What do we still need to know about Ixodes ricinus?

Multiple factors affecting Ixodes ricinus ticks and associated pathogens in European temperate ecosystems (northeastern France)

In Europe, the main vector of tick-borne zoonoses is Ixodes ricinus, which has three life stages. During their development cycle, ticks take three separate blood meals from a wide variety of vertebrate hosts, during which they can acquire and transmit human pathogens such as Borrelia burgdorferi sensu lato, the causative agent of Lyme borreliosis. In this study conducted in Northeastern France, we studied the importance of soil type, land use, forest stand type, and temporal dynamics on the abundance of ticks and their associated pathogens. Negative binomial regression modeling of the results indicated that limestone-based soils were more favorable to ticks than sandstone-based soils. The highest tick abundance was observed in forests, particularly among coniferous and mixed stands. We identified an effect of habitat time dynamics in forests and in wetlands: recent forests and current wetlands supported more ticks than stable forests and former wetlands, respectively. We observed a close association between tick abundance and the abundance of Cervidae, Leporidae, and birds. The tick-borne pathogens responsible for Lyme borreliosis, anaplasmosis, and hard tick relapsing fever showed specific habitat preferences and associations with specific animal families. Machine learning algorithms identified soil related variables as the best predictors of tick and pathogen abundance.

High-Throughput Sequencing Reveals Three Rhabdoviruses Persisting in the IRE/CTVM19 Cell Line

Cell cultures derived from ticks have become a commonly used tool for the isolation and study of tick-borne pathogens and tick biology. The IRE/CTVM19 cell line, originating from embryos of Ixodes ricinus, is one such line. Previously, reovirus-like particles, as well as sequences with similarity to rhabdoviruses and iflaviruses, were detected in the IRE/CTVM19 cell line, suggesting the presence of multiple persisting viruses. Subsequently, the full genome of an IRE/CTVM19-associated rhabdovirus was recovered from a cell culture during the isolation of the Alongshan virus. In the current work, we used high-throughput sequencing to describe a virome of the IRE/CTVM19 cell line. In addition to the previously detected IRE/CTVM19-associated rhabdovirus, two rhabdoviruses were detected: Chimay rhabdovirus and Norway mononegavirus 1. In the follow-up experiments, we were able to detect both positive and negative RNA strands of the IRE/CTVM19-associated rhabdovirus and Norway mononegavirus 1 in the IRE/CTVM19 cells, suggesting their active replication in the cell line. Passaging attempts in cell lines of mammalian origin failed for all three discovered rhabdoviruses.

Land system governance shapes tick-related public and animal health risks

Land cover and land use have established effects on hazard and exposure to vector-borne diseases. While our understanding of the proximate and distant causes and consequences of land use decisions has evolved, the focus on the proximate effects of landscape on disease ecology remains dominant. We argue that land use governance, viewed through a land system lens, affects tick-borne disease risk. Governance affects land use trajectories and potentially shapes landscapes favourable to ticks or increases contact with ticks by structuring human-land interactions. We illustrate the role of land use legacies, trade-offs in land-use decisions, and social inequities in access to land resources, information and decision-making, with three cases: Kyasanur Forest disease in India, Lyme disease in the Outer Hebrides (Scotland), and tick acaricide resistance in cattle in Ecuador. Land use governance is key to managing the risk of tick-borne diseases, by affecting the hazard and exposure. We propose that land use governance should consider unintended consequences on infectious disease risk.

Recreational hazard: Vegetation and host habitat use correlate with changes in tick-borne disease hazard at infrastructure within forest stands

Studies on density and pathogen prevalence of Ixodes ricinus indicate that vegetation and local host community drive much of their variation between green spaces. Contrarily, micro-geographic variation is understudied, although its understanding could reduce disease risk. We studied the density of infectious nymphal Ixodes sp. ("DIN", proxy for disease hazard), density of questing nymphs ("DON") and nymphal infection prevalence ("NIP") near recreational forest infrastructure. Drag sampling within forest stands and at adjacent benches and trails was combined with vegetation surveys, camera trapping hosts and pathogen screening of ticks. We analysed Borrelia burgdorferi s.l. and its genospecies, with complementary analyses on Rickettsia sp., Anaplasma phagocytophilum, Neoehrlichia mikurensis and Borrelia miyamotoi. DIN was highest in forest interior and at trails enclosed by forest. Lower disease hazard was observed at benches and trails at forest edges. This infrastructure effect can be attributed to variation in vegetation characteristics and the habitat use of tick hosts, specifically roe deer, rodents and songbirds. DON is the main driver of DIN at micro-geographic scale and negatively affected by infrastructure and forest edges. A positive association with vegetation cover in understorey and canopy was observed, as were positive trends for local rodent and songbird abundance. NIP of different pathogens was affected by different drivers. Lower B. burgdorferi s.l. prevalence in the interior of forest stands, driven by its most prevalent genospecies B. afzelii, points towards higher density of uninfected hosts there. B. afzelii was positively associated with understorey containing tall species and with high canopy cover, whereas local bird community composition predicts B. garinii prevalence. A positive effect of songbird abundance and a negative effect of pigeons were observed. Our findings support amplification and inhibition mechanisms within forest stands and highlight that the effect of established drivers of DIN may differ based on the considered spatial scale.

Coinfection of Babesia and Borrelia in the Tick Ixodes ricinus-A Neglected Public Health Issue in Europe?

Ixodes ricinus nymphs and adults removed from humans, and larvae and nymphs from birds, have been analysed for infection with Babesia species and Borrelia species previously in separately published studies. Here, we use the same data set to explore the coinfection pattern of Babesia and Borrelia species in the ticks. We also provide an overview of the ecology and potential public health importance in Sweden of I. ricinus infected both with zoonotic Babesia and Borrelia species. Among 1952 nymphs and adult ticks removed from humans, 3.1% were PCR-positive for Babesia spp. Of these Babesia-positive ticks, 43% were simultaneously Borrelia-positive. Among 1046 immatures of I. ricinus removed from birds, 2.5% were Babesia-positive, of which 38% were coinfected with Borrelia species. This study shows that in I. ricinus infesting humans or birds in Sweden, potentially zoonotic Babesia protozoa sometimes co-occur with human-pathogenic Borrelia spp. Diagnostic tests for Babesia spp. infection are rarely performed in Europe, and the medical significance of this pathogen in Europe could be underestimated.

