Natural selection and recombination at host-interacting lipoprotein loci drive genome diversification of Lyme disease and related bacteria

Lyme disease, caused by spirochetes in the Borrelia burgdorferi sensu lato clade within the Borrelia genus, is transmitted by Ixodes ticks and is currently the most prevalent and rapidly expanding tick-borne disease in Europe and North America. We report complete genome sequences of 47 isolates that encompass all established species in this clade while highlighting the diversity of the widespread human pathogenic species B. burgdorferi. A similar set of plasmids has been maintained throughout Borrelia divergence, indicating that they are a key adaptive feature of this genus. Phylogenetic reconstruction of all sequenced Borrelia genomes revealed the original divergence of Eurasian and North American lineages and subsequent dispersals that introduced B. garinii, B. bavariensis, B. lusitaniae, B. valaisiana, and B. afzelii from East Asia to Europe and B. burgdorferi and B. finlandensis from North America to Europe. Molecular phylogenies of the universally present core replicons (chromosome and cp26 and lp54 plasmids) are highly consistent, revealing a strong clonal structure. Nonetheless, numerous inconsistencies between the genome and gene phylogenies indicate species dispersal, genetic exchanges, and rapid sequence evolution at plasmid-borne loci, including key host-interacting lipoprotein genes. While localized recombination occurs uniformly on the main chromosome at a rate comparable to mutation, lipoprotein-encoding loci are recombination hotspots on the plasmids, suggesting adaptive maintenance of recombinant alleles at loci directly interacting with the host. We conclude that within- and between-species recombination facilitates adaptive sequence evolution of host-interacting lipoprotein loci and contributes to human virulence despite a genome-wide clonal structure of its natural populations.

IMPORTANCE

Lyme disease (also called Lyme borreliosis in Europe), a condition caused by spirochete bacteria of the genus Borrelia, transmitted by hard-bodied Ixodes ticks, is currently the most prevalent and rapidly expanding tick-borne disease in the United States and Europe. Borrelia interspecies and intraspecies genome comparisons of Lyme disease-related bacteria are essential to reconstruct their evolutionary origins, track epidemiological spread, identify molecular mechanisms of human pathogenicity, and design molecular and ecological approaches to disease prevention, diagnosis, and treatment. These Lyme disease-associated bacteria harbor complex genomes that encode many genes that do not have homologs in other organisms and are distributed across multiple linear and circular plasmids. The functional significance of most of the plasmid-borne genes and the multipartite genome organization itself remains unknown. Here we sequenced, assembled, and analyzed whole genomes of 47 Borrelia isolates from around the world, including multiple isolates of the human pathogenic species. Our analysis elucidates the evolutionary origins, historical migration, and sources of genomic variability of these clinically important pathogens. We have developed web-based software tools (BorreliaBase.org) to facilitate dissemination and continued comparative analysis of Borrelia genomes to identify determinants of human pathogenicity.

An atlas of human vector-borne microbe interactions reveals pathogenicity mechanisms

Vector-borne diseases are a leading cause of death worldwide and pose a substantial unmet medical need. Pathogens binding to host extracellular proteins (the "exoproteome") represents a crucial interface in the etiology of vector-borne disease. Here, we used bacterial selection to elucidate host-microbe interactions in high throughput (BASEHIT)-a technique enabling interrogation of microbial interactions with 3,324 human exoproteins-to profile the interactomes of 82 human-pathogen samples, including 30 strains of arthropod-borne pathogens and 8 strains of related non-vector-borne pathogens. The resulting atlas revealed 1,303 putative interactions, including hundreds of pairings with potential roles in pathogenesis, including cell invasion, tissue colonization, immune evasion, and host sensing. Subsequent functional investigations uncovered that Lyme disease spirochetes recognize epidermal growth factor as an environmental cue of transcriptional regulation and that conserved interactions between intracellular pathogens and thioredoxins facilitate cell invasion. In summary, this interactome atlas provides molecular-level insights into microbial pathogenesis and reveals potential host-directed targets for next-generation therapeutics.

Development and validation of a multi-target TaqMan qPCR method for detection of Borrelia burgdorferi sensu lato

Reliable detection of bacteria belonging to the Borrelia burgdorferi sensu lato species complex in vertebrate reservoirs, tick vectors, and patients is key to answer questions regarding Lyme borreliosis epidemiology. Nevertheless, the description of characteristics of qPCRs for the detection of B. burgdorferi s. l. are often limited. This study covers the development and validation of two duplex taqman qPCR assays used to target four markers on the chromosome of genospecies of B. burgdorferi s. l. Analytical specificity was determined with a panel of spirochete strains. qPCR characteristics were specified using water or tick DNA spiked with controlled quantities of the targeted DNA sequences of B. afzelii, B. burgdorferi sensu stricto or B. bavariensis. The effectiveness of detection results was finally evaluated using DNA extracted from ticks and biopsies from mammals whose infectious status had been determined by other detection assays. The developed qPCR assays allow exclusive detection of B. burgdorferi s. l. with the exception of the M16 marker which also detect relapsing fever Borreliae. The limit of detection is between 10 and 40 copies per qPCR reaction depending on the sample type, the B. burgdorferi genospecies and the targeted marker. Detection tests performed on various kind of samples illustrated the accuracy and robustness of our qPCR assays. Within the defined limits, this multi-target qPCR method allows a versatile detection of B. burgdorferi s. l., regardless of the genospecies and the sample material analyzed, with a sensitivity that would be compatible with most applications and a reproducibility of 100% under measurement conditions of limits of detection, thereby limiting result ambiguities.

