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Background and Aims: Outbreaks of severe and chronic tick-borne diseases (TBDs) are on the rise. This is through the transmission of infectious disease agents to humans during tick feeding. The transmission rate and extent of microbial exchange, however, vary based on the tick microbiome composition. While select microbes are determined to be members of the normal tick microbiome and others are clearly recognized mammalian and/or avian pathogens, the status of many other members of the tick microbiota with respect to human and alternate host pathogenesis remains unclear. Moreover, the species-level 16S microbiome of prominent TBD vectors, including Ixodes pacificus, have not been extensively studied. To elucidate the I. pacificus microbiome composition, we performed a pan-domain species-specific characterization of the bacterial microbiome on adult I. pacificus ticks collected from two regional parks within Western California. Our methods provide for characterizing nuances within cohort microbiomes and their relationships to geo-locale of origin, surrounding fauna, and prevalences of known and suspected pathogens in relation to current TBD epidemiological zones. Methods: Ninety-two adult I. pacificus bacterial microbiomes were characterized using a high-fidelity, pan-domain, species-specific, full-length 16S rRNA amplification method using circular consensus sequencing performed on the Pacific Biosciences Sequel platform. Data analyses were performed with the MCSMRT data analysis package and database. Results: The species-specific I. pacificus microbiome composition illustrates a complex assortment of microflora, including over 900 eubacterial species with high taxonomic diversity, which was revealed to vary by sex and geo-locale, though the use of full-length 16S gene sequencing. The TBD-associated pathogens, such as Borrelia burgdorferi, Anaplasma phagocytophilum, and Rickettsia monacensis, were identified along with a host of bacteria previously unassociated with ticks. Conclusion: Species-level taxonomic classification of the I. pacificus microbiome revealed that full-length bacterial 16S gene sequencing is required for the granularity to elucidate the microbial diversity within and among ticks based on geo-locale.

During 2013-2019, Borrelia miyamotoi infection was detected in 19 US states. Infection rate was 0.5%-3.2%; of B. miyamotoi-positive ticks, 59.09% had concurrent infections. B. miyamotoi is homogeneous with 1 genotype from Ixodes scapularis ticks in northeastern and midwestern states and 1 from I. pacificus in western states.

In California, the western blacklegged tick, Ixodes pacificus Cooley and Kohls, is the principal vector of the Borrelia burgdorferi sensu lato (sl) complex (Spirochaetales: Spirochaetaceae, Johnson et al.), which includes the causative agent of Lyme disease (B. burgdorferi sensu stricto). Ixodes pacificus nymphs were sampled from 2015 to 2017 at one Sierra Nevada foothill site to evaluate our efficiency in collecting this life stage, characterize nymphal seasonality, and identify environmental factors affecting their abundance and infection with B. burgdorferi sl. To assess sampling success, we compared the density and prevalence of I. pacificus nymphs flagged from four questing substrates (logs, rocks, tree trunks, leaf litter). Habitat characteristics (e.g., canopy cover, tree species) were recorded for each sample, and temperature and relative humidity were measured hourly at one location. Generalized linear mixed models were used to assess environmental factors associated with I. pacificus abundance and B. burgdorferi sl infection. In total, 2,033 substrates were sampled, resulting in the collection of 742 I. pacificus nymphs. Seasonal abundance of nymphs was bimodal with peak activity occurring from late March through April and a secondary peak in June. Substrate type, collection year, month, and canopy cover were all significant predictors of nymphal density and prevalence. Logs, rocks, and tree trunks had significantly greater nymphal densities and prevalences than leaf litter. Cumulative annual vapor pressure deficit was the only significant climatic predictor of overall nymphal I. pacificus density and prevalence. No associations were observed between the presence of B. burgdorferi sl in nymphs and environmental variables.

Over one hundred cases of human rickettsiosis, many fatal, are reported annually across the US-Mexico transboundary region, representing a likely undercount. Although cases are often attributed to Rickettsia rickettsii, the agent of Rocky Mountain spotted fever, multiple other Rickettsia pathogens are present in North America. We conducted multiple-host surveillance of domestic, synanthropic, and sylvatic mammals and their ectoparasites to investigate the ecology of Rickettsia species in this region. A total of 499 mammals, including 83 dogs, 23 wild carnivores, five lagomorphs, and 388 rodents were sampled, and 413 fleas and 447 ticks belonging to 15 and 4 species, respectively, were collected during 2017 and 2018. We detected Rickettsia spp. DNA in one blood sample of coyote (Canis latrans), 11 ear tissues of rodents (10.6%), and 79 ectoparasites (9.5%). Of the 64 Rickettsia-positive fleas, 54 were Echidnophaga gallinacea and 10 were Pulex simulans, while of the 15 ticks, 11 were Rhipicephalus sanguineus s.l. and four Ixodes pacificus. The DNA sequence alignment of gltA and ompB regions revealed one and ten genetic variants of Rickettsia spp., respectively. These variants were clustered in clades of zoonotic species (R. felis, R. massiliae, R. parkeri, R. rickettsii, and R. typhi) and organisms of unknown pathogenic significance (R. asembonensis and Candidatus Rickettsia tarasevichiae). The finding of a coyote infected with R. rickettsii and the multiple zoonotic SFG rickettsial agents in the study area suggest that: 1) wild canids could serve as an amplifying host for RMSF, an alternate host for Rh. sanguineus s.l. ticks, and a means to spread infection and ticks over large areas; and 2) at least some of the human rickettsiosis cases attributed to R. rickettsii could be caused by other Rickettsia species. This study strongly supports the importance of multiple-host and vector eco-epidemiological studies and the One Health approach to better understand disease in a RMSF-epidemic region.

Kwan et al. (2017) published an informative study comparing results obtained by next-generation sequencing (NGS) of mean bacterial genera richness among different life stages, male and female adults, and rearing conditions (field vs. laboratory) for Ixodes pacificus. The current paper examines Kwan et al. (2017) as a case study to provide guidance on statistical design and analysis for estimation of richness, derived from next generation sequencing technology, of the bacterial microbiome in field-collected I. pacificus. Suggestions are provided to further strengthen quantification of microbiome richness in studies in ticks, with focus on sampling design. In-depth treatment is provided of the relative merits of estimating mean richness versus median richness. Research on microbiome diversity in ticks can be made quantitatively rigorous; although, more research on methods is needed.

Lyme disease is the most common vector-borne disease in the United States. While Lyme disease vectors are widespread, high incidence states are concentrated in the Northeast, North Central and Mid-Atlantic regions. Mapping the distribution of Lyme disease spirochetes in ticks may aid in providing data-driven explanations of epidemiological trends and recommendations for targeting prevention strategies to communities at risk. We compiled data from the literature, publicly available tickborne pathogen surveillance databases, and internal CDC pathogen testing databases to map the county-level distribution of Lyme disease spirochetes reported in host-seeking Ixodes pacificus and Ixodes scapularis across the contiguous United States. We report B. burgdorferi s.s.-infected I. scapularis from 384 counties spanning 26 eastern states located primarily in the North Central, Northeastern, and Mid-Atlantic regions, and in I. pacificus from 20 counties spanning 2 western states, with most records reported from northern and north-coastal California. Borrelia mayonii was reported in I. scapularis in 10 counties in Minnesota and Wisconsin in the North Central United States, where records of B. burgdorferi s.s. were also reported. In comparison to a broad distribution of vector ticks, the resulting map shows a more limited distribution of Lyme disease spirochetes.

Tick identification is critical for assessing disease risk from a tick bite and for determining requisite treatment. Data from the University of Rhode Island’s TickEncounter Resource Center’s photo-based surveillance system, TickSpotters, indicate that users incorrectly identified their submitted specimen 83% of the time. Of the top four most commonly submitted tick species, western blacklegged ticks (Ixodes pacificus Cooley & Kohls [Ixodida: Ixodidae]) had the largest proportion of unidentified or misidentified submissions (87.7% incorrectly identified to species), followed by lone star ticks (Amblyomma americanum Linneaus [Ixodida: Ixodidae]; 86.8% incorrect), American dog ticks (Dermacentor variabilis Say [Ixodida: Ixodidae]; 80.7% incorrect), and blacklegged ticks (Ixodes scapularis Say [Ixodida: Ixodidae]; 77.1% incorrect). More than one quarter of participants (26.3%) submitted photographs of ticks that had been feeding for at least 2.5 d, suggesting heightened risk. Logistic regression generalized linear models suggested that participants were significantly more likely to misidentify nymph-stage ticks than adult ticks (odds ratio [OR] = 0.40, 95% confidence interval [CI]: 0.23, 0.68, P < 0.001). Ticks reported on pets were more likely to be identified correctly than those found on humans (OR = 1.07, 95% CI: 1.01–2.04, P < 0.001), and ticks feeding for 2.5 d or longer were more likely to be misidentified than those having fed for one day or less (OR = 0.43, 95% CI: 0.29–0.65, P < 0.001). State and region of residence and season of submission did not contribute significantly to the optimal model. These findings provide targets for future educational efforts and underscore the value of photograph-based tick surveillance to elucidate these knowledge gaps.