Bidirectional tick transport by migratory birds of the African-Western Palearctic flyway over Turkish Thrace: observation of the current situation and future projection

This study was carried out at a vital stopover site of migrating birds in the Turkish Thrace, European part of Turkey, on the Mediterranean/Black Sea Flyway. Ticks were collected from the birds captured in the four migration periods, i.e., autumn 2020, spring 2021, autumn 2021, and spring 2022, and identified morphologically. Throughout the study, 10,651 birds from 77 species were examined, and 671 belonging to 34 species were found infested. The infestation prevalence in total birds and the mean number of ticks per infested bird were 6.3% and 3.8 (range: 1-142), respectively. A total of 2573 ticks were collected with the following species distribution and numbers: Ixodes spp. 70 larvae, I. frontalis 1829 larvae, 337 nymphs, and 30 adults, I. acuminatus 16 nymphs and 42 adults, I. ricinus 39 larvae, 141 nymphs, and one adult, Hyalomma spp. seven larvae and 60 nymphs, and Haemaphysalis sp. one larva. Prevalence, intensity, and species distribution of the ticks in birds varied depending on the month, season, year, and species-specific migration phenology of the birds. The results show that precise determination of the tick-borne risk associated with migratory birds for a particular region necessarily requires long-term and comprehensive studies and indicates that anthropogenic climate change and habitat degradation can significantly differentiate the risk by influencing the migration phenology in birds and by making new regions suitable for the establishment of different ticks.

Efficacy of the Vaccine Candidate Based on the P0 Peptide against Dermacentor nitens and Ixodes ricinus Ticks

The control of ticks through vaccination offers a sustainable alternative to the use of chemicals that cause contamination and the selection of resistant tick strains. However, only a limited number of anti-tick vaccines have reached commercial realization. In this sense, an antigen effective against different tick species is a desirable target for developing such vaccines. A peptide derived from the tick P0 protein (pP0) conjugated to a carrier protein has been demonstrated to be effective against the Rhipicephalus microplus, Rhipicephalus sanguineus, and Amblyomma mixtum tick species. The aim of this work was to assess the efficacy of this peptide when conjugated to the Bm86 protein against Dermacentor nitens and Ixodes ricinus ticks. An RNAi experiment using P0 dsRNA from I. ricinus showed a dramatic reduction in the feeding of injected female ticks on guinea pigs. In the follow-up vaccination experiments, rabbits were immunized with the pP0-Bm86 conjugate and challenged simultaneously with larvae, nymphs, and the adults of I. ricinus ticks. In the same way, horses were immunized with the pP0-Bm86 conjugate and challenged with D. nitens larva. The pP0-Bm86 conjugate showed efficacies of 63% and 55% against I. ricinus and D. nitens ticks, respectively. These results, combined with previous reports of efficacy for this conjugate, show the promising potential for its development as a broad-spectrum anti-tick vaccine.

Can Tick Microbiome Explain Nonlinear Relationship between Tick Abundance and Lyme Disease Incidence?

Ticks (Acari: Ixodida) are hematophagous ectoparasitic arachnids that feed on the blood of vertebrate hosts, posing significant concern due to their unrivaled capacity to transmit various pathogens, which surpasses those of all other known arthropod vectors [...].

Genetic analysis challenges the presence of Ixodes inopinatus in Central Europe: development of a multiplex PCR to distinguish I. inopinatus from I. ricinus

BACKGROUND

Ixodes ricinus is an important vector of several pathogens, primarily in Europe. Recently, Ixodes inopinatus was described from Spain, Portugal, and North Africa and then reported from several European countries. In this study, a multiplex polymerase chain reaction (PCR) was developed to distinguish I. ricinus from I. inopinatus and used in the surveillance of I. inopinatus in Algeria (ALG) and three regions in the Czech Republic (CZ).

METHODS

A multiplex PCR on TROSPA and sequencing of several mitochondrial (16S rDNA, COI) and nuclear markers (TROSPA, ITS2, calreticulin) were used to differentiate these two species and for a subsequent phylogenetic analysis.

RESULTS

Sequencing of TROSPA, COI, and ITS2 separated these two species into two subclades, while 16S rDNA and calreticulin could not distinguish I. ricinus from I. inopinatus. Interestingly, 23 nucleotide positions in the TROSPA gene had consistently double peaks in a subset of ticks from CZ. Cloning of these PCR products led to a clear separation of I. ricinus and I. inopinatus indicating hybridization and introgression between these two tick taxa. Based on a multiplex PCR of TROSPA and analysis of sequences of TROSPA, COI, and ITS2, the majority of ticks in CZ were I. ricinus, no I. inopinatus ticks were found, and 10 specimens showed signs of hybridization. In contrast, most ticks in ALG were I. inopinatus, four ticks were I. ricinus, and no signs of hybridization and introgression were detected.

CONCLUSIONS

We developed a multiplex PCR method based on the TROSPA gene to differentiate I. ricinus and I. inopinatus. We demonstrate the lack of evidence for the presence of I. inopinatus in Central Europe and propose that previous studies be re-examined. Mitochondrial markers are not suitable for distinguishing I. inopinatus from I. ricinus. Furthermore, our data indicate that I. inopinatus and I. ricinus can hybridize, and the hybrids can survive in Europe.

Detection of the Crimean-Congo Hemorrhagic Fever Virus Genome in Questing Ixodes spp. and Haemaphysalis spp. in the Periurban Forestry Areas of Istanbul: Has a New Biorisk Emerged?