A crucial nexus: Phylogenetic versus ecological support of the life-cycle of Ixodes ricinus (Ixodoidea: Ixodidae) and Borrelia spp. amplification

The tick Ixodes ricinus parasitizes a wide range of vertebrates. These hosts vary in the relative contribution to the feeding of the different tick life stages, and their interplay is pivotal in the transmission dynamics of tick-borne pathogens. We aimed to know if there is a phylogenetic signal in the feeding and propagation hosts of I. ricinus, independently of other traits, as well as in the amplification of Borrelia burgdorferi (s.l.) in feeding larvae. We used a compilation of 1127 published field surveys in Europe, providing data for 96,586 hosts, resulting in 265,124 larvae, 72,080 nymphs and 37,726 adults. The load of immature ticks on hosts showed a significant phylogenetic signal towards the genera Psammodromus, Podarcis, and Lacerta (nymphs only). We hypothesize that such signal is the background hallmark of the primitive hosts associations of I. ricinus, probably in the glaciation refugia. A secondary phylogenetic signal for tick immatures appeared for some genera of Rodentia and Eulipotyphla. Results suggest the notion that the tick gained these hosts after spread from glaciation refugia. Analyses support a phylogenetic signal in the tick adults, firmly linked to Cetartiodactyla, but not to Carnivora or Aves. This study provides the first demonstration of host preferences in the generalist tick I. ricinus. We further demonstrate that combinations of vertebrates contribute in different proportions supporting the tick life-cycle in biogeographical regions of the Western Palaearctic as each region has unique combinations of dominant hosts. Analysis of the amplification of B. burgdorferi (s.l.) demonstrated that each genospecies is better amplified by competent reservoirs with which a strong phylogenetic signal exists. These vertebrates are the same along the spatial range: environmental traits do not change the reservoirs along the large territory studied. The transmission of B. burgdorferi (s.l.) is amplified by a few species of vertebrates, that share biogeographical regions with the tick vector in variable proportions.

Prevalence of Lyme Disease and Relapsing Fever Borrelia spp. in Vectors, Animals, and Humans within a One Health Approach in Mediterranean Countries

The genus Borrelia has been divided into Borreliella spp., which can cause Lyme Disease (LD), and Borrelia spp., which can cause Relapsing Fever (RF). The distribution of genus Borrelia has broadened due to factors such as climate change, alterations in land use, and enhanced human and animal mobility. Consequently, there is an increasing necessity for a One Health strategy to identify the key components in the Borrelia transmission cycle by monitoring the human-animal-environment interactions. The aim of this study is to summarize all accessible data to increase our understanding and provide a comprehensive overview of Borrelia distribution in the Mediterranean region. Databases including PubMed, Google Scholar, and Google were searched to determine the presence of Borreliella and Borrelia spp. in vectors, animals, and humans in countries around the Mediterranean Sea. A total of 3026 were identified and screened and after exclusion of papers that did not fulfill the including criteria, 429 were used. After examination of the available literature, it was revealed that various species associated with LD and RF are prevalent in vectors, animals, and humans in Mediterranean countries and should be monitored in order to effectively manage and prevent potential infections.

Unprecedented genetic variability of PFam54 paralogs among Eurasian Lyme borreliosis-causing spirochetes

Lyme borreliosis (LB) is the most common vector-borne disease in the Northern Hemisphere caused by spirochetes belonging to the Borrelia burgdorferi sensu lato (Bbsl) complex. Borrelia spirochetes circulate in obligatory transmission cycles between tick vectors and different vertebrate hosts. To successfully complete this complex transmission cycle, Bbsl encodes for an arsenal of proteins including the PFam54 protein family with known, or proposed, influences to reservoir host and/or vector adaptation. Even so, only fragmentary information is available regarding the naturally occurring level of variation in the PFam54 gene array especially in relation to Eurasian-distributed species. Utilizing whole genome data from isolates (n = 141) originated from three major LB-causing Borrelia species across Eurasia (B. afzelii, B. bavariensis, and B. garinii), we aimed to characterize the diversity of the PFam54 gene array in these isolates to facilitate understanding the evolution of PFam54 paralogs on an intra- and interspecies level. We found an extraordinarily high level of variation in the PFam54 gene array with 39 PFam54 paralogs belonging to 23 orthologous groups including five novel paralogs. Even so, the gene array appears to have remained fairly stable over the evolutionary history of the studied Borrelia species. Interestingly, genes outside Clade IV, which contains genes encoding for proteins associated with Borrelia pathogenesis, more frequently displayed signatures of diversifying selection between clades that differ in hypothesized vector or host species. This could suggest that non-Clade IV paralogs play a more important role in host and/or vector adaptation than previously expected, which would require future lab-based studies to validate.

