Transovarial transmission of Borrelia spirochetes by Ixodes scapularis: a summary of the literature and recent observations

Hitchhiker's Guide to Borrelia burgdorferi

Don't Panic. In the nearly 50 years since the discovery of Lyme disease, Borrelia burgdorferi has emerged as an unlikely workhorse of microbiology. Interest in studying host-pathogen interactions fueled significant progress in making the fastidious microbe approachable in laboratory settings, including the development of culture methods, animal models, and genetic tools. By developing these systems, insight has been gained into how the microbe is able to survive its enzootic cycle and cause human disease. Here, we discuss the discovery of B. burgdorferi and its development as a model organism before diving into the critical lessons we have learned about B. burgdorferi biology at pivotal stages of its lifecycle: gene expression changes during the tick blood meal, colonization of a new vertebrate host, and developing a long-lasting infection in that vertebrate until a new tick feeds. Our goal is to highlight the advancements that have facilitated B. burgdorferi research and identify gaps in our current understanding of the microbe.

Bacterial pathogens in Ixodes ricinus collected from lizards Lacerta agilis and Zootoca vivipara in urban areas of Wrocław, SW Poland- preliminary study

The aim of this study was to determine the level of infection of Ixodes ricinus ticks with pathogens (Borrelia spp., Rickettsia spp., and Anaplasma spp.) collected from Lacerta agilis and Zootoca vivipara lizards in the urban areas of Wrocław (SW Poland). The study was carried out in July-August 2020. Lizards were caught by a noose attached to a pole or by bare hands, identified by species, and examined for the presence of ticks. Each lizard was then released at the site of capture. Ticks were removed with tweezers, identified by species using keys, and molecular tests were performed for the presence of pathogens. From 28 lizards (17 specimens of Z. vivipara and 11 specimens of L. agilis) a total of 445 ticks, including 321 larvae and 124 nymphs, identified as I. ricinus were collected. A larger number of ticks were obtained from L. agilis compared to Z. vivipara. Molecular tests for the presence of pathogens were performed on 445 specimens of I. ricinus. The nested PCR method for the fla gene allowed the detection of Borrelia spp. in 9.4% of ticks, and it was higher in ticks from L. agilis (12.0%) than from Z. vivipara (1.0%). The RFLP method showed the presence of three species, including two belonging to the B. burgdorferi s.l. complex (B. lusitaniae and B. afzelii), and B. miyamotoi. The overall level of infection of Rickettsia spp. was 19.3%, including 27.2% in ticks collected from Z. vivipara and 17.0% from L. agilis. Sequencing of randomly selected samples confirmed the presence of R. helvetica. DNA of Anaplasma spp. was detected only in one pool of larvae collected from L. agilis, and sample sequencing confirmed the presence of (A) phagocytophilum. The research results indicate the important role of lizards as hosts of ticks and their role in maintaining pathogens in the environment including urban agglomeration as evidenced by the first recorded presence of (B) miyamotoi and (A) phagocytophilum in I. ricinus ticks collected from L. agilis. However, confirmation of the role of sand lizards in maintaining (B) miyamotoi and A. phagocytophilum requires more studies and sampling of lizard tissue.

Statewide surveillance of Ixodes scapularis (Acari: Ixodidae) for the presence of the human pathogen Borrelia miyamotoi (Spirochaetales: Spirochaetaceae), a relapsing fever spirochete in Pennsylvania, USA, 2019-2020

Borrelia miyamotoi disease is an emerging tick-borne human illness in the United States caused by Borrelia miyamotoi (Spirochaetales: Spirochaetaceae) bacterium. With Pennsylvania reporting thousands of tick-borne disease cases annually, determining the minimum infection rate (MIR) of B. miyamotoi in Ixodes scapularis (Say, Acari: Ixodidae) adults within Pennsylvania is of utmost importance. Active surveillance was performed from October 2019 to April 2020 to collect a minimum of 50 I. scapularis ticks from every county within Pennsylvania and then screened for B. miyamotoi via qPCR. Ticks were collected from all 67 counties with the majority of those being adult I. scapularis. Additional ticks collected were Dermacentor albipictus (Packard, Acari: Ixodidae), Haemaphysalis longicornis (Neumann, Acari: Ixodidae), and immature I. scapularis. Adult I. scapularis were pooled and tested for B. miyamotoi. MIR for positive B. miyamotoi pools and density of infected adult I. scapularis varied by county, with positive pools from 38 Pennsylvania counties. This is the first statewide evaluation of B. miyamotoi in Pennsylvania in questing adult I. scapularis. These prevalence and distribution data will aid health care practitioners within the state of Pennsylvania and the northeast United States to understand potential risk and bring awareness to the lesser known human Borrelia illness, Borrelia miyamotoi disease.

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.

DnaA modulates gene expression and morphologyof the Lyme Disease spirochete

All bacteria encode a multifunctional DNA-binding protein, DnaA, which initiates chromosomal replication. Despite having the most complex, segmented bacterial genome, little is known about Borrelia burgdorferi DnaA and its role in maintaining the spirochete's physiology. We utilized inducible CRISPR-interference and overexpression to modulate cellular levels of DnaA to better understand this essential protein. Dysregulation of DnaA, either up or down, significantly slowed replication and increased or decreased cell lengths. Using fluorescent microscopy, we found the DnaA CRISPRi mutants had increased numbers of chromosomes with irregular spacing patterns. The DnaA-depleted spirochetes also exhibited a significant defect in helical morphology. RNA-seq of the conditional mutants showed significant changes in the levels of transcripts involved with flagellar synthesis, elongation, cell division, virulence, and other functions. These findings demonstrate that the DnaA plays a commanding role in maintaining borrelial growth dynamics and protein expression, which are essential for the survival of the Lyme disease spirochete.

How do host population dynamics impact Lyme disease risk dynamics in theoretical models?

Lyme disease is the most common wildlife-to-human transmitted disease reported in North America. The study of this disease requires an understanding of the ecology of the complex communities of ticks and host species involved in harboring and transmitting this disease. Much of the ecology of this system is well understood, such as the life cycle of ticks, and how hosts are able to support tick populations and serve as disease reservoirs, but there is much to be explored about how the population dynamics of different host species and communities impact disease risk to humans. In this study, we construct a stage-structured, empirically-informed model with host dynamics to investigate how host population dynamics can affect disease risk to humans. The model describes a tick population and a simplified community of three host species, where primary nymph host populations are made to fluctuate on an annual basis, as commonly observed in host populations. We tested the model under different environmental conditions to examine the effect of environment on the interactions of host dynamics and disease risk. Results show that allowing for host dynamics in the model reduces mean nymphal infection prevalence and increases the maximum annual prevalence of nymphal infection and the density of infected nymphs. Effects of host dynamics on disease measures of nymphal infection prevalence were nonlinear and patterns in the effect of dynamics on amplitude in nymphal infection prevalence varied across environmental conditions. These results highlight the importance of further study of the effect of community dynamics on disease risk. This will involve the construction of further theoretical models and collection of robust field data to inform these models. With a more complete understanding of disease dynamics we can begin to better determine how to predict and manage disease risk using these models.

Peromyscus leucopus , Mus musculus , and humans have distinct transcriptomic responses to larval Ixodes scapularis bites

Ixodes scapularis ticks are an important vector for at least six tick-borne human pathogens, including the predominant North American Lyme disease spirochete Borrelia burgdorferi . The ability for these ticks to survive in nature is credited, in part, to their ability to feed on a variety of hosts without excessive activation of the proinflammatory branch of the vertebrate immune system. While the ability for nymphal ticks to feed on a variety of hosts has been well-documented, the host-parasite interactions between larval I. scapularis and different vertebrate hosts is relatively unexplored. Here we report on the changes in the vertebrate transcriptome present at the larval tick bite site using the natural I. scapularis host Peromyscus leucopus deermouse, a non-natural rodent host Mus musculus (BALB/c), and humans. We note substantially less evidence of activation of canonical proinflammatory pathways in P. leucopus compared to BALB/c mice and pronounced evidence of inflammation in humans. Pathway enrichment analyses revealed a particularly strong signature of interferon gamma, tumor necrosis factor, and interleukin 1 signaling at the BALB/c and human tick bite site. We also note that bite sites on BALB/c mice and humans, but not deermice, show activation of wound-healing pathways. These data provide molecular evidence of the coevolution between larval I. scapularis and P. leucopus as well as expand our overall understanding of I. scapularis feeding.

