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Background: Ixodes scapularis and Ixodes pacificus are important vectors of multiple pathogens in the United States. However, their role in transmission of Bartonella spp., which are commonly reported in rodents and fleas, has been debated. Our previous investigation on Bartonella spp. in host-seeking I. scapularis and I. pacificus showed Bartonella spp. were absent in the ticks, suggesting the two species are unlikely to contribute to Bartonella transmission. It is unclear whether the absence of Bartonella spp. in the host-seeking ticks was attributable to ticks not being exposed to Bartonella in nature or being exposed but unable to acquire or transstadially transmit the bacterium. To assess the likelihood of exposure and acquisition, we tested Ixodes spp. ticks collected from rodents for Bartonella infections. Methods: Blood-fed I. scapularis ticks (n = 792; consisting of 645 larvae and 147 nymphs), I. pacificus ticks (n = 45, all larvae), and Ixodes angustus ticks (n = 16, consisting of 11 larvae and 5 nymphs) collected from rodents from Minnesota and Washington were tested for Bartonella spp. using a quadruplex polymerase chain reaction (PCR) amplicon next-generation sequencing approach that targets Bartonella-specific fragments on gltA, ssrA, rpoB, and groEL. In parallel, rodents and fleas collected from the same field studies were investigated to compare the differences of Bartonella distribution among the ticks, fleas, and rodents. Results: Bartonella spp. were commonly detected in rodents and fleas, with prevalence of 25.6% in rodents and 36.8% in fleas from Minnesota; 27.9% in rodents and 45.2% in fleas from Washington. Of all tested ticks, Bartonella DNA was detected by gltA in only one larval I. scapularis tick from Minnesota. Conclusions: The high prevalence of Bartonella spp. in rodents and fleas coupled with extremely low prevalence of Bartonella spp. in blood-fed ticks suggests that although Ixodes ticks commonly encounter Bartonella in rodents, they rarely acquire the infection through blood feeding. Notably, ticks were at various stages of feeding on rodents when they were collected. Laboratory transmission studies are needed to assess acquisition rates in fully blood-fed ticks and to assess transstadial transmission efficiency if ticks acquire Bartonella infections from feeding to repletion. [ABSTRACT FROM AUTHOR]

Abstract Background Ixodes scapularis and Ixodes pacificus are important vectors of multiple pathogens in the United States. However, their role in transmission of Bartonella spp., which are commonly reported in rodents and fleas, has been debated. Our previous investigation on Bartonella spp. in host-seeking I. scapularis and I. pacificus showed Bartonella spp. were absent in the ticks, suggesting the two species are unlikely to contribute to Bartonella transmission. It is unclear whether the absence of Bartonella spp. in the host-seeking ticks was attributable to ticks not being exposed to Bartonella in nature or being exposed but unable to acquire or transstadially transmit the bacterium. To assess the likelihood of exposure and acquisition, we tested Ixodes spp. ticks collected from rodents for Bartonella infections. Methods Blood-fed I. scapularis ticks (n = 792; consisting of 645 larvae and 147 nymphs), I. pacificus ticks (n = 45, all larvae), and Ixodes angustus ticks (n = 16, consisting of 11 larvae and 5 nymphs) collected from rodents from Minnesota and Washington were tested for Bartonella spp. using a quadruplex polymerase chain reaction (PCR) amplicon next-generation sequencing approach that targets Bartonella-specific fragments on gltA, ssrA, rpoB, and groEL. In parallel, rodents and fleas collected from the same field studies were investigated to compare the differences of Bartonella distribution among the ticks, fleas, and rodents. Results Bartonella spp. were commonly detected in rodents and fleas, with prevalence of 25.6% in rodents and 36.8% in fleas from Minnesota; 27.9% in rodents and 45.2% in fleas from Washington. Of all tested ticks, Bartonella DNA was detected by gltA in only one larval I. scapularis tick from Minnesota. Conclusions The high prevalence of Bartonella spp. in rodents and fleas coupled with extremely low prevalence of Bartonella spp. in blood-fed ticks suggests that although Ixodes ticks commonly encounter Bartonella in rodents, they rarely acquire the infection through blood feeding. Notably, ticks were at various stages of feeding on rodents when they were collected. Laboratory transmission studies are needed to assess acquisition rates in fully blood-fed ticks and to assess transstadial transmission efficiency if ticks acquire Bartonella infections from feeding to repletion. Graphical Abstract

Habitat loss and forest fragmentation are often linked to increased pathogen transmission, but the extent to which habitat isolation and landscape connectivity affect disease dynamics through movement of disease vectors and reservoir hosts has not been well examined. Tick-borne diseases are the most prevalent vector-borne diseases in the United States and on the West Coast, Ixodes pacificus is one of the most epidemiologically important vectors. We investigated the impacts of habitat fragmentation on pathogens transmitted by I. pacificus and sought to disentangle the effects of wildlife communities and landscape metrics predictive of pathogen diversity, prevalence and distribution. We collected pathogen data for four co-occurring bacteria transmitted by I. pacificus and measured wildlife parameters. We also used spatial data and cost-distance analysis integrating expert opinions to assess landscape metrics of habitat fragmentation. We found that landscape metrics were significant predictors of tick density and pathogen prevalence. However, wildlife variables were essential when predicting the prevalence and distribution of pathogens reliant on wildlife reservoir hosts for maintenance. We found that landscape structure was an informative predictor of tick-borne pathogen richness in an urban matrix. Our work highlights the implications of large-scale land management on human disease risk.

