Larvae of Ixodes ricinus transmit Borrelia afzelii and B. miyamotoi to vertebrate hosts

Tick hazard in the South Downs National Park (UK): species, distribution, key locations for future interventions, site density, habitats

Background

South Downs National Park (SDNP) is UK's most visited National Park, and a focus of tick-borne Lyme disease. The first presumed UK autochthonous cases of tick-borne encephalitis and babesiosis were recorded in 2019-20. SDNP aims to conserve wildlife and encourage recreation, so interventions are needed that reduce hazard without negatively affecting ecosystem health. To be successful these require knowledge of site hazards.

Methods

British Deer Society members submitted ticks removed from deer. Key potential intervention sites were selected and six 50 m2 transects drag-sampled per site (mostly twice yearly for 2 years). Ticks were identified in-lab (sex, life stage, species), hazard measured as tick presence, density of ticks (all life stages, DOT), and density of nymphs (DON). Sites and habitat types were analysed for association with hazard. Distribution was mapped by combining our results with records from five other sources.

Results

A total of 87 Ixodes ricinus (all but one adults, 82% F) were removed from 14 deer (10 Dama dama; three Capreolus capreolus; one not recorded; tick burden, 1-35) at 12 locations (commonly woodland). Five key potential intervention sites were identified and drag-sampled 2015-16, collecting 623 ticks (238 on-transects): 53.8% nymphs, 42.5% larvae, 3.7% adults (13 M, 10 F). Ticks were present on-transects at all sites: I. ricinus at three (The Mens (TM); Queen Elizabeth Country Park (QECP); Cowdray Estate (CE)), Haemaphysalis punctata at two (Seven Sisters Country Park (SSCP); Ditchling Beacon Nature Reserve (DBNR)). TM had the highest DOT at 30/300 m2 (DON = 30/300 m2), followed by QECP 22/300 m2 (12/300 m2), CE 8/300 m2 (6/300 m2), and SSCP 1/300 m2 (1/300 m2). For I. ricinus, nymphs predominated in spring, larvae in the second half of summer and early autumn. The overall ranking of site hazard held for DON and DOT from both seasonal sampling periods. DBNR was sampled 2016 only (one adult H. punctata collected). Woodland had significantly greater hazard than downland, but ticks were present at all downland sites. I. ricinus has been identified in 33/37 of SDNPs 10 km2 grid squares, Ixodes hexagonus 10/37, H. punctata 7/37, Dermacentor reticulatus 1/37.

Conclusions

Mapping shows tick hazard broadly distributed across SDNP. I. ricinus was most common, but H. punctata's seeming range expansion is concerning. Recommendations: management of small heavily visited high hazard plots (QECP); post-visit precaution signage (all sites); repellent impregnated clothing for deerstalkers; flock trials to control H. punctata (SSCP, DBNR). Further research at TM may contribute to knowledge on ecological dynamics underlying infection density and predator re-introduction/protection as public health interventions. Ecological research on H. punctata would aid control. SDNP Authority is ideally placed to link and champion policies to reduce hazard, whilst avoiding or reducing conflict between public health and ecosystem health.

Human Tick-Borne Diseases and Advances in Anti-Tick Vaccine Approaches: A Comprehensive Review

This comprehensive review explores the field of anti-tick vaccines, addressing their significance in combating tick-borne diseases of public health concern. The main objectives are to provide a brief epidemiology of diseases affecting humans and a thorough understanding of tick biology, traditional tick control methods, the development and mechanisms of anti-tick vaccines, their efficacy in field applications, associated challenges, and future prospects. Tick-borne diseases (TBDs) pose a significant and escalating threat to global health and the livestock industries due to the widespread distribution of ticks and the multitude of pathogens they transmit. Traditional tick control methods, such as acaricides and repellents, have limitations, including environmental concerns and the emergence of tick resistance. Anti-tick vaccines offer a promising alternative by targeting specific tick proteins crucial for feeding and pathogen transmission. Developing vaccines with antigens based on these essential proteins is likely to disrupt these processes. Indeed, anti-tick vaccines have shown efficacy in laboratory and field trials successfully implemented in livestock, reducing the prevalence of TBDs. However, some challenges still remain, including vaccine efficacy on different hosts, polymorphisms in ticks of the same species, and the economic considerations of adopting large-scale vaccine strategies. Emerging technologies and approaches hold promise for improving anti-tick vaccine development and expanding their impact on public health and agriculture.

Pathogenicity and virulence of Borrelia burgdorferi

Infection with Borrelia burgdorferi often triggers pathophysiologic perturbations that are further augmented by the inflammatory responses of the host, resulting in the severe clinical conditions of Lyme disease. While our apprehension of the spatial and temporal integration of the virulence determinants during the enzootic cycle of B. burgdorferi is constantly being improved, there is still much to be discovered. Many of the novel virulence strategies discussed in this review are undetermined. Lyme disease spirochaetes must surmount numerous molecular and mechanical obstacles in order to establish a disseminated infection in a vertebrate host. These barriers include borrelial relocation from the midgut of the feeding tick to its body cavity and further to the salivary glands, deposition to the skin, haematogenous dissemination, extravasation from blood circulation system, evasion of the host immune responses, localization to protective niches, and establishment of local as well as distal infection in multiple tissues and organs. Here, the various well-defined but also possible novel strategies and virulence mechanisms used by B. burgdorferi to evade obstacles laid out by the tick vector and usually the mammalian host during colonization and infection are reviewed.

Individual heterogeneity in ixodid tick infestation and prevalence of Borrelia burgdorferi sensu lato in a northern community of small mammalian hosts

Heterogeneous aggregation of parasites between individual hosts is common and regarded as an important factor in understanding transmission dynamics of vector-borne diseases. Lyme disease is vectored by generalist tick species, yet we have a limited understanding of how individual heterogeneities within small mammal host populations affect the aggregation of ticks and likelihood of infection. Male hosts often have higher parasite and infection levels than females, but whether this is linked to sexual body size dimorphism remains uncertain. Here, we analysed how host species, sex, and body mass influenced Ixodes ricinus tick infestations and the infection prevalence of Borrelia burgdorferi sensu lato (s.l.) in three species of small mammals involved in the enzootic transmission cycle of Lyme disease in Norway from 2018 to 2022. Larval and nymphal ticks were found on 98% and 34% of all individual hosts, respectively. In bank voles and wood mice, both larval and nymphal tick infestation and infection probability increased with body mass, and it increased more with mass for males than for females. Tick infestation in the common shrew increased with body mass and was higher in males, while pathogen infection was higher in females. Sex-biases in infestation did not correspond with level of sexual body mass dimorphism across species. This study contributes to our understanding of how individual heterogeneity among small mammalian hosts influences I. ricinus tick aggregation and prevalence of B. burgdorferi s.l. at northern latitudes.

Differential associations of horizontally and vertically transmitted symbionts on Ixodes ricinus behaviour and physiology

BACKGROUND

Ixodes ricinus ticks are infected with a large diversity of vertically and horizontally transmitted symbionts. While horizontally transmitted symbionts rely on a vertebrate host for their transmission, vertically transmitted symbionts rely more on the survival of their invertebrate host for transmission. We therefore hypothesized horizontally transmitted symbionts to be associated with increased tick activity to increase host contact rate and vertically transmitted symbionts to be associated with higher tick weight and lipid fraction to promote tick survival.

METHODS

We used a behavioural assay to record the questing activity of I. ricinus ticks. In addition, we measured weight and lipid fraction and determined the presence of ten symbiont species in these ticks using qPCR, of which six were vertically transmitted and four horizontally transmitted.

RESULTS

Vertically transmitted symbionts (e.g. Midichloria mitochondrii) were associated with an increase in tick weight, whereas horizontally transmitted symbionts (e.g. Borrelia burgdorferi sensu lato) were often associated with lower weight and lipid fraction of ticks. Moreover, horizontally transmitted symbionts (e.g. B. burgdorferi s.l.) were associated with increased tick activity, which may benefit pathogen transmission and increases tick-borne disease hazard.

CONCLUSIONS

Our study shows that horizontally and vertically transmitted symbionts differentially influence the behaviour and physiology of I. ricinus and warrants future research to study the underlying mechanisms and effects on transmission dynamics of tick-borne pathogens.