Tick-borne diseases have expanded over the last 2 decades as a result of shifts in tick and pathogen distributions. These shifts have significantly increased the need for accurate portrayal of real-time pathogen distributions and prevalence in hopes of stemming increases in human morbidity. Traditionally, pathogen distribution and prevalence have been monitored through case reports or scientific collections of ticks or reservoir hosts, both of which have challenges that impact the extent, availability, and accuracy of these data. Citizen science tick collections and testing campaigns supplement these data and provide timely estimates of pathogen prevalence and distributions to help characterize and understand tick-borne disease threats to communities. We utilized our national citizen science tick collection and testing program to describe the distribution and prevalence of four Ixodes-borne pathogens, Borrelia burgdorferi sensu lato, Borrelia miyamotoi, Anaplasma phagocytophilum, and Babesia microti, across the continental United States. IMPORTANCE In the 21st century, zoonotic pathogens continue to emerge, while previously discovered pathogens continue to have changes within their distribution and prevalence. Monitoring these pathogens is resource intensive, requiring both field and laboratory support; thus, data sets are often limited within their spatial and temporal extents. Citizen science collections provide a method to harness the general public to collect samples, enabling real-time monitoring of pathogen distribution and prevalence.

Human granulocytic anaplasmosis is an obligate intra-granulocytic parasite that is transmitted by Ixodes scapularis and Ixodes pacificus in North America. We report on the second laboratory-confirmed case of Anaplasma phagocytophilum acquired within the province of Alberta, Canada. A 67-year-old woman from the Edmonton health zone developed nonspecific systemic symptoms including fatigue, night sweats, myalgia, headaches, and fever 6 days after noticing a tick on her left upper arm in May of 2017 (day 0). On day 13, she was found to have thrombocytopenia. Her symptoms progressed until day 16 when she was treated empirically with doxycycline, at which time she slowly improved over the subsequent 2 months. The tick was later identified as a partially engorged female blacklegged tick, I. scapularis, and it was positive for A. phagocytophilum DNA by PCR. Anaplasma serology performed retrospectively on blood samples collected on days 13, 31, and 52 showed a greater than 4-fold increase in A. phagocytophilum (IgG titers from less than 1:64 on day 13 to 1:2048 on days 31 and 52), consistent with an acute infection. Although populations of blacklegged ticks are not yet established in Alberta, suspicion should remain for tick-borne diseases because infected ticks are introduced into the province by migrating birds. This case report highlights the need to remind physicians and other public health professionals that rare, non-endemic tick-borne diseases can occasionally occur in low-risk jurisdictions.

A vector’s susceptibility and ability to transmit a pathogen— termed vector competency—determines disease outcomes, yet the ecological factors influencing tick vector competency remain largely unknown. Ixodes pacificus, the tick vector of Borrelia burgdorferi (Bb) in the western U.S., feeds on rodents, birds, and lizards. Unlike rodents and birds which are reservoirs for Bb and infect juvenile ticks, lizards are refractory to Bb and cannot infect feeding ticks. Additionally, the lizard bloodmeal contains borreliacidal properties, clearing previously infected feeding ticks of their Bb infection. Despite I. pacificus feeding on a range of hosts, it is undetermined how the host identity of the larval bloodmeal affects future nymphal vector competency. We experimentally evaluate the influence of larval host bloodmeal on Bb acquisition by nymphal I. pacificus. Larval I. pacificus were fed on either lizards or mice and after molting, nymphs were fed on Bb-infected mice. We found that lizard-fed larvae were significantly more likely to become infected with Bb during their next bloodmeal than mouse-fed larvae. We also conducted the first RNA-seq analysis on whole-bodied I. pacificus and found significant upregulation of tick antioxidants and antimicrobial peptides in the lizard-fed group. Our results indicate that the lizard bloodmeal significantly alters vector competency and gene regulation in ticks, highlighting the importance of host bloodmeal identity in vector-borne disease transmission and upends prior notions about the role of lizards in Lyme disease community ecology.

A previously unrecognizedRickettsiaspecies was isolated in 1976 from a pool ofIxodes pacificusticks collected in 1967 from Tillamook County, Oregon, USA. The isolate produced low fever and mild scrotal oedema following intraperitoneal injection into male guinea pigs (Cavia porcellus). Subsequent serotyping characterized this isolate as distinct from recognized typhus and spotted fever groupRickettsiaspecies; nonetheless, the isolate remained unevaluated by molecular techniques and was not identified to species level for the subsequent 30 years.Ixodes pacificusis the most frequently identified human-biting tick in the western United States, and as such, formal identification and characterization of this potentially pathogenicRickettsiaspecies is warranted. Whole-genome sequencing of the Tillamook isolate revealed a genome 1.43 Mbp in size with 32.4 mol% G+C content. Maximum-likelihood phylogeny of core proteins places it in the transitional group ofRickettsiabasal to bothRickettsia felisandRickettsia asembonensis. It is distinct from existing named species, with maximum average nucleotide identity of 95.1% toR. asembonensisand maximum digital DNA–DNA hybridization score similarity toR. felisat 80.1%. The closest similarity at the 16S rRNA gene (97.9%) andsca4 (97.5%/97.6% respectively) is toCandidatus‘Rickettsia senegalensis’ andRickettsiasp. cf9, both isolated from cat fleas (Ctenocephalides felis). We characterized growth at various temperatures and in multiple cell lines. The Tillamook isolate grows aerobically in Vero E6, RF/6A and DH82 cells, and growth is rapid at 28 °C and 32 °C. Using accepted genomic criteria, we propose the nameRickettsia tillamookensissp. nov., with the type strain Tillamook 23. Strain Tillamook 23 is available from the Centers for Disease Control and Prevention Rickettsial Isolate Reference Collection (WDCM 1093), Atlanta, GA, USA (CRIRC accession number RTI001T) and the Collection de Souches de l’Unité des Rickettsies (WDCM 875), Marseille, France (CSUR accession number R5043). Using accepted genomic criteria, we propose the nameRickettsia tillamookensissp. nov., with the type strain Tillamook 23 (=CRIRC RTI001=R5043).

Tick-borne diseases in California include Lyme disease (caused by Borrelia burgdorferi), infections with Borrelia miyamotoi, and human granulocytic anaplasmosis (caused by Anaplasma phagocytophilum). We surveyed multiple sites and habitats (woodland, grassland, and coastal chaparral) in California to describe spatial patterns of tick-borne pathogen prevalence in western black-legged ticks (Ixodes pacificus). We found that several species of Borrelia—B. burgdorferi, Borrelia americana, and Borrelia bissettiae—were observed in habitats, such as coastal chaparral, that do not harbor obvious reservoir host candidates. Describing tick-borne pathogen prevalence is strongly influenced by the scale of surveillance: aggregating data from individual sites to match jurisdictional boundaries (e.g., county or state) can lower the reported infection prevalence. Considering multiple pathogen species in the same habitat allows a more cohesive interpretation of local pathogen occurrence. IMPORTANCE Understanding the local host ecology and prevalence of zoonotic diseases is vital for public health. Using tick-borne diseases in California, we show that there is often a bias to our understanding and that studies tend to focus on particular habitats, e.g., Lyme disease in oak woodlands. Other habitats may harbor a surprising diversity of tick-borne pathogens but have been neglected, e.g., coastal chaparral. Explaining pathogen prevalence requires descriptions of data on a local scale; otherwise, aggregating the data can misrepresent the local dynamics of tick-borne diseases.