Male competition conforms to a cost–benefit model, because while aggression may increase reproductive prospects, it can also increase the risk of injury. We hypothesize that an additional cost in aggressive males would be an increase in parasite load associated with a high energy investment into sexual competition. Some of these infections, in turn, may downmodulate the level of host aggression via energetic trade-offs. We staged dyadic male contests in the lab to investigate the relationships of multiple parasites with the agonistic behavior of lizard hosts, Sceloporus occidentalis. We also included both color and behavioral traits from opponents in the analyses because (1) color patches of lizards may serve as intraspecific signals used by conspecifics to assess the quality of opponents, and (2) contests between male lizards fit classical models of escalated aggression, where lizards increase aggression displays in response to an opponent's behavior. The results conform to our hypothesis because male lizards displayed more pushups when they had more ticks. Moreover, some parasites may modulate the levels of aggression because lizards infected by hematic coccidians performed fewer pushups. Interestingly, lizards also displayed fewer pushups when both the chroma and size of the opponent's blue patch were greater. The results thus also supported the role of the blue patch of S. occidentalis as a sexual armament, because it contributed to the deterrence of aggression from opponent lizards. We revealed that natural parasitic infections in lizard hosts can contribute to their agonistic behavior. We encourage future studies to account for parasites in behavioral tests with lizards. [ABSTRACT FROM AUTHOR]

Ixodes pacificus (the western blacklegged tick) occurs in the far western United States (US), where it commonly bites humans. This tick was not considered a species of medical concern until it was implicated in the 1980s as a vector of Lyme disease spirochetes. Later, it was discovered to also be the primary vector to humans in the far western US of agents causing anaplasmosis and hard tick relapsing fever. The core distribution of I. pacificus in the US includes California, western Oregon, and western Washington, with outlier populations reported in Utah and Arizona. In this review, we provide a history of the documented occurrence of I. pacificus in the US from the 1890s to present, and discuss associations of its geographic range with landscape, hosts, and climate. In contrast to Ixodes scapularis (the blacklegged tick) in the eastern US, there is no evidence for a dramatic change in the geographic distribution of I. pacificus over the last half-century. Field surveys in the 1930s and 1940s documented I. pacificus along the Pacific Coast from southern California to northern Washington, in the Sierra Nevada foothills, and in western Utah. County level collection records often included both immatures and adults of I. pacificus, recovered by drag sampling or from humans, domestic animals, and wildlife. The estimated geographic distribution presented for I. pacificus in 1945 by Bishopp and Trembley is similar to that presented in 2022 by the Centers for Disease Control and Prevention. There is no clear evidence of range expansion for I. pacificus, separate from tick records in new areas that could have resulted from newly initiated or intensified surveillance efforts. Moreover, there is no evidence from long-term studies that the density of questing I. pacificus ticks has increased over time in specific areas. It therefore is not surprising that the incidence of Lyme disease has remained stable in the Pacific Coast states from the early 1990s, when it became a notifiable condition, to present. We note that deforestation and deer depredation were less severe in the far western US during the 1800s and early 1900s compared to the eastern US. This likely contributed to I. pacificus maintaining stable, widespread populations across its geographic range in the far western US in the early 1900s, while I. scapularis during the same time period appears to have been restricted to a small number of geographically isolated refugia sites within its present range in the eastern US. The impact that a warming climate may have had on the geographic distribution and local abundance of I. pacificus in recent decades remains unclear.

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.

Infection with Borrelia miyamotoi in California, USA, has been suggested by serologic studies. We diagnosed B. miyamotoi infection in an immunocompromised man in California. Diagnosis was aided by plasma microbial cell–free DNA sequencing. We conclude that the infection was acquired in California.

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.

Nutritive symbiosis between bacteria and ticks is observed across a range of ecological contexts; however, little characterization on the molecular components responsible for this symbiosis has been done. Previous studies in our lab demonstrated that Rickettsia monacensis str. Humboldt (strain Humboldt) can synthesize folate de novo via the folate biosynthesis pathway involving folA, folC, folE, folKP, and ptpS genes. In this study, expression of the strain Humboldt folA gene within a folA mutant Escherichia coli construct was used to functionally characterize the strain Humboldt folA folate gene in vivo. The strain Humboldt folA folate gene was subcloned into a TransBac vector and transformed into a folA mutant E. coli construct. The mutant containing strain Humboldt folA subclone and a pFE604 clone of the knocked-out folA gene was cured of pFE604. Curing of the folA mutant E. coli construct was successful using acridine orange and 43.5 °C incubation temperature. The plasmid curing assay showed curing efficiency of the folA mutant at 100%. Functional complementation was assessed by growth phenotype on minimal media with and without IPTG between strain Humboldt folA and E. coli folA. Large and homogenous wild-type colony growth was observed for both strain Humboldt and E. coli folA on minimal media with 0.1 mM IPTG, wild-type growth for strain Humboldt folA and pin-point growth for E. coli folA on 0.01 mM IPTG, and pin-point growth without IPTG for both strain Humboldt and E. coli folA. This study provides evidence substantiating the in vivo functionality of strain Humboldt folA in producing functional gene products for folate biosynthesis.