Significance

Ixodes scapularis tick bites expose humans to numerous diseases in North America. While larval tick feeding enables pathogens to enter the tick population and eventually spread to humans, how larval ticks interact with mammals has been understudied compared to other tick stages. Here we examined the transcriptomic response of a natural I. scapularis rodent host ( Peromyscus leucopus ), a non-native I. scapularis rodent host ( Mus musculus ), and an incidental host (humans). We find that there are differences in how all three species respond to larval I. scapularis , with the natural host producing the smallest transcriptomic signature of a canonical proinflammatory immune response and the incidental human host producing the most robust signature of inflammation in response to the larval tick. These data expand our understanding of the pressures on ticks in the wild and inform our ability to model these interactions in laboratory settings.

Detection and genetic identification of Borrelia lusitaniae in questing Ixodes inopinatus tick from Tunisia

Background

Until now, there has been limited information on the prevalence and the phylogeny of Borrelia burgdorferi sensu lato in Ixodes ticks in Tunisia, particularly in Ixodes inopinatus.

Methods

The present study aimed to determine the prevalence and the phylogeny of B. burgdorferi s.l., in coexisted I. ricinus and I. inopinatus ticks collected from Northern Tunisia. One hundred questig ticks were collected during winter 2020 by tick-dragging method in Beja gouvernorate located in the north of Tunisia. Real-time PCR panel targeting B. burgdorferi s.l. 23S rRNA gene were performed. Positive DNA samples were subjected to conventional PCRs targeting 457 bp fragment of the Borrelia sp. flagellin (fla) gene using primers FlaF/FlaR. The identified Borrelia sp. isolate underwent partial sequence analysis to determine genospecies and evaluate their phylogenetic position.

Results

The study revealed a prevalence rate of 28% (28/100) for B. burgdorferi sensu lato in the Ixodes ticks. The prevalence rates across tick species and genders did not show significant variations (p > 0.05). Interestingly, the study underlines the coexistence of I. inopinatus and I. ricinus sharing the same geographic areas in Northern Tunisia. Furthermore, DNA of B. lusitaniae was detected in I. inopinatus ticks for the first time in Tunisia. Revealed B. lusitaniae bacterium is similar to previously identified strains in Mediterranean region, but distinct from those isolated exclusively from countries of Eastern and Central Europe, such as Serbia, Romania, and Poland. This study highlights the prevalence of B. burgdorferi s.l. in I. ricinus/I. inopinatus ticks, and reveals B. lusitaniae in I. inopinatus ticks for the first time in Tunisia.

Conclusion

These findings suggest the involvement of I. inopinatus as a potential vector of this pathogenic genospeciess in Tunisia. This may help understanding the ecology of Ixodes ticks, the natural infection and the transmission dynamics of Borrelia species in this country.

Lyme Disease Models of Tick-Mouse Dynamics with Seasonal Variation in Births, Deaths, and Tick Feeding

Lyme disease is the most common vector-borne disease in the United States impacting the Northeast and Midwest at the highest rates. Recently, it has become established in southeastern and south-central regions of Canada. In these regions, Lyme disease is caused by Borrelia burgdorferi, which is transmitted to humans by an infected Ixodes scapularis tick. Understanding the parasite-host interaction is critical as the white-footed mouse is one of the most competent reservoir for B. burgdorferi. The cycle of infection is driven by tick larvae feeding on infected mice that molt into infected nymphs and then transmit the disease to another susceptible host such as mice or humans. Lyme disease in humans is generally caused by the bite of an infected nymph. The main aim of this investigation is to study how diapause delays and demographic and seasonal variability in tick births, deaths, and feedings impact the infection dynamics of the tick-mouse cycle. We model tick-mouse dynamics with fixed diapause delays and more realistic Erlang distributed delays through delay and ordinary differential equations (ODEs). To account for demographic and seasonal variability, the ODEs are generalized to a continuous-time Markov chain (CTMC). The basic reproduction number and parameter sensitivity analysis are computed for the ODEs. The CTMC is used to investigate the probability of Lyme disease emergence when ticks and mice are introduced, a few of which are infected. The probability of disease emergence is highly dependent on the time and the infected species introduced. Infected mice introduced during the summer season result in the highest probability of disease emergence.

Vector competence of Ixodes ricinus instars for the transmission of Borrelia burgdorferi sensu lato in different small mammalian hosts

BACKGROUND

Many pathogens and parasites can infect multiple host species, and the competence of different hosts as pathogen reservoirs is key to understanding their epidemiology. Small mammals are important hosts for the instar stages of Ixodes ricinus ticks, the principal vector of Lyme disease in Europe. Small mammals also act as reservoirs of Borrelia afzelii, the most common genospecies of the Borrelia burgdorferi sensu lato (s.l.) spirochetes causing Lyme disease in Europe. However, we lack quantitative estimates on whether different small mammal species are equally suitable hosts for feeding I. ricinus and whether they show differences in pathogen transmission from host to tick.

METHODS

Here, we analysed the feeding success and prevalence of B. burgdorferi s.l. infections in 12,987 instar I. ricinus found on captured small mammals with known infection status in Norway (2018-2022).

RESULTS

We found that larvae were more likely to acquire a blood meal from common shrews (Sorex araneus, 46%) compared to bank voles (Myodes glareolus, 36%) and wood mice (Apodemus sylvaticus, 31%). Nymphs tended to be more likely to acquire a blood meal from wood mice (66%) compared to bank voles (54%). Common shrews harboured few nymphs (n=19). Furthermore, we found that larvae feeding on infected bank voles (11%) were more likely to be infected with B. burgdorferi s.l. than larvae on infected common shrews (7%) or wood mice (4%).

CONCLUSIONS

Our study provides quantitative evidence of differences in suitability for the instar stages of I. ricinus across taxa of small mammals and highlights how even known small mammal host species can differ in their ability to feed ticks and infect larval ticks with the pathogen causing Lyme disease.

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

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

Female ticks (Ixodes scapularis) infected with Borrelia burgdorferi have increased overwintering survival, with implications for tick population growth

The tick, Ixodes scapularis, vectors pathogens such as Borrelia burgdorferi, the bacterium that causes Lyme disease. Over the last few decades I. scapularis has expanded its range, introducing a novel health threat into these areas. Warming temperatures appear to be one cause of its range expansion to the north. However, other factors are also involved. We show that unfed adult female ticks infected with B. burgdorferi have greater overwintering survival than uninfected female ticks. Locally collected adult female ticks were placed in individual microcosms and allowed to overwinter in both forest and dune grass environments. In the spring we collected the ticks and tested both dead and living ticks for B. burgdorferi DNA. Infected ticks had greater overwintering survival compared with uninfected ticks every winter for three consecutive winters in both forest and dune grass environments. We discuss the most plausible explanations for this result. The increased winter survival of adult female ticks could enhance tick population growth. Our results suggest that, in addition to climate change, B. burgdorferi infection itself may be promoting the northern range expansion of I. scapularis. Our study highlights how pathogens could work synergistically with climate change to promote host range expansion.

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.

Prevalence of five human pathogens in host-seeking Ixodes scapularis and Ixodes pacificus by region, state, and county in the contiguous United States generated through national tick surveillance

The majority of vector-borne disease cases reported in the United States (U.S.) are caused by pathogens spread by the blacklegged tick, Ixodes scapularis. In recent decades, the geographic ranges of the tick and its associated human pathogens have expanded, putting an increasing number of communities at risk for tick-borne infections. In 2018, the U.S. Centers for Disease Control and Prevention (CDC) initiated a national tick surveillance program to monitor changes in the distribution and abundance of ticks and the presence and prevalence of human pathogens in them. We assessed the geographical representativeness of prevalence data submitted to CDC as part of the national tick surveillance effort. We describe county, state, and regional variation in the prevalence of five human pathogens (Borrelia burgdorferi sensu stricto (s.s.), Borrelia mayonii, Borrelia miyamotoi, Anaplasma phagocytophilum, and Babesia microti) in host-seeking I. scapularis and I. pacificus nymphs and adults. Although I. scapularis and I. pacificus are widely distributed in the eastern and western U.S., respectively, pathogen prevalence was estimated predominantly in ticks collected in the Northeast, Ohio Valley, and Upper Midwest regions, where human Lyme disease cases are most commonly reported. Within these regions, we found that state and regional estimates of pathogen prevalence generally reached predictable and stable levels, but variation in prevalence estimates at the sub-state level was considerable. Borrelia burgdorferi s.s. was the most prevalent and widespread pathogen detected. Borrelia miyamotoi and A. phagocytophilum shared a similarly broad geographic range, but were consistently detected at much lower prevalence compared with B. burgdorferi s.s. Babesia microti was detected at similar prevalence to A. phagocytophilum, where both pathogens co-occurred, but was reported over a much more limited geographic range compared with A. phagocytophilum or B. burgdorferi s.s. Borrelia mayonii was identified at very low prevalence with a focal distribution within the Upper Midwest. National assessments of risk for tick-borne diseases need to be improved through collection and testing of ticks in currently under-represented regions, including the West, South, Southeast, and eastern Plains states.