Understanding seasonal patterns of activity, or phenology, of vector species is fundamental to determining seasonality of disease risk and epidemics of vector‐borne disease. Spatiotemporal variation in abiotic conditions can influence variation in phenological patterns and life history events, which can dramatically influence the ecological role and human impact of a species. For arthropod vectors of human diseases such as ticks, these phenological patterns determine human exposure risk, yet how abiotic conditions interact to determine suitable conditions for host‐seeking of vector species is difficult to disentangle. Here, we use MaxEnt to model spatial patterns and differences in host‐seeking phenology of the western blacklegged tick (Ixodes pacificus) in California using spatially and temporally refined adult tick occurrence data and similarly refined climate and environmental data. We empirically validate the model using phenological data from field studies conducted at sites across California's latitudinal gradient. We find adult tick host‐seeking activity varies substantially throughout the year, as well as across the large latitudinal gradient in the state. Suitable conditions for host‐seeking are found earlier in fall and later in the spring in northern than in southern California. These seasonal patterns are primarily associated with monthly precipitation, minimum winter temperature, and winter precipitation, with maximum monthly temperature possibly playing a more prominent role in limiting host‐seeking activity earlier in the spring in southern than northern California. Synthesis and applications. Modelling the seasonal activity of the western blacklegged tick, we find both a longer window for host‐feeding and more protracted risk of human exposure to this vector species in northern than southern California. We further identify key environmental factors associated with these patterns, including precipitation and temperature that are otherwise challenging to elucidate in field and laboratory studies over large spatial scales. Moreover, we illustrate how species distribution models, in combination with temporally refined species occurrence and environmental data, can be used to investigate environmental factors predictive of geographic variation in seasonality or phenology of vector species. This produces not only novel ecological insight, but key information for public health practitioners in managing vector‐borne disease transmission and targeting public outreach and interventions.

Interactions between humans and ticks are often measured indirectly, using surveillance of tick population abundance and pathogen prevalence, or reported human disease data. We used data garnered as part of a free national citizen science research effort to describe actual human exposures to ticks in California. Human-biting ticks (n = 1,905) submitted for identification were predominantly western black-legged ticks (Ixodes pacificus) (68%), American dog ticks (Dermacentor variabilis) (24%), and Pacific Coast ticks (Dermacentor occidentalis) (7%). Tick exposure occurred predominantly during recreational use of the outdoors, rather than exposure near the home environment. Tick submissions peaked in May, but human exposure to ticks occurred throughout the year. Adult I. pacificus were most frequently found on humans during March-May, though previous research demonstrates that questing adults on vegetation are more abundant earlier in the winter.

Vector‐borne diseases constitute a major global health burden and are increasing in geographic range and prevalence. Mounting evidence has demonstrated that the vector microbiome can impact pathogen dynamics, making the microbiome a focal point in vector‐borne disease ecology. However, efforts to generalize preliminary findings across studies and systems and translate these findings into disease control strategies are hindered by a lack of fundamental understanding of the processes shaping the vector microbiome and the interactions therein. Here, we use 16S rRNA sequencing and apply a community ecology framework to analyze microbiome community assembly and interactions in Ixodes pacificus, the Lyme disease vector in the western United States. We find that vertical transmission routes drive population‐level patterns in I. pacificus microbial diversity and composition, but that microbial function and overall abundance do not vary over time or between clutches. Further, we find that the I. pacificus microbiome is not strongly structured based on competition but assembles nonrandomly, potentially due to vector‐specific filtering processes which largely eliminate all but the dominant endosymbiont, Rickettsia. At the scale of the individual I. pacificus, we find support for a highly limited internal microbial community, and hypothesize that the tick endosymbiont may be the most important component of the vector microbiome in influencing pathogen dynamics.

A rickettsial isolate was obtained from a partially engorged Ixodes pacificus female, which was collected from Humboldt County, California. The isolate was provisionally named Rickettsia endosymbiont Ixodes pacificus (REIP). The REIP isolate displayed the highest nucleotide sequence identity to Rickettsia species phylotype G021 in I. pacificus (99%, 99%, and 100% for ompA, 16S rRNA, and gltA, respectively), a bacterium that was previously identified in I. pacifiucs by PCR. Analysis of sequences from complete opening frames of five genes, 16S rRNA, gltA, ompA, ompB, and sca4, provided inference to the bacteria’s classification among other Rickettsia species. The REIP isolate displayed 99.8%, 99.4%, 99.2%, 99.5%, and 99.6% nucleotide sequence identity for 16S rRNA, gltA, ompA, ompB, and sca4 gene, respectively, with genes of ‘R. monacensis’ str. IrR/Munich, indicating the REIP isolate is closely related to ‘R. monacensis’. Our suggestion was further supported by phylogenetic analysis using concatenated sequences of 16S rRNA, gltA, ompA, ompB, and sca4 genes, concatenated sequences of dksA-xerC, mppA-purC, and rpmE-tRNAfMet intergenic spacer regions. Both phylogenetic trees implied that the REIP isolate is most closely related to ‘R. monacensis’ str. IrR/Munich. We propose the bacterium be considered as ‘Rickettsia monacensis’ str. Humboldt for its closest phylogenetic relative (=DSM 103975 T = ATCC TSD-94 T).

The objective of this work was to determine the prevalence of Borrelia and Bartonella species in Ixodes spp. ticks collected from 16 USA states. Genus PCR amplification and sequence analysis of Bartonella and Borrelia 16SsRNA-23SsRNA intergenic regions were performed on DNA extracted from 929 questing adult ticks (671 Ixodes scapularis, 155 Ixodes affinis, and 103 Ixodes pacificus). Overall, 129/929 (13.9%) Ixodes ticks were PCR positive for Borrelia burgdorferi sensu stricto, 48/929 for B. bissettiae whereas 23/929 (2.5%) were PCR positive for a Bartonella henselae. Borrelia bissettiae or B. burgdorferi s.s. and B. henselae co-infections were found in I. affinis from North Carolina at a rate of 4.5%; in a single I. scapularis from Minnesota, but not in I. pacificus. For both bacterial genera, PCR positive rates were highly variable depending on geographic location and tick species, with Ixodes affinis (n = 155) collected from North Carolina, being the tick species with the highest prevalence’s for both Borrelia spp. (63.2%) and B. henselae (10.3%). Based on the results of this and other published studies, improved understanding of the enzootic cycle, transmission dynamics, and vector competence of Ixodes species (especially I. affinis) for transmission of Borrelia spp. and B. henselae should be a public health research priority.

Ixodes scapularis and Ixodes pacificus, are the predominant vectors of multiple human pathogens including Borrelia burgdorferi, one of the causative agents of Lyme disease, in North America. Differences in the habitats and host preferences of these closely-related tick species presents an opportunity to examine key aspects of the tick microbiome. While, advances in sequencing technologies, have accelerated a descriptive understanding of the tick microbiome, molecular and mechanistic insights into the tick microbiome is only beginning to emerge. Progress is stymied by technical difficulties to manipulate the microbiome and by biological variables related to the life cycle of Ixodid ticks. This review will highlight these challenges and examine avenues to understand the significance of the tick microbiome in tick biology.

A vector’s susceptibility and ability to transmit a pathogen— termed vector competency—determines disease outcomes, yet the ecological factors influencing tick vector competency remain largely unknown. Ixodes pacificus, the vector of Borrelia burgdorferi (Bb) in the western U.S., feeds on rodents, birds, and lizards. While rodents and birds are reservoirs for Bb and infect juvenile ticks, lizards are Bb-refractory. Despite I. pacificus feeding on a range of hosts, it is undetermined how larval host bloodmeal identity affects future nymphal vector competency. We experimentally evaluate the influence of larval host bloodmeal on Bb acquisition by nymphal I. pacificus. Larval I. pacificus were fed on either lizards or mice and after molting, nymphs were fed on Bb-infected mice. We found that lizard-fed larvae were significantly more likely to become infected with Bb during their next bloodmeal than mouse-fed larvae. We also conducted the first RNA-seq analysis on whole-bodied I. pacificus and found significant upregulation of antioxidants and antimicrobial peptides in the lizard-fed group. Our results indicate that the lizard bloodmeal significantly alters vector competency and gene regulation in ticks, highlighting the importance of host bloodmeal identity in disease transmission and upends prior notions about the role of lizards in Lyme disease transmission.

Parasites may impair host behavior in ways that reduce host fitness, especially when access to territories or mates becomes disrupted. Western fence lizards (Sceloporus occidentalis) are a key host to western blacklegged ticks (Ixodes pacificus). Males are highly aggressive during the mating season, competing with rivals through displays of color badges, pushups, and other behaviors. We hypothesized that experimental tick infestation diminishes the performance of male western fence lizards in intrasexual contests, via either blood loss, damage to sensory structures, or both. We infested adult males with larval ticks for 7 to 10 days, staged contests between infested and quality-matched control males, and measured their behavior in enclosure arenas. Infested lizards were less aggressive and exhibited decreased hematocrit, compared to non-infested animals. We found no relationships between aggression and either body size or blue ventral badge color traits, for either infested or uninfested males. There was also no effect of tick attachment location and hemoparasite infection on host contest behavior. This is the first demonstration of the impact of I. pacificus parasitism on intraspecific interactions of western fence lizards and suggests that tick infestation has substantial impacts on lizard fitness. Because I. pacificus rely heavily on these lizards for blood meals and dispersal, these impacts could also influence the abundance of ticks and the pathogens they vector. Parasitism frequently impairs host behavior. We investigated the effect of blacklegged tick parasitism on western fence lizard contest behavior using a lab-based manipulation experiment. We demonstrated that tick infestation reduced lizard aggressiveness, which may lead to territory loss and reduced mating opportunities in the wild. We found no evidence that host body size or coloration are correlated with aggression. This study adds to a growing body of evidence that tick parasitism is costly for lizards, while demonstrating the usefulness of manipulation experiments to understand host-parasite interactions.

