Predicting distributions of blacklegged ticks (Ixodes scapularis), Lyme disease spirochetes (Borrelia burgdorferi sensu stricto) and human Lyme disease cases in the eastern United States

Lyme disease is the most commonly reported vector-borne disease in the United States (US), with approximately 300,000 -to- 40,000 cases reported annually. The blacklegged tick, Ixodes scapularis, is the primary vector of the Lyme disease-causing spirochete, Borrelia burgdorferi sensu stricto, in high incidence regions in the upper midwestern and northeastern US. Using county-level records of the presence of I. scapularis or presence of B. burgdorferi s.s. infected host-seeking I. scapularis, we generated habitat suitability consensus maps based on an ensemble of statistical models for both acarological risk metrics. Overall accuracy of these suitability models was high (AUC = 0.76 for I. scapularis and 0.86 for B. burgdorferi s.s. infected-I. scapularis). We sought to compare which acarological risk metric best described the distribution of counties reporting high Lyme disease incidence (≥10 confirmed cases/100,000 population) by setting the models to a fixed omission rate (10%). We compared the percent of high incidence counties correctly classified by the two models. The I. scapularis consensus map correctly classified 53% of high and low incidence counties, while the B. burgdorferi s.s. infected-I. scapularis consensus map classified 83% correctly. Counties classified as suitable by the B. burgdorferi s.s. map showed a 91% overlap with high Lyme disease incidence counties with over a 38-fold difference in Lyme disease incidence between high- and low-suitability counties. A total of 288 counties were classified as highly suitable for B. burgdorferi s.s., but lacked records of infected-I. scapularis and were not classified as high incidence. These counties were considered to represent a leading edge for B. burgdorferi s.s. infection in ticks and humans. They clustered in Illinois, Indiana, Michigan, and Ohio. This information can aid in targeting tick surveillance and prevention education efforts in counties where Lyme disease risk may increase in the future.     

Borrelia species in Ixodes affinis and Ixodes scapularis ticks collected from the coastal plain of North Carolina

Ixodes affinis and I. scapularis are tick species that are widely distributed in the coastal plain region of North Carolina. Both tick species are considered enzootic vectors for spirochetal bacteria of the genus Borrelia and specifically for B. burgdorferi s.s., the pathogen most often attributed as the cause of Lyme disease in the USA. Laboratory testing of individual I. affinis and I. scapularis ticks for the presence of Borrelia DNA was accomplished by PCR, targeting 2 regions of the 16S-23S intergenic spacer. In I. affinis, Borrelia DNA was detected in 63.2% of 155 individual ticks. B. burgdorferi s.s. and B. bissettii were identified by DNA sequencing in 33.5% and 27.9% I. affinis, respectively. Statistical differences were found for sex distribution of Borrelia DNA between I. affinis females (76.8%) and I. affinis males (55.6%) where B. burgdorferi s.s. was more prevalent in females (44.6%) than in males (27.3%). In I. scapularis, 298 individually tested ticks yielded no Borrelia PCR-positive results. This study found a higher incidence of Borrelia spp. in I. affinis collected in coastal North Carolina as compared to previous reports for this tick species in other Southern states, highlighting the potential importance of I. affinis in the maintenance of the enzootic transmission cycle of B. burgdorferi s.l. in North Carolina. The lack of Borrelia DNA in I. scapularis highlights the need for additional studies to better define the transmission cycle for B. burgdorferi s.s. in the southeastern USA and specifically in the state of North Carolina.     