Antigen-Specific CD4 T Cell and B Cell Responses to Borrelia burgdorferi

Long-lived T-dependent B cell responses fail to develop during persistent infection of mice with Borrelia burgdorferi, the causative agent of Lyme disease, raising questions about the induction and/or functionality of anti-B. burgdorferi adaptive immune responses. Yet, a lack of reagents has limited investigations into B. burgdorferi-specific T and B cells. We attempted two approaches to track B. burgdorferi-induced CD4 T cells. First, a B. burgdorferi mutant was generated with an influenza hemagglutinin (HA) peptide, HA111-119, inserted into the B. burgdorferi arthritis-related protein (Arp) locus. Although this B. burgdorferi arp::HA strain remained infectious, peptide-specific TCR transgenic CD4 T cells in vitro, or adoptively transferred into B. burgdorferi arp::HA-infected BALB/c mice, did not clonally expand above those of recipients infected with the parental B. burgdorferi strain or a B. burgdorferi mutant containing an irrelevant peptide. Some expansion, however, occurred in B. burgdorferi arp::HA-infected BALB/c SCID mice. Second, a (to our knowledge) newly identified I-Ab-restricted CD4 T cell epitope, Arp152-166, was used to generate Arp MHC class II tetramers. Flow cytometry showed small numbers of Arp-specific CD4 T cells emerging in mice infected with B. burgdorferi but not with Arp-deficient Borrelia afzelii. Although up to 30% of Arp-specific CD4 T cells were ICOS+PD-1+CXCR5+BCL6+ T follicular helper cells, their numbers declined after day 12, before germinal centers (GCs) are prominent. Although some Arp-specific B cells, identified using fluorochrome-labeled rArp proteins, had the phenotype of GC B cells, their frequencies did not correlate with anti-Arp serum IgG. The data suggest a failure not in the induction, but in the maintenance of GC T follicular helper and/or B cells to B. burgdorferi.

Tick-to-host transmission differs between Borrelia afzelii strains

Many vector-borne pathogens establish multiple-strain infections in the vertebrate host and the arthropod vector. Multiple-strain infections in the host influence strain acquisition by naive vectors. Whether multiple-strain infections in the vector influence strain-specific transmission to naive hosts remains unknown. The spirochete, Borrelia afzelii, causes Lyme borreliosis and multiple-strain infections are common in both the tick vector and vertebrate host. Our study used two B. afzelii strains: Fin-Jyv-A3 and NE4049. Donor mice were infected with Fin-Jyv-A3 alone, NE4049 alone, or with both strains. Larval ticks fed on donor mice and molted into nymphal ticks infected with either strain or both strains. These nymphs were fed on test mice to determine whether multiple-strain infections in the nymph influence nymph-to-host transmission (NHT). Multiple-strain infection in the donor mice reduced the acquisition of both strains by ticks by 23%. Thus, a substantial fraction of infected nymphs from the multiple strain treatment were infected with the "wrong" competitor strain rather than the "right" focal strain. As a result, nymphs from the multiple strain treatment were 46% less likely to infect the test mice with the focal strain compared to nymphs from the single strain treatment. However, multiple-strain infection in the nymphal tick had no effect on the NHT of either strain. The nymphal spirochete load of Fin-Jyv-A3 was 1.9 times higher compared to NE4049. NHT of Fin-Jyv-A3 (79%) was 1.5 times higher compared to NE4049 (53%). Our study suggests that B. afzelii strains with higher nymphal spirochete loads have higher NHT. IMPORTANCE For many vector-borne pathogens, multiple-strain infections in the vertebrate host or arthropod vector are common. Multiple-strain infections in the host reduce strain acquisition by feeding vectors. Whether multiple-strain infections in the vector influence strain transmission to the host remains unknown. In our study, we used two strains of the tick-borne spirochete Borrelia afzelii, which causes Lyme borreliosis, to investigate whether multiple-strain infections in the nymphal tick influenced nymph-to-host transmission (NHT) of strains. Multiple-strain infections in mice reduced the acquisition of both B. afzelii strains by nymphal ticks. As a result, nymphs from the multiple strain treatment were less likely to infect naive test mice with the focal strain. Multiple-strain infection in the nymphal ticks did not influence the NHT of either strain. The strain with the higher bacterial abundance in the nymph had higher NHT. Our study suggests that pathogen abundance in the arthropod vector is important for vector-to-host transmission.

It's about time: small mammal communities and Lyme disease emergence

Theory predicts that biodiversity changes due to climate warming can mediate the rate of disease emergence. The mechanisms linking biodiversity-disease relationships have been described both theoretically and empirically but remain poorly understood. We investigated the relations between host diversity and abundance and Lyme disease risk in southern Quebec, a region where Lyme disease is rapidly emerging. We found that both the abundance of small mammal hosts and the relative abundance of the tick's natural host, the white-footed mouse (Peromyscus leucopus), influenced measures of disease risk in tick vectors (Borrelia burgdorferi infection abundance and prevalence in tick vectors). Our results suggest that the increase in Lyme disease risk is modulated by regional processes involving the abundance and composition of small mammal assemblages. However, the nature and strength of these relationships was dependent both on time and geographic area. The strong effect of P. leucopus abundance on disease risk we report here is of significant concern, as this competent host is predicted to increase in abundance and occurrence in the region, with the northern shift in the range of North American species under climate warming.

Prevalence of Borrelia, Neoehrlichia mikurensis and Babesia in ticks collected from vegetation in eastern Poland

In Poland, tick-borne diseases constitute the majority of diseases related to exposure to biological agents with a predominance of Lyme borreliosis; therefore, research on ticks as a reservoir of various pathogens remains crucial in the epidemiology of human diseases after tick bites. This study aimed to identify the occurrence of Borrelia burgdorferi sensu lato, Borrelia miyamotoi, Neoehrlichia mikurensis, and Babesia spp. in ticks collected from vegetation in eastern Poland. Additionally, the prevalence of co-infections in the adult Ixodes ricinus ticks was determined. Among I. ricinus ticks the predominantly detected pathogen was B. burgdorferi s.l. (23%) with B. burgdorferi sensu stricto as the most frequently identified species, followed by B. garinii. In 2013, the double or triple infections of B. burgdorferi s.s., B. afzelii, and B. garinii species did not exceed 9% in adult ticks, whereas in 2016, the prevalence of mixed infections reached 29%. The prevalence of N. mikurensis and B. miyamotoi in I. ricinus was determined at the same level of 2.8%. Four Babesia species were identified in the examined I. ricinus population: B. microti (1.5%), B. venatorum (1.2%), B. divergens (0.2%), and B. capreoli (0.1%). Co-infections were detected in 10.1% of all infected ticks with the highest prevalence of co-infections with B. burgdorferi s.l. and Babesia species. The changes in the prevalence and the distribution of particular pathogens within tick populations indicate the need for monitoring the current situation related to tick-borne pathogens from the aspect of risk to human health.