Borrelia miyamotoi is a hard tick-associated relapsing fever spirochete that is geographically widespread in Ixodes spp. (Acari: Ixodidae) ticks, but typically occurs at low prevalence. Genetic variability has been described among strains derived from Asia, Europe, and North America, and among tick species that carry the infection, but little variability has been described within foci or tick species. Capitalizing on access to B. miyamotoi nucleic acid extracted from host-seeking Ixodes scapularis Say or Ixodes pacificus Cooley & Kohls from 16 states, we explored genetic variability based on sequence analysis of four amplicons described herein. Consistent with previous studies, we detected significant genetic differences between strains derived from I. scapularis (eastern United States) and I. pacificus (western United States) and identified two distinct sequences in the western United States (Am-West-1 and Am-West-2). Unique to this study, we identified two distinct sequences in the eastern United States (Am-East-1 and Am-East-2). Based on the 161 samples we analyzed, Am-East-1 was the only type represented in 50 B. miyamotoi-infected ticks collected from the Northeast (Vermont, Maine, New York, Connecticut, and Rhode Island), whereas ticks collected from the North-Central and Mid-Atlantic states harbored B. miyamotoi comprised of both Am-East-1 and Am-East-2. Further studies are needed to better characterize the phylogeography of B. miyamotoi and to discern if there are biologically meaningful differences among sequence types. To facilitate further exploration, we developed a polymerase chain reaction (PCR) assay designed to differentiate Am-East-1, Am-East-2, and Am-West sequence types without having to sequence the amplicon.

Burgeoning cases of tick-borne disease present a significant public health problem in the United States. Passive tick surveillance gained traction as an effective way to collect epidemiologic data, and in particular, photograph-based tick surveillance can complement in-hand tick specimen identification to amass distribution data and related encounter demographics. We compared the Federal Information Processing Standards (FIPS) code of tick photos submitted to a free public identification service (TickSpotters) from 2014 to 2019 to published nationwide county reports for three tick species of medical concern: Ixodes scapularis Say (Ixodida: Ixodidae), Ixodes pacificus Cooley and Kohls (Ixodida: Ixodidae), and Amblyomma americanum Linneaus (Ixodida: Ixodidae). We tallied the number of TickSpotters submissions for each tick species according to “Reported” or “Established” criteria per county, and found that TickSpotters submissions represented more than half of the reported counties of documented occurrence, and potentially identified hundreds of new counties with the occurrence of these species. We detected the largest number of new county reports of I. scapularis presence in Michigan, North Carolina, and Texas. Tick image submissions revealed potentially nine new counties of occurrence for I. pacificus, and we documented the largest increase in new county reports of A. americanum in Kentucky, Illinois, Indiana, and Ohio. These findings demonstrate the utility of crowdsourced photograph-based tick surveillance as a complement to other tick surveillance strategies in documenting tick distributions on a nationwide scale, its potential for identifying new foci, and its ability to highlight at-risk localities that might benefit from tick-bite prevention education.

Arthropods pests are most frequently associated with both plants and vertebrate animals. Ticks, in particular the blacklegged ticks Ixodes scapularis Say and Ixodes pacificus Cooley & Kohls (Acari: Ixodidae), are associated with wildlife hosts and are the primary vectors of Lyme disease, the most frequently reported vector-borne disease in the United States. Immature blacklegged ticks in the eastern United States frequently use small mammals from the genus Peromyscus as hosts. These mice are competent reservoirs for Borrelia burgdorferi, the causative agent of Lyme disease, as well as other tick-borne pathogens. To conduct surveillance on immature ticks and pathogen circulation in hosts, capture and handling of these small mammals is required. While protocols for rearing and pest surveillance on plants are common, there are very few protocols aimed at entomologists to conduct research on vertebrate–arthropod relationships. The goal of this manuscript is to provide a practical template for trapping Peromyscus spp. for vector and vector-borne pathogen surveillance and ecology for professionals that may not have a background in wildlife research. Important considerations are highlighted when targeting P. leucopus Rafinesque and P. maniculatus Wagner. Specifically, for tick and tick-borne disease-related projects, materials that may be required are suggested and references and other resources for researchers beginning a trapping study are provided.

Lyme disease is the most common vector-borne disease in temperate zones and a growing public health threat in the United States (US). The life cycles of the tick vectors and spirochete pathogen are highly sensitive to climate, but determining the impact of climate change on Lyme disease burden has been challenging due to the complex ecology of the disease and the presence of multiple, interacting drivers of transmission. Here we incorporated 18 years of annual, county-level Lyme disease case data in a panel data statistical model to investigate prior effects of climate variation on disease incidence while controlling for other putative drivers. We then used these climate-disease relationships to project Lyme disease cases using CMIP5 global climate models and two potential climate scenarios (RCP4.5 and RCP8.5). We find that interannual variation in Lyme disease incidence is associated with climate variation in all US regions encompassing the range of the primary vector species. In all regions, the climate predictors explained less of the variation in Lyme disease incidence than unobserved county-level heterogeneity, but the strongest climate-disease association detected was between warming annual temperatures and increasing incidence in the Northeast. Lyme disease projections indicate that cases in the Northeast will increase significantly by 2050 (23,619 ± 21,607 additional cases), but only under RCP8.5, and with large uncertainty around this projected increase. Significant case changes are not projected for any other region under either climate scenario. The results demonstrate a regionally variable and nuanced relationship between climate change and Lyme disease, indicating possible nonlinear responses of vector ticks and transmission dynamics to projected climate change. Moreover, our results highlight the need for improved preparedness and public health interventions in endemic regions to minimize the impact of further climate change-induced increases in Lyme disease burden.

A diversity of Borrelia burgdorferi sensu lato (Johnson, Schmid, Hyde, Steigerwalt & Brenner) (Spirochaetales: Spirochaetaceae) genomospecies, including the Lyme disease agent, Borrelia burgdorferi sensu stricto (s.s.), have been identified in the western United States. However, enzootic transmission of B. burgdorferi s.l. in small mammals and ticks is poorly characterized throughout much of the region. Here we report prevalence of B. burgdorferi s.l. in small mammal and tick communities in the understudied region of southern California. We found B. burgdorferi s.l. in 1.5% of Ixodes species ticks and 3.6% of small mammals. Infection was uncommon (~0.3%) in Ixodes pacificus Cooley and Kohls (Acari: Ixodidae), the primary vector of the Lyme disease agent to humans in western North America, but a diversity of spirochetes—including Borrelia bissettiae, Borrelia californiensis, Borrelia americana, and B. burgdorferi s.s.—were identified circulating in Ixodes species ticks and their small mammal hosts. Infection with B. burgdorferi s.l. is more common in coastal habitats, where a greater diversity of Ixodes species ticks are found feeding on small mammal hosts (four species when compared with only I. pacificus in other sampled habitats). This provides some preliminary evidence that in southern California, wetter coastal areas might be more favorable for enzootic transmission than hotter and drier climates. Infection patterns confirm that human transmission risk of B. burgdorferi s.s. is low in this region. However, given evidence for local maintenance of B. burgdorferi s.l., more studies of enzootic transmission may be warranted, particularly in understudied regions where the tick vector of B. burgdorferi s.s. occurs.

Tickborne diseases are an increasing public health concern in the United States, where the majority of notifiable cases are caused by pathogens vectored by Ixodes ticks. To better monitor changes in acarological risk of human encounters with these ticks and their associated pathogens, the Centers for Disease Control and Prevention (CDC) recently established a national tick and tickborne pathogen surveillance program. Here, we describe and evaluate a new Multiplex PCR Amplicon Sequencing (MPAS) assay for potential use in surveillance programs targeting two common human-biting vector ticks, Ixodes scapularis and Ixodes pacificus. The ability of the MPAS assay to detect five Ixodes-associated human pathogens (Borrelia burgdorferi sensu stricto, Borrelia mayonii, Borrelia miyamotoi, Anaplasma phagocytophilum and Babesia microti) was compared to that of a previously published and routinely used probe-based (TaqMan) PCR testing algorithm for pathogen detection in Ixodes ticks. Assay performance comparisons included a set of 175 host-seeking Ixodes nymphs collected in Connecticut as well as DNA from our pathogen reference collection. The MPAS assay and the CDC standard TaqMan PCR pathogen testing algorithm were found to have equivalent detection sensitivity for Ixodes-associated human pathogens. However, the MPAS assay was able to detect a broader range of tick-associated microorganisms, more effectively detected co-infections of multiple pathogens in a single tick (including different species within the Borrelia burgdorferi sensu lato complex), and required a smaller volume of test sample (thus preserving more sample for future testing).