Exposure of dogs and cats to Borrelia miyamotoi infected Ixodes ricinus ticks in urban areas of the city of Poznań, west-central Poland

Borrelia miyamotoi is an emerging human pathogen that causes a relapsing fever-like disease named B. miyamotoi disease. The bacterium belongs to the relapsing fever borreliae, and similar to spirochetes of the Borrelia burgdorferi sensu lato group, it is transmitted only by hard ticks of the Ixodes ricinus complex. To date, B. miyamotoi has not been demonstrated to cause illness in dogs or cats, and is poorly documented in veterinary medicine. The aim of this study was to determine the B. miyamotoi presence in (i) host-seeking ticks and (ii) engorged Ixodes sp. ticks collected from dogs and cats during their inspection in veterinary clinics of the city of Poznań, west-central Poland. Host-seeking ticks were sampled in dog walking areas localized in urban forested recreational sites of the city. In this study, 1,059 host-seeking and 837 engorged I. ricinus ticks collected from 680 tick-infested animals (567 dogs and 113 cats) were screened. Additionally, 31 I. hexagonus ticks (one larva, 13 nymphs, and 17 females) were collected from three cats; one larva and one nymph were collected from two dogs; and one dog was infested with a single Dermacentor reticulatus female. Borrelia DNA was identified by the amplification and sequencing of the V4 hypervariable region of the 16S rRNA gene and flaB gene fragments. DNA of B. miyamotoi was detected in 22 (2.1%) of the host-seeking ticks (in all developmental tick stages and in all study areas). In addition, the engorged I. ricinus ticks exhibited a similar B. miyamotoi presence (1.8%). Fifteen I. ricinus ticks collected from animals tested positive for the presence of B. miyamotoi DNA, and the DNA of B. miyamotoi was observed in three (9.1%; one female and two nymphs) I. hexagonus ticks. The single D. reticulatus female collected from a dog tested PCR-negative for the bacterium. The results of this study demonstrated the establishment and broad presence of the bacterium in tick populations from different urban ecosystems of the city of Poznań. The lack of difference in the mean infection presence of animal-derived and host-seeking I. ricinus ticks suggests that the systematic surveillance of pets may be useful for the evaluation of human exposure to B. miyamotoi infected ticks in urban areas. Additional studies are required to further elucidate the role of domestic and wild carnivores in the epidemiology of B. miyamotoi, which remains unknown.

Borrelia miyamotoi a neglected tick-borne relapsing fever spirochete in Thailand

Borrelia miyamotoi is a relapsing fever spirochete that shares the same vector as Lyme disease causing Borrelia. This epidemiological study of B. miyamotoi was conducted in rodent reservoirs, tick vectors and human populations simultaneously. A total of 640 rodents and 43 ticks were collected from Phop Phra district, Tak province, Thailand. The prevalence rate for all Borrelia species was 2.3% and for B. miyamotoi was 1.1% in the rodent population, while the prevalence rate was quite high in ticks collected from rodents with an infection rate of 14.5% (95% CI: 6.3-27.6%). Borrelia miyamotoi was detected in Ixodes granulatus collected from Mus caroli and Berylmys bowersi, and was also detected in several rodent species (Bandicota indica, Mus spp., and Leopoldamys sabanus) that live in a cultivated land, increasing the risk of human exposure. Phylogenetic analysis revealed that the B. miyamotoi isolates detected in rodents and I. granulatus ticks in this study were similar to isolates detected in European countries. Further investigation was conducted to determine the serological reactivity to B. miyamotoi in human samples received from Phop Phra hospital, Tak province and in rodents captured from Phop Phra district using an in-house, direct enzyme-linked immunosorbent assay (ELISA) assay with B. miyamotoi recombinant glycerophosphodiester-phosphodiesterase (rGlpQ) protein as coated antigen. The results showed that 17.9% (15/84) of human patients and 9.0% (41/456) of captured rodents had serological reactivity to B. miyamotoi rGlpQ protein in the study area. While a low level of IgG antibody titers (100-200) was observed in the majority of seroreactive samples, higher titers (400-1,600) were also detected in both humans and rodents. This study provides the first evidence of B. miyamotoi exposure in human and rodent populations in Thailand and the possible roles of local rodent species and Ixodes granulatus tick in its enzootic transmission cycle in nature.

Evaluation of Immunocompetent Mouse Models for Borrelia miyamotoi Infection

Borrelia miyamotoi is a relapsing fever spirochete that is harbored by Ixodes spp. ticks and is virtually uncharacterized, compared to other relapsing fever Borrelia vectored by Ornithodoros spp. ticks. There is not an immunocompetent mouse model for studying B. miyamotoi infection in vivo or for transmission in the vector-host cycle. Our goal was to evaluate B. miyamotoi infections in multiple mouse breeds/strains as a prelude to the ascertainment of the best experimental infection model. Two B. miyamotoi strains, namely, LB-2001 and CT13-2396, as well as three mouse models, namely, CD-1, C3H/HeJ, and BALB/c, were evaluated. We were unable to observe B. miyamotoi LB-2001 spirochetes in the blood via darkfield microscopy or to detect DNA via real-time PCR post needle inoculation in the CD-1 and C3H/HeJ mice. However, LB-2001 DNA was detected via real-time PCR in the blood of the BALB/c mice after needle inoculation, although spirochetes were not observed via microscopy. CD-1, C3H/HeJ, and BALB/c mice generated an antibody response to B. miyamotoi LB-2001 following needle inoculation, but established infections were not detected, and the I. scapularis larvae failed to acquire spirochetes from the exposed CD-1 mice. In contrast, B. miyamotoi CT13-2396 was visualized in the blood of the CD-1 and C3H/HeJ mice via darkfield microscopy and detected by real-time PCR post needle inoculation. Both mouse strains seroconverted. However, no established infection was detected in the mouse organs, and the I. scapularis larvae failed to acquire Borrelia after feeding on CT13-2396 exposed CD-1 or C3H/HeJ mice. These findings underscore the challenges in establishing an experimental B. miyamotoi infection model in immunocompetent laboratory mice. IMPORTANCE Borrelia miyamotoi is a causative agent of hard tick relapsing fever, was first identified in the early 1990s, and was characterized as a human pathogen in 2011. Unlike other relapsing fever Borrelia species, B. miyamotoi spread by means of Ixodes ticks. The relatively recent recognition of this human pathogen means that B. miyamotoi is virtually uncharacterized, compared to other Borrelia species. Currently there is no standard mouse-tick model with which to study the interactions of the pathogen within its vector and hosts. We evaluated two B. miyamotoi isolates and three immunocompetent mouse models to identify an appropriate model with which to study tick-host-pathogen interactions. With the increased prevalence of human exposure to Ixodes ticks, having an appropriate model with which to study B. miyamotoi will be critical for the future development of diagnostics and intervention strategies.

The first detection of relapsing fever spirochete Borrelia miyamotoi in Ixodes ricinus ticks from the northeast Czech Republic

Borrelia miyamotoi, a relapsing fever spirochete, is considered a human pathogen. Knowledge of this borrelia is currently limited. Data about its potential impact on public health, circulation in nature, or its occurrence in natural environments are insufficient. For our study, a total of 505 questing Ixodes ricinus ticks (337 nymphs, 85 females and 83 males) from Hradec Králové Region in the Czech Republic were collected. Additionally, 160 winged Lipoptena deer keds from Hradec Králové Region, from Pardubice Region, Czech Republic, and from one location in western Slovakia were collected. The presence of B. miyamotoi in ticks and deer keds was determined using polymerase chain reaction (PCR) targeting a gene encoding glycerophosphodiester phosphodiesterase (glpQ), antigenic protein specific to the relapsing fever spirochetes. Borrelia miyamotoi was identified in six nymphs and four females of I. ricinus ticks. The overall prevalence was 2%. None of the examined Lipoptena specimens were found to be infected. Although no human case of infection with B. miyamotoi has been reported in the Czech Republic yet, this spirochete is widespread in ticks, and therefore the risk of human infection exists.

Recovery of Partially Engorged Haemaphysalis longicornis (Acari: Ixodidae) Ticks from Active Surveillance

The invasive Asian longhorned tick, Haemaphysalis longicornis, has rapidly spread across the northeastern United States and is associated with pathogens of public health and veterinary concern. Despite its importance in pathogen dynamics, H. longicornis blood-feeding behavior in nature, specifically the likelihood of interrupted feeding, remains poorly documented. Here, we report the recovery of partially engorged, questing H. longicornis from active tick surveillance in Pennsylvania. Significantly more engorged H. longicornis nymphs (1.54%) and adults (3.07%) were recovered compared to Ixodes scapularis nymphs (0.22%) and adults (zero). Mean Scutal Index difference between unengorged and engorged nymph specimens was 0.65 and 0.42 for I. scapularis and H. longicornis, respectively, suggesting the questing, engorged H. longicornis also engorged to a comparatively lesser extent. These data are among the first to document recovery of engorged, host-seeking H. longicornis ticks and provide initial evidence for interrupted feeding and repeated successful questing events bearing implications for pathogen transmission and warranting consideration in vector dynamics models.

A comparison of horizontal and transovarial transmission efficiency of Borrelia miyamotoi by Ixodes scapularis

Borrelia miyamotoi is a relapsing fever spirochete carried by Ixodes spp. ticks throughout the northern hemisphere. The pathogen is acquired either transovarially (vertically) or horizontally through blood-feeding and passed transtadially across life stages. Despite these complementary modes of transmission, infection prevalence of ticks with B. miyamotoi is typically low (<5%) in natural settings and the relative contributions of the two transmission modes have not been studied extensively. Horizontal transmission of B. miyamotoi (strain CT13-2396 or wild type strain) was initiated using infected Ixodes scapularis larvae or nymphs to expose rodents, which included both the immunocompetent CD-1 laboratory mouse (Mus musculus) and a natural reservoir host, the white-footed mouse (Peromyscus. leucopus), to simulate natural enzootic transmission. Transovarial transmission was evaluated using I. scapularis exposed to B. miyamotoi as either larvae or nymphs feeding on immunocompromised SCID mice (M. musculus) and subsequently fed as females on New Zealand white rabbits. Larvae from infected females were qPCR-tested individually to assess transovarial transmission rates. Tissue tropism of B. miyamotoi in infected ticks was demonstrated using in situ hybridization. Between 1 and 12% of ticks were positive (post-molt) for B. miyamotoi after feeding on groups of CD-1 mice or P. leucopus with evidence of infection, indicating that horizontal transmission was inefficient, regardless of whether infected larvae or nymphs were used to challenge the mice. Transovarial transmission occurred in 7 of 10 egg clutches from infected females. Filial infection prevalence in larvae ranged from 3 to 100% (median 71%). Both larval infection prevalence and spirochete load were highly correlated with maternal spirochete load. Spirochetes were disseminated throughout the tissues of all three stages of unfed ticks, including the salivary glands and female ovarian tissue. The results indicate that while multiple transmission routes contribute to enzootic maintenance of B. miyamotoi, transovarial transmission is likely to be the primary source of infected ticks and therefore risk assessment and tick control strategies should target adult female ticks.

