that we explore in this review. Importantly, the incidence of TBE is increasing, and due to global warming, its epidemiology is evolving, with new foci of transmission reported across Europe and in the UK. As such, a better understanding of the symptomatology, diagnostics, treatment, and prevention of TBE is required to inform healthcare professionals going forward, which this review addresses in detail. To this end, the need for robust national surveillance data and randomised control trial data regarding the use of various antivirals (e.g., Galidesivir and 7-deaza-2'-CMA), monoclonal antibodies, and glucocorticoids is required to improve the management and outcomes of TBE.

Arboviral vaccines for use in pregnant travelers

Pregnant women traveling abroad can be exposed to a variety of arboviruses, primarily spread by mosquitoes or ticks. Some arboviral infections can be of particular concern for pregnant women or their fetuses. Vaccination is one preventive measure that can reduce the risk for infection. Several arboviral vaccines have been licensed for many years and can be used to prevent infection in travelers, namely Japanese encephalitis, yellow fever, and tick-borne encephalitis vaccines. Recommendations on use of these vaccines in pregnancy vary. Other arboviral vaccines have been licensed but are not indicated for use in pregnant travelers (e.g., dengue vaccines) or are in development (e.g., chikungunya, Zika vaccines). This review describes arboviral vaccines for travelers, focusing on women who are pregnant and those planning travel during pregnancy.

Tick-Borne Encephalitis Virus: A Comprehensive Review of Transmission, Pathogenesis, Epidemiology, Clinical Manifestations, Diagnosis, and Prevention

Tick-borne encephalitis virus (TBEV), a member of the Flaviviridae family, can cause serious infection of the central nervous system in humans, resulting in potential neurological complications and fatal outcomes. TBEV is primarily transmitted to humans through infected tick bites, and the viral agent circulates between ticks and animals, such as deer and small mammals. The occurrence of the infection aligns with the seasonal activity of ticks. As no specific antiviral therapy exists for TBEV infection, treatment approaches primarily focus on symptomatic relief and support. Active immunization is highly effective, especially for individuals in endemic areas. The burden of TBEV infections is increasing, posing a growing health concern. Reported incidence rates rose from 0.4 to 0.9 cases per 100,000 people between 2015 and 2020. The Baltic and Central European countries have the highest incidence, but TBE is endemic across a wide geographic area. Various factors, including social and environmental aspects, improved medical awareness, and advanced diagnostics, have contributed to the observed increase. Diagnosing TBEV infection can be challenging due to the non-specific nature of the initial symptoms and potential co-infections. Accurate diagnosis is crucial for appropriate management, prevention of complications, and effective control measures. In this comprehensive review, we summarize the molecular structure of TBEV, its transmission and circulation in natural environments, the pathogenesis of TBEV infection, the epidemiology and global distribution of the virus, associated risk factors, clinical manifestations, and diagnostic approaches. By improving understanding of these aspects, we aim to enhance knowledge and promote strategies for timely and accurate diagnosis, appropriate management, and the implementation of effective control measures against TBEV infections.

The tick saliva peptide HIDfsin2 promotes the tick-borne virus SFTSV replication in vitro by enhancing p38 signal pathway

Pathogens co-evolved with ticks to facilitate blood collection and pathogen transmission. Although tick saliva was recently found to be rich in bioactive peptides, it is still elusive which saliva peptide promotes virus transmission and which pathways are invovled. Here, we used a saliva peptide HIDfsin2 and a severe fever with thrombocytopenia syndrome virus (SFTSV) both carried by the tick Haemaphysalis longicornis to elucidate the relationship between tick saliva components and tick-borne viruses. HIDfsin2 was found to promote the replication of SFTSV in a dose-dependent manner in vitro. HIDfsin2 was further revealed to MKK3/6-dependently magnify the activation of p38 MAPK. The overexpression, knockdown and phosphorylation site mutation of p38α indicated that p38 MAPK activation facilitated SFTSV infection in A549 cells. Moreover, the blockade of p38 MAPK activation significantly suppressed SFTSV replication. Differently, HIDfsin2 or pharmacological inhibition of p38 MAPK activation had no effect on a mosquito-borne Zika virus (ZIKV). All these results showed that HIDfsin2 specifically promoted SFTSV replication through the MKK3/6-dependent enhancement of p38 MAPK activation. Our study provides a new perspective on the transmission of tick-borne viruses under natural conditions, and supports that the blockade of p38 MAPK activation can be a promising strategy against the mortal tick-borne virus SFTSV.