Global environmental change is having profound effects on the ecology of infectious disease systems, which are widely anticipated to become more pronounced under future climate and land use change. Arthropod vectors of disease are particularly sensitive to changes in abiotic conditions such as temperature and moisture availability. Recent research has focused on shifting environmental suitability for, and geographic distribution of, vector species under projected climate change scenarios. However, shifts in seasonal activity patterns, or phenology, may also have dramatic consequences for human exposure risk, local vector abundance and pathogen transmission dynamics. Moreover, changes in land use are likely to alter human-vector contact rates in ways that models of changing climate suitability are unlikely to capture. Here we used climate and land use projections for California coupled with seasonal species distribution models to explore the response of the western blacklegged tick (Ixodes pacificus), the primary Lyme disease vector in western North America, to projected climate and land use change. Specifically, we investigated how environmental suitability for tick host-seeking changes seasonally, how the magnitude and direction of changing seasonal suitability differs regionally across California, and how land use change shifts human tick-encounter risk across the state. We found vector responses to changing climate and land use vary regionally within California under different future scenarios. Under a hotter, drier scenario and more extreme land use change, the duration and extent of seasonal host-seeking activity increases in northern California, but declines in the south. In contrast, under a hotter, wetter scenario seasonal host-seeking declines in northern California, but increases in the south. Notably, regardless of future scenario, projected increases in developed land adjacent to current human population centers substantially increase potential human-vector encounter risk across the state. These results highlight regional variability and potential nonlinearity in the response of disease vectors to environmental change.

Vector populations in Washington, USA, are infected with several disease-causing pathogens., Tickborne diseases are rare in Washington, USA, and the ecology of these pathogens is poorly understood. We integrated surveillance data from humans and ticks to better describe their epidemiology and ecology. During 2011–2016, a total of 202 tickborne disease cases were reported in Washington residents. Of these, 68 (34%) were autochthonous, including cases of Lyme disease, Rocky Mountain spotted fever, tickborne relapsing fever, and tularemia. During May 2011–December 2016, we collected 977 host-seeking ticks, including Ixodes pacificus, I. angustus, I. spinipalpis, I. auritulus, Dermacentor andersoni, and D. variabilis ticks. The prevalence of Borrelia burgdorferi sensu stricto in I. pacificus ticks was 4.0%; of B. burgdorferi sensu lato, 3.8%; of B. miyamotoi, 4.4%; and of Anaplasma phagocytophilum, 1.9%. We did not detect Rickettsia rickettsii in either Dermacentor species. Case-patient histories and detection of pathogens in field-collected ticks indicate that several tickborne pathogens are endemic to Washington.

It is generally accepted that parasites exert negative effects on their hosts and that natural selection favors specific host responses that mitigate this impact. It is also known that some components of the host immune system often co-evolve with parasite antigens resulting in a host-parasite arms race. In addition to immunological components of the anti-parasitic response, host behavioral responses are also important in this arms race and natural selection may favor avoidance strategies that preclude contact with parasites, or shifts in the host's thermoregulatory strategy to combat active infections (e.g., behavioral fever). Ticks are widespread parasites with direct and indirect costs on their vertebrate hosts. Their saliva provokes hemolysis in the blood of their hosts and can transmit a plethora of tick-borne pathogens. We enquired whether tick infestation by Ixodes pacificus can provoke a thermoregulatory response in Sceloporus occidentalis. For this, we compared the thermoregulatory behavior of tick-infested lizards against tick-infested lizards co-infected with two different species of coccidians (Lankesterella occidentalis and Acroeimeria sceloporis). After this, lizards were kept in individual terraria with a basking spot and fed ad libitum. We found that tick-infested lizards sought cooler temperatures in proportion to their tick load, and this response was independent of the co-infection status by L. occidentalis. This was consistent in April and June (when tick loads were significantly lower) and suggests a conservative strategy to save energy which might have been selected to overcome tick infestations during phenological peaks of this parasite. However, this behavior was not observed in lizards co-infected with A. sceloporis, suggesting that co-infection with this intestinal parasite prompt lizards to be active. Cost of tick infestation was confirmed because housed lizards lost weight at a constant ratio to initial tick load, independently of other infections. The broader implications of these findings are discussed in the context of climate change., Research at UCSC was supported by an NSF grant (EF-1241848 to BS). Spanish Ministerio de Economía y Competitividad provided support (BES-2010-038427 and EEBB-I-14-08326 to RM-P).

Rickettsia buchneri (formerly Rickettsia endosymbiont of Ixodes scapularis, or REIS) is an obligate intracellular endoparasite of the black-legged tick, the primary vector of Lyme disease in North America. It is noteworthy among the rickettsiae for its relatively large genome (1.8 Mb) and extraordinary proliferation of mobile genetic elements (MGEs), which comprise nearly 35% of its genome. Previous analysis of the R. buchneri genome identified several integrative conjugative elements named Rickettsiales amplified genomic elements (RAGEs); the composition of these RAGEs suggests that continued genomic invasions by MGEs facilitated the proliferation of rickettsial genes related to an intracellular lifestyle. In this study, we compare the genomic diversity at RAGE loci among sequenced rickettsiae that infect three related Ixodes spp., including two strains of R. buchneri and Rickettsia endosymbiont of Ixodes pacificus strain Humboldt, as well as a closely related species R. tamurae infecting Amblyomma testudinarium ticks. We further develop a novel multiplex droplet digital PCR assay and use it to quantify copy number ratios of chromosomal R. buchneri RAGE-A and RAGE-B to the single-copy gene gltA within natural populations of I. scapularis. Our results reveal substantial diversity among R. buchneri at these loci, both within individual ticks as well as in the I. scapularis population at large, demonstrating that genomic rearrangement of MGEs is an active process in these intracellular bacteria.

Tick-borne disease surveillance in North America has long focused on Lyme disease, though there is currently a significant shift towards comprehensive pathogen surveillance in ticks. Central California has often been overlooked in regular tick-borne pathogen surveillance despite the presence of numerous medically important tick species. The bacterial genus Rickettsia contains tick-borne species that are known pathogens, such as those in the spotted fever group; nonpathogenic endosymbionts; and many species with unknown pathogenic potential. Five common tick species (Ixodes pacificus Cooley and Kohls [Acari: Ixodidae], Dermacentor occidentalis Marx [Acari: Ixodidae], D. variabilis Say, Rhipicephalus sanguineus Latreille [Acari: Ixodidae], and Ornithodoros parkeri Cooley [Acari: Argasidae]) of California were collected by both traditional and modern techniques, and subsequently screened for Rickettsia spp. Many individuals from all five tick species were PCR positive for Rickettsia spp., and a combination of species-specific primers, a restriction fragment length polymorphism assay, and DNA sequencing was used to further characterize the species composition in these ticks. Probable Rickettsia philipii (Rickettsia 364D) was detected in one (1.56%) D. occidentalis collected in Fresno County; R. rhipicephali was detected in 23.4% of D. occidentalis from Fresno Co.; R. bellii was detected in 88.2% of D. variabilis, 7.8% of D. occidentalis, and in one R. rhipicephalus (1.1%) from Fresno Co.; R. monacensis str. Humboldt was detected in three (100%) of I. pacificus collected in both Fresno and Madera Co.; and an uncharacterized Rickettsia was detected in (26.4%) of O. parkeri collected in both Fresno and Madera Co. The findings in this study highlight the need for ongoing surveillance in this region of California.

Although nonpathogenic bacterial endosymbionts have been shown to contribute to their arthropod host’s fitness by supplying them with essential vitamins and amino acids, little is known about the nutritional basis for the symbiotic relationship of endosymbionts in ticks. Our lab has previously reported that Rickettsia species phylotype G021 in Ixodes pacificus carries all five genes for de novo folate synthesis, and that these genes are monophyletic with homologs from other Rickettsia species. In this study, the rickettsial folate synthesis folA gene, coding for dihydrofolate reductase, was PCR amplified, cloned into an expression vector, and overexpressed in E. coli. Bioinformatic analysis identified that the FolA protein of phylotype G021 has the conserved DHFR domain, NADP binding sites, and substrate binding sites of bacterial dihydrofolate reductase. SDS-PAGE results showed that recombinant rickettsial FolA protein was overexpressed in BL21(DE3) E. coli in its soluble form. Affinity chromatography was used to purify the protein, and in vitro enzyme assays were performed to assess the biochemical activity of dihydrofolate reductase. The specific activity of recombinant FolA from phylotype G021 was determined to be 16.1 U/mg. This study has revealed that Rickettsia species phylotype G021 of I. pacificus is capable of producing a functional enzyme of the folate biosynthesis pathway, addressing the nutritional interactions behind the symbiosis between Rickettsia species phylotype G021 and its host.