Seasonal dynamics of tick burden and associated Borrelia burgdorferi s.l. and Borrelia miyamotoi infections in rodents in a Dutch forest ecosystem

Ixodes ricinus ticks transmit Borrelia burgdorferi sensu lato (s.l.) as well as Borrelia miyamotoi. Larvae become infected when feeding on infected rodents, with horizontal transmission of B. burgdorferi and horizontal and vertical transmission of B. miyamotoi. We studied seasonal dynamics of infection rates of I. ricinus and their rodent hosts, and hence transmission risk of these two distinctly different Borrelia species. Rodents were live-trapped and inspected for ticks from May to November in 2013 and 2014 in a forest in The Netherlands. Trapped rodents were temporarily housed in the laboratory and detached ticks were collected. Borrelia infections were determined from the trapped rodents and collected ticks. Borrelia burgdorferi s.l. and B. miyamotoi were found in ticks as well as in rodents. Rodent density was higher in 2014, whereas tick burden as well as the Borrelia infection rates in rodents were higher in 2013. The density of B. miyamotoi-infected nymphs did not differ between the years. Tick burdens were higher on Apodemus sylvaticus than on Myodes glareolus, and higher on males than on females. Borrelia-infection rate of rodents varied strongly seasonally, peaking in summer. As the larval tick burden also peaked in summer, the generation of infected nymphs was highest in summer. We conclude that the heterogeneity of environmental and host-specific factors affects the seasonal transmission of Borrelia spp., and that these effects act more strongly on horizontally transmitted B. burgdorferi spp. than on the vertically transmitted B. miyamotoi.

The evolving story of Borrelia burgdorferi sensu lato transmission in Europe

Beside mosquitoes, ticks are well-known vectors of different human pathogens. In the Northern Hemisphere, Lyme borreliosis (Eurasia, LB) or Lyme disease (North America, LD) is the most commonly occurring vector-borne infectious disease caused by bacteria of the genus Borrelia which are transmitted by hard ticks of the genus Ixodes. The reported incidence of LB in Europe is about 22.6 cases per 100,000 inhabitants annually with a broad range depending on the geographical area analyzed. However, the epidemiological data are largely incomplete, because LB is not notifiable in all European countries. Furthermore, not only differ reporting procedures between countries, there is also variation in case definitions and diagnostic procedures. Lyme borreliosis is caused by several species of the Borrelia (B.) burgdorferi sensu lato (s.l.) complex which are maintained in complex networks including ixodid ticks and different reservoir hosts. Vector and host influence each other and are affected by multiple factors including climate that have a major impact on their habitats and ecology. To classify factors that influence the risk of transmission of B. burgdorferi s.l. to their different vertebrate hosts as well as to humans, we briefly summarize the current knowledge about the pathogens including their astonishing ability to overcome various host immune responses, regarding the main vector in Europe Ixodes ricinus, and the disease caused by borreliae. The research shows, that a higher standardization of case definition, diagnostic procedures, and standardized, long-term surveillance systems across Europe is necessary to improve clinical and epidemiological data.

Transovarial Transmission of Borrelia hermsii by Its Tick Vector and Reservoir Host Ornithodoros hermsi

Transovarial passage of relapsing fever spirochetes (Borrelia species) by infected female argasid ticks to their progeny is a widespread phenomenon. Yet this form of vertical inheritance has been considered rare for the North American tick Ornithodoros hermsi infected with Borrelia hermsii. A laboratory colony of O. hermsi was established from a single infected female and two infected males that produced a population of ticks with a high prevalence of transovarial transmission based on infection assays of single and pooled ticks feeding on mice and immunofluorescence microscopy of eggs and larvae. Thirty-eight of forty-five (84.4%) larval cohorts (groups of larvae originating from the same egg clutch) transmitted B. hermsii to mice over four and a half years, and one hundred and three single and one hundred and fifty-three pooled nymphal and adult ticks transmitted spirochetes during two hundred and fourteen of two hundred and fifty-six (83.6%) feedings on mice over seven and a half years. The perpetuation of B. hermsii for many years by infected ticks only (without acquisition of spirochetes from vertebrate hosts) demonstrates the reservoir competence of O. hermsi. B. hermsii produced the variable tick protein in eggs and unfed larvae infected by transovarial transmission, leading to speculation of the possible steps in the evolution of borreliae from a tick-borne symbiont to a tick-transmitted parasite of vertebrates.

Bactericidal activity of avian complement: a contribution to understand avian-host tropism of Lyme borreliae

Complement has been considered as an important factor impacting the host-pathogen association of spirochetes belonging to the Borrelia burgdorferi sensu lato complex, and may play a role in the spirochete's ecology. Birds are known to be important hosts for ticks and in the maintenance of borreliae. Recent field surveys and laboratory transmission studies indicated that certain avian species act as reservoir hosts for different Borrelia species. Nevertheless, our current understanding of the molecular mechanisms determining host tropism of Borrelia is still in its fledgling stage. Concerning the role of complement in avian-host tropism, only a few bird species and Borrelia species have been analysed so far. Here, we performed in vitro serum bactericidal assays with serum samples collected from four bird species including the European robin Erithacus rubecula, the great tit Parus major, the Eurasian blackbird Turdus merula, and the racing pigeon Columba livia, as well as four Borrelia species (B. afzelii, B. garinii, B. valaisiana, and B. burgdorferi sensu stricto). From July to September 2019, juvenile wild birds were caught using mist nets in Portugal. Racing pigeons were sampled in a loft in October 2019. Independent of the bird species analysed, all Borrelia species displayed an intermediate serum-resistant or serum-resistant phenotype except for B. afzelii challenged with serum from blackbirds. This genospecies was efficiently killed by avian complement, suggesting that blackbirds served as dead-end hosts for B. afzelii. In summary, these findings suggest that complement contributes in the avian-spirochete-tick infection cycle and in Borrelia-host tropism.

The Contribution of Wildlife Hosts to the Rise of Ticks and Tick-Borne Diseases in North America

Wildlife vertebrate hosts are integral to enzootic cycles of tick-borne pathogens, and in some cases have played key roles in the recent rise of ticks and tick-borne diseases in North America. In this forum article, we highlight roles that wildlife hosts play in the maintenance and transmission of zoonotic, companion animal, livestock, and wildlife tick-borne pathogens. We begin by illustrating how wildlife contribute directly and indirectly to the increase and geographic expansion of ticks and their associated pathogens. Wildlife provide blood meals for tick growth and reproduction; serve as pathogen reservoirs; and can disperse ticks and pathogens-either through natural movement (e.g., avian migration) or through human-facilitated movement (e.g., wildlife translocations and trade). We then discuss opportunities to manage tick-borne disease through actions directed at wildlife hosts. To conclude, we highlight key gaps in our understanding of the ecology of tick-host interactions, emphasizing that wildlife host communities are themselves a very dynamic component of tick-pathogen-host systems and therefore complicate management of tick-borne diseases, and should be taken into account when considering host-targeted approaches. Effective management of wildlife to reduce tick-borne disease risk further requires consideration of the 'human dimensions' of wildlife management. This includes understanding the public's diverse views and values about wildlife and wildlife impacts-including the perceived role of wildlife in fostering tick-borne diseases. Public health agencies should capitalize on the expertise of wildlife agencies when developing strategies to reduce tick-borne disease risks.

Wild ungulate species differ in their contribution to the transmission of Ixodes ricinus-borne pathogens

BACKGROUND

Several ungulate species are feeding and propagation hosts for the tick Ixodes ricinus as well as hosts to a wide range of zoonotic pathogens. Here, we focus on Anaplasma phagocytophilum and Borrelia burgdorferi (s.l.), two important pathogens for which ungulates are amplifying and dilution hosts, respectively. Ungulate management is one of the main tools to mitigate human health risks associated with these tick-borne pathogens. Across Europe, different species of ungulates are expanding their ranges and increasing in numbers. It is currently unclear if and how the relative contribution to the life-cycle of I. ricinus and the transmission cycles of tick-borne pathogens differ among these species. In this study, we aimed to identify these relative contributions for five European ungulate species.

METHODS

We quantified the tick load and collected ticks and spleen samples from hunted fallow deer (Dama dama, n = 131), moose (Alces alces, n = 15), red deer (Cervus elaphus, n = 61), roe deer (Capreolus capreolus, n = 30) and wild boar (Sus scrofa, n = 87) in south-central Sweden. We investigated the presence of tick-borne pathogens in ticks and spleen samples using real-time PCR. We determined if ungulate species differed in tick load (prevalence and intensity) and in infection prevalence in their tissue as well as in the ticks feeding on them.