Background: Istanbul is one of the world's most densely populated metropolitan cities, with various geographical areas that possess distinct characteristics. These areas have different climates, vegetation, and host populations that can support the survival of diverse tick species. Turkey is a region with a high risk of Crimean-Congo hemorrhagic fever (CCHF) disease, making it crucial to screen for this risk, especially in densely populated regions such as Istanbul and its surrounding areas. However, the presence of potential vectors for CCHF virus (CCHFV) in these areas has not been studied in the past 12 years. Materials and Methods: In this study, a total of 676 ticks were collected from 6 centers using the flagging and dragging method. Ticks were identified as Ixodes ricinus (7.85%), Ixodes spp. immature form (73.22%), Haemaphysalis parva (0.89%), Haemaphysalis inermis (0.15%), and Haemaphysalis spp. immature form (17.89%). Pooled samples were screened for the CCHFV genome (S segment) by RT-nested PCR. Results: A total of 14.28% of the samples were found as positive. Phylogenetic analysis revealed that all the CCHFV sequences obtained from the positive samples were clustered in clade V: Europe/Turkey genogroup. Conclusion: This study suggests that ixodes spp. and Haemaphysalis spp. may have the potential to pose a biorisk for Crimean-Congo hemorrhagic fever.

Tick Activity, Host Range, and Tick-Borne Pathogen Prevalence in Mountain Habitats of the Western Carpathians, Poland

In mountainous regions, diverse ecosystems provide a habitat for numerous species of organisms. In this study, we focused on ixodid ticks and their presence in the Western Carpathians, Poland. Our objectives were to investigate the impact of environmental factors on tick occurrence and activity, the prevalence of vectored pathogens, and tick hosts, and their role as reservoir organisms for tick-borne pathogens (TBPs). To this end, we collected ticks from the vegetation and from animals (Apodemus agrarius, A. flavicollis, Capreolus capreolus, Microtus spp., Myodes glareolus, Ovis aries). In addition, we collected blood samples from rodents. The collected material underwent molecular analysis, utilizing the high-throughput microfluidic real-time PCR technique, to detect the presence of TBPs. Our findings confirmed the occurrence of only two species of ixodid ticks in the study area: the dominant Ixodes ricinus, and Dermacentor reticulatus with very limited abundance. Temperature significantly influenced tick activity, and the number of I. ricinus nymphs varied with altitude. We also observed a circadian pattern of questing activity in I. ricinus ticks. The main hosts for juvenile tick stages were M. glareolus and A. agrarius, while adult stages were frequently found on C. capreolus. I. ricinus ticks collected from the vegetation were often infected with Rickettsia helvetica (up to 35.71%), Borrelia afzelii (up to 28.57%), and Ehrlichia spp. (up to 9.52%). In contrast, juvenile stages frequently carried Bartonella spp. (up to 10.00%), Mycoplasma spp. (up to 16.67%) and R. helvetica (up to 16.67%). Moreover, we detected genetic material of Mycoplasma spp. (up to 100.00%), Ehrlichia spp. (up to 35.71%), Bartonella spp. (up to 25.00%), and Borrelia spp. (up to 6.25%) in rodent blood samples. The obtained results indicate A. agrarius and M. glareolus as reservoir animals for TBPs in the studied region.

Wild ungulates as sentinels of flaviviruses and tick-borne zoonotic pathogen circulation: an Italian perspective

BACKGROUND

Vector-borne zoonotic diseases are a concerning issue in Europe. Lyme disease and tick-borne encephalitis virus (TBEV) have been reported in several countries with a large impact on public health; other emerging pathogens, such as Rickettsiales, and mosquito-borne flaviviruses have been increasingly reported. All these pathogens are linked to wild ungulates playing roles as tick feeders, spreaders, and sentinels for pathogen circulation. This study evaluated the prevalence of TBEV, Borrelia burgdorferi sensu lato, Rickettsia spp., Ehrlichia spp., and Coxiella spp. by biomolecular screening of blood samples and ticks collected from wild ungulates. Ungulates were also screened by ELISA and virus neutralization tests for flaviviral antibody detection.

RESULTS

A total of 274 blood samples were collected from several wild ungulate species, as well as 406 Ixodes ricinus, which were feeding on them. Blood samples tested positive for B. burgdorferi s.l. (1.1%; 0-2.3%) and Rickettsia spp. (1.1%; 0-2.3%) and showed an overall flaviviral seroprevalence of 30.6% (22.1-39.2%): 26.1% (17.9-34.3%) for TBEV, 3.6% (0.1-7.1%) for Usutu virus and 0.9% (0-2.7%) for West Nile virus. Ticks were pooled when possible and yielded 331 tick samples that tested positive for B. burgdorferi s.l. (8.8%; 5.8-11.8%), Rickettsia spp. (26.6%; 21.8-31.2%) and Neoehrlichia mikurensis (1.2%; 0-2.4%). TBEV and Coxiella spp. were not detected in either blood or tick samples.

CONCLUSIONS

This research highlighted a high prevalence of several tick-borne zoonotic pathogens and high seroprevalence for flaviviruses in both hilly and alpine areas. For the first time, an alpine chamois tested positive for anti-TBEV antibodies. Ungulate species are of particular interest due to their sentinel role in flavivirus circulation and their indirect role in tick-borne diseases and maintenance as Ixodes feeders and spreaders.