In the western United States, Ixodes pacificus Cooley & Kohls (Acari: Ixodidae) is the primary vector of the agents causing Lyme disease and granulocytic anaplasmosis in humans. The geographic distribution of the tick is associated with climatic variables that include temperature, precipitation, and humidity, and biotic factors such as the spatial distribution of its primary vertebrate hosts. Here, we explore (1) how climate change may alter the geographic distribution of I. pacificus in California, USA, during the 21st century, and (2) the spatial overlap among predicted changes in tick habitat suitability, land access, and ownership. Maps of potential future suitability for I. pacificus were generated by applying climate-based species distribution models to a multi-model ensemble of climate change projections for the Representative Concentration Pathway (RCP) 4.5 (moderate emission) and 8.5 (high emission) scenarios for two future periods: mid-century (2026-2045) and end-of-century (2086-2099). Areas climatically-suitable for I. pacificus are projected to expand by 23% (mid-century RCP 4.5) to 86% (end-of-century RCP 8.5) across California, compared to the historical period (1980-2014), with future estimates of total suitable land area ranging from about 88 to 133 thousand km2, or up to about a third of California. Regions projected to have the largest area increases in suitability by end-of-century are in northwestern California and the south central and southern coastal ranges. Over a third of the future suitable habitat is on lands currently designated as open access (i.e. publicly available), and by 2100, the amount of these lands that are suitable habitat for I. pacificus is projected to more than double under the most extreme emissions scenario (from ~23,000 to >51,000 km2). Of this area, most is federally-owned (>45,000 km2). By the end of the century, 26% of all federal land in the state is predicted to be suitable habitat for I. pacificus. The resulting maps may facilitate regional planning and preparedness by informing public health and vector control decision-makers.

Vector-borne pathogens are increasingly found to interact with the vector's microbiome, influencing disease transmission dynamics. However, the processes that regulate the formation and development of the microbiome are largely unexplored for most tick species, an emerging group of disease vectors. It is not known how much of the tick microbiome is acquired through vertical transmission vs. horizontally from the environment or interactions with bloodmeal sources. Using 16S rRNA sequencing, we examined the microbiome of Ixodes pacificus, the vector of Lyme disease in the western USA, across life stages and infection status. We also characterized microbiome diversity in field and laboratory-collected nymphal ticks to determine how the surrounding environment affects microbiome diversity. We found a decrease in both species richness and evenness as the tick matures from larva to adult. When the dominant Rickettsial endosymbiont was computationally removed from the tick microbial community, we found that infected nymphs had lower species evenness than uninfected ticks, suggesting that lower microbiome diversity is associated with pathogen transmission in wild-type ticks. Furthermore, laboratory-reared nymph microbiome diversity was found to be compositionally distinct and significantly depauperate relative to field-collected nymphs. These results highlight unique patterns in the microbial community of I. pacificus that is distinct from other tick species. We provide strong evidence that ticks acquire a significant portion of their microbiome through exposure to their environment despite a loss of overall diversity through life stages. We provide evidence that loss of microbial diversity is at least in part due to elimination of microbial diversity with bloodmeal feeding but other factors may also play a role.

Ixodes pacificus is a host of many bacteria including Rickettsia species phylotypes G021 and G022. As part of the overall goal of understanding interactions of phylotypes with their tick host, this study focused on molecular detection of rickettsiae in ovarian and midgut tissue of I. pacificus by fluorescent in situ hybridization (FISH), PCR, and ultrastructural analysis. Of three embedding media (Technovit 8100, Unicryl, and paraffin) tested for generating thin sections, tissues embedded in paraffin resulted in the visualization of bacteria with low autofluorescence in FISH. Digoxigenin-labeled probes were used in FISH to intensify bacterial hybridization signals using Tyramide Signal Amplification reaction. Using this technique, rickettsiae were detected in the cytoplasm of oocytes of I. pacificus. The presence of rickettsiae in the ovary and midgut was further confirmed by PCR and transmission electron microscopic analysis. Overall, the methods in this study can be used to identify locations of tick-borne bacteria in tick tissues and understand transmission routes of bacterial species in ticks.

Lyme disease is a serious health problem, with many patients requiring in-depth clinical assessment and extended treatment. In the present study, we provide the first records of the western blacklegged tick, Ixodes pacificus , and Ixodes spinipalpis parasitizing eastern cottontails, Sylvilagus floridanus . We also documented a triple co-infestation of 3 tick species (Ixodes angustus, I. pacificus , I. spinipalpis) feeding on an eastern cottontail. Notably, we discovered a unique member of the Lyme disease bacterium, Borrelia burgdorferi sensu lato (s.l.) in Canada. Ixodes spinipalpis ticks, which were collected from an eastern cottontail on Vancouver Island, British Columbia (BC), were positive for B. burgdorferi s.l. With the use of polymerase chain reaction amplification on the tick extracts and DNA sequencing on the borrelial amplicons, we detected Borrelia genomospecies 2, a novel subgroup of the B. burgdorferi s.l. complex. Based on 416 nucleotides of the flagellin B (flaB) gene, our amplicons are identical to the Borrelia genomospecies 2 type strain CA28. Borrelia genomospecies 2 is closely related genetically to other B. burgdorferi s.l. genospecies, namely Borrelia americana, Borrelia andersonii, and B. burgdorferi sensu stricto (s.s.) that cause Lyme disease. Like some other borrelial strains, Borrelia genomospecies 2 can be missed by current Lyme disease serology. Health-care providers must be aware that Borrelia genomospecies 2 is present in I. pacificus and I. spinipalpis ticks in far-western North America, and patients with clinical symptoms of Lyme disease need to be assessed for potential infection with this pathogen.

Insight into the composition and function of the tick microbiome has expanded considerably in recent years. Thus far, tick microbiome studies have focused on species and life stages that are responsible for transmitting disease. In this study we conducted extensive field sampling of six tick species in the far-western United States to comparatively examine the microbial composition of sympatric tick species: Ixodes pacificus, Ixodes angustus, Dermacentor variabilis, Dermacentor occidentalis, Dermacentor albipictus, and Haemaphysalis leporispalustris. These species represent both common vectors of disease and species that rarely encounter humans, exhibiting a range of host preferences and natural history. We found significant differences in microbial species diversity and composition by tick species and life stage. The microbiome of most species examined were dominated by a few primary endosymbionts. Across all species, the relative abundance of these endosymbionts increased with life stage while species richness and diversity decreased with development. Only one species, I. angustus, did not show the presence of a single dominant microbial species indicating the unique physiology of this species or its interaction with the surrounding environment. Tick species that specialize in a small number of host species or habitat ranges exhibited lower microbiome diversity, suggesting that exposure to environmental conditions or host blood meal diversity can affect the tick microbiome which in turn may affect pathogen transmission. These findings reveal important associations between ticks and their microbial community and improve our understanding of the function of non-pathogenic microbiomes in tick physiology and pathogen transmission.

Lyme disease is the most prevalent vector-borne disease in the United States, yet critical gaps remain in our understanding of tick and host interactions that shape disease dynamics. Rodents such as deer mice (Peromyscus spp.) and dusky-footed woodrats (Neotoma fuscipes) are key reservoirs for Borrelia burgdorferi, the etiological bacterium of Lyme disease, and can vary greatly in abundance between habitats. The aggregation of Ixodes pacificus, the western black-legged tick, on rodent hosts is often assumed to be constant across various habitats and not dependent on the rodent or predator communities; however, this is rarely tested. The factors that determine tick burdens on key reservoir hosts are important in estimating Lyme disease risk because larger tick burdens can amplify pathogen transmission. This study is the first to empirically measure I. pacificus larval burdens on competent reservoir hosts as a function of community factors such as rodent diversity, predator diversity, and questing tick abundance. Rodents were live trapped at oak woodland sites to collect tick burdens and tissue samples to test for infection with Borrelia burgdorferi sensu lato. We found that N. fuscipes tick burdens were negatively correlated with predator diversity, but positively correlated with questing I. pacificus larvae. In addition, rodent hosts that were infected with B. burgdorferi sensu lato tend to have higher burdens of larval ticks. These results demonstrate that tick burdens can be shaped by variability between individuals, species, and the broader host community with consequences for transmission and prevalence of tick-borne pathogens.