RESULTS

Wild boar hosted fewer adult female ticks than any of the deer species, indicating that deer are more important as propagation hosts. Among the deer species, moose had the lowest number of female ticks, while there was no difference among the other deer species. Given the low number of infected nymphs, the relative contribution of all ungulate species to the transmission of B. burgdorferi (s.l.) was low. Fallow deer, red deer and roe deer contributed more to the transmission of A. phagocytophilum than wild boar.

CONCLUSIONS

The ungulate species clearly differed in their role as a propagation host and in the transmission of B. burgdorferi and A. phagocytophilum. This study provides crucial information for ungulate management as a tool to mitigate zoonotic disease risk and argues for adapting management approaches to the local ungulate species composition and the pathogen(s) of concern.

Long-term study of Borrelia and Babesia prevalence and co-infection in Ixodes ricinus and Dermacentor recticulatus ticks removed from humans in Poland, 2016-2019

Background

Lyme borreliosis (LB) is the most common vector-borne disease in Europe. Monitoring changes in the prevalence of different Borrelia species in ticks may be an important indicator of risk assessment and of differences in pathogenicity in humans. The objective of our study was to assess the prevalence, co-infection and distribution of Borrelia and Babesia species in ticks removed from humans in a large sample collected during a study period of 4 years.

Methods

The ticks were collected throughout Poland from March to November over 4-year period from 2016 to 2019. All ticks (n = 1953) were morphologically identified in terms of species and developmental stage. Molecular screening for Borrelia and Babesia by amplification of the flagellin gene (flaB) or 18S rRNA marker was performed. Pathogen identity was confirmed by Sanger sequencing or PCR–restriction fragment length polymorphism analysis.

Results

The ticks removed from humans in Poland during this study belonged to two species: Ixodes ricinus (97%) and Dermacentor reticulatus (3%). High Borrelia prevalence (25.3%), including B. miyamotoi (8.4%), was confirmed in Ixodes ricinus ticks removed from humans, as was the change in frequency of occurrence of Borrelia species during the 4-year study. Despite Babesia prevalence being relatively low (1.3%), the majority of tested isolates are considered to be pathogenic to humans. Babesia infection was observed more frequently among Borrelia-positive ticks (2.7%) than among ticks uninfected with Borrelia (0.8%). The most frequent dual co-infections were between Borrelia afzelii and Babesia microti. The presence of Borrelia was also confirmed in D. reticulatus (12.7%); however the role of these ticks in spirochete transmission to susceptible hosts is still unclear.

Conclusions

Although the overall risk of developing LB after a tick bite is low in Europe, knowledge of the prevalence and distribution of Borrelia and Babesia species in ticks might be an important indicator of the risk of both these tick-borne diseases.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04849-5.

Host association of Borrelia burgdorferi sensu lato: A review

Borrelia burgdorferi sensu lato (Bbsl) is a bacterial species complex that includes the etiological agents of the most frequently reported vector-borne disease in the Northern hemisphere, Lyme borreliosis. It currently comprises > 20 named and proposed genospecies that use vertebrate hosts and tick vectors for transmission in the Americas and Eurasia. Host (and vector) associations influence geographic distribution and speciation in Bbsl, which is of particular relevance to human health. To target gaps in knowledge for future efforts to understand broad patterns of the Bbsl-tick-host system and how they relate to human health, the present review aims to give a comprehensive summary of the literature on host association in Bbsl. Of 465 papers consulted (404 after exclusion criteria were applied), 96 sought to experimentally establish reservoir competence of 143 vertebrate host species for Bbsl. We recognize xenodiagnosis as the strongest method used, however it is infrequent (20% of studies) probably due to difficulties in maintaining tick vectors and/or wild host species in the lab. Some well-established associations were not experimentally confirmed according to our definition (ex: Borrelia garinii, Ixodes uriae and sea birds). We conclude that our current knowledge on host association in Bbsl is mostly derived from a subset of host, vector and bacterial species involved, providing an incomplete knowledge of the physiology, ecology and evolutionary history of these interactions. More studies are needed on all host, vector and bacterial species globally involved with a focus on non-rodent hosts and Asian Bbsl complex species, especially with experimental research that uses xenodiagnosis and genomics to analyze existing host associations in different ecosystems.

Ticking on Pandora's box: a prospective case-control study into 'other' tick-borne diseases

BACKGROUND

Tick-borne pathogens other than Borrelia burgdorferi sensu lato - the causative agent of Lyme borreliosis - are common in Ixodes ricinus ticks. How often these pathogens cause human disease is unknown. In addition, diagnostic tools to identify such diseases are lacking or reserved to research laboratories. To elucidate their prevalence and disease burden, the study 'Ticking on Pandora's Box' has been initiated, a collaborative effort between Amsterdam University Medical Center and the National Institute for Public Health and the Environment.

METHODS

The study investigates how often the tick-borne pathogens Anaplasma phagocytophilum, Babesia species, Borrelia miyamotoi, Neoehrlichia mikurensis, spotted fever group Rickettsia species and/or tick-borne encephalitis virus cause an acute febrile illness after tick-bite. We aim to determine the impact and severity of these tick-borne diseases in the Netherlands by measuring their prevalence and describing their clinical picture and course of disease. The study is designed as a prospective case-control study. We aim to include 150 cases - individuals clinically suspected of a tick-borne disease - and 3 matched healthy control groups of 200 persons each. The controls consist respectively of a group of individuals with either a tick-bite without complaints, the general population and of healthy blood donors. During a one-year follow-up we will acquire blood, urine and skin biopsy samples and ticks at baseline, 4 and 12 weeks. Additionally, participants answer modified versions of validated questionnaires to assess self-reported symptoms, among which the SF-36, on a 3 monthly basis.

DISCUSSION

This article describes the background and design of the study protocol of 'Ticking on Pandora's Box'. With our study we hope to provide insight into the prevalence, clinical presentation and disease burden of the tick-borne diseases anaplasmosis, babesiosis, B. miyamotoi disease, neoehrlichiosis, rickettsiosis and tick-borne encephalitis and to assist in test development as well as provide recommendations for national guidelines.

TRIAL REGISTRATION

NL9258 (retrospectively registered at Netherlands Trial Register, trialregister.nl in in February 2021).

Masting by beech trees predicts the risk of Lyme disease

BACKGROUND

The incidence of Lyme borreliosis and other tick-borne diseases is increasing in Europe and North America. There is currently much interest in identifying the ecological factors that determine the density of infected ticks as this variable determines the risk of Lyme borreliosis to vertebrate hosts, including humans. Lyme borreliosis is caused by the bacterium Borrelia burgdorferi sensu lato (s.l.) and in western Europe, the hard tick Ixodes ricinus is the most important vector.

METHODS

Over a 15-year period (2004-2018), we monitored the monthly abundance of I. ricinus ticks (nymphs and adults) and their B. burgdorferi s.l. infection status at four different elevations on a mountain in western Switzerland. We collected climate variables in the field and from nearby weather stations. We obtained data on beech tree seed production (masting) from the literature, as the abundance of Ixodes nymphs can increase dramatically 2 years after a masting event. We used generalized linear mixed effects models and AIC-based model selection to identify the ecological factors that influence inter-annual variation in the nymphal infection prevalence (NIP) and the density of infected nymphs (DIN).

RESULTS

We found that the NIP decreased by 78% over the study period. Inter-annual variation in the NIP was explained by the mean precipitation in the present year, and the duration that the DNA extraction was stored in the freezer prior to pathogen detection. The DIN decreased over the study period at all four elevation sites, and the decrease was significant at the top elevation. Inter-annual variation in the DIN was best explained by elevation site, year, beech tree masting index 2 years prior and the mean relative humidity in the present year. This is the first study in Europe to demonstrate that seed production by deciduous trees influences the density of nymphs infected with B. burgdorferi s.l. and hence the risk of Lyme borreliosis.

CONCLUSIONS

Public health officials in Europe should be aware that masting by deciduous trees is an important predictor of the risk of Lyme borreliosis.

Spatial and temporal patterns of the emerging tick-borne pathogen Borrelia miyamotoi in blacklegged ticks (Ixodes scapularis) in New York

Borrelia miyamotoi, a bacterium that causes relapsing fever, is found in ixodid ticks throughout the northern hemisphere. The first cases of human infection with B. miyamotoi were identified in 2011. In the eastern USA, blacklegged ticks (Ixodes scapularis) become infected by feeding on an infected vertebrate host, or through transovarial transmission. We surveyed B. miyamotoi prevalence in ticks within forested habitats in Dutchess County, New York, and identified possible reservoir hosts. To assess spatial variation in infection, we collected questing nymphal ticks at > 150 sites. To assess temporal variation in infection, we collected questing nymphs for 8 years at a single study site. We collected questing larval ticks from nine plots to estimate the amount of transovarial transmission. To evaluate potential reservoir hosts, we captured 14 species of mammal and bird hosts naturally infested with larval blacklegged ticks and held these hosts in the laboratory until ticks fed to repletion and molted to nymphs. We determined infection for all ticks using quantitative polymerase chain reaction. The overall infection prevalence of questing nymphal ticks across all sites was ~ 1%, but prevalence at individual sites was as high as 9.1%. We detected no significant increase in infection through time. Only 0.4% of questing larval ticks were infected. Ticks having fed as larvae from short-tailed shrews, red squirrels, and opossums tended to have higher infection prevalence than did ticks having fed on other hosts. Further studies of the role of hosts in transmission are warranted. The locally high prevalence of B. miyamotoi in the New York/New England landscape suggests the importance of vigilance by health practitioners and the public.