Tick populations and molecular detection of selected tick-borne pathogens in questing ticks from northern and central Tanzania

Ticks are vectors and reservoirs of a variety of pathogens including protozoa, bacteria and viruses which cause tick-borne diseases (TBDs) in humans and livestock. TBDs pose serious constraints to the improvement of livestock production in tropical and subtropical regions of the world. Despite their wide distribution, information on the tick and pathogen relationship is scarce in Tanzania. We used nested PCR and sequencing to screen pathogens of public and veterinary health importance in ticks collected by flagging from four districts of Tanzania. In total, 2021 ticks comprising nine species were identified. DNA from ticks was pooled according to tick species, developmental stage, and location, then screened for Babesia bigemina, Babesia bovis, Theileria parva and Coxiella burnetii. Out of 377 pools, 34.7% were positive for at least one pathogen. Theileria parva was the most abundant with a minimum infection rate (MIR) of 2.8%, followed by B. bigemina (MIR = 1.8%) and B. bovis (MIR = 0.8%). Multiple pathogens detection was observed in 7.2% of the tested pools. However, PCR screening of individual tick DNA revealed that only 0.3% of the examined pools had co-infection. DNA of C. burnetii was never detected in any tick DNA pool. The MIR of tick-borne pathogens (TBPs) differed significantly among districts, seasons, tick species, and tick developmental stages. Sequence analysis showed that B. bigemina RAP-1a, B. bovis SBP-4, and T. parva p104 genes were conserved among pathogens in the four districts. Despite the absence of C. burnetii in ticks, considering its pathogenic potential, it is essential to continue monitoring for its possible recurrence in ticks. This information adds to the knowledge of TBPs epidemiology and will contribute to the scientific basis for planning future control strategies.

Spotted Fever Group Rickettsiae in Ticks and Small Mammals from Grassland and Forest Habitats in Central Germany

Rickettsiae of the spotted fever group (SFG) are zoonotic tick-borne pathogens. Small mammals are important hosts for the immature life stages of two of the most common tick species in Europe, Ixodes ricinus and Dermacentor reticulatus. These hosts and vectors can be found in diverse habitats with different vegetation types like grasslands and forests. To investigate the influence of environmental and individual factors on Rickettsia prevalence, this study aimed to analyse the prevalence of SFG rickettsiae in ticks and small mammals in different small-scale habitats in central Germany for the first time. Small mammals of ten species and ticks of two species were collected from grasslands and forests in the Hainich-Dün region, central Germany. After species identification, DNA samples from 1098 ticks and ear snips of 1167 small mammals were screened for Rickettsia DNA by qPCR targeting the gltA gene. Positive samples were retested by conventional PCR targeting the ompB gene and sequencing. Rickettsia DNA was detected in eight out of ten small mammal species. Small mammal hosts from forests (14.0%) were significantly more often infected than those from grasslands (4.4%) (p < 0.001). The highest prevalence was found in the mostly forest-inhabiting genus Apodemus (14.8%) and the lowest in Microtus (6.6%), which inhabits grasslands. The prevalence was higher in D. reticulatus (46.3%) than in the I. ricinus complex (8.6%). Adult ticks were more often infected than nymphs (p = 0.0199). All sequenced rickettsiae in I. ricinus complex ticks were R. helvetica, and the ones in D. reticulatus were R. raoultii. Unlike adults, questing nymphs have had only one blood meal, which explains the higher prevalence in I. ricinus adults. Interestingly, habitat type did influence infection probability in small mammals, but did not in ticks. A possible explanation may be the high prevalence in Apodemus flavicollis and A. sylvaticus which were more abundant in the forest.

Surveillance and Risk Analysis for Bovine Babesiosis in England and Wales to Inform Disease Distribution

Babesia divergens is a zoonotic piroplasm that infects both cattle and humans in Europe. Disease transmission occurs through Ixodes ricinus tick bites, a species that is increasing in abundance and distribution across Europe in response to climate and land-use changes. Developments in agri-environment policy and changing consumer demands may also have unintended consequences on tick-borne disease rates. Currently, B. divergens surveillance in British cattle is limited, rendering temporal trend analysis and the detection of potential zoonotic hotspots impossible. The objective of this study was to assess syndromic surveillance as a means of determining babesiosis distribution in British cattle, and to evaluate the intrinsic disease risk factors in order to respond to disease threats posed by changing environments. Samples from 95 clinically affected cattle on 70 unique holdings were screened for Babesia spp., using established blood smear examination techniques and a B. divergens-specific PCR method, between April and December 2021. B. divergens was detected in 45/95 animals (47.4%), with PCR offering the advantage of identification at species level. Infection with Anaplasma phagocytophilum was detected in 19/95 animals (20%). Co-infection was detected in five animals. The cases were recorded across multiple geographic regions and throughout the sampling period. Univariate logistic regression analysis failed to identify any statistically significant risk factors for B. divergens presence. This study demonstrates that bovine babesiosis is geographically widespread throughout England and Wales, placing a large proportion of the cattle population at risk of infection, with the potential for zoonotic transmission to humans.

Universal Tick Vaccines: Candidates and Remaining Challenges

Recent advancements in molecular biology, particularly regarding massively parallel sequencing technologies, have enabled scientists to gain more insight into the physiology of ticks. While there has been progress in identifying tick proteins and the pathways they are involved in, the specificities of tick-host interaction at the molecular level are not yet fully understood. Indeed, the development of effective commercial tick vaccines has been slower than expected. While omics studies have pointed to some potential vaccine immunogens, selecting suitable antigens for a multi-antigenic vaccine is very complex due to the participation of redundant molecules in biological pathways. The expansion of ticks and their pathogens into new territories and exposure to new hosts makes it necessary to evaluate vaccine efficacy in unusual and non-domestic host species. This situation makes ticks and tick-borne diseases an increasing threat to animal and human health globally, demanding an urgent availability of vaccines against multiple tick species and their pathogens. This review discusses the challenges and advancements in the search for universal tick vaccines, including promising new antigen candidates, and indicates future directions in this crucial research field.

Biodiversity in the Lyme-light: ecological restoration and tick-borne diseases in Europe

Biodiversity loss and the emergence of zoonotic diseases are two major global challenges. An urgent question is how ecosystems and wildlife communities can be restored whilst minimizing the risk of zoonotic diseases carried by wildlife. Here, we evaluate how current ambitions to restore Europe's natural ecosystems may affect the hazard of diseases vectored by the tick Ixodes ricinus at different scales. We find that effects of restoration efforts on tick abundance are relatively straightforward but that the interacting effects of vertebrate diversity and abundance on pathogen transmission are insufficiently known. Long-term integrated surveillance of wildlife communities, ticks, and their pathogens is needed to understand their interactions and to prevent nature restoration from increasing tick-borne disease (TBD) hazard.