BS, Raghuraman V, Rajyaguru U, Llamera KE, Andrew L, Anderson AS, Hovius JW, Liberator PA, Simon R, Hao L. Development of a sequence-based in silico OspA typing method for Borrelia burgdorferi sensu lato

Lyme disease (LD), caused by spirochete bacteria of the genus Borrelia burgdorferi sensu lato, remains the most common vector-borne disease in the northern hemisphere. Borrelia outer surface protein A (OspA) is an integral surface protein expressed during the tick cycle, and a validated vaccine target. There are at least 20 recognized Borrelia genospecies, that vary in OspA serotype. This study presents a new in silico sequence-based method for OspA typing using next-generation sequence data. Using a compiled database of over 400 Borrelia genomes encompassing the 4 most common disease-causing genospecies, we characterized OspA diversity in a manner that can accommodate existing and new OspA types and then defined boundaries for classification and assignment of OspA types based on the sequence similarity. To accommodate potential novel OspA types, we have developed a new nomenclature: OspA in silico type (IST). Beyond the ISTs that corresponded to existing OspA serotypes 1-8, we identified nine additional ISTs that cover new OspA variants in B. bavariensis (IST9-10), B. garinii (IST11-12), and other Borrelia genospecies (IST13-17). The IST typing scheme and associated OspA variants are available as part of the PubMLST Borrelia spp. database. Compared to traditional OspA serotyping methods, this new computational pipeline provides a more comprehensive and broadly applicable approach for characterization of OspA type and Borrelia genospecies to support vaccine development.

A high fidelity approach to assembling the complex Borrelia genome

BACKGROUND

Bacteria of the Borrelia burgdorferi sensu lato (s.l.) complex can cause Lyme borreliosis. Different B. burgdorferi s.l. genospecies vary in their host and vector associations and human pathogenicity but the genetic basis for these adaptations is unresolved and requires completed and reliable genomes for comparative analyses. The de novo assembly of a complete Borrelia genome is challenging due to the high levels of complexity, represented by a high number of circular and linear plasmids that are dynamic, showing mosaic structure and sequence homology. Previous work demonstrated that even advanced approaches, such as a combination of short-read and long-read data, might lead to incomplete plasmid reconstruction. Here, using recently developed high-fidelity (HiFi) PacBio sequencing, we explored strategies to obtain gap-free, complete and high quality Borrelia genome assemblies. Optimizing genome assembly, quality control and refinement steps, we critically appraised existing techniques to create a workflow that lead to improved genome reconstruction.

RESULTS

Despite the latest available technologies, stand-alone sequencing and assembly methods are insufficient for the generation of complete and high quality Borrelia genome assemblies. We developed a workflow pipeline for the de novo genome assembly for Borrelia using several types of sequence data and incorporating multiple assemblers to recover the complete genome including both circular and linear plasmid sequences.

CONCLUSION

Our study demonstrates that, with HiFi data and an ensemble reconstruction pipeline with refinement steps, chromosomal and plasmid sequences can be fully resolved, even for complex genomes such as Borrelia. The presented pipeline may be of interest for the assembly of further complex microbial genomes.

First molecular confirmation of multiple zoonotic vector-borne diseases in pet dogs and cats of Hong Kong SAR

In recent years, the incidence of vector-borne diseases (VBDs) has increased throughout the globe. In particular, tick-borne diseases (e.g., caused by Ehrlichia canis, E. ewingii, Anaplasma phagocytophilum, A. platys, Borrelia burgdorferi sensu stricto (s.s.) and Babesia gibsoni) and mosquito-borne diseases (e.g., caused by Dirofilaria immitis) diseases pose a burden on animal health. Nevertheless, there have been no studies undertaken on the occurrence of VBDs in pet dogs and cats in Hong Kong SAR. This study fills this gap, and is the first to determine the seroprevalence of major VBDs, such as those caused by D. immitis, E. canis, E. ewingii, A. phagocytophilum, A. platys and B. burgdorferi s.s, in dogs and cats through commercially available SNAP 4Dx plus testing. Infection by all these pathogens and Babesia sp. was further assessed through PCR and DNA sequencing. A total of 224 blood samples were collected from domestic dogs (n = 159) and cats (n = 65) in Hong Kong SAR during summer 2022. Hematocrit and platelet counts were determined in each blood sample and other hematological parameters were assessed using an automatic hematology analyzer and vortex the specimen for one to two minutes at or near the highest setting to minimize the clumping. All cat sera samples were negative for tested pathogens, but antibodies against some of the pathogens were detected in dog sera samples. Here, the highest figures were recorded for seroprevalence of E. canis/E. ewingii (10.7%), followed by D. immitis (5.7%), and A. phagocytophilum/A. platys (2.5%). No B. burgdorferi s.s. antibodies were detected in any of the dogs tested. Through molecular diagnostics, we detected the presence of B. gibsoni (3.7%), E. canis (3.1%), D. immitis (5.7%), and A. phagocytophilum (1.3%). Neighbor-Joining phylogenetic trees for vector-borne pathogens (i.e., genus Anaplasma sp.) showed 100% clustering to Japan, the USA and Germany, whereas genus Ehrlichia sp. showed 100% clustering to China, Turkey, Cuba, and Greece. Similarly, genus Babesia sp. clustered 100% to India, Sri Lanka and Austria, while D. immitis clustered in Iraq, South Korea, Portugal, France, the USA and Italy. This study provides the first evidence on the occurrence of tick-borne pathogens in pet dogs in Hong Kong SAR. Based on these findings, it is recommended that appropriate screening should be undertaken in domestic dogs to evaluate the prevalence of these pathogens and promote the timely control of VBDs.