Borrelia miyamotoi-An Emerging Human Tick-Borne Pathogen in Europe

Borrelia miyamotoi is classified as a relapsing fever spirochete. Although B. miyamotoi is genetically and ecologically distinct from Borrelia burgdorferi sensu lato, both microorganisms are transmitted by the same Ixodes tick species. B. miyamotoi was detected in I. persulcatus ticks in 1994 in Japan. A phylogenetic analysis based on selected sequences of B. miyamotoi genome revealed genetic differences between isolates from Asia, North America, and Europe, which are clearly separated into three genotypes. Symptomatic human cases of Borrelia miyamotoi disease (BMD) were first reported in 2011 in Russia and then in North America, Europe, and Asia. The most common clinical manifestation of BMD is fever with flu-like symptoms. Several differences in rare symptoms (thrombocytopenia, monocytosis, cerebrospinal fluid pleocytosis, or symptoms related to the central nervous system) have been noted among cases caused by Asian, European, and American types of B. miyamotoi. BMD should be considered in the diagnosis of patients after tick bites, particularly with meningoencephalitis, without anti-Borrelia antibodies in the cerebrospinal fluid. This review describes the biology, ecology, and potential of B. miyamotoi as a tick-borne pathogen of public health concern, with particular emphasis on Europe.

Perpetuation of Borreliae

With one exception (epidemic relapsing fever), borreliae are obligately maintained in nature by ticks. Although some Borrelia spp. may be vertically transmitted to subsequent generations of ticks, most require amplification by a vertebrate host because inheritance is not stable. Enzootic cycles of borreliae have been found globally; those receiving the most attention from researchers are those whose vectors have some degree of anthropophily and, thus, cause zoonoses such as Lyme disease or relapsing fever. To some extent, our views on the synecology of the borreliae has been dominated by an applied focus, viz., analyses that seek to understand the elements of human risk for borreliosis. But, the elements of borrelial perpetuation do not necessarily bear upon risk, nor do our concepts of risk provide the best structure for analyzing perpetuation. We identify the major global themes for the perpetuation of borreliae, and summarize local variations on those themes, focusing on key literature to outline the factors that serve as the basis for the distribution and abundance of borreliae.

Seasonality of acarological risk of exposure to Borrelia miyamotoi from questing life stages of Ixodes scapularis collected from Wisconsin and Massachusetts, USA

Measures of acarological risk of exposure to Ixodes scapularis-borne disease agents typically focus on nymphs; however, the relapsing fever group spirochete Borrelia miyamotoi can be passed transovarially, and I. scapularis larvae are capable of transmitting B. miyamotoi to their hosts. To quantify the larval contribution to acarological risk, relative to nymphs and adults, we collected questing I. scapularis for 3 yr at Fort McCoy, Wisconsin (WI, n = 23,367 ticks), and Cape Cod, Massachusetts (MA, n = 4190) in the United States. Borrelia miyamotoi infection prevalence was estimated for I. scapularis larvae, nymphs, females, and males, respectively, as 0.88, 2.05, 0.63, and 1.22 % from the WI site and 0.33, 2.32, 2.83, and 2.11 % from the MA site. Densities of B. miyamotoi-infected ticks (DIT, per 1000 m²) were estimated for larvae, nymphs, females, and males, respectively, as 0.36, 0.14, 0.01, and 0.03 from the WI site and 0.05, 0.06, 0.03, and 0.02 from the MA site. Thus, although larval infection prevalence with B. miyamotoi was significantly lower than that of nymphs and similar to that of adults, because of their higher abundance, the larval contribution to the overall DIT was similar to that of nymphs and trended towards a greater contribution than adults. Assuming homogenous contact rates with humans, these results suggest that eco-epidemiological investigations of B. miyamotoi disease in North America should include larvae. A fuller appreciation of the epidemiological implications of these results, therefore, requires an examination of the heterogeneity in contact rates with humans among life stages.

Longitudinal study of prevalence and spatio-temporal distribution of Borrelia burgdorferi sensu lato in ticks from three defined habitats in Latvia, 1999-2010

Members of the Borrelia burgdorferi sensu lato (s.l.) species complex are known to cause human Lyme borreliosis. Because of longevity of some reservoir hosts and the Ixodes tick vectors' life cycle, long-term studies are required to better understand species and population dynamics of these bacteria in their natural habitats. Ticks were collected between 1999 and 2010 in three ecologically different habitats in Latvia. We used multilocus sequence typing utilizing eight chromosomally located housekeeping genes to obtain information about species and population fluctuations and/or stability of B. burgdorferi s.l. in these habitats. The average prevalence over all years was 18.9%. From initial high-infection prevalences of 25.5%, 33.1% and 31.8%, from 2002 onwards the infection rates steadily decreased to 7.3%. Borrelia afzelii and Borrelia garinii were the most commonly found genospecies but striking local differences were obvious. In one habitat, a significant shift from rodent-associated to bird-associated Borrelia species was noted whilst in the other habitats, Borrelia species composition was relatively stable over time. Sequence types (STs) showed a random spatial and temporal distribution. These results demonstrated that there are temporal regional changes and extrapolations from one habitat to the next are not possible.

A new Borrelia on the block: Borrelia miyamotoi - a human health risk?

Background

Borrelia miyamotoi clusters phylogenetically among relapsing fever borreliae, but is transmitted by hard ticks. Recent recognition as a human pathogen has intensified research into its ecology and pathogenic potential.

Aims

We aimed to provide a timely critical integrative evaluation of our knowledge on B. miyamotoi, to assess its public health relevance and guide future research.

Methods

This narrative review used peer-reviewed literature in English from January 1994 to December 2018.

Results

Borrelia miyamotoi occurs in the world’s northern hemisphere where it co-circulates with B. burgdorferi sensu lato, which causes Lyme disease. The two borreliae have overlapping vertebrate and tick hosts. While ticks serve as vectors for both species, they are also reservoirs for B. miyamotoi. Three B. miyamotoi genotypes are described, but further diversity is being recognised. The lack of sufficient cultivable isolates and vertebrate models compromise investigation of human infection and its consequences. Our understanding mainly originates from limited case series. In these, human infections mostly present as influenza-like illness, with relapsing fever in sporadic cases and neurological disease reported in immunocompromised patients. Unspecific clinical presentation, also occasionally resulting from Lyme- or other co-infections, complicates diagnosis, likely contributing to under-reporting. Diagnostics mainly employ PCR and serology. Borrelia miyamotoi infections are treated with antimicrobials according to regimes used for Lyme disease.

Conclusions

With co-infection of tick-borne pathogens being commonplace, diagnostic improvements remain important. Developing in vivo models might allow more insight into human pathogenesis. Continued ecological and human case studies are key to better epidemiological understanding, guiding intervention strategies.

Rejection of the name Borreliella and all proposed species comb. nov. placed therein

Rejection (nomen rejiciendum) of the name Borreliella and all new combinations therein is being requested on grounds of risk to human health and patient safety (Principle 1, subprinciple 2 and Rule 56a) and violation to aim for stability of names, to avoid useless creation of names (Principle 1, subprinciple 1 and 3) and that names should not be changed without sufficient reason (Principle 9 of the International Code of Nomenclature of Prokaryotes).

A novel Borrelia species, intermediate between Lyme disease and relapsing fever groups, in neotropical passerine-associated ticks

Lyme disease (LD) and relapsing fevers (RF) are vector-borne diseases caused by bacteria of the Borrelia genus. Here, we report on the widespread infection by a non-described Borrelia species in passerine-associated ticks in tropical rainforests of French Guiana, South America. This novel Borrelia species is common in two tick species, Amblyomma longirostre and A. geayi, which feed on a broad variety of neotropical mammal and bird species, including migratory species moving to North America. The novel Borrelia species is divergent from the LD and RF species, and is more closely related to the reptile- and echidna-associated Borrelia group that was recently described. Genome sequencing showed that this novel Borrelia sp. has a relatively small genome consisting of a 0.9-Mb-large chromosome and an additional 0.3 Mb dispersed on plasmids. It harbors an RF-like genomic organization but with a unique mixture of LD- and RF-specific genes, including genes used by RF Borrelia for the multiphasic antigen-switching system and a number of immune-reactive protein genes used for the diagnosis of LD. Overall, our data indicate that this novel Borrelia is an intermediate taxon between the LD and RF species that may impact a large host spectrum, including American mammals. The designation "Candidatus Borrelia mahuryensis" is proposed for this species.

Interrupted Blood Feeding in Ticks: Causes and Consequences

Ticks are obligate hematophagous arthropods and act as vectors for a great variety of pathogens, including viruses, bacteria, protozoa, and helminths. Some tick-borne viruses, such as Powassan virus and tick-borne encephalitis virus, are transmissible within 15–60 min after tick attachment. However, a minimum of 3–24 h of tick attachment is necessary to effectively transmit bacterial agents such as Ehrlichia spp., Anaplasma spp., and Rickettsia spp. to a new host. Longer transmission periods were reported for Borrelia spp. and protozoans such as Babesia spp., which require a minimum duration of 24–48 h of tick attachment for maturation and migration of the pathogen. Laboratory observations indicate that the probability of transmission of tick-borne pathogens increases with the duration an infected tick is allowed to remain attached to the host. However, the transmission time may be shortened when partially fed infected ticks detach from their initial host and reattach to a new host, on which they complete their engorgement. For example, early transmission of tick-borne pathogens (e.g., Rickettsia rickettsii, Borrelia burgdorferi, and Babesia canis) and a significantly shorter transmission time were demonstrated in laboratory experiments by interrupted blood feeding. The relevance of such situations under field conditions remains poorly documented. In this review, we explore parameters of, and causes leading to, spontaneous interrupted feeding in nature, as well as the effects of this behavior on the minimum time required for transmission of tick-borne pathogens.