‘Candidatus Ehrlichia khabarensis’ was first described from rodents and insectivores in the Far East territory of Khabarovsk on the Russian Pacific Coast. Here we report the detection of DNA from this microorganism in rodents and fed ticks collected from rodents in British Columbia, Canada in 2013–2014. ‘Candidatus Ehrlichia khabarensis’ was detected in (i) a female Ixodes angustus tick collected from a Peromyscus maniculatus; (ii) a female Dermacentor andersoni tick collected from a Perognathus parvus; (iii) a pool of 2 larval Ixodes pacificus ticks collected from a single P. maniculatus; and (iv) a pool of 3 nymphal I. pacificus ticks collected from a single P. maniculatus. Three of these four rodents (2 P. maniculatus and 1 P. parvus) with infected ticks also had evidence of ‘Candidatus Ehrlichia khabarensis’ in at least one tissue type. The infected P. maniculatus and Ixodes ticks came from the Vancouver area in western British Columbia and the P. parvus and Dermacentor tick from an inland site in central British Columbia. Although it remains to be determined whether ‘Candidatus Ehrlichia khabarensis’ has any negative impacts on wildlife, domestic animals or humans, we note that all three tick species found to contain the DNA of this microorganism are known to bite humans. Future detection of this microorganism either in ticks collected from rodents and allowed to molt to the next life stage prior to being tested, or from host-seeking ticks, is required to determine if it can survive the tick’s molt after being ingested via an infectious blood meal.

In the twenty-first century, ticks and tick-borne diseases have expanded their ranges and impact across the US. With this spread, it has become vital to monitor vector and disease distributions, as these shifts have public health implications. Typically, tick-borne disease surveillance (e.g., Lyme disease) is passive and relies on case reports, while disease risk is calculated using active surveillance, where researchers collect ticks from the environment. Case reports provide the basis for estimating the number of cases; however, they provide minimal information on vector population or pathogen dynamics. Active surveillance monitors ticks and sylvatic pathogens at local scales, but it is resource-intensive. As a result, data are often sparse and aggregated across time and space to increase statistical power to model or identify range changes. Engaging public participation in surveillance efforts allows spatially and temporally diverse samples to be collected with minimal effort. These citizen-driven tick collections have the potential to provide a powerful tool for tracking vector and pathogen changes. We used MaxEnt species distribution models to predict the current and future distribution of Ixodes pacificus across the Western US through the use of a nationwide citizen science tick collection program. Here, we present niche models produced through citizen science tick collections over two years. Despite obvious limitations with citizen science collections, the models are consistent with previously-predicted species ranges in California that utilized more than thirty years of traditional surveillance data. Additionally, citizen science allows for an expanded understanding of I. pacificus distribution in Oregon and Washington. With the potential for rapid environmental changes instigated by a burgeoning human population and rapid climate change, the development of tools, concepts, and methodologies that provide rapid, current, and accurate assessment of important ecological qualities will be invaluable for monitoring and predicting disease across time and space.

The western blacklegged tick, Ixodes pacificus, an important vector in the western United States of two zoonotic spirochetes: Borrelia burgdorferi (also called Borreliella burgdorferi), causing Lyme disease, and Borrelia miyamotoi, causing a relapsing fever-type illness. Human cases of Lyme disease are well-documented in California, with increased risk in the north coastal areas and western slopes of the Sierra Nevada range. Despite the established presence of B. miyamotoi in the human-biting I. pacificus tick in California, clinical cases with this spirochete have not been well studied. To assess exposure to B. burgdorferi and B. miyamotoi in California, and to address the hypothesis that B. miyamotoi exposure in humans is similar in geographic range to B. burgdorferi, 1,700 blood donor sera from California were tested for antibodies to both pathogens. Sampling was from high endemic and low endemic counties for Lyme disease in California. All sera were screened using the C6 ELISA. All C6 positive and equivocal samples and nine randomly chosen C6 negative samples were further analyzed for B. burgdorferi antibody using IgG western blot and a modified two ELISA test system and for B. miyamotoi antibody using the GlpQ ELISA and B. miyamotoi whole cell sonicate western blot. Of the 1,700 samples tested in series, eight tested positive for antibodies to B. burgdorferi (0.47%, Exact 95% CI: 0.20, 0.93) and two tested positive for antibodies to B. miyamotoi (0.12%, Exact 95% CI: 0.01, 0.42). There was no statistically significant difference in seroprevalence for either pathogen between high and low Lyme disease endemic counties. Our results confirm a low frequency of Lyme disease and an even lower frequency of B. miyamotoi exposure among adult blood donors in California; however, our findings reinforce public health messaging that there is risk of infection by these emerging diseases in the state.

Lyme disease, a zoonotic disease, is the most prevalent vector-borne disease in the Northern Hemisphere. Diversity of the vector (tick) microbiome can impact pathogen transmission, yet the biotic and abiotic factors that drive microbiome diversity are largely unresolved, especially under natural, field conditions. We describe the microbiome of Ixodes pacificus ticks, the vector for Lyme disease in the western United States, and show a strong impact of host blood meal identity on tick microbiome species richness and composition. Western fence lizards, a host that is refractory to the Lyme disease pathogen, significantly reduces microbiome diversity in ticks relative to ticks that feed on a mammalian reservoir host. Host blood meal-driven reduction of tick microbiome diversity may have lifelong repercussions on I. pacificus vector competency and ultimately disease dynamics.

Ixodes ricinus L. is the commonest tick encountered by humans in Central Europe and the most important vector of tick-borne diseases (TBD) in Europe. Foresters represent a group at elevated risk of exposure to I. ricinus throughout Europe. Here, we employed an experimental design similar to one used previously to determine what behavioral factors heighten exposure to questing Ixodes pacificus nymphs in a hardwood forest in the far-western United States. Five activities were evaluated systematically in a coniferous forest and a mixed broadleaved forest in west-central Poland: (i) walking in low-cut running shoes; (ii) sitting atop logs; (iii) sitting on leaf litter; (iv) sitting against tree trunks; and (v) gathering firewood. In total, 540 I. ricinus ticks were collected (198 larvae, 327 nymphs, 15 adults) in spring and late summer. Many more ticks were collected in spring than in late summer, and in the mixed broadleaved forest versus the coniferous forest. The riskiest behavior for acquiring nymphs in both spring and late summer was gathering firewood, though sitting atop logs was nearly as perilous in late summer. In contrast, the riskiest behaviors for encountering larvae in spring and late summer were sitting against trunks and sitting on leaf litter, respectively. Pairwise comparisons revealed only two significant associations among the variables explored for the nymphs: season and human gender, and season and type of forest. For larval ticks, the most significant associations were gender and sitting behaviors and for type of forest and sitting on leaf litter or against tree trunks. The most commonly infested body region was the lower legs.

Borrelia miyamotoi is an emerging, tick-borne human pathogen. In North America, it is primarily associated with Ixodes scapularis and Ixodes pacificus, two species known to bite humans. Here we describe the development and evaluation of a pair of real-time TaqMan PCR assays designed to detect B. miyamotoi in North American ticks. We sought to achieve sensitivity to B. miyamotoi strains associated with ticks throughout North America, the full genetic diversity of which is unknown, by targeting sequences that are largely conserved between B. miyamotoi strains from the eastern United States and genetically distinct B. miyamotoi strains from Japan. The two assays target different loci on the B. miyamotoi chromosome and can be run side by side under identical cycling conditions. One of the assays also includes a tick DNA target that can be used to verify the integrity of tick-derived samples. Using both recombinant plasmid controls and genomic DNA from North American and Japanese strains, we determined that both assays reliably detect as few as 5 copies of the B. miyamotoi genome. We verified that neither detects B. burgdorferi, B. lonestari or B. turicatae. This sensitive and specific pair of assays successfully detected B. miyamotoi in naturally-infected, colony-reared nymphs and in field-collected I. scapularis and I. pacificus from the Northeast and the Pacific Northwest respectively. These assays will be useful in screening field-collected Ixodes spp. from varied regions of North America to assess the risk of human exposure to this emerging pathogen.