Detection, identification and genotyping of Borrelia spp. in ticks of Coastal-Karst and Littoral-Inner Carniola regions in Slovenia

The density and spread of tick vector species have increased throughout Europe in the last 30 years, leading to an increase of Lyme borreliosis cases, including in Slovenia. The aim of this study was to isolate Borrelia strains and determine the prevalence of B. burgdorferi sensu lato and B. miyamotoi in adults of Ixodes ricinus (Linnaeus) collected in 2019 in the two regions of the country (Coastal-Karst and Littoral-Inner Carniola) by cultivation and PCR. We isolated B. burgdorferi s.l. by culture method in 28/559 (5%) ticks from both regions. Culture-negative samples (531/559, i.e., 95%) were additionally tested by real-time PCR. In 155/531 (29.2%) PCR-positive samples, a fragment of flaB or glpQ was amplified and further sequenced to identify species of the Borrelia. Using both methods, cultivation and PCR, Borrelia spp. prevalence was 32.7% in the Coastal-Karst region and 33.0% in the Littoral-Inner Carniola region. Genotyping of the Borrelia spp. isolates revealed that 17/28 (60%) were B. garinii subtype Mlg2. Of all tick samples tested for B. miyamotoi 8/398 (2%) were PCR positive. Based on previous studies in these regions, we had expected more ticks to be infected with B. afzelii, but genotyping revealed that B. garinii was the most abundant.

Out of Asia? Expansion of Eurasian Lyme borreliosis causing genospecies display unique evolutionary trajectories

Vector-borne pathogens exist in obligate transmission cycles between vector and reservoir host species. Host and vector shifts can lead to geographic expansion of infectious agents and the emergence of new diseases in susceptible individuals. Three bacterial genospecies (Borrelia afzelii, Borrelia bavariensis, and Borrelia garinii) predominantly utilize two distinct tick species as vectors in Asia (Ixodes persulcatus) and Europe (Ixodes ricinus). Through these vectors, the bacteria can infect various vertebrate groups (e.g., rodents, birds) including humans where they cause Lyme borreliosis, the most common vector-borne disease in the Northern hemisphere. Yet, how and in which order the three Borrelia genospecies colonized each continent remains unclear including the evolutionary consequences of this geographic expansion. Here, by reconstructing the evolutionary history of 142 Eurasian isolates, we found evidence that the ancestors of each of the three genospecies probably have an Asian origin. Even so, each genospecies studied displayed a unique substructuring and evolutionary response to the colonization of Europe. The pattern of allele sharing between continents is consistent with the dispersal rate of the respective vertebrate hosts, supporting the concept that adaptation of Borrelia genospecies to the host is important for pathogen dispersal. Our results highlight that Eurasian Lyme borreliosis agents are all capable of geographic expansion with host association influencing their dispersal; further displaying the importance of host and vector association to the geographic expansion of vector-borne pathogens and potentially conditioning their capacity as emergent pathogens.

Allelic Variants of P66 Gene in Borrelia bavariensis Isolates from Patients with Ixodid Tick-Borne Borreliosis

Protein P66 is one of the crucial virulence factors of Borrelia, inducing the production of specific antibodies in patients with ixodid tick-borne borreliosis (ITBB). Various species of Borrelia are characterized by genetic variability of the surface-exposed loop of P66. However, little is known about this variability in Borrelia bavariensis. Here we describe the variability of the nucleotide sequences of P66 gene locus in isolates of B. bavariensis. Analysis of nucleotide sequences of P66 in 27 isolates of B. bavariensis from ITBB patients revealed three allelic variants of this gene. The alignment score of amino acid sequences in the isolates showed amino acid replacements in various positions confirming the presence of three allelic variants. Two of them are characteristic only for some isolates of B. bavariensis of the Eurasian gene pool from various parts of the geographic ranges of B. bavariensis from various samples. At least three allelic variants of P66 B. bavariensis have been identified, which have different amino acid expression, occur with different frequency in ITBB patients and, presumably, can have different effects on the course of the infection.