Despite increasing severity and frequency of wildfires, knowledge about how fire impacts the ecology of tickborne pathogens is limited. In 2018, the River Fire burned a forest in the far-western U.S.A. where the ecology of tick-borne pathogens had been studied for decades. Forest structure, avifauna, large and small mammals, lizards, ticks, and tick-borne pathogens (Anaplasma phagocytophilum, Borrelia burgdorferi, Borrelia miyamotoi) were assessed after the wildfire in 2019 and 2020. Burning reduced canopy cover and eliminated the layer of thick leaf litter that hosted free-living ticks, which over time was replaced by forbs and grasses. Tick abundance and the vertebrate host community changed dramatically. Avian species adapted to cavity nesting became most prevalent, while the number of foliage-foraging species increased by 83% as vegetation regenerated. Nine mammalian species were observed on camera traps, including sentinel (black-tailed jackrabbits) and reservoir hosts (western gray squirrels) of B. burgdorferi. One Peromyscus sp. mouse was captured in 2019 but by 2020, numbers were rebounding (n=37), although tick infestations on rodents remained sparse (0.2/rodent). However, western fence lizards (n=19) hosted 8.6 ticks on average in 2020. Assays for pathogens found no B. miyamotoi in either questing or host-feeding ticks, A. phagocytophilum DNA in 4% (1/23) in 2019, and 17% (29/173) in 2020 for questing and host-feeding ticks combined, and B. burgdorferi DNA in just 1% of all ticks collected in 2020 (2/173). We conclude that a moderately severe wildfire can have dramatic impacts on the ecology of tick-borne pathogens, with changes posited to continue for multiple years. [ABSTRACT FROM AUTHOR]

Background: Ixodes scapularis and Ixodes pacificus ticks are the principal vectors of the agent of Lyme disease and several other tick-borne diseases in Canada. Tick surveillance data can be used to identify local tick-borne disease risk areas and direct public health interventions. The objective of this article is to describe the seasonal and spatial characteristics of the main Lyme disease vectors in Canada, and the tick-borne pathogens they carry, using passive and active surveillance data from 2020. Methods: Passive and active surveillance data were compiled from the National Microbiology Laboratory Branch (Public Health Agency of Canada), provincial and local public health authorities, and eTick (an online, image-based platform). Seasonal and spatial analyses of ticks and their associated pathogens are presented, including infection prevalence estimates. Results: In passive surveillance, I. scapularis (n=7,534) were submitted from all provinces except Manitoba and British Columbia, while I. pacificus (n=718) were submitted only from British Columbia. No ticks were submitted from the Territories. The seasonal distribution of I. scapularis submissions was bimodal, but unimodal for I. pacificus. Four tick-borne pathogens were identified in I. scapularis (Borrelia burgdorferi, Anaplasma phagocytophilum, Babesia microti and Borrelia miyamotoi) and one in I. pacificus (B. miyamotoi). In active surveillance, I. scapularis (n=688) were collected in Ontario, Québec and New Brunswick. Five tick-borne pathogens were identified: B. burgdorferi, A. phagocytophilum, B. microti, B. miyamotoi and Powassan virus. Conclusion: This article provides a snapshot of the distribution of I. scapularis and I. pacificus and their associated human pathogens in Canada in 2020, which can help assess the risk of exposure to tick-borne pathogens in different provinces. [ABSTRACT FROM AUTHOR]

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.

Lyme disease is the most common vector-borne disease in the United States and Canada. The causative agent of Lyme disease in North America is the spirochete Borrelia burgdorferi. In western North America, the primary vector of Borrelia burgdorferi is the western black-legged tick, Ixodes pacificus. Surveillance and modelling efforts indicate that I. pacificus is primarily found in coastal California, Oregon, Washington and the southern coastal regions of British Columbia However, infection rates with B. burgdorferi among I. pacificus ticks remain low, ranging from 0.6% to 9.9%. Lyme disease case numbers in western North America are also relatively low compared to eastern North America. Enzootic maintenance of B. burgdorferi by hosts in natural environments and climatic factors may influence Lyme disease risk. The borreliacidal western fence lizard, Sceloporus occidentalis, may contribute to the low infection rates observed in I. pacificus ticks, while the migratory nature of avian hosts can allow for long-distance tick dispersal. Moderately warm and moist environments and protection from sunlight define the suitable habitats of I. pacificus ticks. In this review, we discuss the ecology and epidemiology of Lyme disease in relation to I. pacificus, as well as the need for more studies in western North America.

Simple Summary: Lyme disease affects about half a million Americans every year, and cases in Canada are rising. However, the number of studies focusing on the epidemiology of Lyme disease in western North America has been relatively low compared to that of eastern North America. Here, we aim to summarize the current state of knowledge regarding Lyme disease epidemiology in western North America, which includes current surveillance efforts tracking Lyme disease cases, modelling studies clarifying the geographic distributions of vectors for Borrelia burgdorferi, and the dynamics required to maintain and transmit B. burgdorferi in the natural environment. In providing a comprehensive picture of the state of Lyme disease in western North America, this review may be particularly helpful for future studies in the region. Lyme disease is the most common vector-borne disease in the United States and Canada. The causative agent of Lyme disease in North America is the spirochete Borrelia burgdorferi. In western North America, the primary vector of Borrelia burgdorferi is the western black-legged tick, Ixodes pacificus. Surveillance and modelling efforts indicate that I. pacificus is primarily found in coastal California, Oregon, Washington and the southern coastal regions of British Columbia However, infection rates with B. burgdorferi among I. pacificus ticks remain low, ranging from 0.6% to 9.9%. Lyme disease case numbers in western North America are also relatively low compared to eastern North America. Enzootic maintenance of B. burgdorferi by hosts in natural environments and climatic factors may influence Lyme disease risk. The borreliacidal western fence lizard, Sceloporus occidentalis, may contribute to the low infection rates observed in I. pacificus ticks, while the migratory nature of avian hosts can allow for long-distance tick dispersal. Moderately warm and moist environments and protection from sunlight define the suitable habitats of I. pacificus ticks. In this review, we discuss the ecology and epidemiology of Lyme disease in relation to I. pacificus, as well as the need for more studies in western North America. [ABSTRACT FROM AUTHOR]

