Tick-Borne Encephalitis Virus: Seasonal and Annual Variation of Epidemiological Parameters Related to Nymph-to-Larva Transmission and Exposure of Small Mammals

Environmental and host factors underlying tick-borne virus infection in wild animals: Investigation of the emerging Yezo virus in Hokkaido, Japan

Yezo virus (YEZV) is an emerging tick-borne virus that causes acute febrile illness. It has been continuously reported in patients and ticks in Japan and China since its first identification in Hokkaido, Japan. While serological tests have demonstrated that YEZV infections are prevalent in wild animals, such as raccoons (Procyon lotor), the determinants of infection in wild animals remain largely unknown. We examined the prevalence of YEZV in invasive raccoons, native tanukis (raccoon dogs, Nyctereutes procyonoides albus), and ticks in six study areas in Hokkaido between 2018 and 2023 to identify ecological factors underlying YEZV infection in wild animals. YEZV RNA fragments were detected in 0.22% of the 1,857 questing ticks. Anti-YEZV antibodies were detected in 32 of the 514 (6.2%) raccoon serum samples and in 5 of the 40 (12.5%) tanuki serum samples. Notably, the seroprevalence in raccoons varied significantly in one of the study areas over the years, that is, 0.0%, 60.0%, and 28.6% in 2021, 2022, and 2023, respectively, implying the temporary emergence of YEZV microfoci. By analyzing the tick load and YEZV seropositivity in raccoons in a field-based setting, we found a positive correlation between adult Ixodes ovatus load and YEZV-antibody positivity, highlighting the importance of I. ovatus in YEZV infection in wild animals. We also explored the environmental and host factors influencing YEZV seropositivity in raccoons and tanukis and found that landscape factors, such as the size of forest area around the trap site, were crucial for YEZV seropositivity in these animals. The significant variables for YEZV seropositivity in raccoons were partially different from those affecting tick infestation intensity in raccoons. The present results extend our understanding of tick-borne virus circulation in the field, emphasizing the unique ecology of the emerging YEZV.

Vectorial dynamics underpinning current and future tick-borne virus emergence in Europe

Tick-borne diseases pose a growing threat to human and animal health in Europe, with tick-borne encephalitis virus (TBEV) and Crimean-Congo haemorrhagic fever virus (CCHFV), vectored by Ixodes ricinus and Hyalomma marginatum, respectively, emerging as primary public health concerns. The ability of ticks to transmit pathogens to multiple hosts and maintain infections across life stages makes them highly efficient vectors. However, many aspects of tick ecology and vectorial capacity remain understudied. This review examines key factors contributing to the vectorial competence of European ticks and their associated viruses. We first explore the influence of climate change on vector and disease ecology, using TBEV and CCHFV as case studies. We then analyse the role of the tick antiviral response in shaping vector competence. By integrating these elements, this review aims to enhance our understanding of tick-borne viral diseases and support the development of public health strategies, particularly through the One Health framework, to mitigate their impact in Europe.

Arthropod-Borne Viruses of Human and Animal Importance: Overwintering in Temperate Regions of Europe during an Era of Climate Change

The past three decades have seen an increasing number of emerging arthropod-borne viruses in temperate regions This process is ongoing, driven by human activities such as inter-continental travel, combined with the parallel emergence of invasive arthropods and an underlying change in climate that can increase the risk of virus transmission and persistence. In addition, natural events such as bird migration can introduce viruses to new regions. Despite the apparent regularity of virus emergence, arthropod-borne viruses circulating in temperate regions face the challenge of the late autumn and winter months where the arthropod vector is inactive. Viruses therefore need mechanisms to overwinter or they will fail to establish in temperate zones. Prolonged survival of arthropod-borne viruses within the environment, outside of both vertebrate host and arthropod vector, is not thought to occur and therefore is unlikely to contribute to overwintering in temperate zones. One potential mechanism is continued infection of a vertebrate host. However, infection is generally acute, with the host either dying or producing an effective immune response that rapidly clears the virus. There are few exceptions to this, although prolonged infection associated with orbiviruses such as bluetongue virus occurs in certain mammals, and viraemic vertebrate hosts therefore can, in certain circumstances, provide a route for long-term viral persistence in the absence of active vectors. Alternatively, a virus can persist in the arthropod vector as a mechanism for overwintering. However, this is entirely dependent on the ecology of the vector itself and can be influenced by changes in the climate during the winter months. This review considers the mechanisms for virus overwintering in several key arthropod vectors in temperate areas. We also consider how this will be influenced in a warming climate.

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.