Mites, rodents, and pathogens: A global review for a multi-species interaction in disease ecology

More than ever, there is a need to understand how pathogens, vectors and hosts occur temporally and spatially to predict the occurrence of zoonotic outbreaks. Related to this, mites of the Mesostigmata and Trombidiformes orders have the potential to transmit several diseases, yet their information of occurrence, distribution and zoonotic accompanying agents have not been systematically organized. We conducted a systematic review using a combination of words through the Sysrev platform, as well as literature searches in specialised databases to identify global patterns of infections, trends in mite-rodent-pathogen research and existing knowledge gaps. The inclusion criterion was the detection of pathogens in mites, either by molecular or serological techniques or by direct observation under the microscope, which rendered 125 papers. Most works have been carried out in Asia, mainly around the transmission of Orientia tsutsugamushi. Interestingly, co-infections, i.e., the presence of more than one pathogen in an individual, are common in other Acari groups such as ticks. Moreover, this is not the case for Trombidiformes and Mesostigmata as only 4.7 percent of the articles reviewed detected more than one pathogen in these mites. It is important to include a multi-host, multi-vector and multi-pathogen approaches to understand complex systems in disease ecology. A synergy between mite taxonomists, physicians and veterinarians, decision-makers, governmental organisations, and society is needed to address the emergence of mite-borne new or neglected diseases.

Diversity of Borrelia burgdorferi sensu lato in ticks and small mammals from different habitats

BACKGROUND

Ixodid ticks are important vectors for zoonotic pathogens, with Ixodes ricinus being the most important in Europe. Rodents are hosts of immature life stages of I. ricinus ticks and are considered main reservoirs for tick-borne pathogens, e.g. Borrelia burgdorferi. The aim of this study was to analyse the prevalence as well as genospecies and sequence type (ST) diversity of Borrelia burgdorferi sensu lato in ticks and small mammals from central Germany and to elaborate on the influence of environmental and/or individual host and vector factors on Borrelia prevalence.

METHODS

After species identification, 1167 small mammal skin samples and 1094 ticks from vegetation were screened by B. burgdorferi sensu lato real-time polymerase chain reaction, and positive samples were characterized by multilocus sequence typing. Generalized linear (mixed) models were used to estimate how seasonality, small mammal species/tick life stage and habitat affect individual infection status.

RESULTS

In total, 10 small mammal species and three tick species, Ixodes ricinus, Ixodes inopinatus (both considered members of the I. ricinus complex) and Dermacentor reticulatus, were investigated. Borrelia DNA was detected in eight host species, i.e. the striped field mouse (Apodemus agrarius), the yellow-necked field mouse (Apodemus flavicollis), the wood mouse (Apodemus sylvaticus), the water vole (Arvicola amphibius), the bank vole (Clethrionomys glareolus), the field vole (Microtus agrestis), the common vole (Microtus arvalis), and the common shrew (Sorex araneus). Two species were Borrelia negative, the greater white-toothed shrew (Crocidura russula) and the pygmy shrew (Sorex minutus). The average prevalence was 6.2%, with two genospecies detected, Borrelia afzelii and Borrelia garinii, and at least three STs that had not been previously reported in small mammals. Borrelia prevalence in small mammals did not differ between seasons. Six genospecies of Borrelia-Borrelia afzelii, Borrelia valaisiana, Borrelia garinii, Borrelia lusitaniae, Borrelia spielmanii, and Borrelia burgdorferi sensu stricto-and 25 STs of Borrelia, of which 12 have not been previously described at all and five have not been previously reported in Germany, were detected in 13% of I. ricinus complex ticks. Prevalence was highest in adult females (25.3%) and lowest in nymphs (11.4%). Prevalence was significantly higher in ticks from grassland (16.8%) compared to forests (11.4%).

CONCLUSIONS

The high level of small mammal diversity in this region of Germany seems to be reflected in a wide variety of genospecies and STs of B. burgdorferi.

Molecular detection of Borrelia burgdorferi (sensu lato) and Rickettsia spp. in hard ticks distributed in Tokachi District, eastern Hokkaido, Japan