Foraging ticks reportedly exploit diverse cues to locate their hosts. Here, we tested the hypothesis that host-seeking Western black-legged ticks, Ixodes pacificus, and black-legged ticks, I. scapularis, respond to microbes dwelling in sebaceous gland secretions of white-tailed deer, Odocoileus virginianus, the ticks' preferred host. Using sterile wet cotton swabs, microbes were collected from the pelage of a sedated deer near forehead, preorbital, tarsal, metatarsal and interdigital glands. Swabs were plated on agar, and isolated microbes were identified by 16S rRNA amplicon sequencing. Of 31 microbial isolates tested in still-air olfactometers, 10 microbes induced positive arrestment responses by ticks, whereas 10 others were deterrent. Of the 10 microbes prompting arrestment by ticks, four microbes—including Bacillus aryabhattai (isolates A4)—also attracted ticks in moving-air Y-tube olfactometers. All four of these microbes emitted carbon dioxide and ammonia as well as volatile blends with overlapping blend constituents. The headspace volatile extract (HVE) of B. aryabhattai (HVE-A4) synergistically enhanced the attraction of I. pacificus to CO2. A synthetic blend of HVE-A4 headspace volatiles in combination with CO2 synergistically attracted more ticks than CO2 alone. Future research should aim to develop a least complex host volatile blend that is attractive to diverse tick taxa.

The movement of Ixodes pacificus and Dermacentor occidentalis adult ticks down a hill slope under natural conditions was investigated using the mark-release-recapture (MRR) method. We evaluated the movement of host-seeking adult ticks down a hill slope to determine if ticks travel downhill from an uphill area to trail margins below. During the tick seasons in 2016-2018, the ticks were collected by flagging, marked with paint, released at a predetermined location, and recaptured by weekly flagging. Of the 188 female and 114 male I. pacificus marked and released 30 m above the trail in 2016, 11 (5.8%) females and one (0.9%) male were recaptured at the trail. In 2017, of the 71 female and 52 male I. pacificus, none were recaptured. In 2018, nine (3.6%) female and one (0.4%) male I. pacificus were recaptured of the 247 female and 287 males marked and released. Fifteen (18.5%) female and six (7.3%) male D. occidentalis were recaptured of the 81 females and 82 males marked and released in 2017. The trail deterred further movement with only 0.6% of I. pacificus and 8.3% of D. occidentalis recaptured on the opposite side of the trail. This study demonstrated that some I. pacificus and D. occidentalis adults found along the uphill side of trails may have originated from an area 30 m from the trail margin, some travelling at a rate of 1.6-1.9 m/day under natural conditions in chaparral. [ABSTRACT FROM AUTHOR]

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.

Habitat loss and forest fragmentation are often linked to increased pathogen transmission, but the extent to which habitat isolation and landscape connectivity affect disease dynamics through movement of disease vectors and reservoir hosts has not been well examined. Tick-borne diseases are the most prevalent vector-borne diseases in the United States and on the West Coast, Ixodes pacificus is one of the most epidemiologically important vectors. We investigated the impacts of habitat fragmentation on pathogens transmitted by I. pacificus and sought to disentangle the effects of wildlife communities and landscape metrics predictive of pathogen diversity, prevalence and distribution. We collected pathogen data for four co-occurring bacteria transmitted by I. pacificus and measured wildlife parameters. We also used spatial data and cost-distance analysis integrating expert opinions to assess landscape metrics of habitat fragmentation. We found that landscape metrics were significant predictors of tick density and pathogen prevalence. However, wildlife variables were essential when predicting the prevalence and distribution of pathogens reliant on wildlife reservoir hosts for maintenance. We found that landscape structure was an informative predictor of tick-borne pathogen richness in an urban matrix. Our work highlights the implications of large-scale land management on human disease risk., Competing Interests: We declare we have no competing interests., (© 2024 The Authors.)

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.

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. Rodents and birds are reservoirs for Bb and infect juvenile ticks, while 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. [ABSTRACT FROM AUTHOR]

Background: The primary vectors of the agent of Lyme disease in Canada are Ixodes scapularis and Ixodes pacificus ticks. Surveillance for ticks and the pathogens they can transmit can inform local tick-borne disease risk and guide public health interventions. The objective of this article is to characterize passive and active surveillance of the main Lyme disease tick vectors in Canada in 2019 and the tick-borne pathogens they carry. Methods: Passive surveillance data were compiled from the National Microbiology Laboratory Branch and provincial public health data sources. Active surveillance was conducted in selected sentinel sites in all provinces. Descriptive analysis of ticks submitted and infection prevalence of tick-borne pathogens are presented. Seasonal and spatial trends are also described. Results: In passive surveillance, specimens of I. scapularis (n=9,858) were submitted from all provinces except British Columbia and I. pacificus (n=691) were submitted in British Columbia and Alberta. No ticks were submitted from the territories. The seasonal distribution pattern was bimodal for I. scapularis adults, but unimodal for I. pacificus adults. Borrelia burgdorferi was the most prevalent pathogen in I. scapularis (18.8%) and I. pacificus (0.3%). In active surveillance, B. burgdorferi was identified in 26.2% of I. scapularis; Anaplasma phagocytophilum in 3.4% of I. scapularis, and Borrelia miyamotoi and Powassan virus in 0.5% or fewer of I. scapularis. These same tick-borne pathogens were not found in the small number of I. pacificus tested. Conclusion: This surveillance article provides a snapshot of the main Lyme disease vectors in Canada and their associated pathogens, which can be used to monitor emerging risk areas for exposure to tick-borne pathogens. [ABSTRACT FROM AUTHOR]