Ecological and environmental factors affecting the risk of tick-borne encephalitis in Europe, 2017 to 2021

BackgroundTick-borne encephalitis (TBE) is a disease which can lead to severe neurological symptoms, caused by the TBE virus (TBEV). The natural transmission cycle occurs in foci and involves ticks as vectors and several key hosts that act as reservoirs and amplifiers of the infection spread. Recently, the incidence of TBE in Europe has been rising in both endemic and new regions.AimIn this study we want to provide comprehensive understanding of the main ecological and environmental factors that affect TBE spread across Europe.MethodsWe searched available literature on covariates linked with the circulation of TBEV in Europe. We then assessed the best predictors for TBE incidence in 11 European countries by means of statistical regression, using data on human infections provided by the European Surveillance System (TESSy), averaged between 2017 and 2021.ResultsWe retrieved data from 62 full-text articles and identified 31 different covariates associated with TBE occurrence. Finally, we selected eight variables from the best model, including factors linked to vegetation cover, climate, and the presence of tick hosts.DiscussionThe existing literature is heterogeneous, both in study design and covariate types. Here, we summarised and statistically validated the covariates affecting the variability of TBEV across Europe. The analysis of the factors enhancing disease emergence is a fundamental step towards the identification of potential hotspots of viral circulation. Hence, our results can support modelling efforts to estimate the risk of TBEV infections and help decision-makers implement surveillance and prevention campaigns.

Seroprevalence for Borrelia burgdorferi sensu lato and tick-borne encephalitis virus antibodies and associated risk factors among forestry workers in northern France, 2019 to 2020

BackgroundLyme borreliosis (LB) is the most common tick-borne disease (TBD) in France. Forestry workers are at high risk of TBD because of frequent exposure to tick bites.AimWe aimed to estimate the seroprevalence of Borrelia burgdorferi sensu lato and tick-borne encephalitis virus (TBEV) antibodies among forestry workers in northern France. We compared seroprevalence by geographical area and assessed factors associated with seropositivity.MethodsBetween 2019 and 2020, we conducted a randomised cross-sectional seroprevalence survey. Borrelia burgdorferi sl seropositivity was defined as positive ELISA and positive or equivocal result in western blot. Seropositivity for TBEV was defined as positive result from two ELISA tests, confirmed by serum neutralisation. We calculated weighted seroprevalence and adjusted prevalence ratios to determine association between potential risk factors and seropositivity.ResultsA total of 1,778 forestry workers participated. Seroprevalence for B. burgdorferi sl was 15.5% (95% confidence interval (CI): 13.9-17.3), 3.5 times higher in the eastern regions than in the western and increased with seniority and with weekly time in a forest environment. Seroprevalence was 2.5 times higher in forestry workers reporting a tick bite during past years and reporting usually not removing ticks rapidly. Seroprevalence for TBEV was 0.14% (95% CI: 0.05-0.42).ConclusionWe assessed for the first time seroprevalence of B. burgdorferi sl and TBEV antibodies among forestry workers in northern France. These results will be used, together with data on LB and tick-borne encephalitis (TBE) incidence and on exposure to tick-bites, to target prevention programmes.

Virome diversity of Hyalomma dromedarii ticks collected from camels in the United Arab Emirates

Background and Aim: Viruses are important components of the microbiome of ticks. Ticks are capable of transmitting several serious viral diseases to humans and animals. Hitherto, the composition of viral communities in Hyalomma dromedarii ticks associated with camels in the United Arab Emirates (UAE) remains unexplored. This study aimed to characterize the RNA virome diversity in male and female H. dromedarii ticks collected from camels in Al Ain, UAE. Materials and Methods: We collected ticks, extracted, and sequenced RNA, using Illumina (NovaSeq 6000) and Oxford Nanopore (MinION). Results: From the total generated sequencing reads, 180,559 (~0.35%) and 197,801 (~0.34%) reads were identified as virus-related reads in male and female tick samples, respectively. Taxonomic assignment of the viral sequencing reads was accomplished based on bioinformatic analyses. Further, viral reads were classified into 39 viral families. Poxiviridae, Phycodnaviridae, Phenuiviridae, Mimiviridae, and Polydnaviridae were the most abundant families in the tick viromes. Notably, we assembled the genomes of three RNA viruses, which were placed by phylogenetic analyses in clades that included the Bole tick virus. Conclusion: Overall, this study attempts to elucidate the RNA virome of ticks associated with camels in the UAE and the results obtained from this study improve the knowledge of the diversity of viruses in H. dromedarii ticks. Keywords: camels, Hyalomma dromedarii, nanopore technology, UAE, viral diversity, virome analysis, whole genome sequencing.