Capybara density and climatic factors as modulators of Ehrlichia prevalence in questing ticks in the Iberá wetlands, Argentina

We evaluated the presence of Ehrlichia spp. in unfed capybara ticks, Amblyomma dubitatum, and explored its association with capybaras density, ticks density and environmental variables. We observed that in the Iberá wetlands ecoregion A. dubitatum is infected by "Candidatus Ehrlichia hydrochoerus" and in a lesser extent with an Ehrlichia species closely related to Ehrlichia chaffeensis. The frequency of "Ca. Ehrlichia hydrochoerus" presence in A. dubitatum was not associated with vector abundance, but the probability of finding "Ca. Ehrlichia hydrochoerus"-infected ticks increased when the density of capybaras was low two months before. We hypothesize that when the density of capybaras decreases, A. dubitatum immature stages may seek out alternative hosts one of which could exhibit high realized reservoir competence for "Ca. Ehrlichia hydrochoerus", leading to an increased prevalence of this ehrlichiae in questing A. dubitatum. High minimum temperatures and high cumulative rainfall in the time period previous to tick collection (15 to 60 days) were positively correlated with the prevalence of "Ca. Ehrlichia hydrochoerus" infection in A. dubitatum. Our results suggest that a combination of factors (both biological and abiotic) could raise the risk of human exposure to tick-borne Ehrlichia in the Iberá wetlands ecoregion.

Spatiotemporal distribution of Borrelia miyamotoi (Spirochaetales: Spirochaetaceae) and coinfection with other tick-borne pathogens in host-seeking Ixodes scapularis (Acari: Ixodidae) from New York State, USA

Blacklegged ticks (Ixodes scapularis Say, Acari: Ixodidae) were collected from 432 locations across New York State (NYS) during the summer and autumn of 2015-2020 to determine the prevalence and geographic distribution of Borrelia miyamotoi (Spirochaetales: Spirochaetaceae) and coinfections with other tick-borne pathogens. A total of 48,386 I. scapularis were individually analyzed using a multiplex real-time polymerase chain reaction assay to simultaneously detect the presence of Bo. miyamotoi, Borrelia burgdorferi (Spirochaetales: Spirochaetaceae), Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae), and Babesia microti (Piroplasmida: Babesiidae). Overall prevalence of Bo. miyamotoi in host-seeking nymphs and adults varied geographically and temporally at the regional level. The rate of polymicrobial infection in Bo. miyamotoi-infected ticks varied by developmental stage, with certain co-infections occurring more frequently than expected by chance. Entomological risk of exposure to Bo. miyamotoi-infected nymphal and adult ticks (entomological risk index [ERI]) across NYS regions in relation to human cases of Bo. miyamotoi disease identified during the study period demonstrated spatial and temporal variation. The relationship between select environmental factors and Bo. miyamotoi ERI was explored using generalized linear mixed effects models, resulting in different factors significantly impacting ERI for nymphs and adult ticks. These results can inform estimates of Bo. miyamotoi disease risk and further our understanding of Bo. miyamotoi ecological dynamics in regions where this pathogen is known to occur.

Social transmission of bacterial symbionts homogenizes the microbiome within and across generations of group-living spiders

Disentangling modes and fidelity of symbiont transmission are key for understanding host-symbiont associations in wild populations. In group-living animals, social transmission may evolve to ensure high-fidelity transmission of symbionts, since non-reproducing helpers constitute a dead-end for vertical transmission. We investigated symbiont transmission in the social spider Stegodyphus dumicola, which lives in family groups where the majority of females are non-reproducing helpers, females feed offspring by regurgitation, and individuals feed communally on insect prey. Group members share temporally stable microbiomes across generations, while distinct variation in microbiome composition exists between groups. We hypothesized that horizontal transmission of symbionts is enhanced by social interactions, and investigated transmission routes within (horizontal) and across (vertical) generations using bacterial 16S rRNA gene amplicon sequencing in three experiments: (i) individuals were sampled at all life stages to assess at which life stage the microbiome is acquired. (ii) a cross-fostering design was employed to test whether offspring carry the microbiome from their natal nest, or acquire the microbiome of the foster nest via social transmission. (iii) adult spiders with different microbiome compositions were mixed to assess whether social transmission homogenizes microbiome composition among group members. We demonstrate that offspring hatch symbiont-free, and bacterial symbionts are transmitted vertically across generations by social interactions with the onset of regurgitation feeding by (foster)mothers in an early life stage. Social transmission governs horizontal inter-individual mixing and homogenization of microbiome composition among nest mates. We conclude that temporally stable host-symbiont associations in social species can be facilitated and maintained by high-fidelity social transmission.

Multiomics Reveals Symbionts, Pathogens, and Tissue-Specific Microbiome of Blacklegged Ticks (Ixodes scapularis) from a Lyme Disease Hot Spot in Southeastern Ontario, Canada

Ticks in the family Ixodidae are important vectors of zoonoses, including Lyme disease (LD), which is caused by spirochete bacteria from the Borreliella (Borrelia) burgdorferi sensu lato complex. The blacklegged tick (Ixodes scapularis) continues to expand across Canada, creating hot spots of elevated LD risk at the leading edge of its expanding range. Current efforts to understand the risk of pathogen transmission associated with I. scapularis in Canada focus primarily on targeted screens, while natural variation in the tick microbiome remains poorly understood. Using multiomics consisting of 16S metabarcoding and ribosome-depleted, whole-shotgun RNA transcriptome sequencing, we examined the microbial communities associated with adult I. scapularis (n = 32), sampled from four tissue types (whole tick, salivary glands, midgut, and viscera) and three geographical locations within a LD hot spot near Kingston, Ontario, Canada. The communities consisted of both endosymbiotic and known or potentially pathogenic microbes, including RNA viruses, bacteria, and a Babesia sp. intracellular parasite. We show that β-diversity is significantly higher between the bacterial communities of individual tick salivary glands and midguts than that of whole ticks. Linear discriminant analysis effect size (LEfSe) determined that the three potentially pathogenic bacteria detected by V4 16S rRNA sequencing also differed among dissected tissues only, including a Borrelia strain from the B. burgdorferi sensu lato complex, Borrelia miyamotoi, and Anaplasma phagocytophilum. Importantly, we find coinfection of I. scapularis by multiple microbes, in contrast to diagnostic protocols for LD, which typically focus on infection from a single pathogen of interest (B. burgdorferi sensu stricto). IMPORTANCE As a vector of human health concern, blacklegged ticks (Ixodes scapularis) transmit pathogens that cause tick-borne diseases (TBDs), including Lyme disease (LD). Several hot spots of elevated LD risk have emerged across Canada as I. scapularis expands its range. Focusing on a hot spot in southeastern Ontario, we used high-throughput sequencing to characterize the microbiome of whole ticks and dissected salivary glands and midguts. Compared with whole ticks, salivary glands and midguts were more diverse and associated with distinct bacterial communities that are less dominated by Rickettsia endosymbiont bacteria and are enriched for pathogenic bacteria, including a B. burgdorferi sensu lato-associated Borrelia sp., Borrelia miyamotoi, and Anaplasma phagocytophilum. We also found evidence of coinfection of I. scapularis by multiple pathogens. Overall, our study highlights the challenges and opportunities associated with the surveillance of the microbiome of I. scapularis for pathogen detection using metabarcoding and metatranscriptome approaches.

Identifying suitable habitat for Ixodes scapularis (Acari: Ixodidae) infected with Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae), Babesia microti (Piroplasmida: Babesiidae), and Borrelia miyamotoi (Spirochaetales: Spirochaetaceae) to guide surveillance efforts in the eastern United States

Understanding the distribution of infected ticks is informative for the estimation of risk for tickborne diseases. The blacklegged tick, Ixodes scapularis (Acari: Ixodidae), is the primary vector for 7 medically significant pathogens in United States. However, knowledge of the ranges of these pathogens in host-seeking ticks is incomplete, particularly for those occurring at low prevalence. To aid in prioritizing costly field sampling efforts, we estimated ranges of suitable habitat for Anaplasma phagocytophilum, Babesia microti, and Borrelia miyamotoi in the eastern United States based on existing county-level surveillance records. The resulting suitability maps were compared against those developed previously for Bo. burgdorferi s.s., which shares similar ecology but has been detected in a greater number of counties. The overall accuracy of the habitat suitability models was high (AUC ≥ 0.92) for all 4 pathogens. The most important predictors were related to temperature and moisture. The upper midwestern and northeastern states were predicted to be highly suitable for all 4 pathogens. Based on our models, we prioritized sampling in 431, 275, and 539 counties currently lacking pathogen records that our models classified as suitable for A. phagocytophilum, Ba. microti, and Bo. miyamotoi, respectively. As a second-tier priority, we identified 311 (A. phagocytophilum), 590 (Ba. microti), and 252 (Bo. miyamotoi) counties, based on high suitability scores for Bo. burgdorferi. Our models can be used to improve cost-effectiveness of field sampling efforts aimed at improving accuracy and completeness of pathogen distribution maps.