The biology of Ixodes ricinus with emphasis on its ecology

Prior to its identification as the vector of Lyme borreliosis spirochaetes in Europe in 1983, interest in Ixodes ricinus (L.) was moderate and mainly concerned the transmission of pathogens to farm animals and of tick-borne encephalitis virus to humans. The situation now is very different, and more papers have been published on I. ricinus than on any other ixodid tick species. However, this large literature is scattered and in recent years has become dominated by the molecular detection and characterization of the many pathogens that I. ricinus transmits. Several decades have now elapsed since a review addressing its basic biology and ecology appeared, and the present publication seeks to present basic aspects of its biology and ecology that are related to its role as a vector of disease agents, including its life cycle, feeding behaviour, host relations, survival off the host, and the impact of weather and climate.

The emergence and shift in seasonality of Lyme borreliosis in Northern Europe

Climate change has had a major impact on seasonal weather patterns, resulting in marked phenological changes in a wide range of taxa. However, empirical studies of how changes in seasonality impact the emergence and seasonal dynamics of vector-borne diseases have been limited. Lyme borreliosis, a bacterial infection spread by hard-bodied ticks, is the most common vector-borne disease in the northern hemisphere and has been rapidly increasing in both incidence and geographical distribution in many regions of Europe and North America. By analysis of long-term surveillance data (1995-2019) from across Norway (latitude 57°58'-71°08' N), we demonstrate a marked change in the within-year timing of Lyme borreliosis cases accompanying an increase in the annual number of cases. The seasonal peak in cases is now six weeks earlier than 25 years ago, exceeding seasonal shifts in plant phenology and previous model predictions. The seasonal shift occurred predominantly in the first 10 years of the study period. The concurrent upsurgence in case number and shift in case timing indicate a major change in the Lyme borreliosis disease system over recent decades. This study highlights the potential for climate change to shape the seasonal dynamics of vector-borne disease systems.

Revealing the Tick Microbiome: Insights into Midgut and Salivary Gland Microbiota of Female Ixodes ricinus Ticks

The ectoparasite Ixodes ricinus is an important vector for many tick-borne diseases (TBD) in the northern hemisphere, such as Lyme borreliosis, rickettsiosis, human granulocytic anaplasmosis, or tick-borne encephalitis virus. As climate change will lead to rising temperatures in the next years, we expect an increase in tick activity, tick population, and thus in the spread of TBD. Consequently, it has never been more critical to understand relationships within the microbial communities in ticks that might contribute to the tick's fitness and the occurrence of TBD. Therefore, we analyzed the microbiota in different tick tissues such as midgut, salivary glands, and residual tick material, as well as the microbiota in complete Ixodes ricinus ticks using 16S rRNA gene amplicon sequencing. By using a newly developed DNA extraction protocol for tick tissue samples and a self-designed mock community, we were able to detect endosymbionts and pathogens that have been described in the literature previously. Further, this study displayed the usefulness of including a mock community during bioinformatic analysis to identify essential bacteria within the tick.

Factors responsible for Ixodes ricinus presence and abundance across a natural-urban gradient

To better understand the spatial distribution of the common tick Ixodes ricinus, we investigated how local site factors and landscape characteristics influence tick presence and abundance in different greenspaces along the natural-urban gradient in Stockholm County, Sweden. Ticks and field data were collected in 2017 and 2019 and analyzed in relation to habitat type distributions estimated from land cover maps using geographical information system (GIS). A total of 1378 (992 larvae, 370 nymphs, 13 females, and 3 males) questing ticks were collected from 295 sampling plots in 47 different greenspaces. Ticks were present in 41 of the 47 greenspaces and our results show that both local site features such as vegetation height, and landscape characteristics like the amount of mixed coniferous forest, significantly affect tick abundance. Tick abundance was highest in rural areas with large natural and seminatural habitats, but ticks were also present in parks and gardens in highly urbanized areas. Greenspaces along the natural-urban gradient should be included in surveillance for ticks and tick-borne diseases, including highly urbanized sites that may be perceived by the public as areas with low risk for tick encounters.

A Retrospective Study with a Commercial Vaccine against Lyme Borreliosis in Dogs Using Two Different Vaccination Schedules: Characterization of the Humoral Immune Response

Lyme borreliosis, a multisystemic disease caused by spirochetes of the genus Borrelia, is the most common tick-borne disease in the northern hemisphere. Differently from human medicine, several vaccines are available for dogs. To provide the best protection possible, vaccination schemes should be adapted regularly to meet the needs resulting from an increased tick exposure risk due to an inescapable climate change. In this retrospective study, a total of 183 vaccinations were performed with a commercial, multivalent vaccine against Lyme borreliosis, and vaccinated dogs were monitored over an observation period of 13 months. Dogs were either vaccinated on days 0 and 21 and a booster on day 365 (standard vaccination schedule), or with an additional booster vaccination on day 180. Canine serum samples were then tested for their borrelia-specific antibody levels using a two-tiered test system consisting of a kinetic ELISA followed by a line immunoassay. Dogs vaccinated with the standard vaccination schedule displayed decreasing antibody levels between days 120 and 360, which is probably insufficient to prevent an infection with borreliae. In contrast, the additional booster vaccination received on day 180 intercepts this decline in antibody levels between days 225 and 360, providing a sufficient immunity to prevent infection. The results from this retrospective study allow us to recommend a basic vaccination schedule with an additional booster vaccination on day 180 to ensure the best possible protection for dogs against Lyme borreliosis.

What do we know about the microbiome of I. ricinus?