Liver microbiome of Peromyscus leucopus,a key reservoir host species for emerging infectious diseases in North America

Microbiome studies generally focus on the gut microbiome, which is composed of a large proportion of commensal bacteria. Here we propose a first analysis of the liver microbiome using next generation sequencing as a tool to detect potentially pathogenic strains. We used Peromyscus leucopus, the main reservoir host species of Lyme disease in eastern North America, as a model and sequenced V5-V6 regions of the 16S gene from 18 populations in southern Quebec (Canada). The Lactobacillus genus was found to dominate the liver microbiome. We also detected a large proportion of individuals infected by Bartonella vinsonii arupensis, a human pathogenic bacteria responsible for endocarditis, as well as Borrelia burgdorferi, the pathogen responsible for Lyme disease in North America. We then compared the microbiomes among two P. leucopus genetic clusters occurring on either side of the St. Lawrence River, and did not detect any effect of the host genotype on their liver microbiome assemblage. Finally, we report, for the first time, the presence of B. burgdorferi in a small mammal host from the northern side of the St. Lawrence River, in support of models that have predicted the northern spread of Lyme disease in Canada.     

我国莱姆病的流行病学和病原学研究

１９８７－１９９６年对２２个省市（市、区）的６０个县、区进行了莱姆病的调查研究。血清流行病学证实２２个省市（市、区）林区人群均存在莱姆病感染，感染率平均为５．０６％（１７２４／３４１０４）。病原学证实１７个省（市、区）存在莱姆病自然疫源地。１１个省（市、区）有典型病例存在。     

Mapping distributions of the Lyme disease vector,Ixodes scapularis,and spirochete,Borrelia burgdorferi,in Kentucky using passive and active surveillance

Lyme disease is the most common tick-borne illness in the United States and is becoming more prevalent each year. It is transmitted to humans and animals through the bites of Ixodes scapularis ticks infected with Borrelia burgdorferi in the eastern United States, I. pacificus in the western U.S, and I. ricinus in Europe and Asia. In Kentucky, where Lyme disease is non-endemic, the number of reported human cases in 2010 totaled five. In 2019, that number had increased by over 300%. Identifying the distribution of I. scapularis populations infected with B. burgdorferi is important data for effective prevention strategies and an important first step in monitoring disease spread. In collaboration with the Kentucky Department for Public Health, we performed surveillance for I. scapularis throughout the state of Kentucky using both active and passive surveillance methods. Diagnostic testing for the identification of B. burgdorferi (sensu stricto) was also conducted. We identified 457 I. scapularis ticks from March 2019 to December 2020 from 32 counties in Kentucky. B. burgdorferi was detected in I. scapularis populations collected from 14 different counties. These results add to the little data that exists in Kentucky on I. scapularis and B. burgdorferi distribution.     

Deer management generally reduces densities of nymphal Ixodes scapularis,but not prevalence of infection with Borrelia burgdorferi sensu stricto

Human Lyme disease–primarily caused by the bacterium Borrelia burgdorferi sensu stricto (s.s.) in North America–is the most common vector-borne disease in the United States. Research on risk mitigation strategies during the last three decades has emphasized methods to reduce densities of the primary vector in eastern North America, the blacklegged tick (Ixodes scapularis). Controlling white-tailed deer populations has been considered a potential method for reducing tick densities, as white-tailed deer are important hosts for blacklegged tick reproduction. However, the feasibility and efficacy of white-tailed deer management to impact acarological risk of encountering infected ticks (namely, density of host-seeking infected nymphs; DIN) is unclear. We investigated the effect of white-tailed deer density and management on the density of host-seeking nymphs and B. burgdorferi s.s. infection prevalence using surveillance data from eight national parks and park regions in the eastern United States from 2014–2022. We found that deer density was significantly positively correlated with the density of nymphs (nymph density increased by 49% with a 1 standard deviation increase in deer density) but was not strongly correlated with the prevalence of B. burgdorferi s.s. infection in nymphal ticks. Further, while white-tailed deer reduction efforts were followed by a decrease in the density of I. scapularis nymphs in parks, deer removal had variable effects on B. burgdorferi s.s. infection prevalence, with some parks experiencing slight declines and others slight increases in prevalence. Our findings suggest that managing white-tailed deer densities alone may not be effective in reducing DIN in all situations but may be a useful tool when implemented in integrated management regimes.     