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.

Human Borrelia miyamotoi Infection in North America

Borrelia miyamotoi is an emerging pathogen that causes a febrile illness and is transmitted by the same hard-bodied (ixodid) ticks that transmit several other pathogens, including Borrelia species that cause Lyme disease. B. miyamotoi was discovered in 1994 in Ixodes persulcatus ticks in Japan. It was first reported in humans in 2011 in Russia. It has subsequently been reported in North America, Europe, and Asia. B. miyamotoi infection is widespread in Ixodes ticks in the northeastern, northern Midwestern, and far western United States and in Canada. In endemic areas, human B. miyamotoi seroprevalence averages from 1 to 3% of the population, compared with 15 to 20% for B. burgdorferi. The most common clinical manifestations of B. miyamotoi infection are fever, fatigue, headache, chills, myalgia, arthralgia, and nausea. Complications include relapsing fever and rarely, meningoencephalitis. Because clinical manifestations are nonspecific, diagnosis requires laboratory confirmation by PCR or blood smear examination. Antibiotics are effective in clearing infection and are the same as those used for Lyme disease, including doxycycline, tetracycline, erythromycin, penicillin, and ceftriaxone. Preventive measures include avoiding areas where B. miyamotoi-infected ticks are found, landscape management, and personal protective strategies such as protective clothing, use of acaricides, and tick checks with rapid removal of embedded ticks.

Borrelia miyamotoi: A Comprehensive Review

Borrelia miyamotoi is an emerging tick-borne pathogen in the Northern Hemisphere and is the causative agent of Borrelia miyamotoi disease (BMD). Borrelia miyamotoi is vectored by the same hard-bodied ticks as Lyme disease Borrelia, yet phylogenetically groups with relapsing fever Borrelia, and thus, has been uniquely labeled a hard tick-borne relapsing fever Borrelia. Burgeoning research has uncovered new aspects of B. miyamotoi in human patients, nature, and the lab. Of particular interest are novel findings on disease pathology, prevalence, diagnostic methods, ecological maintenance, transmission, and genetic characteristics. Herein, we review recent literature on B. miyamotoi, discuss how findings adapt to current Borrelia doctrines, and briefly consider what remains unknown about B. miyamotoi.

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.

Precipitation as a Possible Risk Factor for Development of Lyme Nephritis in Dogs

To determine whether increased precipitation in the months before diagnosis is predictive of Lyme nephritis in dogs, a symmetrical bidirectional case-crossover study was performed on 87 dogs with presumptive Lyme nephritis. Cases were geographically linked to the nearest precipitation monitoring station, and monthly total precipitation for the 4 mo preceding diagnosis was compared to monthly total precipitation 1 yr before and 1 yr after. Breeds affected included Labrador retrievers (n = 31), mixed-breed dogs (n = 19), and golden retrievers (n = 10), with a possible seasonal bimodal peak in spring and fall. In age-adjusted analysis, each additional inch of precipitation 3 mo before diagnosis was associated with an increase in the odds of developing Lyme nephritis (odds ratio 1.13, 95% confidence interval 1.01-1.25). The results suggest that increased precipitation is a possible risk factor for Lyme nephritis in dogs. These findings may provide useful information for the accurate diagnosis of dogs with protein-losing nephropathy and may guide future studies of risk factors for Lyme nephritis. The methodology employed here reinforces the concept that case-crossover designs have utility beyond acute exposures and outcomes and offer an attractive alternative for studies in companion animals.

Use of mammalian museum specimens to test hypotheses about the geographic expansion of Lyme disease in the southeastern United States

Lyme disease, caused primarily in North America by the bacterium Borrelia burgdorferi sensu stricto, is the most frequently reported vector-borne disease in North America and its geographic extent is increasing in all directions from foci in the northeastern and north central United States. Several southeastern states, including Virginia and North Carolina, have experienced large increases in Lyme disease incidence in the past two decades, with the biggest changes in incidence occurring in the western portion of each state. We tested the hypothesis that B. burgdorferi s.s. was present in western Virginia and North Carolina Peromyscus leucopus populations prior to the recent emergence of Lyme disease. Specifically, we examined archived P. leucopus museum specimens, sampled between 1900 and 2000, for B. burgdorferi s.s. DNA. After confirming viability of DNA extracted from ear punch biopsies from P. leucopus study skins collected between 1945 and 2000 in 19 Virginia counties and 17 North Carolina counties, we used qPCR of two species-specific loci to test for the presence of B. burgdorferi s.s. DNA. Ten mice, all collected from the Eastern Shore of Virginia in 1989, tested positive for presence of B. burgdorferi; all of the remaining 344 specimens were B. burgdorferi-negative. Our results suggest that B. burgdorferi s.s was not common in western Virginia or North Carolina prior to the emergence of Lyme disease cases in the past two decades. Rather, the emergence of Lyme disease in this region has likely been driven by the relatively recent expansion of B. burgdorferi s.s. in southward-moving ticks and reservoir hosts in the mountainous counties of these two states.

When to Think About Other Borreliae:: Hard Tick Relapsing Fever (Borrelia miyamotoi), Borrelia mayonii, and Beyond

In North America, several hard tick-transmitted Borrelia species other than Borrelia burgdorferi cause human disease, including Borrelia miyamotoi, Borrelia mayonii, and possibly Borrelia bissettii. Due to overlapping clinical syndromes, nonspecific tickborne disease (TBD) testing strategies, and shared treatment approaches, infections with these lesser known Borrelia are likely under-reported. In this article, we describe the epidemiology, clinical manifestations, diagnosis, and treatment of these less common Borrelia pathogens.

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.

Relevance of Spatial and Temporal Trends in Nymphal Tick Density and Infection Prevalence for Public Health and Surveillance Practice in Long-Term Endemic Areas: A Case Study in Monmouth County, NJ

Tick-borne diseases are a growing public health problem in the United States, and the US northeast has reported consistently high case rates for decades. Monmouth County, New Jersey, was one of the earliest jurisdictions to report Lyme disease cases in 1979 and reports several hundred cases per year nearly 40 yr later. In the time since, however, tick-borne health risks have expanded far beyond Lyme disease to include a variety of other bacterial pathogens and viruses, and additional vectors, necessitating a continually evolving approach to tick surveillance. In 2017, Monmouth County initiated an active surveillance program targeting sites across three ecological regions for collection of Ixodes scapularis Say (Acari: Ixodidae) and Amblyomma americanum L. (Acari: Ixodidae) as well as testing via qPCR for associated bacterial pathogens. During the first five years of this program (2017-2021), we report high levels of spatiotemporal variability in nymphal density and infection prevalence in both species, limiting the granularity with which human risk can be predicted from acarological data. Nonetheless, broader patterns emerged, including an ongoing trend of A. americanum dominance, risks posed by Borrelia miyamotoi, and the frequency of coinfected ticks. We present some of the first county-level, systematic surveillance of nymphal A. americanum density and infection prevalence in the northeastern US. We also documented a temporary decline in Borrelia burgdorferi that could relate to unmeasured trends in reservoir host populations. We discuss the implications of our findings for tick-borne disease ecology, public health communication, and tick surveillance strategies in endemic areas.

Lyme Carditis: From Pathophysiology to Clinical Management

Cardiac involvement is a rare but relevant manifestation of Lyme disease that frequently presents as atrioventricular block (AVB). Immune-mediated injury has been implicated in the pathogenesis of Lyme carditis due to possible cross-reaction between Borrelia burgdorferi antigens and cardiac epitopes. The degree of the AVB can fluctuate rapidly, with two-thirds of patients progressing to complete AVB. Thus, continuous heart rhythm monitoring is essential, and a temporary pacemaker may be necessary. Routinely permanent pacemaker implantation, however, is contraindicated because of the frequent transient nature of the condition. Antibiotic therapy should be initiated as soon as the clinical suspicion of Lyme carditis arises to reduce the duration of the disease and minimize the risk of complications. Diagnosis is challenging and is based on geographical epidemiology, clinical history, signs and symptoms, serological testing, ECG and echocardiographic findings, and exclusion of other pathologies. This paper aims to explain the pathophysiological basis of Lyme carditis, describe its clinical features, and delineate the treatment principles.