Seroprevalence of Tick-Borne Encephalitis (TBE) Virus Antibodies in Wild Rodents from Two Natural TBE Foci in Bavaria, Germany

Tick-borne encephalitis (TBE) is Eurasia's most important tick-borne viral disease. Rodents play an important role as natural hosts. Longitudinal studies on the dynamics of the seroprevalence rates in wild rodents in natural foci over the year are rare, and the dynamics of the transmission cycle still need to be understood. To better understand the infection dynamics, rodents were captured in a capture-mark-release-recapture-study in two natural foci in Bavaria, Germany, monthly from March 2019 to October 2022. Overall, 651 blood and thoracic lavage samples from 478 different wild rodents (Clethrionomys glareolus and Apodemus flavicollis) were analyzed for antibodies against tick-borne encephalitis virus (TBEV) by indirect immunofluorescence assay (IIFA) and confirmed using a serum neutralization test (SNT). Furthermore, a generalized linear mixed model (GLMM) analysis was performed to investigate ecological and individual factors for the probability of infection in rodents. Clethrionomys glareolus (19.4%) had a higher seroprevalence than A. flavicollis (10.5%). Within Cl. glareolus, more males (40.4%) than females (15.6%) were affected, and more adults (25.4%) than juveniles (9.8%). The probability of infection of rodents rather depends on factors such as species, sex, and age than on the study site of a natural focus, year, and season. The high incidence rates of rodents, particularly male adult bank voles, highlight their critical role in the transmission cycle of TBEV in a natural focus and demonstrate that serologically positive rodents can be reliably detected in a natural focus regardless of season or year. In addition, these data contribute to a better understanding of the TBEV cycle and thus could improve preventive strategies for human infections.

Tick-Borne Encephalitis Virus Prevalence in Sheep, Wild Boar and Ticks in Belgium

Tick-borne encephalitis virus (TBEV) is the most important tick-borne zoonotic virus in Europe. In Belgium, antibodies to TBEV have already been detected in wildlife and domestic animals, but up-to-date prevalence data for TBEV are lacking, and no studies have assessed its seroprevalence in sheep. Serum samples of 480 sheep from all over Belgium and 831 wild boar hunted in Flanders (northern Belgium) were therefore screened for TBEV antibodies by ELISA and plaque reduction neutralization test (PRNT), respectively. The specificity of positive samples was assessed by PRNTs for TBEV and the Louping Ill, West Nile, and Usutu viruses. TBEV seroprevalence was 0.42% (2/480, CI 95%: 0.11-1.51) in sheep and 9.27% (77/831, CI 95%: 7.48-11.43) in wild boar. TBEV seroprevalence in wild boar from the province of Flemish Brabant was significantly higher (22.38%, 15/67) compared to Limburg (7.74%, 34/439) and Antwerp (8.61%, 28/325). Oud-Heverlee was the hunting area harboring the highest TBEV seroprevalence (33.33%, 11/33). In an attempt to obtain a Belgian TBEV isolate, 1983 ticks collected in areas showing the highest TBEV seroprevalence in wild boars were tested by real-time qPCR. No TBEV-RNA-positive tick was detected. The results of this study suggest an increase in TBEV prevalence over the last decade and highlight the need for One-Health surveillance in Belgium.

A Systematic Review of the Distribution of Tick-Borne Pathogens in Wild Animals and Their Ticks in the Mediterranean Rim between 2000 and 2021

Tick-borne pathogens (TBPs) can be divided into three groups: bacteria, parasites, and viruses. They are transmitted by a wide range of tick species and cause a variety of human, animal, and zoonotic diseases. A total of 148 publications were found on tick-borne pathogens in wild animals, reporting on 85 species of pathogens from 35 tick species and 17 wild animal hosts between 2000 and February 2021. The main TBPs reported were of bacterial origin, including Anaplasma spp. and Rickettsia spp. A total of 72.2% of the TBPs came from infected ticks collected from wild animals. The main tick genus positive for TBPs was Ixodes. This genus was mainly reported in Western Europe, which was the focus of most of the publications (66.9%). It was followed by the Hyalomma genus, which was mainly reported in other areas of the Mediterranean Rim. These TBPs and TBP-positive tick genera were reported to have come from a total of 17 wild animal hosts. The main hosts reported were game mammals such as red deer and wild boars, but small vertebrates such as birds and rodents were also found to be infected. Of the 148 publications, 12.8% investigated publications on Mediterranean islands, and 36.8% of all the TBPs were reported in seven tick genera and 11 wild animal hosts there. The main TBP-positive wild animals and tick genera reported on these islands were birds and Hyalomma spp. Despite the small percentage of publications focusing on ticks, they reveal the importance of islands when monitoring TBPs in wild animals. This is especially true for wild birds, which may disseminate their ticks and TBPs along their migration path.