I. ricinus is an obligate hematophagous parasitic arthropod that is responsible for the transmission of a wide range of zoonotic pathogens including spirochetes of the genus Borrelia, Rickettsia spp., C. burnetii, Anaplasma phagocytophilum and Francisella tularensis, which are part the tick´s microbiome. Most of the studies focus on "pathogens" and only very few elucidate the role of "non-pathogenic" symbiotic microorganisms in I. ricinus. While most of the members of the microbiome are leading an intracellular lifestyle, they are able to complement tick´s nutrition and stress response having a great impact on tick´s survival and transmission of pathogens. The composition of the tick´s microbiome is not consistent and can be tied to the environment, tick species, developmental stage, or specific organ or tissue. Ovarian tissue harbors a stable microbiome consisting mainly but not exclusively of endosymbiotic bacteria, while the microbiome of the digestive system is rather unstable, and together with salivary glands, is mostly comprised of pathogens. The most prevalent endosymbionts found in ticks are Rickettsia spp., Ricketsiella spp., Coxiella-like and Francisella-like endosymbionts, Spiroplasma spp. and Candidatus Midichloria spp. Since microorganisms can modify ticks' behavior, such as mobility, feeding or saliva production, which results in increased survival rates, we aimed to elucidate the potential, tight relationship, and interaction between bacteria of the I. ricinus microbiome. Here we show that endosymbionts including Coxiella-like spp., can provide I. ricinus with different types of vitamin B (B2, B6, B7, B9) essential for eukaryotic organisms. Furthermore, we hypothesize that survival of Wolbachia spp., or the bacterial pathogen A. phagocytophilum can be supported by the tick itself since coinfection with symbiotic Spiroplasma ixodetis provides I. ricinus with complete metabolic pathway of folate biosynthesis necessary for DNA synthesis and cell division. Manipulation of tick´s endosymbiotic microbiome could present a perspective way of I. ricinus control and regulation of spread of emerging bacterial pathogens.

Survival in the understorey: Testing direct and indirect effects of microclimatological changes on Ixodes ricinus

The distribution of ticks in the Ixodes ricinus species complex is partly driven by climate, with temperature and relative humidity affecting survival. These variables are driven by macroclimate, but vary locally due to microclimate buffering. This buffering has been suggested to be one of the driving forces behind variation in tick survival and density in time and space. In order to understand the role of the herb layer with respect to this variation, we deployed I. ricinus within an existing experimental setup studying the response of forest understorey to micrometeorological changes. This allowed for the analysis of both direct effects of warming on tick survival in controlled field conditions, as well as indirect effects through changes in herb layer biomass. Herb layer biomass estimates were observed to be higher in plots that had been experimentally warmed, with a trend towards higher survival in these warmed plots. This marginal increase in survival rate may be due to increased microclimate buffering. Comparing our results to literature implies that canopy and shrub layer vegetation have a larger effect on climate buffering, and therefore also on tick survival. Since the herb layer biomass is expected to increase due to global warming and increased frequency of disturbance-induced canopy gaps, survival in forested habitats may increase in the future. This would increase the difference in survival compared to that in open habitats.

Recreation reduces tick density through fine-scale risk effects on deer space-use

Altered interactions between pathogens, their hosts and vectors have potential consequences for human disease risk. Notably, tick-borne pathogens, many of which are associated with growing deer abundance, show global increasing prevalence and pose increasing challenges for disease prevention. Human activities can largely affect the patterns of deer space-use and can therefore be potential management tools to alleviate human-wildlife conflicts. Here, we tested how deer space-use patterns are influenced by human recreational activities, and how this in turn affects the spatial distribution of the sheep tick (Ixodes ricinus), a relevant disease vector of zoonoses such as Lyme borrelioses. We compared deer dropping and questing tick density on transects near (20 m) and further away from (100 m) forest trails that were either frequently used (open for recreation) or infrequently used (closed for recreation, but used by park managers). In contrast to infrequently used trails, deer dropping density was 31% lower near (20 m) than further away from (100 m) frequently used trails. Similarly, ticks were 62% less abundant near (20 m) frequently used trails compared to further away from (100 m) these trails, while this decline in tick numbers was only 14% near infrequently used trails. The avoidance by deer of areas close to human-used trails was thus associated with a similar reduction in questing tick density near these trails. As tick abundance generally correlates to pathogen prevalence, the use of trails for recreation may reduce tick-borne disease risk for humans on and near these trails. Our study reveals an unexplored effect of human activities on ecosystems and how this knowledge could be potentially used to mitigate zoonotic disease risk.

Diversity of Tick-Borne Pathogens in Tick Larvae Feeding on Breeding Birds in France

Birds play a role in maintaining tick-borne diseases by contributing to the multiplication of ticks and pathogens on a local scale during the breeding season. In the present study, we describe the diversity of tick and pathogen species of medical and veterinary importance in Europe hosted by 1040 captured birds (56 species) during their breeding season in France. Of the 3114 ticks collected, Ixodes ricinus was the most prevalent species (89.5%), followed by I. frontalis (0.8%), I. arboricola (0.7%), Haemaphysalis concinna (0.5%), H. punctata (0.5%), Hyalomma spp. (0.2%), and Rhipicephalus spp. (0.06%). Because they may be representative of the bird infection status for some pathogen species, 1106 engorged tick larvae were screened for pathogens. Borrelia burgdorferi sensu lato was the most prevalent pathogen genus in bird-feeding larvae (11.7%), followed by Rickettsia spp. (7.4%), Anaplasma spp. (5.7%), Babesia spp. (2.3%), Ehrlichia spp. (1.4%), and B. miyamotoi (1%). Turdidae birds (Turdus merula and T. philomelos), Troglodytes troglodytes, and Anthus trivialis had a significantly higher prevalence of B. burgdorferi s.l.-infected larvae than other pathogen genera. This suggests that these bird species could act as reservoir hosts for B. burgdorferi s.l. during their breeding season, and thus play an important role in acarological risk.

New Cell Lines Derived from European Tick Species

Tick cell lines are important tools for research on ticks and the pathogens they transmit. Here, we report the establishment of ten new cell lines from European ticks of the genera Argas, Dermacentor, Hyalomma, Ixodes and Rhipicephalus originating from Germany and Spain. For each cell line, the method used to generate the primary culture, a morphological description of the cells and species confirmation by sequencing of the partial 16S rRNA gene are presented. Further molecular analysis of the two new Ixodes ricinus cell lines and three existing cell lines of the same species revealed genetic variation between cell lines derived from ticks collected in the same or nearby locations. Collectively, these new cell lines will support research into a wide range of viral, bacterial and protozoal tick-borne diseases prevalent in Europe.