Evolutionary genomics of Borrelia burgdorferi sensu lato: Findings,hypotheses, and the rise of hybrids

Borrelia burgdorferi sensu lato (B. burgdorferi s.l.), the group of bacterial species represented by Lyme disease pathogens, has one of the most complex and variable genomic architectures among prokaryotes. Showing frequent recombination within and limited gene flow among geographic populations, the B. burgdorferi s.l. genomes provide an excellent window into the processes of bacterial evolution at both within- and between-population levels. Comparative analyses of B. burgdorferi s.l. genomes revealed a highly dynamic plasmid composition but a conservative gene repertoire. Gene duplication and loss as well as sequence variations at loci encoding surface-localized lipoproteins (e.g., the PF54 genes) are strongly associated with adaptive differences between species. There are a great many conserved intergenic spacer sequences that are candidates for cis-regulatory elements and non-coding RNAs. Recombination among coexisting strains occurs at a rate approximately three times the mutation rate. The coexistence of a large number of genomic groups within local B. burgdorferi s.l. populations may be driven by immune-mediated diversifying selection targeting major antigen loci as well as by adaptation to multiple host species. Questions remain regarding the ecological causes (e.g., climate change, host movements, or new adaptations) of the ongoing range expansion of B. burgdorferi s.l. and on the genomic variations associated with its ecological and clinical variability. Anticipating an explosive growth of the number of B. burgdorferi s.l. genomes sampled from both within and among species, we propose genome-based methods to test adaptive mechanisms and to identify molecular bases of phenotypic variations. Genome sequencing is also necessary for monitoring a likely increase of genetic admixture of previously isolated species and populations in North America and elsewhere.     

Identification of TROSPA homologue in Ixodes persulcatus Schulze,the specific vector for human Lyme borreliosis in Japan

The tick receptor for outer surface protein A (TROSPA) is an Ixodes scapularis (I. scapularis) receptor for Borrelia burgdorferi (B. burgdorferi), the causative agent of Lyme disease in North America. The blockade of TROSPA has been shown to reduce B. burgdorferi adherence to the I. scapularis gut in vivo. Thus, TROSPA is one of the potential targets for the development of vector–antigen-based vaccines to prevent the transmission of B. burgdorferi. The aim of this study is to identify the TROSPA gene in I. persulcatus Schulze, the specific vector for human Lyme borreliosis in Japan. The cDNA clone encoding the TROSPA-like sequence with 483 nucleotides was obtained from whole-body homogenates of fed nymphs of I. persulcatus. The putative amino acid sequence of I. persulcatus TROSPA was 88.2% and 87.8% identical to that of I. scapularis and I. ricinus, respectively. This finding will facilitate investigations on the role of I. persulcatus TROSPA and its interaction with Borrelia spp. and will have important implications on endeavors to develop a tick vaccine.     

Comparison of Lyme Disease in the United States and Europe

Lyme disease, or Lyme borreliosis, is the most common tickborne disease in the United States and Europe. In both locations, Ixodes species ticks transmit the Borrelia burgdorferi sensu lato bacteria species responsible for causing the infection. The diversity of Borrelia species that cause human infection is greater in Europe; the 2 B. burgdorferi s.l. species collectively responsible for most infections in Europe, B. afzelii and B. garinii, are not found in the United States, where most infections are caused by B. burgdorferi sensu stricto. Strain differences seem to explain some of the variation in the clinical manifestations of Lyme disease, which are both minor and substantive, between the United States and Europe. Future studies should attempt to delineate the specific virulence factors of the different species of B. burgdorferi s.l. responsible for these variations in clinical features.     