Potential Mechanisms of Transmission of Tick-Borne Viruses at the Virus-Tick Interface

Ticks (Acari; Ixodidae) are the second most important vector for transmission of pathogens to humans, livestock, and wildlife. Ticks as vectors for viruses have been reported many times over the last 100 years. Tick-borne viruses (TBVs) belong to two orders (Bunyavirales and Mononegavirales) containing nine families (Bunyaviridae, Rhabdoviridae, Asfarviridae, Orthomyxovirida, Reoviridae, Flaviviridae, Phenuviridae, Nyamiviridae, and Nairoviridae). Among these TBVs, some are very pathogenic, causing huge mortality, and hence, deserve to be covered under the umbrella of one health. About 38 viral species are being transmitted by <10% of the tick species of the families Ixodidae and Argasidae. All TBVs are RNA viruses except for the African swine fever virus from the family Asfarviridae. Tick-borne viral diseases have also been classified as an emerging threat to public health and animals, especially in resource-poor communities of the developing world. Tick-host interaction plays an important role in the successful transmission of pathogens. The ticks' salivary glands are the main cellular machinery involved in the uptake, settlement, and multiplication of viruses, which are required for successful transmission into the final host. Furthermore, tick saliva also participates as an augmenting tool during the physiological process of transmission. Tick saliva is an important key element in the successful transmission of pathogens and contains different antimicrobial proteins, e.g., defensin, serine, proteases, and cement protein, which are key players in tick-virus interaction. While tick-virus interaction is a crucial factor in the propagation of tick-borne viral diseases, other factors (physiological, immunological, and gut flora) are also involved. Some immunological factors, e.g., toll-like receptors, scavenger receptors, Janus-kinase (JAK-STAT) pathway, and immunodeficiency (IMD) pathway are involved in tick-virus interaction by helping in virus assembly and acting to increase transmission. Ticks also harbor some endogenous viruses as internal microbial faunas, which also play a significant role in tick-virus interaction. Studies focusing on tick saliva and its role in pathogen transmission, tick feeding, and control of ticks using functional genomics all point toward solutions to this emerging threat. Information regarding tick-virus interaction is somewhat lacking; however, this information is necessary for a complete understanding of transmission TBVs and their persistence in nature. This review encompasses insight into the ecology and vectorial capacity of tick vectors, as well as our current understanding of the predisposing, enabling, precipitating, and reinforcing factors that influence TBV epidemics. The review explores the cellular, biochemical, and immunological tools which ensure and augment successful evading of the ticks' defense systems and transmission of the viruses to the final hosts at the virus-vector interface. The role of functional genomics, proteomics, and metabolomics in profiling tick-virus interaction is also discussed. This review is an initial attempt to comprehensively elaborate on the epidemiological determinants of TBVs with a focus on intra-vector physiological processes involved in the successful execution of the docking, uptake, settlement, replication, and transmission processes of arboviruses. This adds valuable data to the existing bank of knowledge for global stakeholders, policymakers, and the scientific community working to devise appropriate strategies to control ticks and TBVs.

A One-Health Approach to Investigating an Outbreak of Alimentary Tick-Borne Encephalitis in a Non-endemic Area in France (Ain, Eastern France): A Longitudinal Serological Study in Livestock, Detection in Ticks, and the First Tick-Borne Encephalitis Virus Isolation and Molecular Characterisation

Tick-borne encephalitis virus’ (TBEV) geographic range and the human incidence are increasing throughout Europe, putting a number of non-endemic regions and countries at risk of outbreaks. In spring 2020, there was an outbreak of tick-born encephalitis (TBE) in Ain, Eastern France, where the virus had never been detected before. All patients but one had consumed traditional unpasteurised raw goat cheese from a local producer. We conducted an investigation in the suspected farm using an integrative One Health approach. Our methodology included (i) the detection of virus in cheese and milk products, (ii) serological testing of all animals in the suspected farm and surrounding farms, (iii) an analysis of the landscape and localisation of wooded area, (iv) the capture of questing ticks and small mammals for virus detection and estimating enzootic hazard, and (v) virus isolation and genome sequencing. This approach allowed us to confirm the alimentary origin of the TBE outbreak and witness in real-time the seroconversion of recently exposed individuals and excretion of virus in goat milk. In addition, we identified a wooded focus area where and around which there is a risk of TBEV exposure. We provide the first TBEV isolate responsible for the first alimentary-transmitted TBE in France, obtained its full-length genome sequence, and found that it belongs to the European subtype of TBEV. TBEV is now a notifiable human disease in France, which should facilitate surveillance of its incidence and distribution throughout France.

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.