Ticks climb the mountains: Ixodid tick infestation and infection by tick-borne pathogens in the Western Alps

In mountain areas of northwestern Italy, ticks were rarely collected in the past. In recent years, a marked increase in tick abundance has been observed in several Alpine valleys, together with more frequent reports of Lyme borreliosis. We then carried out a four-year study to assess the distribution and abundance of ticks and transmitted pathogens and determine their altitudinal limit in a natural park area in Piedmont region. Ixodes ricinus and Dermacentor marginatus were collected from both the vegetation and hunted wild ungulates. Tick abundance was significantly associated with altitude, habitat type and signs of animal presence, roe deer's in particular. Ixodes ricinus prevailed in distribution and abundance and, although their numbers decreased with increasing altitude, we recorded the presence of all active life stages of up to around 1700 m a.s.l., with conifers as the second most infested habitat after deciduous woods. Molecular analyses demonstrated the infection of questing I. ricinus nymphs with B. burgdorferi sensu lato (15.5 %), Rickettsia helvetica and R. monacensis (20.7 %), Anaplasma phagocytophilum (1.9 %), Borrelia miyamotoi (0.5 %) and Neoehrlichia mikurensis (0.5 %). One third of the questing D. marginatus were infected with R. slovaca. We observed a spatial aggregation of study sites infested by B. burgdorferi s.l. infected ticks below 1400 m. Borrelia-infected nymphs prevailed in open areas, while SFG rickettsiae prevalence was higher in coniferous and deciduous woods. Interestingly, prevalence of SFG rickettsiae in ticks doubled above 1400 m, and R. helvetica was the only pathogen detected above 1800 m a.s.l. Tick infestation on hunted wild ungulates indicated the persistence of tick activity during winter months and, when compared to past studies, confirmed the recent spread of I. ricinus in the area. Our study provides new insights into the population dynamics of ticks in the Alps and confirms a further expansion of ticks to higher altitudes in Europe. We underline the importance of adopting a multidisciplinary approach in order to develop effective strategies for the surveillance of tick-borne diseases, and inform the public about the hazard posed by ticks, especially in recently invaded areas.

Multiple Tick-Borne Pathogens in Ixodes ricinus Ticks Collected from Humans in Romania

Ticks are medically important vectors of infectious diseases that are able to transmit pathogens to humans and animals. Tick-borne diseases represent a major health concern, posing an increasing risk to the public health during the last century and affecting millions of people. The aim of the current study was to provide epidemiological data regarding the presence of certain tick-borne pathogens in ticks feeding on humans in Romania. Overall, 522 Ixodes ricinus ticks collected from humans were screened for six pathogens: Borrelia spp., Neoehrlichia mikurensis, Babesia spp., Coxiella spp., Bartonella spp., and Francisella tularensis. Ticks attached to humans were collected between 2013-2015 in Cluj County, Romania. Conventional, nested and quantitative PCR were used to detect specific genetic sequences of each pathogen. For identifying the infectious agents, positive samples were sequenced. The infection prevalence was 21.07% from which 8.18% were mixed infections. The detected agents were Borrelia spp., N. mikurensis and Babesia spp. The present data reveal the endemic occurrence of potentially zoonotic pathogens in Romania. Revealing the current distribution of tick-borne pathogens in ticks collected from humans may provide new insights in understanding the complex ecology of tick-borne diseases and enlightens current knowledge about the infection prevalence at local, regional and national levels.

Tripartite Interactions among Ixodiphagus hookeri, Ixodes ricinus and Deer: Differential Interference with Transmission Cycles of Tick-Borne Pathogens

For the development of sustainable control of tick-borne diseases, insight is needed in biological factors that affect tick populations. Here, the ecological interactions among Ixodiphagus hookeri, Ixodes ricinus, and two vertebrate species groups were investigated in relation to their effects on tick-borne disease risk. In 1129 questing ticks, I. hookeri DNA was detected more often in I. ricinus nymphs (4.4%) than in larvae (0.5%) and not in adults. Therefore, we determined the infestation rate of I. hookeri in nymphs from 19 forest sites, where vertebrate, tick, and tick-borne pathogen communities had been previously quantified. We found higher than expected co-occurrence rates of I. hookeri with deer-associated Anaplasma phagocytophilum, and lower than expected rates with rodent-associated Borrelia afzelii and Neoehrlichia mikurensis. The prevalence of I. hookeri in nymphs varied between 0% and 16% and was positively correlated with the encounter probability of ungulates and the densities of all life stages of I. ricinus. Lastly, we investigated the emergence of I. hookeri from artificially fed, field-collected nymphs. Adult wasps emerged from seven of the 172 fed nymphs. From these observations, we inferred that I. hookeri is parasitizing I. ricinus larvae that are feeding on deer, rather than on rodents or in the vegetation. Since I. hookeri populations depend on deer abundance, the main propagation host of I. ricinus, these wasps have no apparent effect on tick populations. The presence of I. hookeri may directly interfere with the transmission cycle of A. phagocytophilum, but not with that of B. afzelii or N. mikurensis.

Transovarial transmission of Borrelia spp., Rickettsia spp. and Anaplasma phagocytophilum in Ixodes ricinus under field conditions extrapolated from DNA detection in questing larvae

Background

Ixodes ricinus constitutes the main European vector tick for the Lyme borreliosis pathogen Borrelia burgdorferi (sensu lato), the relapsing fever borrelia Borrelia miyamotoi, as well as Anaplasma phagocytophilum and several Rickettsia species. Under laboratory conditions, a transovarial transmission to the next tick generation is described for Rickettsia spp. and Borrelia spp., especially regarding B. miyamotoi, whereas the efficiency of transovarial transfer under field conditions is largely unstudied.

Methods

In order to better estimate the potential infection risk by tick larvae for humans and animals, 1500 I. ricinus larvae from 50 collected “nests” (larvae adhering to the flag in a clumped manner) were individually examined for Borrelia, Rickettsia and A. phagocytophilum DNA using quantitative real-time PCR (qPCR).

Results

Thirty-nine of 50 nests each (78.0%, 95% CI: 64.0–88.5%) were positive for Borrelia spp. and Rickettsia spp. DNA, and in three nests (6.0%, 95% CI: 1.3–16.5%) A. phagocytophilum DNA was detected. Overall, DNA from at least one pathogen could be detected in 90.0% (45/50, 95% CI: 78.2–96.7%) of the nests. Of the 1500 larvae, 137 were positive for Borrelia spp. DNA (9.1%, 95% CI: 7.7–10.7%), 341 for Rickettsia spp. DNA (22.7%, 95% CI: 20.6–24.9%) and three for A. phagocytophilum DNA (0.2%, 95% CI: 0–0.6%). Quantity of Borrelia spp. and Anaplasma spp. DNA in positive larvae was low, with 2.7 × 10⁰Borrelia 5S-23S gene copies and 2.4 × 10¹A. phagocytophilum msp2/p44 gene copies detected on average, while Rickettsia-positive samples contained on average 5.4 × 10²gltA gene copies. Coinfections were found in 66.0% (33/50, 95% CI: 51.2–78.8%) of the nests and 8.6% (38/443, 95% CI: 6.1–11.6%) of positive larvae. In fact, larvae had a significantly higher probability of being infected with Borrelia spp. or Rickettsia spp. when both pathogens were present in the nest.

Conclusions

This study provides evidence for transovarial transmission of Rickettsia spp. and Borrelia spp. in I. ricinus under field conditions, possibly facilitating pathogen persistence in the ecosystem and reducing the dependence on the presence of suitable reservoir hosts. Further studies are needed to prove transovarial transmission and to explain the surprisingly high proportion of nests containing Rickettsia and/or Borrelia DNA-positive larvae compared to infection rates in adult ticks commonly reported in other studies.

Evaluation of Disease Causality of Rare Ixodes ricinus-Borne Infections in Europe

In Europe, Ixodes ricinus ticks transmit pathogens such as Borrelia burgdorferi sensu lato and tick-borne encephalitis virus (TBEV). In addition, there is evidence for transmission to humans from I. ricinus of Anaplasma phagocytophilum, Babesia divergens, Babesia microti, Babesia venatorum, Borrelia miyamotoi, Neoehrlichia mikurensis, Rickettsia helvetica and Rickettsia monacensis. However, whether infection with these potential tick-borne pathogens results in human disease has not been fully demonstrated for all of these tick-borne microorganisms. To evaluate the available evidence for a causative relation between infection and disease, the current study analyses European case reports published from 2008 to 2018, supplemented with information derived from epidemiological and experimental studies. The evidence for human disease causality in Europe found in this review appeared to be strongest for A. phagocytophilum and B. divergens. Nonetheless, some knowledge gaps still exist. Importantly, comprehensive evidence for pathogenicity is lacking for the remaining tick-borne microorganisms. Such evidence could be gathered best through prospective studies, for example, studies enrolling patients with a fever after a tick bite, the development of specific new serological tools, isolation of these microorganisms from ticks and patients and propagation in vitro, and through experimental studies.