Development of a baited oral vaccine for use in reservoir-targeted strategies against Lyme disease

Lyme disease is a major human health problem which continues to increase in incidence and geographic distribution. As a vector-borne zoonotic disease, Lyme disease may be amenable to reservoir targeted strategies for control. We have previously reported that a vaccinia virus (VV) based vaccine expressing outer surface protein A (OspA) of Borrelia burgdorferi, the causative agent of Lyme disease, protects inbred strains of laboratory mice against infection by feeding ticks and clears the ticks of infection when administered by gavage. Here we extend these studies to develop an effective bait formulation for delivery of the VV based vaccine and test its characteristics under simulated environmental conditions. We show that this vaccine is efficacious in decreasing acquisition of B. burgdorferi by uninfected larval ticks as well as in decreasing transmission from infected ticks to its natural reservoir, Peromyscus leucopus, when fed to mice in oral baits. Using live, in vivo imaging techniques, we describe the distribution of vaccinia virus infection after ingestion of the baited vaccines and establish the use of in vivo imaging technology for optimization of bait delivery. In summary, a VV based OspA vaccine is stable in an oral bait preparation and provides protection against infection for both the natural reservoir and the tick vector of Lyme disease.     

Prevalence of Borrelia burgdorferi and diversity of its outer surface protein C (ospC) alleles in blacklegged ticks (Ixodes scapularis) in Delaware

Characterizing the diversity of genes associated with virulence and transmission of a pathogen across the pathogen's distribution can inform our understanding of host infection risk. Borrelia burgdorferi is a vector-borne bacterium that causes Lyme disease in humans and is common in the United States. The outer surface protein C (ospC) gene of B. burgdorferi exhibits substantial genetic variation across the pathogen's distribution and plays a critical role in virulence and transmission in vertebrate hosts. In fact, B. burgdorferi infections that disseminate across host tissues in humans are associated with only a subset of ospC alleles. Delaware has a high incidence of Lyme disease, but the diversity of ospC in B. burgdorferi in the state has not been evaluated. We used PCR to amplify ospC in B. burgdorferi-infected blacklegged ticks (Ixodes scapularis) in sites statewide and used short-read sequencing to identify ospC alleles. B. burgdorferi prevalence in blacklegged ticks varied across sites, but not significantly so. We identified 15 previously characterized ospC alleles accounting for nearly all of the expected diversity of alleles across the sites as estimated using the Chao1 index. Nearly 40% of sequenced infections (23/58) had more than one ospC allele present suggesting mixed strain infections and the relative frequencies of alleles in single infections were positively correlated with their relative frequencies in mixed infections. Turnover of ospC alleles was positively related to distance between sites with closer sites having more similar allele compositions than more distant sites. This suggests a degree of B. burgdorferi dispersal limitation or habitat specialization. OspC alleles known to cause disseminated infections in humans were found at the highest frequencies across sites, corresponding to Delaware's high incidence of Lyme disease.     

Borrelia miyamotoi sensu lato Seroreactivity and Seroprevalence in the Northeastern United States

Borrelia miyamotoi sensu lato, a relapsing fever Borrelia sp., is transmitted by the same ticks that transmit B. burgdorferi (the Lyme disease pathogen) and occurs in all Lyme disease�?"endemic areas of the United States. To determine the seroprevalence of IgG against B. miyamotoi sensu lato in the northeastern United States and assess whether serum from B. miyamotoi sensu lato�?"infected persons is reactive to B. burgdorferi antigens, we tested archived serum samples from area residents during 1991�?"2012. Of 639 samples from healthy persons, 25 were positive for B. miyamotoi sensu lato and 60 for B. burgdorferi. Samples from �%^10% of B. miyamotoi sensu lato�?"seropositive persons without a recent history of Lyme disease were seropositive for B. burgdorferi. Our resultsA suggest thatA human B. miyamotoiA sensu latoA infection may be common in southern New England and that B. burgdorferi antibody testing is not an effective surrogate for detecting B. miyamotoi sensu lato infection.     