Case Report: Lyme Borreliosis and Pregnancy - Our Experience

Lyme Borreliosis (LB) is an infection transmitted by Ixodes sp. ticks. Its early manifestation includes erythema migrans rash. Since the discovery of LB in 1975, the question arose as to whether this infection could be vertically transmitted from mother to fetus during pregnancy, as transplacental transmission has already been known for other spirochetoses, such as syphilis, relapsing fever and leptospirosis. The first confirmed case with positive Lyme serology was described in 1985 in a 28-year- old mother who had acquired Lyme in the first trimester and then developed an erythema migrans rash. Subsequently, transmission of Borrelia burgdorferi sl. in humans from mother to fetus has been documented through identification of Borrelia spirochetes in fetal tissues/and or placenta by various methods including culture, PCR and indirect immunofluorescence. Adverse birth outcomes, which are limited in case of prompt LB treatment, included spontaneous miscarriage, preterm birth and hyperbilirubinemia, but also cardiac involvement and cutaneous angiomas have been documented although rarely. No significant associations were found between adverse outcomes at birth and the trimester of infection. Patients treated for gestational LB had a lower frequency of miscarriages and premature births, as also the frequency of congenital malformations was similar to that observed in the normal population. The recommended treatment for LB in pregnancy is Amoxicillin, 1 g 3 times a day for 14-21 days. In the present study, we report our case series, which includes 11 pregnant women, 6 of which developed erythema migrans during pregnancy (between week 8 and 34), 3 had myoarticular or neurological symptoms and 2 had positive serology, but did not develop any clinical symptoms. Our data stress on the importance of early antibiotic treatment also in seropositive gestating women without symptoms in order to avoid any possible complication to fetus and newborns.

Transmission patterns of tick-borne pathogens among birds and rodents in a forested park in southeastern Canada

Ixodes scapularis ticks are expanding their range in parts of northeastern North America, bringing with them pathogens of public health concern. While rodents like the white-footed mouse, Peromyscus leucopus, are considered the primary reservoir of many emerging tick-borne pathogens, the contribution of birds, as alternative hosts and reservoirs, to local transmission cycles has not yet been firmly established. From 2016 to 2018, we collected host-seeking ticks and examined rodent and bird hosts for ticks at 48 sites in a park where blacklegged ticks are established in Quebec, Canada, in order to characterize the distribution of pathogens in ticks and mammalian and avian hosts. We found nearly one third of captured birds (n = 849) and 70% of small mammals (n = 694) were infested with I. scapularis. Five bird and three mammal species transmitted Borrelia burgdorferi to feeding larvae (n larvae tested = 2257) and we estimated that about one fifth of the B. burgdorferi-infected questing nymphs in the park acquired their infection from birds, the remaining being attributable to mice. Ground-foraging bird species were more parasitized than other birds, and species that inhabited open habitat were more frequently infested and were more likely to transmit B. burgdorferi to larval ticks feeding upon them. Female birds were more likely to transmit infection than males, without age differentiation, whereas in mice, adult males were more likely to transmit infection than juveniles and females. We also detected Borrelia miyamotoi in larvae collected from birds, and Anaplasma phagocytophilum from a larva collected from a white-footed mouse. This study highlights the importance of characterising the reservoir potential of alternative reservoir hosts and to quantify their contribution to transmission dynamics in different species assemblages. This information is key to identifying the most effective host-targeted risk mitigation actions.

Exploring the Effects of Prescribed Fire on Tick Spread and Propagation in a Spatial Setting

Lyme disease is one of the most prominent tick-borne diseases in the United States, and prevalence of the disease has been steadily increasing over the past several decades due to a number of factors, including climate change. Methods for control of the disease have been considered, one of which is prescribed burning. In this paper, the effects of prescribed burns on the abundance of ticks present in a spatial domain are assessed. A spatial stage-structured tick-host model with an impulsive differential equation system is developed to simulate the effect that controlled burning has on tick populations. Subsequently, a global sensitivity analysis is performed to evaluate the effect of various model parameters on the prevalence of infectious nymphs. Results indicate that while ticks can recover relatively quickly following a burn, yearly, high-intensity prescribed burns can reduce the prevalence of ticks in and around the area that is burned. The use of prescribed burns in preventing the establishment of ticks into new areas is also explored, and it is observed that frequent burning can slow establishment considerably.

Borrelia burgdorferi (Spirochaetales: Spirochaetaceae) Infection Prevalence and Host Associations of Ticks Found on Peromyscus spp. in Maryland

Lyme disease, caused by Borrelia burgdorferi sensu stricto and most commonly transmitted by Ixodes scapularis Say (Ixodida: Ixodidae), is the most common tick-borne disease in Maryland. Because B. burgdorferi s.s. is maintained in enzootic cycles among wild mice (Peromyscus spp) and Ixodes spp ticks, differing patterns of parasitism of ticks on mice could impact the infection prevalence with B. burgdorferi. We determined the infection prevalence of Peromyscus spp as well as questing and partially engorged nymphal ticks collected at six sites on private land in five counties in Maryland from May to August 2020. Questing nymph infection prevalence (NIP) was 14%. We trapped 1258 mice and collected 554 ticks and 413 ear tissue samples. The prevalence of infested Peromyscus spp varied based on host age and sex, with older and male mice more likely to be infested. We detected a significant difference amongst the proportion of attached Ixodes and the location of trapping. Similarly, the prevalence of B. burgdorferi infected Peromyscus spp mice varied between locations (average mouse infection prevalence was 40%), with the highest prevalence in locations where Ixodes were the most commonly found ticks. The B. burgdorferi infection prevalence in partially engorged I. scapularis nymphs retrieved from Peromyscus spp was ~36% which lends further support to the host infection prevalence. Local differences in distribution of infected vectors and reservoirs are important factors to consider when planning interventions to reduce Lyme disease risk.

Emergence of Ixodes scapularis (Acari: Ixodidae) in a Small Mammal Population in a Coastal Oak-Pine Forest, Maine, USA

In the United States, surveillance has been key to tracking spatiotemporal emergence of blacklegged ticks [Ixodes scapularis Say (Ixodida:Ixodidae)] and their pathogens such as Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner (Spirochaetales: Spirochaetaceae), the agent of Lyme disease. On the Holt Research Forest in midcoastal Maine, collection of feeding ticks from live-trapped small mammal hosts allowed us to track the emergence and establishment of I. scapularis, 1989-2019. From 1989-1995, we collected only I. angustus Neumann (Ixodida: Ixodidae)(vole tick), Dermacentor variabilis Say (Ixodida: Ixodidae) (American dog tick), and I. marxi Banks (Ixodida: Ixodidae) (squirrel tick) from seven species of small mammals. The most abundant tick host was the white-footed mouse [Peromyscus leucopus Rafinesque (Rodentia:Cricetidae)] followed by the red-backed vole (Myodes gapperi Vigors (Rodentia: Cricetidae)). Emergence of I. scapularis was signaled via the appearance of subadult I. scapularis in 1996. Emergence of B. burgdorferi was signaled through its appearance in I. scapularis feeding on mice in 2005. There was a substantial increase in I. scapularis prevalence (proportion of hosts parasitized) and burdens (ticks/host) on white-footed mice and red-backed voles in 2007. The ~11-yr time-to-establishment for I. scapularis was consistent with that seen in other studies. White-footed mice comprised 65.9% of all captures and hosted 94.1% of the total I. scapularis burden. The white-footed mouse population fluctuated interannually, but did not trend up as did I. scapularis prevalence and burdens. There were concurrent declines in I. angustus and D. variabilis. We discuss these results in the broader context of regional I. scapularis range expansion.