One Health Approach to Tick and Tick-Borne Disease Surveillance in the United Kingdom

Where ticks are found, tick-borne diseases can present a threat to human and animal health. The aetiology of many of these important diseases, including Lyme disease, bovine babesiosis, tick-borne fever and louping ill, have been known for decades whilst others have only recently been documented in the United Kingdom (UK). Further threats such as the importation of exotic ticks through human activity or bird migration, combined with changes to either the habitat or climate could increase the risk of tick-borne disease persistence and transmission. Prevention of tick-borne diseases for the human population and animals (both livestock and companion) is dependent on a thorough understanding of where and when pathogen transmission occurs. This information can only be gained through surveillance that seeks to identify where tick populations are distributed, which pathogens are present within those populations, and the periods of the year when ticks are active. To achieve this, a variety of approaches can be applied to enhance knowledge utilising a diverse range of stakeholders (public health professionals and veterinarians through to citizen scientists). Without this information, the application of mitigation strategies to reduce pathogen transmission and impact is compromised and the ability to monitor the effects of climate change or landscape modification on the risk of tick-borne disease is more challenging. However, as with many public and animal health interventions, there needs to be a cost-benefit assessment on the most appropriate intervention applied. This review will assess the challenges of tick-borne diseases in the UK and argue for a cross-disciplinary approach to their surveillance and control.

Model-based extrapolation of ecological systems under future climate scenarios: The example of Ixodes ricinus ticks

Models can be applied to extrapolate consequences of climate change for complex ecological systems in the future. The acknowledged systems' behaviour at present is projected into the future considering climate projection data. Such an approach can be used to addresses the future activity and density of the castor bean tick Ixodes ricinus, the most widespread tick species in Europe. It is an important vector of pathogens causing Lyme borreliosis and tick-borne encephalitis. The population dynamics depend on several biotic and abiotic factors. Such complexity makes it difficult to predict the future dynamics and density of I. ricinus and associated health risk for humans. The objective of this study is to force ecological models with high-resolution climate projection data to extrapolate I. ricinus tick density and activity patterns into the future. We used climate projection data of temperature, precipitation, and relative humidity for the period 1971-2099 from 15 different climate models. Tick activity was investigated using a climate-driven cohort-based population model. We simulated the seasonal population dynamics using climate data between 1971 and 2099 and observed weather data since 1949 at a specific location in southern Germany. We evaluated derived quantities of local tick ecology, e.g. the maximum questing activity of the nymphal stage. Furthermore, we predicted spatial density changes by extrapolating a German-wide tick density model. We compared the tick density of the reference period (1971-2000) with the counter-factual densities under the near-term scenario (2012-2041), mid-term scenario (2050-2079) and long-term scenario (2070-2099). We found that the nymphal questing peak would shift towards early seasons of the year. Also, we found high spatial heterogeneity across Germany, with predicted hotspots of up to 2,000 nymphs per 100 m2 and coldspots with constant density. As our results suggest extreme changes in tick behaviour and density, we discuss why caution is needed when extrapolating climate data-driven models into the distant future when data on future climate drive the model projection.

An eco-epidemiological modeling approach to investigate dilution effect in two different tick-borne pathosystems

Disease (re)emergence appears to be driven by biodiversity decline and environmental change. As a result, it is increasingly important to study host-pathogen interactions within the context of their ecology and evolution. The dilution effect is the concept that higher biodiversity decreases pathogen transmission. It has been observed especially in zoonotic vector-borne pathosystems, yet evidence against it has been found. In particular, it is still debated how the community (dis)assembly assumptions and the degree of generalism of vectors and pathogens affect the direction of the biodiversity-pathogen transmission relationship. The aim of this study was to use empirical data and mechanistic models to investigate dilution mechanisms in two rodent-tick-pathogen systems differing in their vector degree of generalism. A community was assembled to include ecological interactions that expand from purely additive to purely substitutive. Such systems are excellent candidates to analyze the link between vector ecology, community (dis)assembly dynamics, and pathogen transmission. To base our mechanistic models on empirical data, rodent live-trapping, including tick sampling, was conducted in Wales across two seasons for three consecutive years. We have developed a deterministic single-vector, multi-host compartmental model that includes ecological relationships with non-host species, uniquely integrating theoretical and observational approaches. To describe pathogen transmission across a gradient of community diversity, the model was populated with parameters describing five different scenarios differing in ecological complexity; each based around one of the pathosystems: Ixodes ricinus (generalist tick)-Borrelia burgdorferi and I. trianguliceps (small mammals specialist tick)-Babesia microti. The results suggested that community composition and interspecific dynamics affected pathogen transmission with different dilution outcomes depending on the vector degree of generalism. The model provides evidence that dilution and amplification effects are not mutually exclusive in the same community but depend on vector ecology and the epidemiological output considered (i.e., the "risk" of interest). In our scenarios, more functionally diverse communities resulted in fewer infectious rodents, supporting the dilution effect. In the pathosystem with generalist vector we identified a hump shaped relationship between diversity and infections in hosts, while for that characterized by specialist tick, this relationship was more complex and more dependent upon specific parameter values.

Incidence of Tick-Borne Encephalitis during the COVID-19 Pandemic in Selected European Countries

Ixodes ricinus ticks are one of the most important vectors and reservoirs of infectious diseases in Europe, and tick-borne encephalitis (TBE) is one of the most dangerous human diseases transmitted by these vectors. The aim of the present study was to investigate the TBE incidence in some European countries during the COVID-19 pandemic. To this end, we analyzed the data published by the European Center for Disease Prevention and Control (ECDC) and Eurostat on the number of reported TBE and COVID-19 cases in 2020 and TBE cases in 2015–2019 (reference period). Significant differences in the TBE incidence were found between the analyzed countries. The highest TBE incidence was found in Lithuania (25.45/100,000 inhabitants). A high TBE incidence was also observed in Central European countries. In 12 of the 23 analyzed countries, there was significant increase in TBE incidence during the COVID-19 pandemic during 2020 compared to 2015–2019. There was no correlation between the incidence of COVID-19 and TBE and between the availability of medical personnel and TBE incidence in the studied countries. In conclusion, Central Europe and the Baltic countries are areas with a high risk of TBE infection. Despite the COVID-19 pandemic and imposed restrictions, the incidence of TBE is increasing in more than half of the analyzed countries.