Background: Lyme disease (LD) is a multisystem infection that can affect the skin, heart, joints and nervous system. In Canada, the incidence of LD cases has increased over the past decade making this a disease of public health concern. The objective of this study is to summarize the epidemiology of LD cases reported in Canada from 2009 through 2019. Methods: Incidence over time, case classification (confirmed and probable), seasonal and geographic distribution, demographic and clinical characteristics of reported LD cases were determined. Logistic regression was used to explore potential demographic risk factors for the occurrence of LD. Results: During 2009–2019, a total of 10,150 LD cases were reported by the provinces to the Public Health Agency of Canada, of which 7,242 (71.3%) were confirmed and 2,908 (28.7%) were probable cases. The annual count increased from 144 in 2009 to 2,634 in 2019, mainly due to an increase in locally acquired infections, from 65.3% to 93.6%, respectively. The majority of cases (92.1%) were reported from three provinces: Ontario (46.0%); Nova Scotia (28.0%); and Québec (18.1%). Most of the locally acquired cases (74.0%) were reported in the summer months of June (20.0%), July (35.4%) and August (18.6%). The highest incidence rates (cases per 100,000 population) were in children aged 5–9 years (45.0) and in adults aged 65–69 years (74.3), with 57.3% of all reported cases occurring among males. The most common presenting symptoms were single erythema migrans rash (75.1%) and arthritis (34.1%). The frequency of reported clinical manifestations varied among age groups and seasons with erythema migrans and arthritis at presentation reported more frequently in children than older patients. Conclusion: The results of this report highlight the continued emergence of LD in Canada and the need for further development and implementation of targeted awareness campaigns designed to minimize the burden of LD. [ABSTRACT FROM AUTHOR]

Abstract Background Understanding the drivers of Lyme disease incidence at broad spatial scales is critical for predicting and mitigating human disease risk. Previous studies have identified vector phenology and behavior, host community composition, and landscape features as drivers of variable Lyme disease risk. However, while the Lyme disease transmission cycles in the eastern and western USA involve different vector species (Ixodes scapularis and Ixodes pacificus, respectively), the role of vector-specific differences in transmission efficiency has not been directly examined. By comparing the performance of traits involved in vector competence between these two species, this study aims to identify how vector competence contributes to variable Lyme disease risk. Methods We used a suite of laboratory experiments to compare the performance of traits related to vector competence for the two USA Lyme disease vectors. For each species, we measured the rate of attachment to a common rodent host, the engorgement weight, and the efficiency of pathogen acquisition (host to tick) and pathogen transmission (tick to host) from laboratory mice. In measuring pathogen acquisition and transmission, we used two different pathogen strains, one sympatric with I. scapularis and one sympatric with I. pacificus, to assess the importance of vector-pathogen coevolutionary history in transmission dynamics. Results We found I. pacificus had significantly higher host attachment success and engorgement weights, but significantly lower pathogen transmission efficiency relative to I. scapularis. Molting success and pathogen acquisition did not differ between these two species. However, pathogen acquisition efficiency was significantly higher for both sympatric vector and pathogen strains than the allopatric pairings. Conclusions This study identified species-specific vector traits as a potential driver of broad scale variation in Lyme disease risk in the USA. In particular, the exceedingly low rates of pathogen transmission from tick to host observed for I. pacificus may limit Lyme disease transmission efficiency in the western USA. Further, observed variation in pathogen acquisition between sympatric and allopatric vector-pathogen strains indicate that vector-pathogen coevolutionary history may play a key role in transmission dynamics. These findings underscore the need to consider vector traits and vector-pathogen coevolution as important factors governing regional Lyme disease risk.

Introduction: Lyme disease is the most common vector-borne disease in the United States and Canada. The primary vector for the causative agent of Lyme disease, Borrelia burgdorferi, in the Pacific Northwest is the western blacklegged tick, Ixodes pacificus. Materials and Methods: Using active tick surveillance data from British Columbia, Canada, and Washington State, USA, habitat suitability models using MaxEnt (maximum entropy) were developed for I. pacificus to predict its current and mid-century geographic distributions. Passive surveillance data both from BC and WA were also visualized. Results: According to the constructed models, the number of frost-free days during the winter is the most relevant predictor of its habitat suitability, followed by summer climate moisture, ecoregion, and mean minimum fall temperature. The ensemble geographic distribution map predicts that the coastal regions and inland valleys of British Columbia and the Puget Lowlands of Washington State provide the most suitable habitats for I. pacificus . The density map of ticks submitted from passive surveillance data was overlaid on the current distribution map and demonstrates the correlation between numbers of submissions and habitat suitability. Mid-century projections, based on current climate change predictions, indicate a range expansion, especially of low and moderate suitability, from current distribution. Regarding Lyme disease risk, I. pacificus identified from both active and passive surveillance and tested positive for B. burgdorferi were found to be in areas of moderate to very high suitability for I. pacificus . Conclusion: According to developed models, the total suitable habitat area for I. pacificus will expand in the interior regions of British Columbia and Washington State. However, the risk remains small given relatively low infection rates among I. pacificus . Further studies are required to better understand how this might change in the future.