Ticks transmit various pathogens, including parasites, bacteria and viruses to humans and animals. To investigate the ticks and the potentially zoonotic pathogens that they may carry, questing ticks were collected in 2017 from 7 sites in Tokachi District, eastern Hokkaido, Japan. A total of 1563 ticks including adults (male and female), nymphs and larvae were collected. Four species of ticks were identified: Ixodes ovatus, Ixodes persulcatus, Haemaphysalis japonica and Haemaphysalis megaspinosa. Of the 1563 ticks, 1155 were used for DNA extraction. In total, 527 individual tick DNA samples prepared from adults (n = 484), nymphs (n = 41) and larvae (n = 2); and 67 pooled tick DNA samples prepared from larval stages (n = 628) were examined using PCR methods and sequencing to detect Borrelia burgdorferi (sensu lato) and Rickettsia spp. The phylogenetic analysis of Borrelia spp. flaB gene sequences showed the presence of the human pathogenic B. burgdorferi (s.l.) species (Borrelia garinii, Borrelia bavariensis and Borrelia afzelii) in I. persulcatus, whereas the non-pathogenic species Borrelia japonica was found only in I. ovatus. In I. persulcatus, B. garinii and/or its closely related species B. bavariensis was detected in both adults and nymphs at a prevalence of 21.9% whereas B. afzelii was found only in adults (1.8%). The prevalence of B. japonica in adult I. ovatus was 21.8%. Rickettsia species were identified through phylogenetic analysis based on gltA, 16S rRNA, ompB and sca4 genes. Four genotypes were detected in the samples which were classified into three species. The prevalence of human pathogenic Rickettsia helvetica was 26.0% in I. persulcatus adults and nymphs, 55.6% in I. persulcatus larval pools, and 1.7% in H. megaspinosa larval pools. The prevalence of “Candidatus R. tarasevichiae” was 15.4% in I. persulcatus adults and nymphs and 33.3% in I. persulcatus larval pools. The prevalence of “Candidatus R. principis” in H. megaspinosa adults and nymphs was 11.1% whereas it was detected in 3.4% of the H. megaspinosa larval pools. These results indicate that most of the risks of Lyme borreliosis and spotted fever group rickettsiosis infection in eastern Hokkaido, Japan, are restricted to I. persulcatus.

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Four species of ticks were collected in Tokachi District, eastern Hokkaido, Japan.

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Tick DNA samples were subjected to PCR to detect Borrelia spp. and Rickettsia spp.

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Borrelia garinii/Borrelia bavariensis and Borrelia afzelii were detected in Ixodes persulcatus.

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Rickettsia helvetica and “Candidatus R. tarasevichiae” were detected in I. persulcatus.

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First molecular detection of “Candidatus R. principis” in larvae of Haemaphysalis megaspinosa.

Utilizing Two Borrelia bavariensis Isolates Naturally Lacking the PFam54 Gene Array To Elucidate the Roles of PFam54-Encoded Proteins

Lyme borreliosis is the most common vector-borne disease in the Northern Hemisphere, caused by spirochetes belonging to the Borrelia burgdorferi sensu lato species complex, which are transmitted by ixodid ticks. B. burgdorferi sensu lato species produce a family of proteins on the linear plasmid 54 (PFam54), some of which confer the functions of cell adhesion and inactivation of complement, the first line of host defense. However, the impact of PFam54 in promoting B. burgdorferi sensu lato pathogenesis remains unclear because of the hurdles to simultaneously knock out all PFam54 proteins in a spirochete. Here, we describe two Borrelia bavariensis strains, PBN and PNi, isolated from patients naturally lacking PFam54 but maintaining the rest of the genome with greater than 95% identity to the reference B. bavariensis strain, PBi. We found that PBN and PNi less efficiently survive in human serum than PBi. Such defects were restored by introducing two B. bavariensis PFam54 recombinant proteins, BGA66 and BGA71, confirming the role of these proteins in providing complement evasion of B. bavariensis. Further, we found that all three strains remain detectable in various murine tissues 21 days post-subcutaneous infection, supporting the nonessential role of B. bavariensis PFam54 in promoting spirochete persistence. This study identified and utilized isolates deficient in PFam54 to associate the defects with the absence of these proteins, building the foundation to further study the role of each PFam54 protein in contributing to B. burgdorferi sensu lato pathogenesis. IMPORTANCE To establish infections, Lyme borreliae utilize various means to overcome the host's immune system. Proteins encoded by the PFam54 gene array play a role in spirochete survival in vitro and in vivo. Moreover, this gene array has been described in all currently available Lyme borreliae genomes. By investigating the first two Borrelia bavariensis isolates naturally lacking the entire PFam54 gene array, we showed that both patient isolates display an increased susceptibility to human serum, which can be rescued in the presence of two PFam54 recombinant proteins. However, both isolates remain infectious to mice after intradermal inoculation, suggesting the nonessential role of PFam54 during the long-term, but may differ slightly in the colonization of specific tissues. Furthermore, these isolates show high genomic similarity to type strain PBi (>95%) and could be used in future studies investigating the role of each PFam54 protein in Lyme borreliosis pathogenesis.

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.

The Role of Ticks in the Emergence of Borrelia burgdorferi as a Zoonotic Pathogen and Its Vector Control: A Global Systemic Review

Ticks are widely distributed across the globe, serving as hosts for numerous pathogens that make them major contributors to zoonotic parasitosis. Borrelia burgdorferi is a bacterial species that causes an emerging zoonotic tick-borne disease known as Lyme borreliosis. The role of ticks in the transmission of this pathogen was explored in this study. According to this systematic review, undertaken according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, 19 tick species are known to carry Borrelia burgdorferi, with more than half of the recorded cases in the last two decades related to Ixodes ricinus and Ixodes scapularis ticks. Forty-six studies from four continents, Europe, North America, Asia, and Africa, reported this pathogen in ticks collected from vegetation, animals, and humans. This study highlights an increasing distribution of tick-associated Borrelia burgdorferi, likely driven by accelerated tick population increases in response to climate change coupled with tick dispersal via migratory birds. This updated catalogue helps in compiling all tick species responsible for the transmission of B. burgdorferi across the globe. Gaps in research exist on Borrelia burgdorferi in continents such as Asia and Africa, and in considering environmentally friendly vector control strategies in Europe and North America.