Context-dependent host dispersal and habitat fragmentation determine heterogeneity in infected tick burdens: an agent-based modelling study

As the incidence of tick-borne diseases has sharply increased over the past decade, with serious consequences for human and animal health, there is a need to identify ecological drivers contributing to heterogeneity in tick-borne disease risk. In particular, the relative importance of animal host dispersal behaviour in its three context-dependent phases of emigration, transfer and settlement is relatively unexplored. We built a spatially explicit agent-based model to investigate how the host dispersal process, in concert with the tick and host demographic processes, habitat fragmentation and the pathogen transmission process, affects infected tick distributions among hosts. A sensitivity analysis explored the impacts of different input parameters on infected tick burdens on hosts and infected tick distributions among hosts. Our simulations indicate that ecological predictors of infected tick burdens differed among the post-egg life stages of ticks, with tick attachment and detachment, tick questing activity and pathogen transmission dynamics identified as key processes, in a coherent way. We also found that the type of host settlement strategy and the proportion of habitat suitable for hosts determined super-spreading of infected ticks. We developed a theoretical mechanistic framework that can serve as a first step towards applied studies of on-the-ground public health intervention strategies.

Context-dependent host dispersal and habitat fragmentation determine heterogeneity in infected tick burdens: an agent-based modelling study

As the incidence of tick-borne diseases has sharply increased over the past decade, with serious consequences for human and animal health, there is a need to identify ecological drivers contributing to heterogeneity in tick-borne disease risk. In particular, the relative importance of animal host dispersal behaviour in its three context-dependent phases of emigration, transfer and settlement is relatively unexplored. We built a spatially explicit agent-based model to investigate how the host dispersal process, in concert with the tick and host demographic processes, habitat fragmentation and the pathogen transmission process, affects infected tick distributions among hosts. A sensitivity analysis explored the impacts of different input parameters on infected tick burdens on hosts and infected tick distributions among hosts. Our simulations indicate that ecological predictors of infected tick burdens differed among the post-egg life stages of ticks, with tick attachment and detachment, tick questing activity and pathogen transmission dynamics identified as key processes, in a coherent way. We also found that the type of host settlement strategy and the proportion of habitat suitable for hosts determined super-spreading of infected ticks. We developed a theoretical mechanistic framework that can serve as a first step towards applied studies of on-the-ground public health intervention strategies.

Borrelia burgdorferi and Borrelia miyamotoi in Atlantic Canadian wildlife

Borrelia burgdorferi and Borrelia miyamotoi are tick-vectored zoonotic pathogens maintained in wildlife species. Tick populations are establishing in new areas globally in response to climate change and other factors. New Brunswick is a Canadian maritime province at the advancing front of tick population establishment and has seen increasing numbers of ticks carrying B. burgdorferi, and more recently B. miyamotoi. Further, it is part of a region of Atlantic Canada with wildlife species composition differing from much of continental North America and little information exists as to the presence and frequency of infection of Borrelia spp. in wildlife in this region. We used a citizen science approach to collect a wide range of animals including migratory birds, medium-sized mammals, and small mammals. In total we tested 339 animals representing 20 species for the presence of B. burgdorferi and B. miyamotoi. We have developed new nested PCR primers and a protocol with excellent specificity for detecting both of these Borrelia species, both single and double infections, in tissues and organs of various wildlife species. The positive animals were primarily small non-migratory mammals, approximately twice as many were infected with B. burgdorferi than B. miyamotoi and one animal was found infected with both. In addition to established reservoir species, the jumping mouse (Napaeozapus insignis) was found frequently infected; this species had the highest infection prevalence for both B. burgdorferi and B. miyamotoi and has not previously been identified as an important carrier for either Borrelia species. Comprehensive testing of tissues found that all instances of B. burgdorferi infection were limited to one tissue within the host, whereas two of the five B. miyamotoi infections were diffuse and found in multiple systems. In the one coinfected specimen, two fetuses were also recovered and found infected with B. miyamotoi. This presumptive transplacental transmission suggests that vertical transmission in mammals is possible. This finding implies that B. miyamotoi could rapidly spread into wildlife populations, as well as having potential human health implications.

Ecology of Ixodes pacificus Ticks and Associated Pathogens in the Western United States

Lyme disease is the most important vector-borne disease in the United States and is increasing in incidence and geographic range. In the Pacific west, the western black-legged tick, Ixodes pacificus Cooley and Kohls, 1943 is an important vector of the causative agent of Lyme disease, the spirochete, Borrelia burgdorferi. Ixodes pacificus life cycle is expected to be more than a year long, and all three stages (larva, nymph, and adult) overlap in spring. The optimal habitat consists of forest cover, cooler temperatures, and annual precipitation in the range of 200–500 mm. Therefore, the coastal areas of California, Oregon, and Washington are well suited for these ticks. Immature stages commonly parasitize Western fence lizards (Sceloporus occidentalis) and gray squirrels (Sciurus griseus), while adults often feed on deer mice (Peromyscus maniculatus) and black-tailed deer (Odocoileus h. columbianus). Ixodes pacificus carry several pathogens of human significance, such as Borrelia burgdorferi, Bartonella, and Rickettsiales. These pathogens are maintained in the environment by many hosts, including small mammals, birds, livestock, and domestic animals. Although a great deal of work has been carried out on Ixodes ticks and the pathogens they transmit, understanding I. pacificus ecology outside California still lags. Additionally, the dynamic vector–host–pathogen system means that new factors will continue to arise and shift the epidemiological patterns within specific areas. Here, we review the ecology of I. pacificus and the pathogens this tick is known to carry to identify gaps in our knowledge.

Extermination Speed of an Imidacloprid and Flumethrin Polymer Matrix Collar against Larvae, Nymphs and Adults of Haemaphysalis longicornis

The objective of this study was to evaluate the efficacy of an imidacloprid 10% and flumethrin 4.5% polymer matrix collar against the developmental stages of Haemaphysalis longicornis infesting dogs using the hair from treated dogs in a semi-in-vitro assay set. When incubated with 0.5 g of the hair collected from the dogs installed with the drug-embedded collar after 10 days, average death rate of the larval, nymphal, and adult H. longicornis was 21.5%, 77.9%, and 100% at 30 min, 1 hr, and 2 hr, respectively. This study showed the larval stages as well as the nymphal and adult stages of H. longicornis ticks are killed upon contact with the hair from dogs treated with the collar within 2 hr.

Borreliae Part 2: Borrelia Relapsing Fever Group and Unclassified Borrelia

Borreliae of the relapsing fever group (RFG) are heterogenous and can be divided mainly into three groups according to vectors, namely the soft-tick-borne relapsing fever (STBRF) Borreliae, the hard-tick-borne relapsing fever (HTBRF) Borreliae, the louse-borne relapsing fever (LBRF) Borreliae, and the avian relapsing fever ones. With respect to the geographical distribution, the STBRF Borreliae are further subdivided into Old World and New World strains. Except for the Avian relapsing fever group Borreliae, which cause avian spirochetosis, all the others share infectivity in humans. They are indeed the etiological agent of both endemic and epidemic forms of relapsing fever, causing high spirochaetemia and fever. Vectors are primarily soft ticks of Ornithodoros spp. in the STBRF group; hard ticks, notably Ixodes sp., Amblyomma sp., Dermacentor sp., and Rhipicephalus sp., in the HTBRF group; and the louse pediculus humanus humanus in the TBRF one. A recent hypothesis was supported for a common ancestor of RFG Borreliae, transmitted at the beginning by hard-body ticks. Accordingly, STBRF Borreliae switched to use soft-bodied ticks as a vector, which was followed by the use of lice by Borrelia recurrentis. There are also new candidate species of Borreliae, at present unclassified, which are also described in this review.

Borreliae Part 1: Borrelia Lyme Group and Echidna-Reptile Group

Simple Summary

Borreliae are spirochaetes, which represent a heterogeneous phylum within bacteria. Spirochaetes are indeed distinguished from other bacteria for their spiral shape, which also characterizes Borreliae. This review describes briefly the organization of the phylum Spirocheteales with a digression about its pathogenicity and historical information about bacteria isolation and characterization. Among spirochaetes, Borrelia genus is here divided into three groups, namely the Lyme group (LG), the Echidna-Reptile group (REPG) and the Relapsing Fever group (RFG). Borreliae Part 1 deals with Lyme group and Echidna-Reptile group Borreliae, while the subject of Borreliae Part 2 is Relapsing Fever group and unclassified Borreliae. Lyme group Borreliae is organized here in sections describing ecology, namely tick vectors and animal hosts, epidemiology, microbiology, and Borrelia genome organization and antigen characterization. Furthermore, the main clinical manifestations in Lyme borreliosis are also described. Although included in the Lyme group due to their particular clinical features, Borrelia causing Baggio Yoshinari syndrome and Borrelia mayonii are described in dedicated paragraphs. The Borrelia Echidna-Reptile group has been recently characterized including spirochaetes that apparently are not pathogenic to humans, but infect reptiles and amphibians. The paragraph dedicated to this group of Borreliae describes their vectors, hosts, geographical distribution and their characteristics.