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 present 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 are only beginning to emerge. Progress is stymied by technical difficulties in manipulating the microbiome and by biological variables related to the life cycle of Ixodid ticks. This review highlights these challenges and examines avenues to understand the significance of the tick microbiome in tick biology. Ixodes scapularis and Ixodes pacificus harbor a simple primary microbiome composed predominantly of the endosymbiont Rickettsia buchneri. The tick microbiome also includes microbiota acquired from the environment and represents a transient microbiome. The tick microbiome composition is in a state of flux and is likely influenced by multiple biotic and abiotic factors. The microbiome composition is regulated by immune responses of the tick at the vector–host and environment–vector interfaces. Understanding the interactions between the tick microbiota, tick-borne pathogens and tick immune responses will reveal new insights in tick biology. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Ixodes pacificus (the western blacklegged tick) occurs in the far western United States (US), where it commonly bites humans. This tick was not considered a species of medical concern until it was implicated in the 1980s as a vector of Lyme disease spirochetes. Later, it was discovered to also be the primary vector to humans in the far western US of agents causing anaplasmosis and hard tick relapsing fever. The core distribution of I. pacificus in the US includes California, western Oregon, and western Washington, with outlier populations reported in Utah and Arizona. In this review, we provide a history of the documented occurrence of I. pacificus in the US from the 1890s to present, and discuss associations of its geographic range with landscape, hosts, and climate. In contrast to Ixodes scapularis (the blacklegged tick) in the eastern US, there is no evidence for a dramatic change in the geographic distribution of I. pacificus over the last half-century. Field surveys in the 1930s and 1940s documented I. pacificus along the Pacific Coast from southern California to northern Washington, in the Sierra Nevada foothills, and in western Utah. County level collection records often included both immatures and adults of I. pacificus , recovered by drag sampling or from humans, domestic animals, and wildlife. The estimated geographic distribution presented for I. pacificus in 1945 by Bishopp and Trembley is similar to that presented in 2022 by the Centers for Disease Control and Prevention. There is no clear evidence of range expansion for I. pacificus , separate from tick records in new areas that could have resulted from newly initiated or intensified surveillance efforts. Moreover, there is no evidence from long-term studies that the density of questing I. pacificus ticks has increased over time in specific areas. It therefore is not surprising that the incidence of Lyme disease has remained stable in the Pacific Coast states from the early 1990s, when it became a notifiable condition, to present. We note that deforestation and deer depredation were less severe in the far western US during the 1800s and early 1900s compared to the eastern US. This likely contributed to I. pacificus maintaining stable, widespread populations across its geographic range in the far western US in the early 1900s, while I. scapularis during the same time period appears to have been restricted to a small number of geographically isolated refugia sites within its present range in the eastern US. The impact that a warming climate may have had on the geographic distribution and local abundance of I. pacificus in recent decades remains unclear. [ABSTRACT FROM AUTHOR]

The emerging tick-borne pathogen Anaplasma phagocytophilum infects humans, domestic animals, and wildlife throughout the Holarctic. In the western US, the ecology of A. phagocytophilum is particularly complex, with multiple pathogen strains, tick vectors, and reservoir hosts. A recent phylogenetic analysis of A. phagocytophilum strains isolated from various small mammal hosts in California documented distinct clustering of woodrat strains separate from sciurid (chipmunk and squirrel) strains. Here, we identified strains of A. phagocytophilum in various Ixodes tick species in California and related these genotypes to those found among reservoir and clinical hosts from the same areas. The sequences from all of the nidicolous (nest-dwelling) Ixodes ticks grouped within a clade that also contained all of the woodrat-origin A. phagocytophilum strains. Two of the I. pacificus sequences were also grouped within this woodrat clade, while the remaining five belonged to a less genetically diverse clade that included several sciurid-origin strains as well as a dog, a horse, and a human strain. By comparing A. phagocytophilum strains from multiple sources concurrently, we were able to gain a clearer picture of how A. phagocytophilum strains in the western US are partitioned, which hosts and vectors are most likely to be infected with a particular strain, and which tick species and reservoir hosts pose the greatest health risk to humans and domestic animals.

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. [ABSTRACT FROM AUTHOR]

Ticks (Chelicerata, Ixodida) are blood-feeding ectoparasites believed to have evolved at least about 120 millions of years ago and found worldwide. However, many aspects of their unique life cycle and anatomy, including their mechanical properties, remain to be understood. Here, we compared the mechanical properties of the cuticle of the argasid tick Ornithodoros moubata to those of two species of ixodid tick, Amblyomma hebraeum and Ixodes pacificus that we explored in our earlier studies of the tick exoskeleton. Significant differences were expected given the substantial difference in life cycle, including a five-fold increase during the repeated adult blood meal for female O. moubata vs. 70- to 120-fold during the single feeding of the adult female A. hebraeum and I. pacificus. We demonstrate here that the layered structure and mechanical properties (stiffness and viscosity) of the cuticle show minor differences, but the difference in cuticle thickness is substantial. Ductility is lost during feeding; reduced pH restores ductility. Previous work suggests that this occurs in vivo in engorged ixodid ticks; there is no evidence of this occurring in vivo in O. moubata. Thinning of cuticle in O. moubata fed females is consistent with the predicted stretch of cuticle due to the blood meal; there is no evidence of cuticle synthesis during the short feeding period. Dimensional analysis suggests that the soft feel of argasid ticks is related to cuticle thickness, not cuticle stiffness. Relative to argasid ticks, the hard ixodid ticks accommodate a ca. 20-fold higher size of blood meal by starting with a thicker cuticle and growing much additional cuticle during engorgement. [ABSTRACT FROM AUTHOR]

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

Borrelia burdorferi genotype in the northeastern United States is associated with Lyme borreliosis severity. Analysis of DNA sequences of the outer surface protein C gene and rrs-rrlA intergenic spacer from extracts of Ixodes spp. ticks in 3 US regions showed linkage disequilibrium between the 2 loci within a region but not consistently between regions.