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.

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.

Recent Progress in Lyme Disease and Remaining Challenges

Lyme disease (also known as Lyme borreliosis) is the most common vector-borne disease in the United States with an estimated 476,000 cases per year. While historically, the long-term impact of Lyme disease on patients has been controversial, mounting evidence supports the idea that a substantial number of patients experience persistent symptoms following treatment. The research community has largely lacked the necessary funding to properly advance the scientific and clinical understanding of the disease, or to develop and evaluate innovative approaches for prevention, diagnosis, and treatment. Given the many outstanding questions raised into the diagnosis, clinical presentation and treatment of Lyme disease, and the underlying molecular mechanisms that trigger persistent disease, there is an urgent need for more support. This review article summarizes progress over the past 5 years in our understanding of Lyme and tick-borne diseases in the United States and highlights remaining challenges.

Unexpected failure of Ixodes scapularis nymphs to transmit a North American Borrelia bissettiae strain

Globally, the Borrelia burgdorferi (sensu lato) complex comprises more than 21 species of spirochetes. Although the USA is home to a diverse fauna of Lyme disease group Borrelia species, only two are considered responsible for human clinical disease: Borrelia burgdorferi (sensu stricto) and Borrelia mayonii. However, evidence has implicated additional B. burgdorferi (s.l.) species in human illness elsewhere. While much research has focused on the B. burgdorferi (s.s.)-tick interface, tick vectors for most of the other North American Lyme disease group Borrelia species remain experimentally unconfirmed. In this report we document the ability of Ixodes scapularis to acquire but not transmit a single strain of Borrelia bissettiae, a potential human pathogen, in a murine infection model. Pathogen-free I. scapularis larvae were allowed to feed on mice with disseminated B. burgdorferi (s.s.) or B. bissettiae infections. Molted infected nymphs were then allowed to feed on naïve mice to assess transmission to a susceptible host through spirochete culture and qPCR throughout in ticks collected at various developmental stages (fed larvae and nymphs, molted nymphs, and adults). In this study, similar proportions of I. scapularis larvae acquired B. bissettiae and B. burgdorferi (s.s.) but transstadial passage to the nymphal stage was less effective for B. bissettiae. Furthermore, B. bissettiae-infected nymphs did not transmit B. bissettiae infection to naïve susceptible mice as determined by tissue culture and serology. In the tick, B. bissettiae spirochete levels slightly increased from fed larvae to molted and then fed nymphs, yet the bacteria were absent in molted adults. Moreover, in contrast to B. burgdorferi (s.s.), B. bissettiae failed to exponentially increase in upon completion of feeding in our transmission experiment. In this specific model, I. scapularis was unable to support B. bissettiae throughout its life-cycle, and while live spirochetes were detected in B. bissettiae-infected ticks fed on naïve mice, there was no evidence of murine infection. These data question the vector competence of Ixodes scapularis for B. bissettiae. More importantly, this specific B. bissettiae-I. scapularis model may provide a tool for researchers to delineate details on mechanisms involved in Borrelia-tick compatibility.

Borrelia burgdorferi Sensu Lato in Questing and Engorged Ticks from Different Habitat Types in Southern Germany

Borrelia burgdorferi sensu lato (s.l.) causes the most common tick-borne infection in Europe, with Germany being amongst the countries with the highest incidences in humans. This study aimed at (1) comparing infection rates of B. burgdorferi s.l. in questing Ixodes ricinus ticks from different habitat types in Southern Germany, (2) analysing genospecies distribution by habitat type, and (3) testing tissue and ticks from hosts for B. burgdorferi s.l. Questing ticks from urban, pasture, and natural habitats together with feeding ticks from cattle (pasture) and ticks and tissue samples from wild boars and roe deer (natural site) were tested by PCR and RFLP for species differentiation. B. burgdorferi s.l. was found in 29.8% questing adults and 15% nymphs. Prevalence was lower at the urban sites with occurrence of roe deer than where these were absent. Borrelia burgdorferi s.l. DNA was found in 4.8% ticks from roe deer, 6.3% from wild boar, and 7.8% from cattle. Six genospecies were identified in unfed ticks: Borrelia afzelii (48.6%), Borrelia burgdorferi sensu stricto (16%), Borrelia garinii (13.2%), Borrelia valaisiana (7.5%), Borrelia spielmanii (6.2%), and Borrelia bavariensis (0.9%). This study shows high infection levels and a great diversity of Borrelia in questing ticks. The presence of roe deer seems to reduce B. burgdorferi s.l. infection rates in tick populations.

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.