A spatially-explicit model of acarological risk of exposure to Borrelia burgdorferi-infected Ixodes pacificus nymphs in northwestern California based on woodland type,temperature, and water vapor

In the far-western United States, the nymphal stage of the western black-legged tick, Ixodes pacificus, has been implicated as the primary vector to humans of Borrelia burgdorferi sensu stricto (hereinafter referred to as B. burgdorferi), the causative agent of Lyme borreliosis in North America. In the present study, we sought to determine if infection prevalence with B. burgdorferi in I. pacificus nymphs and the density of infected nymphs differ between dense-woodland types within Mendocino County, California, and to develop and evaluate a spatially-explicit model for density of infected nymphs in dense woodlands within this high-incidence area for Lyme borreliosis. In total, 4.9% (264) of 5431 I. pacificus nymphs tested for the presence of B. burgdorferi were infected. Among the 78 sampling sites, infection prevalence ranged from 0% to 22% and density of infected nymphs from 0 to 2.04 per 100 m². Infection prevalence was highest in woodlands dominated by hardwoods (6.2%) and lowest for redwood (1.9%) and coastal pine (0%). Density of infected nymphs also was higher in hardwood-dominated woodlands than in conifer-dominated ones that included redwood or pine. Our spatial risk model, which yielded an overall accuracy of 85%, indicated that warmer areas with less variation between maximum and minimum monthly water vapor in the air were more likely to include woodlands with elevated acarological risk of exposure to infected nymphs. We found that 37% of dense woodlands in the county were predicted to pose an elevated risk of exposure to infected nymphs, and that 94% of the dense-woodland areas that were predicted to harbor elevated densities of infected nymphs were located on privately-owned land.     

Lyme Disease,Virginia, USA, 2000–2011

Lyme disease, caused by the bacterium Borrelia burgdorferi and transmitted in the eastern United States by the black-legged tick (Ixodes scapularis), is increasing in incidence and expanding geographically. Recent environmental modeling based on extensive field collections of host-seeking I. scapularis ticks predicted a coastal distribution of ticks in mid-Atlantic states and an elevational limit of 510 m. However, human Lyme disease cases are increasing most dramatically at higher elevations in Virginia, a state where Lyme disease is rapidly emerging. Our goal was to explore the apparent incongruity, during 2000–2011, between human Lyme disease data and predicted and observed I. scapularis distribution. We found significantly higher densities of infected ticks at our highest elevation site than at lower elevation sites. We also found that I. scapularis ticks in Virginia are more closely related to northern than to southern tick populations. Clinicians and epidemiologists should be vigilant in light of the changing spatial distributions of risk.     

Detection of spirochaetes of Borrelia burgdorferi complexe in the skin of cervids by PCR and culture

To determine whether deer may play a role in the cycle of the Lyme disease spirochete Borrelia burgdorferi, we sought evidence for the presence of the pathogen in skin of deer and roe deer. Biopsies of 2 mm³ were taken at four different levels from nail to tarsus. A total of 50 animals (200 biopsies) were shot in the Lyme disease foci of Rambouillet during the hunting season 1995–1996 and 1996–1997, from the begining of November to the end of February. Borrelia burgdorferi s.l. DNA was detected by PCR in 18 biopsies from 14 animals (28%). Borrelia burgdorferi s.str. was predominant (50%), followed by B. garinii (30%) and B. afzelii (10%). Multiple infections were detected in four animals: same species at different levels or two different species from the same biopsy or from different biopsies from the same foot. A total of 125 biopsies were cultivated on BSKH medium. Cultures at 160 days revealed immobile spiralled forms in 10 cultures. One, from a deer killed at the end of December, was confirmed by PCR as B. burgdorferi s.str. These results, frequency of detection of spirochetes by PCR in the skin, multiple infections and alive spirochetes in biopsies taken out side the season of activity of ticks strongly suggest an affinity of Lyme disease spirochetes for skin of cervids.     