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. [ABSTRACT FROM AUTHOR]

Background: Understanding the drivers of Lyme disease incidence at broad spatial scales is critical for predicting and mitigating human disease risk. Previous studies have identified vector phenology and behavior, host community composition, and landscape features as drivers of variable Lyme disease risk. However, while the Lyme disease transmission cycles in the eastern and western USA involve different vector species (Ixodes scapularis and Ixodes pacificus, respectively), the role of vector-specific differences in transmission efficiency has not been directly examined. By comparing the performance of traits involved in vector competence between these two species, this study aims to identify how vector competence contributes to variable Lyme disease risk. Methods: We used a suite of laboratory experiments to compare the performance of traits related to vector competence for the two USA Lyme disease vectors. For each species, we measured the rate of attachment to a common rodent host, the engorgement weight, and the efficiency of pathogen acquisition (host to tick) and pathogen transmission (tick to host) from laboratory mice. In measuring pathogen acquisition and transmission, we used two different pathogen strains, one sympatric with I. scapularis and one sympatric with I. pacificus, to assess the importance of vector-pathogen coevolutionary history in transmission dynamics. Results: We found I. pacificus had significantly higher host attachment success and engorgement weights, but significantly lower pathogen transmission efficiency relative to I. scapularis. Molting success and pathogen acquisition did not differ between these two species. However, pathogen acquisition efficiency was significantly higher for both sympatric vector and pathogen strains than the allopatric pairings. Conclusions: This study identified species-specific vector traits as a potential driver of broad scale variation in Lyme disease risk in the USA. In particular, the exceedingly low rates of pathogen transmission from tick to host observed for I. pacificus may limit Lyme disease transmission efficiency in the western USA. Further, observed variation in pathogen acquisition between sympatric and allopatric vector-pathogen strains indicate that vector-pathogen coevolutionary history may play a key role in transmission dynamics. These findings underscore the need to consider vector traits and vector-pathogen coevolution as important factors governing regional Lyme disease risk. [ABSTRACT FROM AUTHOR]

Lyme disease (LD), caused by the spirochete Borrelia burgdorferi (Bb), is the most commonly reported vector-borne disease in the United States. We have been studying the ecology, epidemiology, and prevention of LD since 1982. The primary objectives of this research are intended to clarify the transmission cycles of Bb and closely related spirochetes in diverse habitats; to determine what behaviors and environmental factors elevate the risk of human exposure to Bb-infected ticks; and to evaluate preventive methods. Several genospecies of LD spirochetes are maintained in transmission cycles involving western gray squirrels, dusky-footed wood rats, California kangaroo rats or deer mice, and three species of Ixodes ticks. The western gray squirrel has been implicated as a primary reservoir host of Bb in dense woodlands. The western black-legged tick, Ixodes pacificus, especially the nymphal stage, is the primary bridging vector from wildlife reservoirs to people. In Mendocino County, the prevalence of spirochete-infection in the nymphs typically averages between 5 and 10% (range,

Granulocytic anaplasmosis (GA) is a disease of humans, domestic animals, and wildlife caused by Anaplasma phagocytophilum, formerly Ehrlichia phagocytophila, E. equi, and the unnamed agents of “human granulocytic ehrlichiosis” (HGE). This pathogen is inoculated into host skin by the bite of Ixodes spp. ticks, including I. pacificus in California and I. scapularis in the eastern U.S. After inoculation, A. phagocytophilum disseminates to the blood and is phagocytosed into host neutrophils. The clinical characteristics of GA in people vary from no symptoms to fever, headache, neurological symptoms, and occasionally death. Horses with GA may experience high fever, depression, reduced ability to eat, limb edema, jaundice, and ataxia. GA is an emerging disease in the eastern U.S. but only a handful of human cases have been reported in the western U.S., despite relatively common reports of disease in horses and dogs. Wildlife and dog-sentinel studies have clarified that infection is common in the coast range mountains and Sierra Nevada foothills, with ongoing research focusing on ecological determinants that can modify the prevalence of infection in any particular area. One known reservoir is the dusky-footed woodrat. However, important, poorly understood ecological determinants serve to modify the probability of rodent as well as human infections, including climate, vegetation, and possibly the presence of other reservoir-competent rodents and nidicolous woodrat-specialist ticks such as I. spinipalpis.

Whole-genome sequences are presented for three Borrelia burgdorferi , a causative agent of Lyme disease in North America, isolated from Ixodes pacificus ticks collected in British Columbia, Canada. Shotgun DNA libraries were prepared with Illumina DNA Prep and sequenced using the MiniSeq platform. Genome assemblies enabled multilocus sequence typing and ospC typing., Competing Interests: The authors declare no conflict of interest.