T Cells in Tick-Borne Flavivirus Encephalitis: A Review of Current Paradigms in Protection and Disease Pathology

The family Flaviviridae is comprised of a diverse group of arthropod-borne viruses that are the etiological agents of globally relevant diseases in humans. Among these, infection with several of these flaviviruses-including West Nile virus (WNV), Zika virus (ZIKV), Japanese encephalitis virus (JEV), tick-borne encephalitis virus (TBEV), and Powassan virus (POWV)-can result in neuroinvasive disease presenting as meningitis or encephalitis. Factors contributing to the development and resolution of tick-borne flavivirus (TBEV, POWV) infection and neuropathology remain unclear, though many recently undertaken studies have described the virus-host interactions underlying encephalitic disease. With access to neural tissues despite the selectively permeable blood-brain barrier, T cells have emerged as one notable contributor to neuroinflammation. The goal of this review is to summarize the recent advances in tick-borne flavivirus immunology-particularly with respect to T cells-as it pertains to the development of encephalitis. We found that although T cell responses are rarely evaluated in a clinical setting, they are integral in conjunction with antibody responses to restricting the entry of TBFV into the CNS. The extent and means by which they can drive immune pathology, however, merits further study. Understanding the role of the T cell compartment in tick-borne flavivirus encephalitis is instrumental for improving vaccine safety and efficacy, and has implications for treatments and interventions for human disease.

Fatal Case of Imported Tick-Borne Encephalitis in South Serbia

Tick-borne encephalitis (TBE) is vaccine-preventable neglected zoonotic neuroinvasive disease, caused by tick-borne encephalitis virus (TBEV). Many of the Central and Eastern European countries are affected by TBE, which is often poorly perceived by tourists visiting endemic territories. Here we are reporting a fatal case of imported TBE in Serbian resident who was exposed to a tick bite during a visit to Switzerland.

Comparison of laboratory and immune characteristics of the initial and second phase of tick-borne encephalitis

Tick-borne encephalitis (TBE) usually has a biphasic course which begins with unspecific febrile illness, followed by central nervous system involvement. Because TBE is not yet suspected during the initial phase, knowledge of early TBE pathogenesis is incomplete. Herein we evaluated laboratory and immune findings in the initial and second (meningoencephalitic) phase of TBE in 88 well-defined adult patients. Comparison of nine laboratory blood parameters in both phases of TBE revealed that laboratory abnormalities, consisting of low leukocyte and platelet counts and increased liver enzymes levels, were predominately associated with the initial phase of TBE and resolved thereafter. Assessment of 29 immune mediators in serum during the initial phase, and in serum and cerebrospinal fluid (CSF) during the second phase of TBE revealed highly distinct clustering patterns among the three groups. In the initial phase of TBE, the primary finding in serum was a rather heterogeneous immune response involving innate (CXCL11), B cell (CXCL13, BAFF), and T cell mediators (IL-27 and IL-4). During the second phase of TBE, growth factors associated with angiogenesis (GRO-α and VEGF-A) were the predominant characteristic in serum, whereas innate and Th1 mediators were the defining feature of immune responses in CSF. These findings imply that distinct immune processes play a role in the pathophysiology of different phases of TBE and in different compartments.

Quantification of Antiviral Cytokines in Serum, Cerebrospinal Fluid and Urine of Patients with Tick-Borne Encephalitis in Croatia

BACKGROUND

Tick-borne encephalitis virus (TBEV) is one of the most significant arboviruses affecting the human central nervous system (CNS) in Europe. Data on cytokine response in TBEV infection are limited.