Beech tree masting explains the inter-annual variation in the fall and spring peaks of Ixodes ricinus ticks with different time lags

BACKGROUND

The tick Ixodes ricinus is an important vector of tick-borne diseases including Lyme borreliosis. In continental Europe, the nymphal stage of I. ricinus often has a bimodal phenology with a large spring peak and a smaller fall peak. There is consensus about the origin of the spring nymphal peak, but there are two alternative hypotheses for the fall nymphal peak. In the direct development hypothesis, larvae quest as nymphs in the fall of the same year that they obtained their larval blood meal. In the developmental diapause hypothesis, larvae overwinter in the engorged state and quest as nymphs one year after they obtained their larval blood meal. These two hypotheses make different predictions about the time lags that separate the larval blood meal and the density of questing nymphs (DON) in the spring and fall.

METHODS

Inter-annual variation in seed production (masting) by deciduous trees is a time-lagged index for the density of vertebrate hosts (e.g., rodents) which provide blood meals for larval ticks. We used a long-term data set on the masting of the European beech tree and a 15-year study on the DON at 4 different elevation sites in western Switzerland to differentiate between the two alternative hypotheses for the origin of the fall nymphal peak.

RESULTS

Questing I. ricinus nymphs had a bimodal phenology at the three lower elevation sites, but a unimodal phenology at the top elevation site. At the lower elevation sites, the DON in the fall was strongly correlated with the DON in the spring of the following year. The inter-annual variation in the densities of I. ricinus nymphs in the fall and spring was best explained by a 1-year versus a 2-year time lag with the beech tree masting index. Fall nymphs had higher fat content than spring nymphs indicating that they were younger. All these observations are consistent with the direct development hypothesis for the fall peak of I. ricinus nymphs at our study site. Our study provides new insight into the complex bimodal phenology of this important disease vector.

CONCLUSIONS

Public health officials in Europe should be aware that following a strong mast year, the DON will increase 1 year later in the fall and 2 years later in the spring. Studies of I. ricinus populations with a bimodal phenology should consider that the spring and fall peak in the same calendar year represent different generations of ticks.

Associating Land Cover Changes with Patterns of Incidences of Climate-Sensitive Infections: An Example on Tick-Borne Diseases in the Nordic Area

Some of the climate-sensitive infections (CSIs) affecting humans are zoonotic vector-borne diseases, such as Lyme borreliosis (BOR) and tick-borne encephalitis (TBE), mostly linked to various species of ticks as vectors. Due to climate change, the geographical distribution of tick species, their hosts, and the prevalence of pathogens are likely to change. A recent increase in human incidences of these CSIs in the Nordic regions might indicate an expansion of the range of ticks and hosts, with vegetation changes acting as potential predictors linked to habitat suitability. In this paper, we study districts in Fennoscandia and Russia where incidences of BOR and TBE have steadily increased over the 1995-2015 period (defined as 'Well Increasing districts'). This selection is taken as a proxy for increasing the prevalence of tick-borne pathogens due to increased habitat suitability for ticks and hosts, thus simplifying the multiple factors that explain incidence variations. This approach allows vegetation types and strengths of correlation specific to the WI districts to be differentiated and compared with associations found over all districts. Land cover types and their changes found to be associated with increasing human disease incidence are described, indicating zones with potential future higher risk of these diseases. Combining vegetation cover and climate variables in regression models shows the interplay of biotic and abiotic factors linked to CSI incidences and identifies some differences between BOR and TBE. Regression model projections up until 2070 under different climate scenarios depict possible CSI progressions within the studied area and are consistent with the observed changes over the past 20 years.

Human Babesiosis in Europe

Babesiosis is attracting increasing attention as a worldwide emerging zoonosis. The first case of human babesiosis in Europe was described in the late 1950s and since then more than 60 cases have been reported in Europe. While the disease is relatively rare in Europe, it is significant because the majority of cases present as life-threatening fulminant infections, mainly in immunocompromised patients. Although appearing clinically similar to human babesiosis elsewhere, particularly in the USA, most European forms of the disease are distinct entities, especially concerning epidemiology, human susceptibility to infection and clinical management. This paper describes the history of the disease and reviews all published cases that have occurred in Europe with regard to the identity and genetic characteristics of the etiological agents, pathogenesis, aspects of epidemiology including the eco-epidemiology of the vectors, the clinical courses of infection, diagnostic tools and clinical management and treatment.

Pitfalls in Tick and Tick-Borne Pathogens Research, Some Recommendations and a Call for Data Sharing

An understanding of the relationships of ticks and tick-borne pathogens can only be achieved by integrating data from multiple studies. The publication of raw material is a necessary step for wide-area meta-analyses and study design, data collection and reporting require harmonization. This is an opinion paper, not a consensus position, and is open to debate. This work reflects our view about how data should be communicated in mainstream journals. We indicate rules that should be observed in recording weather data, to avoid serendipitous correlations between the density of ticks and climate variables and recommend the inclusion of raw data in reports. We stress the need for standardized methods to collect ticks that cannot be obtained by standard flagging. The reporting of infection rates of pathogens in ticks should avoid conclusions based on pure molecular findings in feeding ticks. Studies demonstrating the vectorial capacity of ticks should not be supported only by molecular surveys of feeding ticks. Vacuous conclusions about vectorial or reservoir status based solely on the finding of genomic material of a pathogen should be discouraged. We stress that phylogenetic studies based on random selection of sequences from GenBank are unsuitable. We firmly support the development of a dedicated server of curated sequences of ticks and pathogens as a standard for future studies.