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. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

Nutritive symbiosis between bacteria and ticks is observed across a range of ecological contexts; however, little characterization on the molecular components responsible for this symbiosis has been done. Previous studies in our lab demonstrated that Rickettsia monacensis str. Humboldt (strain Humboldt) can synthesize folate de novo via the folate biosynthesis pathway involving folA, folC, folE, folKP , and ptpS genes. In this study, expression of the strain Humboldt folA gene within a folA mutant Escherichia coli construct was used to functionally characterize the strain Humboldt folA folate gene in vivo. The strain Humboldt folA folate gene was subcloned into a TransBac vector and transformed into a folA mutant E. coli construct. The mutant containing strain Humboldt folA subclone and a pFE604 clone of the knocked-out folA gene was cured of pFE604. Curing of the folA mutant E. coli construct was successful using acridine orange and 43.5 °C incubation temperature. The plasmid curing assay showed curing efficiency of the folA mutant at 100%. Functional complementation was assessed by growth phenotype on minimal media with and without IPTG between strain Humboldt folA and E. coli folA. Large and homogenous wild-type colony growth was observed for both strain Humboldt and E. coli folA on minimal media with 0.1 mM IPTG, wild-type growth for strain Humboldt folA and pin-point growth for E. coli folA on 0.01 mM IPTG, and pin-point growth without IPTG for both strain Humboldt and E. coli folA. This study provides evidence substantiating the in vivo functionality of strain Humboldt folA in producing functional gene products for folate biosynthesis. [ABSTRACT FROM AUTHOR]

Abstract Identifying the effects of human‐driven perturbations, such as species introductions or habitat fragmentation, on the ecology and dynamics of infectious disease has become a central focus of disease ecologists. Yet, comparatively little is known about how the ecology of zoonotic disease systems responds to catastrophic disturbance events such as wildfires or hurricanes. In California, wildfire disturbance is centrally important to the ecology of forests and oak woodlands and is projected to increase in severity and extent under future climate change. Here, taking advantage of a recent wildfire as a natural experiment, we investigate the effects of wildfire disturbance on the ecology of tick‐borne disease in California oak woodlands. We find that wildfire leads to elevated abundance of questing adult and nymphal western blacklegged ticks (Ixodes pacificus) in the year following fire, relative to unburned control plots, but that vector tick abundance declines sharply in the following two years. We find that the abundance of non‐competent hosts (western fence lizards) for the Lyme disease bacterium is unaffected by fire, but that the abundances of important reproductive hosts (deer) for ticks and reservoir hosts (dusky‐footed woodrats) for tick‐borne pathogens are significantly negatively affected by fire. We found ticks and hosts infected with Borrelia burgdorferi sensu lato only within the burn extent and only in the year following the wildfire, though rates of infection were exceedingly low representing little risk to humans. In aggregate, due to the differential effects of this catastrophic disturbance event on different key host species and vector tick life stages in the transmission of tick‐borne pathogens, we conclude that wildfire may potentially increase risk of exposure to vector ticks in the first year following wildfire in California, but that risk is dampened substantially in following years due to tick population declines and loss of key reservoir hosts from the system.

The mechanical properties of the cuticle of Ixodes pacificus (Ip) are compared to those of Amblyomma hebraeum (Ah) from our earlier work. The 10-fold size difference between the species is expected to lead to significant differences in mechanical properties, because cuticular stretch depends on high internal hydrostatic pressure during the rapid phase of engorgement. We demonstrate here: (1) The cuticle of partially fed Ip is less stiff and viscous than that of Ah. (2) A stretch-recoil cycle in both ticks consists of recoverable deformation (ESv) and permanent deformation (ESp); ESp is higher in Ip, and increases sharply during the slow phase of engorgement, but not in Ah. (3) Injected dopamine (DA) increases ESp and reduces all measures of stiffness and viscosity, suggesting that a catecholaminergic neurotransmitter plays a fundamental role in modulating mechanical properties of the cuticle. However, unlike Ah, DA’s effect was not different from that of the control (1.2% NaCl). Mere insertion of the needle may have punctured the gut, causing the release of perhaps a catecholamine that increases ESp, an hypothesis supported by the fact that inserting a needle without any injection also caused an increase in ESp. (4) Stretch reduces ESp, but subjecting loops to pH 6.5 in vitro restores it. (5) Despite the smaller size of Ip, later onset of the rapid phase of engorgement, a thinner cuticle and different mechanical properties all reduce the internal pressure needed for stretch. [ABSTRACT FROM AUTHOR]

Identifying the effects of human‐driven perturbations, such as species introductions or habitat fragmentation, on the ecology and dynamics of infectious disease has become a central focus of disease ecologists. Yet, comparatively little is known about how the ecology of zoonotic disease systems responds to catastrophic disturbance events such as wildfires or hurricanes. In California, wildfire disturbance is centrally important to the ecology of forests and oak woodlands and is projected to increase in severity and extent under future climate change. Here, taking advantage of a recent wildfire as a natural experiment, we investigate the effects of wildfire disturbance on the ecology of tick‐borne disease in California oak woodlands. We find that wildfire leads to elevated abundance of questing adult and nymphal western blacklegged ticks (Ixodes pacificus) in the year following fire, relative to unburned control plots, but that vector tick abundance declines sharply in the following two years. We find that the abundance of non‐competent hosts (western fence lizards) for the Lyme disease bacterium is unaffected by fire, but that the abundances of important reproductive hosts (deer) for ticks and reservoir hosts (dusky‐footed woodrats) for tick‐borne pathogens are significantly negatively affected by fire. We found ticks and hosts infected with Borrelia burgdorferi sensu lato only within the burn extent and only in the year following the wildfire, though rates of infection were exceedingly low representing little risk to humans. In aggregate, due to the differential effects of this catastrophic disturbance event on different key host species and vector tick life stages in the transmission of tick‐borne pathogens, we conclude that wildfire may potentially increase risk of exposure to vector ticks in the first year following wildfire in California, but that risk is dampened substantially in following years due to tick population declines and loss of key reservoir hosts from the system. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]