Clobetasol increases the abundance of Borrelia burgdorferi in the skin 70 times more in male mice compared to female mice

Lyme borreliosis is caused by the spirochete Borrelia burgdorferi and is transmitted among vertebrate hosts by Ixodes scapularis ticks in eastern North America. Treatment with topical corticosteroids increases the abundance of B. burgdorferi in the skin of lab mice that have been experimentally infected via needle inoculation. In the present study, female and male C3H/HeJ mice were infected with B. burgdorferi via nymphal tick bite. Infected mice were treated with clobetasol on the skin of the right hindleg on days 35 and 36 post-infection and euthanized at days -2, 1, 3, 5, and 7 post-treatment; a group of control mice was infected but not treated with clobetasol. The spirochete abundance was quantified in 8 mouse tissues including bladder, heart, left hindleg skin, right hindleg skin, dorsal skin, ventral skin, left ear and right ear. Averaged across the 8 mouse tissues, the abundance of B. burgdorferi on days 3 and 5 were 21.4x and 14.4x higher in mice treated with clobetasol compared to the untreated control mice, but there were large differences among tissues. There was a dramatic sex-specific effect of the clobetasol treatment; the peak abundance of B. burgdorferi in the skin (left hindleg, right hindleg, dorsal, ventral) was 72.6x higher in male mice compared to female mice. In contrast, there was little difference between the sexes in the tissue spirochete load in the ears, bladder, and heart. Topical application of clobetasol could increase the sensitivity of direct diagnostic methods (e.g., culture, PCR) to detect B. burgdorferi in host skin biopsies.     

Evolution of Northeastern and Midwestern Borrelia burgdorferi, United States

The per capita incidence of human Lyme disease in the northeastern United States is more than twice that in the Midwest. However, the prevalence of Borrelia burgdorferi, the bacterium that causes Lyme disease, in the tick vector is nearly identical in the 2 regions. The disparity in human Lyme disease incidence may result from a disparity in the human invasiveness of the bacteria in the Northeast and Midwest caused by fundamentally different evolutionary histories. B. burgdorferi populations in the Northeast and Midwest are geographically isolated, enabling evolutionary divergence in human invasiveness. However, we found that B. burgdorferi populations in the Northeast and Midwest shared a recent common ancestor, which suggests that substantial evolutionary divergence in human invasiveness has not occurred. We propose that differences in either animal ecology or human behavior are the root cause of the differences in human incidence between the 2 regions.     

Failure of the Asian longhorned tick,Haemaphysalis longicornis,to serve as an experimental vector of the Lyme disease spirochete,Borrelia burgdorferi sensu stricto

The invasive, human-biting Asian longhorned tick, Haemaphysalis longicornis, was detected in New Jersey in the eastern United States in August of 2017 and by November of 2018 this tick had been recorded from 45 counties across 9 states, primarily along the Eastern Seaboard. The establishment of H. longicornis in the United States has raised the questions of how commonly it will bite humans and which native pathogens may naturally infect this tick. There also is a need for experimental vector competence studies with native pathogens to determine if H. longicornis can acquire a given pathogen while feeding, pass it transstadially, and then transmit the pathogen in the next life stage. In this experimental study, we evaluated the vector competence of a population of H. longicornis originating from the United States (New York) for a native isolate (B31) of the Lyme disease spirochete, Borrelia burgdorferi sensu stricto (s.s.). In agreement with a previous experimental study on the vector competence of H. longicornis for Borrelia garinii, we found that uninfected H. longicornis larvae could acquire B. burgdorferi s.s. while feeding on infected Mus musculus mice (infection prevalence >50% in freshly fed larvae) but that the infection was lost during the molt to the nymphal stage. None of 520 tested molted nymphs were found to be infected, indicating that transstadial passage of B. burgdorferi s.s. is absent or rare in H. longicornis; and based on the potential error associated with the number of nymphs testing negative in this study, we estimate that the upper 95% limit for infection prevalence was 0.73%. An Ixodes scapularis process control showed both effective acquisition of B. burgdorferi s.s. from infected mice by uninfected larvae and transstadial passage to the nymphal stage (infection prevalence of 80–82% for both freshly fed larvae and molted nymphs). We also observed that although H. longicornis larvae could be compelled to feed on mice by placing the ticks within feeding capsules, attachment and feeding success was minimal (<0.5%) when larvae were placed freely on the fur of the mice. We conclude that H. longicornis is unlikely to contribute more than minimally, if at all, to transmission of Lyme disease spirochetes in the United States.     