Effect of rodent density on tick and tick-borne pathogen populations: consequences for infectious disease risk

BACKGROUND

Rodents are considered to contribute strongly to the risk of tick-borne diseases by feeding Ixodes ricinus larvae and by acting as amplifying hosts for pathogens. Here, we tested to what extent these two processes depend on rodent density, and for which pathogen species rodents synergistically contribute to the local disease risk, i.e. the density of infected nymphs (DIN).

METHODS

In a natural woodland, we manipulated rodent densities in plots of 2500 m² by either supplementing a critical food source (acorns) or by removing rodents during two years. Untreated plots were used as controls. Collected nymphs and rodent ear biopsies were tested for the presence of seven tick-borne microorganisms. Linear models were used to capture associations between rodents, nymphs, and pathogens.

RESULTS

Investigation of data from all plots, irrespective of the treatment, revealed a strong positive association between rodent density and nymphal density, nymphal infection prevalence (NIP) with Borrelia afzelii and Neoehrlichia mikurensis, and hence DIN's of these pathogens in the following year. The NIP, but not the DIN, of the bird-associated Borrelia garinii, decreased with increasing rodent density. The NIPs of Borrelia miyamotoi and Rickettsia helvetica were independent of rodent density, and increasing rodent density moderately increased the DINs. In addition, NIPs of Babesia microti and Spiroplasma ixodetis decreased with increasing rodent density, which had a non-linear association with DINs of these microorganisms.

CONCLUSIONS

A positive density dependence for all rodent- and tick-associated tick-borne pathogens was found, despite the observation that some of them decreased in prevalence. The effects on the DINs were variable among microorganisms, more than likely due to contrasts in their biology (including transmission modes, host specificity and transmission efficiency). The strongest associations were found in rodent-associated pathogens that most heavily rely on horizontal transmission. Our results draw attention to the importance of considering transmission mode of a pathogen while developing preventative measures to successfully reduce the burden of disease.

Novel targets and strategies to combat borreliosis

Lyme borreliosis is a bacterial infection that can be spread to humans by infected ticks and may severely affect many organs and tissues. Nearly four decades have elapsed since the discovery of the disease agent called Borrelia burgdorferi. Although there is a plethora of knowledge on the infectious agent and thousands of scientific publications, an effective way on how to combat and prevent Lyme borreliosis has not been found yet. There is no vaccine for humans available, and only one active vaccine program in clinical development is currently running. A spirited search for possible disease interventions is of high public interest as surveillance data indicates that the number of cases of Lyme borreliosis is steadily increasing in Europe and North America. This review provides a condensed digest of the history of vaccine development up to new promising vaccine candidates and strategies that are targeted against Lyme borreliosis, including elements of the tick vector, the reservoir hosts, and the Borrelia pathogen itself.

Host dispersal shapes the population structure of a tick-borne bacterial pathogen

Birds are hosts for several zoonotic pathogens. Because of their high mobility, especially of longdistance migrants, birds can disperse these pathogens, affecting their distribution and phylogeography. We focused on Borrelia burgdorferi sensu lato, which includes the causative agents of Lyme borreliosis, as an example for tick-borne pathogens, to address the role of birds as propagation hosts of zoonotic agents at a large geographical scale. We collected ticks from passerine birds in 11 European countries. B. burgdorferi s.l. prevalence in Ixodes spp. was 37% and increased with latitude. The fieldfare Turdus pilaris and the blackbird T. merula carried ticks with the highest Borrelia prevalence (92 and 58%, respectively), whereas robin Erithacus rubecula ticks were the least infected (3.8%). Borrelia garinii was the most prevalent genospecies (61%), followed by B. valaisiana (24%), B. afzelii (9%), B. turdi (5%) and B. lusitaniae (0.5%). A novel Borrelia genospecies "Candidatus Borrelia aligera" was also detected. Multilocus sequence typing (MLST) analysis of B. garinii isolates together with the global collection of B. garinii genotypes obtained from the Borrelia MLST public database revealed that: (a) there was little overlap among genotypes from different continents, (b) there was no geographical structuring within Europe, and (c) there was no evident association pattern detectable among B. garinii genotypes from ticks feeding on birds, questing ticks or human isolates. These findings strengthen the hypothesis that the population structure and evolutionary biology of tick-borne pathogens are shaped by their host associations and the movement patterns of these hosts.

Whole genome sequencing of Borrelia miyamotoi isolate Izh-4: reference for a complex bacterial genome

BACKGROUND

The genus Borrelia comprises spirochaetal bacteria maintained in natural transmission cycles by tick vectors and vertebrate reservoir hosts. The main groups are represented by a species complex including the causative agents of Lyme borreliosis and relapsing fever group Borrelia. Borrelia miyamotoi belongs to the relapsing fever group of spirochetes and forms distinct populations in North America, Asia, and Europe. As all Borrelia species B. miyamotoi possess an unusual and complex genome consisting of a linear chromosome and a number of linear and circular plasmids. The species is considered an emerging human pathogen and an increasing number of human cases are being described in the Northern hemisphere. The aim of this study was to produce a high quality reference genome that will facilitate future studies into genetic differences between different populations and the genome plasticity of B. miyamotoi.

RESULTS

We used multiple available sequencing methods, including Pacific Bioscience single-molecule real-time technology (SMRT) and Oxford Nanopore technology (ONT) supplemented with highly accurate Illumina sequences, to explore the suitability for whole genome assembly of the Russian B. miyamotoi isolate, Izh-4. Plasmids were typed according to their potential plasmid partitioning genes (PF32, 49, 50, 57/62). Comparing and combining results of both long-read (SMRT and ONT) and short-read methods (Illumina), we determined that the genome of the isolate Izh-4 consisted of one linear chromosome, 12 linear and two circular plasmids. Whilst the majority of plasmids had corresponding contigs in the Asian B. miyamotoi isolate FR64b, there were only four that matched plasmids of the North American isolate CT13-2396, indicating differences between B. miyamotoi populations. Several plasmids, e.g. lp41, lp29, lp23, and lp24, were found to carry variable major proteins. Amongst those were variable large proteins (Vlp) subtype Vlp-α, Vlp-γ, Vlp-δ and also Vlp-β. Phylogenetic analysis of common plasmids types showed the uniqueness in Russian/Asian isolates of B. miyamotoi compared to other isolates.

CONCLUSIONS

We here describe the genome of a Russian B. miyamotoi clinical isolate, providing a solid basis for future comparative genomics of B. miyamotoi isolates. This will be a great impetus for further basic, molecular and epidemiological research on this emerging tick-borne pathogen.

Vector competence studies with hard ticks and Borrelia burgdorferi sensu lato spirochetes: A review

Use of emerging technology allowing for identification of genetic material from pathogens and endosymbionts in ticks collected from humans, domestic animals, wildlife, or the environment has resulted in an avalanche of new data on tick-microorganism associations. This rapidly growing stream of new information is a tremendous resource but also presents challenges, including how detection of pathogen genetic material in ticks should best be interpreted. There is a tendency in the more recent published literature to incorrectly use the term “vector” based on detection of pathogen genetic material from tick species not experimentally confirmed to serve as vectors of the pathogen in question. To serve as a vector of a horizontally maintained pathogen, such as a Borrelia burgdorferi sensu lato (s.l.) Lyme borreliosis spirochete, the tick species in question must be capable of acquiring the pathogen while feeding in the larval or nymphal stage on an infectious host, maintaining it transstadially through the molt, and then transmitting the pathogen to a naïve host while feeding in the subsequent nymphal or adult stage. This review examines the experimental evidence for and against species of hard (ixodid) ticks from different genera to serve as vectors of B. burgdorferi s.l. spirochetes. Of the 18 Ixodes species ticks evaluated to date, 13 were experimentally confirmed as vectors of B. burgdorferi s.l. spirochetes. These studies focused primarily on the three major Lyme borreliosis agents: Borrelia burgdorferi sensu stricto, Borrelia afzelii, and Borrelia garinii. In striking contrast, none of 8 tick species from other genera (1 Amblyomma species, 5 Dermacentor species, and 2 Haemaphysalis species) evaluated to date were unequivocally experimentally confirmed as vectors of B. burgdorferi s.l. spirochetes. The strength of the evidence for or against each tick species to serve as a vector of B. burgdorferi s.l. spirochetes is discussed together with key knowledge gaps and research challenges.

A three-years assessment of Ixodes ricinus-borne pathogens in a French peri-urban forest

BACKGROUND

Ixodes ricinus is the predominant tick species in Europe and the primary pathogen vector for both humans and animals. These ticks are frequently involved in the transmission of Borrelia burgdorferi (sensu lato), the causative agents of Lyme borreliosis. While much more is known about I. ricinus tick-borne pathogen composition, information about temporal tick-borne pathogen patterns remain scarce. These data are crucial for predicting seasonal/annual patterns which could improve understanding and prevent tick-borne diseases.

METHODS

We examined tick-borne pathogen (TBP) dynamics in I. ricinus collected monthly in a peri-urban forest over three consecutive years. In total, 998 nymphs were screened for 31 pathogenic species using high-throughput microfluidic real-time PCR.