The Population Structure of Borrelia lusitaniae Is Reflected by a Population Division of Its Ixodes Vector

Populations of vector-borne pathogens are shaped by the distribution and movement of vector and reservoir hosts. To study what impact host and vector association have on tick-borne pathogens, we investigated the population structure of Borrelia lusitaniae using multilocus sequence typing (MLST). Novel sequences were acquired from questing ticks collected in multiple North African and European locations and were supplemented by publicly available sequences at the Borrelia Pubmlst database (accessed on 11 February 2020). Population structure of B. lusitaniae was inferred using clustering and network analyses. Maximum likelihood phylogenies for two molecular tick markers (the mitochondrial 16S rRNA locus and a nuclear locus, Tick-receptor of outer surface protein A, trospA) were used to confirm the morphological species identification of collected ticks. Our results confirmed that B. lusitaniae does indeed form two distinguishable populations: one containing mostly European samples and the other mostly Portuguese and North African samples. Of interest, Portuguese samples clustered largely based on being from north (European) or south (North African) of the river Targus. As two different Ixodes species (i.e., I. ricinus and I. inopinatus) may vector Borrelia in these regions, reference samples were included for I. inopinatus but did not form monophyletic clades in either tree, suggesting some misidentification. Even so, the trospA phylogeny showed a monophyletic clade containing tick samples from Northern Africa and Portugal south of the river Tagus suggesting a population division in Ixodes on this locus. The pattern mirrored the clustering of B. lusitaniae samples, suggesting a potential co-evolution between tick and Borrelia populations that deserve further investigation.

The need to unravel the twisted nature of the Borrelia burgdorferi sensu lato complex across Europe

Lyme borreliosis is a vector-borne infection caused by bacteria under the Borrelia burgdorferi sensu lato complex, both in Europe and North America. Differential gene expression at different times throughout its infectious cycle allows the spirochete to survive very diverse environments within different mammalian hosts as well as the tick vector. To date, the vast majority of data about spirochetal proteins and their functions are from genetic studies carried out on North American strains of a single species, i.e. B. burgdorferi sensu stricto. The whole-genome sequences recently obtained for several European species/strains make it feasible to adapt and use genetic techniques to study inherent differences between them. This review highlights the crucial need to undertake independent studies of genospecies within Europe, given their varying genetic content and pathogenic potential, and differences in clinical manifestation.

Hedgehogs, Squirrels, and Blackbirds as Sentinel Hosts for Active Surveillance of Borrelia miyamotoi and Borrelia burgdorferi Complex in Urban and Rural Environments

Lyme borreliosis (LB), caused by spirochetes of the Borrelia burgdorferi sensu lato (s.l.) complex, is one of the most common vector-borne zoonotic diseases in Europe. Knowledge about the enzootic circulation of Borrelia pathogens between ticks and their vertebrate hosts is epidemiologically important and enables assessment of the health risk for the human population. In our project, we focused on the following vertebrate species: European hedgehog (Erinaceus europaeus), Northern white-breasted hedgehog (E. roumanicus), Eurasian red squirrel (Sciurus vulgaris), and Common blackbird (Turdus merula). The cadavers of accidentally killed animals used in this study constitute an available source of biological material, and we have confirmed its potential for wide monitoring of B. burgdorferi s.l. presence and genospecies diversity in the urban environment. High infection rates (90% for E. erinaceus, 73% for E. roumanicus, 91% for S. vulgaris, and 68% for T. merula) were observed in all four target host species; mixed infections by several genospecies were detected on the level of individuals, as well as in particular tissue samples. These findings show the usefulness of multiple tissue sampling as tool for revealing the occurrence of several genospecies within one animal and the risk of missing particular B. burgdorferi s.l. genospecies when looking in one organ alone.

Baltic Group Tick-Borne Encephalitis Virus Phylogeography: Systemic Inconsistency Pattern between Genetic and Geographic Distances

Tick-Borne Encephalitis Virus (TBEV) is a dangerous arbovirus widely distributed in Northern Eurasia. The area of this pathogen changes over time. At the beginning of the 2000s, the Ixodes tick populations in Karelia increased. At the same time, the area of I. persulcatus, the main vector of the Siberian TBEV subtype, also expanded. Herein, we sequenced 10 viruses isolated from ticks collected in three locations from the Karelia region in 2008-2018. PCR positive samples were passaged in suckling mice or pig embryo kidney cells (PEK). After the second passage in suckling, mice viral RNA was isolated and E-gene fragment was sequenced. Viral sequences were expected to be similar or nearly identical. Instead, there was up to a 4.8% difference in nucleotide sequence, comparable with the most diverse viruses belonging to the Baltic subgroup in Siberian TBEV subtype (Baltic TBEV-Sib). To reveal whether this was systemic or incidental, a comprehensive phylogeographical analysis was conducted. Interestingly, viruses within each geographic region demonstrated comparable diversity to the whole Baltic TBEV-Sib. Moreover, Baltic TBEV-Sib has a distribution area limited by three ecological regions. This means that active virus mixing occurs in the vast geographic area forming one common virus pool. The most plausible explanation is the involvement of flying animals in the TBEV spread.

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.