Outer surface protein E (OspE) mediates Borrelia burgdorferi sensu stricto strain-specific complement evasion in the eastern fence lizard,Sceloporus undulatus

In North America, Lyme disease is primarily caused by the spirochetal bacterium Borrelia burgdorferi sensu stricto (Bb), which is transmitted between multiple vertebrate hosts and ixodid ticks, and is a model commonly used to study host-pathogen interactions. While Bb is consistently observed in its mammalian and avian reservoirs, the bacterium is rarely isolated from North American reptiles. Two closely related lizard species, the eastern fence lizard (Sceloporus undulatus) and the western fence lizard (Sceloporus occidentalis), are examples of reptiles parasitized by Ixodes ticks. Vertebrates are known to generate complement as an innate defense mechanism, which can be activated before Bb disseminate to distal tissues. Complement from western fence lizards has proven lethal against one Bb strain, implying the role of complement in making those lizards unable to serve as hosts to Bb. However, Bb DNA is occasionally identified in distal tissues of field-collected eastern fence lizards, suggesting some Bb strains may overcome complement-mediated clearance in these lizards. These findings raise questions regarding the role of complement and its impact on Bb interactions with North American lizards. In this study, we found Bb seropositivity in a small population of wild-caught eastern fence lizards and observed Bb strain-specific survivability in lizard sera. We also found that a Bb outer surface protein, OspE, from Bb strains viable in sera, promotes lizard serum survivability and binds to a complement inhibitor, factor H, from eastern fence lizards. Our data thus identify bacterial and host determinants of eastern fence lizard complement evasion, providing insights into the role of complement influencing Bb interactions with North American lizards.     

Active and Passive Surveillance and Phylogenetic Analysis of Borrelia burgdorferi Elucidate the Process of Lyme Disease Risk Emergence in Canada

Background

Northward expansion of the tick Ixodes scapularis is driving Lyme disease (LD) emergence in Canada. Information on mechanisms involved is needed to enhance surveillance and identify where LD risk is emerging.

Objectives

We used passive and active surveillance and phylogeographic analysis of Borrelia burgdorferi to investigate LD risk emergence in Quebec.

Methods

In active surveillance, we collected ticks from the environment and from captured rodents. B. burgdorferi transmission was detected by serological analysis of rodents and by polymerase chain reaction assays of ticks. Spatiotemporal trends in passive surveillance data assisted interpretation of active surveillance. Multilocus sequence typing (MLST) of B. burgdorferi in ticks identified likely source locations of B. burgdorferi.

Results

In active surveillance, we found I. scapularis at 55% of sites, and we were more likely to find them at sites with a warmer climate. B. burgdorferi was identified at 13 I. scapularis–positive sites, but infection prevalence in ticks and animal hosts was low. Low infection prevalence in ticks submitted in passive surveillance after 2004—from the tick-positive regions identified in active surveillance—coincided with an exponential increase in tick submissions during this time. MLST analysis suggested recent introduction of B. burgdorferi from the northeastern United States.

Conclusions

These data are consistent with I. scapularis ticks dispersed from the United States by migratory birds, founding populations where the climate is warmest, and then establishment of B. burgdorferi from the United States several years after I. scapularis have established. These observations provide vital information for public health to minimize the impact of LD in Canada.