METHODS

We analyzed the cytokine response in serum, cerebrospinal fluid (CSF) and urine samples of patients with TBE. The control group consisted of patients with 'febrile headache' who had normal CSF cytology. The panel included 12 cytokines: TNF-α, IL-6, Th1 (IL-2, IFN-γ), Th2 (IL-4, IL-5, IL-13), Th9 (IL-9), Th17 (IL-17A, IL-17F), Th22 (IL-22) cytokines and IL-10.

RESULTS

TBE patients were more likely to have increased levels of IL-6 and IFN-γ in CSF compared to controls (85.7% vs. 58.8% and 85.7% vs. 47.1%, respectively). However, concentrations of IL-6 (the most abundant cytokine in the CSF of both groups), IL-10 and IL-9 were lower in TBEV patients compared with controls, but the difference was statistically significant for IL-9 only (p = 0.001). By analyzing the cytokine levels in different clinical samples, all measured cytokines were detected in the serum, with the highest concentrations found for IFN-γ, TNF-α, IL-10, IL-17F and IL-22. Higher concentrations of cytokines in the CSF compared with serum were observed for IL-5, IL-6 and IL-22. All cytokines except IL-13 were detectable in urine but in a small proportion of patients, except for IL-22, which was detectable in 95.8% of patients.

CONCLUSIONS

Cytokine composition in different clinical samples of TBE patients reveals a different network of early innate immune response cytokines, Th1, Th2, Th9, Th22, Th17 and anti-inflammatory cytokines.

Recommendations to Improve Tick-Borne Encephalitis Surveillance and Vaccine Uptake in Europe

There has been an increase in reported TBE cases in Europe since 2015, reaching a peak in some countries in 2020, highlighting the need for better management of TBE risk in Europe. TBE surveillance is currently limited, in part, due to varying diagnostic guidelines, access to testing, and awareness of TBE. Consequently, TBE prevalence is underestimated and vaccination recommendations inadequate. TBE vaccine uptake is unsatisfactory in many TBE-endemic European countries. This review summarizes the findings of a scientific workshop of experts to improve TBE surveillance and vaccine uptake in Europe. Strategies to improve TBE surveillance and vaccine uptake should focus on: aligning diagnostic criteria and testing across Europe; expanding current vaccine recommendations and reducing their complexity; and increasing public education of the potential risks posed by TBEV infection.

Complete Genome Sequencing of Tick-Borne Encephalitis Virus Directly from Clinical Samples: Comparison of Shotgun Metagenomic and Targeted Amplicon-Based Sequencing

The clinical presentation of tick-borne encephalitis virus (TBEV) infection varies from asymptomatic to severe meningoencephalitis or meningoencephalomyelitis. The TBEV subtype has been suggested as one of the most important risk factors for disease severity, but TBEV genetic characterization is difficult. Infection is usually diagnosed in the post-viremic phase, and so relevant clinical samples of TBEV are extremely rare and, when present, are associated with low viral loads. To date, only two complete TBEV genomes sequenced directly from patient clinical samples are publicly available. The aim of this study was to develop novel protocols for the direct sequencing of the TBEV genome, enabling studies of viral genetic determinants that influence disease severity. We developed a novel oligonucleotide primer scheme for amplification of the complete TBEV genome. The primer set was tested on 21 clinical samples with various viral loads and collected over a 15-year period using the two most common sequencing platforms. The amplicon-based strategy was compared to direct shotgun sequencing. Using the novel primer set, we successfully obtained nearly complete TBEV genomes (>90% of genome) from all clinical samples, including those with extremely low viral loads. Comparison of consensus sequences of the TBEV genome generated using the novel amplicon-based strategy and shotgun sequencing showed no difference. We conclude that the novel primer set is a powerful tool for future studies on genetic determinants of TBEV that influence disease severity and will lead to a better understanding of TBE pathogenesis.