Four members of the Ixodes ricinus species complex, Ixodes pacificus, Ixodes persulcatus, Ixodes ricinus and Ixodes scapularis, have, between them, a worldwide distribution within the northern hemisphere. They are responsible for the transmission of several animal and human pathogens, including the causal agents of Lyme borreliosis, tick-borne encephalitis, human granulocytic anaplasmosis and human babesiosis. Despite the importance of these ticks as vectors, the knowledge and understanding of the role that diapause plays in their complex life cycles are confused and incomplete. In view of the continuing geographic spread of these tick species, as well as the effects of climate change on vector-borne diseases, it is timely to encourage research on diapause phenomena to improve understanding of their biology and of pathogen transmission dynamics. In our review we seek to clarify thinking on the topic and to address gaps in our knowledge that require the attention of researchers.

In addition to serving as vectors of several other human pathogens, the black-legged tick, Ixodes scapularis Say, and western black-legged tick, Ixodes pacificus Cooley and Kohls, are the primary vectors of the spirochete (Borrelia burgdorferi) that causes Lyme disease, the most common vector-borne disease in the United States. Over the past two decades, the geographic range of I. pacificus has changed modestly while, in contrast, the I. scapularis range has expanded substantially, which likely contributes to the concurrent expansion in the distribution of human Lyme disease cases in the Northeastern, North-Central and Mid-Atlantic states. Identifying counties that contain suitable habitat for these ticks that have not yet reported established vector populations can aid in targeting limited vector surveillance resources to areas where tick invasion and potential human risk are likely to occur. We used county-level vector distribution information and ensemble modeling to map the potential distribution of I. scapularis and I. pacificus in the contiguous United States as a function of climate, elevation, and forest cover. Results show that I. pacificus is currently present within much of the range classified by our model as suitable for establishment. In contrast, environmental conditions are suitable for I. scapularis to continue expanding its range into northwestern Minnesota, central and northern Michigan, within the Ohio River Valley, and inland from the southeastern and Gulf coasts. Overall, our ensemble models show suitable habitat for I. scapularis in 441 eastern counties and for I. pacificus in 11 western counties where surveillance records have not yet supported classification of the counties as established.

Patterns of seasonal activity and density of host-seeking western blacklegged ticks, Ixodes pacificus, were investigated in central and southern California. Weekly to monthly drag sampling was undertaken at two sites in Santa Barbara County and one site in Los Angeles County over multiple years. Adult I. pacificus became active in the winter (late November) and were rare or absent by late April to early May. Nymphal ticks became active in early to late February, were absent by early May to early June, and were rarely encountered using the drag method throughout their period of peak seasonal activity. Larval ticks became active earlier in the season, or at the same time as nymphs (early to late February) and were absent by early May. These results suggest a highly truncated period of I. pacificus seasonal questing activity, particularly apparent in the juvenile tick stages, in central and southern California relative to observed patterns in Lyme-endemic northwestern California. Notably, the highly truncated period of questing activity of the juvenile stages has important implications for pathogen transmission dynamics in that there exists only a brief window for horizontally transmitted pathogens to be acquired by one tick cohort and subsequently transmitted, through hosts, to the next tick cohort in this system. The broader patterns observed also suggest low human risk of tick-borne disease in central and southern California, and have implications for reduced tick-borne disease risk in the western US more generally under projected climate change.

The expanding geographic ranges of tick species that are known pathogen vectors can have implications for human, domestic animal, and wildlife health. Although Alaska is home to several hard tick species, it has historically been outside of the range of the most common medically important ticks in the contiguous United States and western Canada. To assess the status of tick species establishment in the state and to provide a baseline for tracking future change in the distribution of ticks, we reviewed and compiled historical tick records and summarized recent tick occurrence records collected through the development of the Alaska Submit-A-Tick Program and through tick drag sampling at sentinel sites in southcentral Alaska. Between 1909–2019, there were 1190 tick records representing 4588 individual ticks across 15 species in Alaska. The majority of ticks were species historically found in Alaska: Haemaphysalis leporispalustris, Ixodes angustus, Ixodes auritulus, Ixodes howelli, Ixodes signatus, and Ixodes uriae. Over half of all tick records in the state were collected in the last 10 yr. During this time, the number of tick records and the number of tick species recorded in Alaska each year has increased substantially. Between 2010–2019, there were 611 tick records representing 1921 individual ticks. The most common hosts for reported ticks were domestic animals (n = 343, 56 %) followed by small wild mammals (n = 147, 24 %), humans (n = 49, 8%), and wild birds (n = 31, 5%). Less than 5% of records (n = 25) were of unattached ticks found in the environment. Since 2007, non-native tick species have been documented in the state every year, including Amblyomma americanum, Dermacentor andersoni, Dermacentor occidentalis, Dermacentor variabilis, Ixodes pacificus, Ixodes ricinus, Ixodes scapularis, Ixodes texanus, and Rhipicephalus sanguineus sensu lato (s.l.). Almost half of the records (n = 68, 48 %) of non-native tick species from 2010 to 2019 represented ticks found on a host (usually a dog or a human) that had traveled outside of Alaska in the two weeks prior to collection. However, A. americanum, D. variabilis, I. pacificus, I. texanus, and R. sanguineus s.l. have been found on humans and domestic animals in Alaska without reported recent travel. In particular, there is evidence to suggest that there is local establishment of R. sanguineus s.l. in Alaska. A tick species historically found in the state, I. angustus was frequently found on human and dogs, suggesting a potential role as a bridge vector of pathogens. Given the inconsistency of tick monitoring in Alaska over the past century, it is difficult to draw many conclusions from temporal trends in the data. Continued monitoring through the Alaska Submit-A-Tick Program will allow a more accurate assessment of the changing risk of ticks and tick-borne diseases in the state and provide information for setting clinical and public health guidelines for tick-borne disease prevention.

Lyme disease, the most commonly reported vector-borne disease in North America, is caused by the spirochete Borrelia burgdorferi sensu stricto, which is transmitted by Ixodes scapularis in eastern Canada and Ixodes pacificus in western Canada. Recently, the northward range expansion of I. scapularis ticks, in south-eastern Canada, has resulted in a dramatic increase in the incidence of human Lyme disease. Detecting emerging areas of Lyme disease risk allows public health to target disease prevention efforts. We analysed passive tick surveillance data from Ontario and Manitoba to i) assess the relationship between the total numbers of I. scapularis submissions in passive surveillance from humans, and the number of human Lyme disease cases, and ii) develop province-specific acarological indicators of risk that can be used to generate surveillance-based risk maps. We also assessed associations between numbers of nymphal I. scapularis tick submissions only and Lyme disease case incidence. Using General Estimating Equation regression, the relationship between I. scapularis submissions (total numbers and numbers of nymphs only) in each census sub-division (CSD) and the number of reported Lyme disease cases was positively correlated and highly significant in the two provinces (P ≤ 0.001). The numbers of I. scapularis submissions over five years discriminated CSDs with ≥ 3 Lyme disease cases from those with < 3 cases with high accuracy when using total numbers of tick submission (Receiver Operating Characteristics area under the curve [AUC] = 0.89) and moderate accuracy (AUC = 0.78) when using nymphal tick submissions only. In Ontario the optimal cut-off point was a total 12 tick submissions from a CSD over five years (Sensitivity = 0.82, Specificity = 0.84), while in Manitoba the cut-off point was five ticks (Sensitivity = 0.71, Specificity = 0.79) suggesting regional variability of the risk of acquiring Lyme disease from an I. scapularis bite. The performances of the acarological indicators developed in this study for Ontario and Manitoba support the ability of passive tick surveillance to provide an early signal of the existence Lyme disease risk areas in regions where ticks and the pathogens they transmit are expanding their range.

Ixodes pacificus Cooley & Kohls (Acari: Ixodidae), the primary vector of Lyme disease spirochetes to humans in the far-western United States, is broadly distributed across Pacific Coast states, but its distribution is not uniform within this large, ecologically diverse region. To identify areas of suitable habitat, we assembled records of locations throughout California where two or more I. pacificus were collected from vegetation from 1980 to 2014. We then employed ensemble species distribution modeling to identify suitable climatic conditions for the tick and restricted the results to land cover classes where these ticks are typically encountered (i.e., forest, grass, scrub-shrub, riparian). Cold-season temperature and rainfall are particularly important abiotic drivers of suitability, explaining between 50 and 99% of the spatial variability across California among models. The likelihood of an area being classified as suitable increases steadily with increasing temperatures >0°C during the coldest quarter of the year, and further increases when precipitation amounts range from 400 to 800 mm during the coldest quarter, indicating that areas in California with relatively warm and wet winters typically are most suitable for I. pacificus. Other consistent predictors of suitability include increasing autumn humidity, temperatures in the warmest month between 23 and 33°C, and low-temperature variability throughout the year. The resultant climatic suitability maps indicate that coastal California, especially the northern coast, and the western Sierra Nevada foothills have the highest probability of I. pacificus presence.