RESULTS

We detected DNA from Anaplasma phagocytophilum (5.3%), Rickettsia helvetica (4.5%), Borrelia burgdorferi (s.l.) (3.7%), Borrelia miyamotoi (1.2%), Babesia venatorum (1.5%) and Rickettsia felis (0.1%). Among all analysed ticks, 15.9% were infected by at least one of these microorganisms, and 1.3% were co-infected. Co-infections with B. afzeli/B. garinii and B. garinii/B. spielmanii were significantly over-represented. Moreover, significant variations in seasonal and/or inter-annual prevalence were observed for several pathogens (R. helvetica, B. burgdorferi (s.l.), B. miyamotoi and A. phagocytophilum).

CONCLUSIONS

Analysing TBP prevalence in monthly sampled tick over three years allowed us to assess seasonal and inter-annual fluctuations of the prevalence of TBPs known to circulate in the sampled area, but also to detect less common species. All these data emphasize that sporadic tick samplings are not sufficient to determine TBP prevalence and that regular monitoring is necessary.

Controversies in bacterial taxonomy: The example of the genus Borrelia

In this paper we survey key issues in bacterial taxonomy and review the literature regarding the recent genus separation proposed for the genus Borrelia. We discuss how information on members of the genus Borrelia is increasing but detailed knowledge on the relevant features is available only for a small subset of species. The data accumulated here show that there is considerable overlap in ecology, clinical aspects and molecular features between clades that argue against splitting of the genus Borrelia.

Borrelia spp. in small mammals in Romania

BACKGROUND

Small mammals play an important role in the life-cycle of ticks and are reservoirs for several zoonotic pathogens. The aim of this study was to provide epidemiological data regarding the presence of Borrelia spp. in tissues of small mammals from Romania.

METHODS

We examined 401 individuals belonging to 11 small mammal species collected in Romania. Collections cover the largest effort to survey these reservoirs in the country. Tissue samples were analyzed by multiplex qPCR targeting the ospA gene of Borrelia burgdorferi (s.l.) and a part of the flaB gene of B. miyamotoi. Positive samples were further analysed by conventional PCR and sequenced.

RESULTS

The overall prevalence of infection with Borrelia spp. in small mammal tissues was 4.9%. The most commonly detected species were B. afzelii, followed by B. garinii/B. bavariensis, B. miyamotoi and B. burgdorferi (s.s.). To our knowledge, we report for the first time the detection of Borrelia spp. in Crocidura leucodon and C. suaveolens, and B. miyamotoi in the liver of Myodes glareolus.

CONCLUSIONS

To our knowledge, our study evaluates for the first time the occurrence of Borrelia spp. in small mammals in Romania, contributing to a better knowledge of the distribution of these bacteria. This survey upgrades previous data on the spatial distribution of the pathogens and reveals the importance of animal surveillance regarding Lyme borreliosis and relapsing fever caused by B. miyamotoi.

Counterattacking the tick bite: towards a rational design of anti-tick vaccines targeting pathogen transmission

Hematophagous arthropods are responsible for the transmission of a variety of pathogens that cause disease in humans and animals. Ticks of the Ixodes ricinus complex are vectors for some of the most frequently occurring human tick-borne diseases, particularly Lyme borreliosis and tick-borne encephalitis virus (TBEV). The search for vaccines against these diseases is ongoing. Efforts during the last few decades have primarily focused on understanding the biology of the transmitted viruses, bacteria and protozoans, with the goal of identifying targets for intervention. Successful vaccines have been developed against TBEV and Lyme borreliosis, although the latter is no longer available for humans. More recently, the focus of intervention has shifted back to where it was initially being studied which is the vector. State of the art technologies are being used for the identification of potential vaccine candidates for anti-tick vaccines that could be used either in humans or animals. The study of the interrelationship between ticks and the pathogens they transmit, including mechanisms of acquisition, persistence and transmission have come to the fore, as this knowledge may lead to the identification of critical elements of the pathogens' life-cycle that could be targeted by vaccines. Here, we review the status of our current knowledge on the triangular relationships between ticks, the pathogens they carry and the mammalian hosts, as well as methods that are being used to identify anti-tick vaccine candidates that can prevent the transmission of tick-borne pathogens.

Prevalence and Identification of Borrelia burgdorferi Sensu Lato Genospecies in Ticks from Northeastern China

Lyme disease is considered as one of important tick-transmitted zoonosis in northeastern China, where the causative agents, the Borrelia burgdorferi sensu lato (s.l.) complex, remain poorly characterized. The purpose of the present study was to determine the prevalence and genospecies of B. burgdorferi s.l. in ticks in northeastern China. In May, 2015, a total of 2785 unfed adult ticks were collected in the Jilin and Heilongjiang provinces of northeastern China, with the predominant tick species of Ixodes persulcatus (59.9%), followed by Haemaphysalis concinna (14.8%), Haemaphysalis longicornis (8.9%), Dermacentor nuttalli (9.4%), and Dermacentor silvarum (7.0%). Only I. persulcatus was tested positive for Borrelia spirochetes DNA by PCR, targeting the 5S-23S rRNA intergenic spacer and 16S rRNA genes, with a prevalence of 1.9%. Phylogenetic analysis based on the partial 5S-23S rRNA intergenic spacer and 16S rRNA genes showed that these positive samples were grouped into four pathogenic genospecies for humans, including Borrelia garinii (2.8%), Borrelia afzelii (0.2%), Borrelia bavariensis (0.1%), and Borrelia bissettii (0.1%). These results showed that B. garinii is the predominant genospecies and I. persulcatus is the main tick host and carrier in northeastern China. To our knowledge, B. bissettii were detected for the first time in China.

Vertical transmission rates of Borrelia miyamotoi in Ixodes scapularis collected from white-tailed deer

Borrelia miyamotoi is a relapsing fever spirochete transmitted by ticks in the Ixodes ricinus complex. In the eastern United States, B. miyamotoi is transmitted by I. scapularis, which also vectors several other pathogens including B. burgdorferi sensu stricto. In contrast to Lyme borreliae, B. miyamotoi can be transmitted vertically from infected female ticks to their progeny. Therefore, in addition to nymphs and adults, larvae can vector B. miyamotoi to wildlife and human hosts. Two widely varying filial infection prevalence (FIP) estimates - 6% and 73% - have been reported previously from two vertically infected larval clutches; to our knowledge, no other estimates of FIP or transovarial transmission (TOT) rates for B. miyamotoi have been described in the literature. Thus, we investigated TOT and FIP of larval clutches derived from engorged females collected from hunter-harvested white-tailed deer in 2015 (n = 664) and 2016 (n = 599) from Maine, New Hampshire, Tennessee, and Wisconsin. After engorged females oviposited in the lab, they (n = 492) were tested for B. miyamotoi infection by PCR. Subsequently, from each clutch produced by an infected female, larval pools, as well as 100 individual eggs or larvae, were tested. The TOT rate of the 11 infected females was 90.9% (95% CI; 57.1-99.5%) and the mean FIP of the resulting larval clutches was 84.4% (95% CI; 68.1-100%). Even though the overall observed vertical transmission rate (the product of TOT and FIP; 76.7%, 95% CI; 44.6-93.3%) was high, additional horizontal transmission may be required for enzootic maintenance of B. miyamotoi based on the results of a previously published deterministic model. Further investigation of TOT and FIP variability and the underlying mechanisms, both in nature and the laboratory, will be needed to resolve this question. Meanwhile, studies quantifying the acarological risk of Borrelia miyamotoi disease need to consider not only nymphs and adults, but larval I. scapularis as well.

Genome-wide analysis of Borrelia turcica and 'Candidatus Borrelia tachyglossi' shows relapsing fever-like genomes with unique genomic links to Lyme disease Borrelia

Borrelia are tick-borne bacteria that in humans are the aetiological agents of Lyme disease and relapsing fever. Here we present the first genomes of B. turcica and B. tachyglossi, members of a recently described and rapidly expanding Borrelia clade associated with reptile (B. turcica) or echidna (B. tachyglossi) hosts, transmitted by hard ticks, and of unknown pathogenicity. Borrelia tachyglossi and B. turcica genomes are similar to those of relapsing fever Borrelia species, containing a linear ~ 900 kb chromosome, a single long (> 70 kb) linear plasmid, and numerous short (< 40 kb) linear and circular plasmids, as well as a suite of housekeeping and macronutrient biosynthesis genes which are not found in Lyme disease Borrelia. Additionally, both B. tachyglossi and B. turcica contain paralogous vsp and vlp proteins homologous to those used in the multiphasic antigen-switching system used by relapsing fever Borrelia to evade vertebrate immune responses, although their number was greatly reduced compared to human-infectious species. However, B. tachyglossi and B. turcica chromosomes also contain numerous genes orthologous to Lyme disease Borrelia-specific genes, demonstrating a unique evolutionary, and potentially phenotypic link between these groups. Borrelia tachyglossi and B. turcica genomes also have unique genetic features, including degraded and deleted tRNA modification genes, and an expanded range of macronutrient salvage and biosynthesis genes compared to relapsing fever and Lyme disease Borrelia. These genomes and genomic comparisons provide an insight into the biology and evolutionary origin of these Borrelia, and provide a valuable resource for future work.