Abstract

Borreliae are divided into three groups, namely the Lyme group (LG), the Echidna-Reptile group (REPG) and the Relapsing Fever group (RFG). Currently, only Borrelia of the Lyme and RF groups (not all) cause infection in humans. Borreliae of the Echidna-Reptile group represent a new monophyletic group of spirochaetes, which infect amphibians and reptiles. In addition to a general description of the phylum Spirochaetales, including a brief historical digression on spirochaetosis, in the present review Borreliae of Lyme and Echidna-Reptile groups are described, discussing the ecology with vectors and hosts as well as microbiological features and molecular characterization. Furthermore, differences between LG and RFG are discussed with respect to the clinical manifestations. In humans, LG Borreliae are organotropic and cause erythema migrans in the early phase of the disease, while RFG Borreliae give high spirochaetemia with fever, without the development of erythema migrans. With respect of LG Borreliae, recently Borrelia mayonii, with intermediate characteristics between LG and RFG, has been identified. As part of the LG, it gives erythema migrans but also high spirochaetemia with fever. Hard ticks are vectors for both LG and REPG groups, but in LG they are mostly Ixodes sp. ticks, while in REPG vectors do not belong to that genus.

Monitoring Risk: Tick and Borrelia burgdorferi Public Participatory Surveillance in the Canadian Maritimes, 2012-2020

Ticks are vectors of many diseases, including Lyme disease (Ld). Lyme disease is an emerging disease in Canada caused by infection with the Lyme borreliosis (Lb) members of the Borrelia genus of spirochaete bacteria, of which Borrelia&nbsp;burgdorferi is regionally the most prevalent. The primary tick vector in central and eastern Canada, Ixodes scapularis, is increasing in numbers and in the geographical extent of established populations. This study documents the distribution of ticks recovered by passive surveillance, and their B.&nbsp;burgdorferi infection prevalence, in three Canadian Maritime provinces from 2012-2020. These regions represent areas in which tick populations are widely established, establishing, and considered non-established. Using a community science approach by partnering with veterinarians and members of the public, we collected over 7000 ticks from the 3 provinces. The three species found most often on companion animals and humans were I. scapularis (76.9%), Ixodes cookei (10.4%) and Dermacentor variabilis (8.9%). The most common hosts were dogs (60.5%), cats (16.8%) and humans (17.6%). As is typical of passive surveillance tick collections, the majority of ticks recovered were adult females; for I. scapularis 90.2%, 5.3%, 3.9% and 0.6% of the total of 5630 ticks recovered for this species were adult females, adult males, nymphs and larvae, respectively. The majority of B. burgdorferi-infected ticks were I. scapularis, as expected. Borrelia infection prevalence in I scapularis was higher in Nova Scotia (20.9%), the province with the most endemic regions, than New Brunswick (14.1%) and Prince Edward Island (9.1%), provinces thought to have established and non-established tick populations, respectively. The province-wide Borrelia infection prevalence generally increased in these latter tow provinces over the course of the study. The host did not have a significant effect on B. burgdorferi infection prevalence; I. scapularis ticks from dogs, cats, humans was, 13.3% (n = 3622), 15.6% (n = 817), 17.9% (n = 730), respectively. No I. scapularis larvae were found infected (n = 33) but B. burgdorferi was detected in 14.8% of both adults (n = 5140) and nymphs (n = 215). The incidence of B. burgdorferi infection also did not differ by engorgement status 15.0% (n = 367), 15.1% (n = 3101) and 14.4% (n = 1958) of non-engorged, engorged and highly engorged ticks, respectively, were infected. In New Brunswick, at the advancing front of tick population establishment, the province-wide infection percentages generally increased over the nine-year study period and all health district regions showed increased tick recoveries and a trend of increased percentages of Borrelia-infected ticks over the course of the study. Within New Brunswick, tick recoveries but not Borrelia infection prevalence were significantly different from endemic and non-endemic regions, suggesting cryptic endemic regions existed prior to their designation as a risk area. Over the 9 years of the study, tick recoveries increased in New Brunswick, the primary study region, and I. scapularis recoveries spread northwards and along the coast, most but not all new sites of recoveries were predicted by climate-based models, indicating that ongoing tick surveillance is necessary to accurately detect all areas of risk. Comparison of tick recoveries and public health risk areas indicates a lag in identification of risk areas. Accurate and timely information on tick distribution and the incidence of Borrelia and other infections are essential for keeping the public informed of risk and to support disease prevention behaviors.

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.

Borrelia afzelii Infection in the Rodent Host Has Dramatic Effects on the Bacterial Microbiome of Ixodes ricinus Ticks

The microbiome of blood-sucking arthropods can shape their competence to acquire and maintain infections with vector-borne pathogens. We used a controlled study to investigate the interactions between Borrelia afzelii, which causes Lyme borreliosis in Europe, and the bacterial microbiome of Ixodes ricinus, its primary tick vector. We applied a surface sterilization treatment to I. ricinus eggs to produce dysbiosed tick larvae that had a low bacterial abundance and a changed bacterial microbiome compared to those of the control larvae. Dysbiosed and control larvae fed on B. afzelii-infected mice and uninfected control mice, and the engorged larvae were left to molt into nymphs. The nymphs were tested for B. afzelii infection, and their bacterial microbiome underwent 16S rRNA amplicon sequencing. Surprisingly, larval dysbiosis had no effect on the vector competence of I. ricinus for B. afzelii, as the nymphal infection prevalence and the nymphal spirochete load were the same between the dysbiosed group and the control group. The strong effect of egg surface sterilization on the tick bacterial microbiome largely disappeared once the larvae molted into nymphs. The most important determinant of the bacterial microbiome of I. ricinus nymphs was the B. afzelii infection status of the mouse on which the nymphs had fed as larvae. Nymphs that had taken their larval blood meal from an infected mouse had a less abundant but more diverse bacterial microbiome than the control nymphs. Our study demonstrates that vector-borne infections in the vertebrate host shape the microbiome of the arthropod vector. IMPORTANCE Many blood-sucking arthropods transmit pathogens that cause infectious disease. For example, Ixodes ricinus ticks transmit the bacterium Borrelia afzelii, which causes Lyme disease in humans. Ticks also have a microbiome, which can influence their ability to acquire and transmit tick-borne pathogens such as B. afzelii. We sterilized I. ricinus eggs with bleach, and the tick larvae that hatched from these eggs had a dramatically reduced and changed bacterial microbiome compared to that of control larvae. These larvae fed on B. afzelii-infected mice, and the resultant nymphs were tested for B. afzelii and for their bacterial microbiome. We found that our manipulation of the bacterial microbiome had no effect on the ability of the tick larvae to acquire and maintain populations of B. afzelii. In contrast, we found that B. afzelii infection had dramatic effects on the bacterial microbiome of I. ricinus nymphs. Our study demonstrates that infections in the vertebrate host can shape the tick microbiome.

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.

Competition between strains of Borrelia afzelii in the host tissues and consequences for transmission to ticks

Pathogen species often consist of genetically distinct strains, which can establish mixed infections or coinfections in the host. In coinfections, interactions between pathogen strains can have important consequences for their transmission success. We used the tick-borne bacterium Borrelia afzelii, which is the most common cause of Lyme disease in Europe, as a model multi-strain pathogen to investigate the relationship between coinfection, competition between strains, and strain-specific transmission success. Mus musculus mice were infected with one or two strains of B. afzelii, strain transmission success was measured by feeding ticks on mice, and the distribution of each strain in six different mouse organs and the ticks was measured using qPCR. Coinfection and competition reduced the tissue infection prevalence of both strains and changed their bacterial abundance in some tissues. Coinfection and competition also reduced the transmission success of the B. afzelii strains from the infected hosts to feeding ticks. The ability of the B. afzelii strains to establish infection in the host tissues was strongly correlated with their transmission success to the tick vector. Our study demonstrates that coinfection and competition between pathogen strains inside the host tissues can have major consequences for their transmission success.