Reported County-Level Distribution of Seven Human Pathogens Detected in Host-Seeking Ixodes scapularis and Ixodes pacificus (Acari: Ixodidae) in the Contiguous United States

Geographic variation in the distribution of Anaplasma phagocytophilum variants in host-seeking Ixodes scapularis nymphs and adults in the eastern United States elucidated using next generation sequencing

Human anaplasmosis cases, caused by Anaplasma phagocytophilum, are increasing in the United States. This trend is explained, in part, by expansion in the geographic range of the primary vector, Ixodes scapularis. Multiple variants of A. phagocytophilum have been identified in field collected ticks, but only a single variant (human active, or "Ap-ha," variant) has been shown to be pathogenic in humans. Until recently, laboratory methods used to differentiate variants were cumbersome and seldomly used in large scale assessments of the pathogen's geographic distribution. As a result, many surveys reported A. phagocytophilum without segregating variants. Lack of discrimination among A. phagocytophilum variants could lead to overestimation of anaplasmosis risk to humans. Next Generation Sequencing (NGS) assays were recently developed to efficiently detect multiple Ixodes scapularis-borne human pathogens including Ap-ha. In this study, we utilized NGS to detect and differentiate A. phagocytophilum variants (Ap-ha vs. non ha) in host-seeking I. scapularis nymphs and adults collected across 23 states in the eastern United States from 2012 to 2023 as part of national tick surveillance efforts and research studies. Many of the included ticks were tested previously using a TaqMan PCR assay that could detect A. phagocytophilum but could not differentiate variants. We retested A. phagocytophilum infected ticks with NGS to differentiate variants. Anaplasma phagocytophilum (any variant) was identified in 165 (35 %) of 471 counties from which ticks were tested, whereas Ap-ha was detected in 70 (15 %) of 469 counties where variants were differentiated. Both variants were identified in 32 % (n = 40) of 126 counties with either variant detected. Among states where A. phagocytophilum (any variant) was detected, prevalence ranged from 2 % to 19 % in unfed adults and from 0.2 % to 7.8 % in unfed nymphs; prevalence of Ap-ha variant ranged from 0.0 % to 16 % in adults, and 0.0 % to 4.6 % in nymphs.

A next generation sequencing assay combining Ixodes species identification with pathogen detection to support tick surveillance efforts in the United States

The burden of tick-borne diseases continues to increase in the United States. Tick surveillance has been implemented to monitor changes in the distribution and prevalence of human disease-causing pathogens in ticks that frequently bite humans. Such efforts require accurate identification of ticks to species and highly sensitive and specific assays that can detect and differentiate pathogens from genetically similar microbes in ticks that have not been demonstrated to be pathogenic in humans. We describe a modification to a next generation sequencing pathogen detection assay that includes a target that accurately identifies Ixodes ticks to species. We show that the replacement of internal control primers used to ensure assay performance with primers that also act as an internal control and can additionally differentiate tick species, retains high sensitivity and specificity, improves efficiency, and reduces costs by eliminating the need to run separate assays to screen for pathogens and for tick identification.

Density of host-seeking Ixodes scapularis nymphs by region, state, and county in the contiguous United States generated through national tick surveillance

The majority of vector-borne disease cases reported annually in the United States are caused by pathogens spread by the blacklegged tick, Ixodes scapularis. The number and geographic distribution of cases have increased as the geographic range and abundance of the tick have expanded in recent decades. A large proportion of Lyme disease and other I. scapularis-borne diseases are associated with nymphal tick bites; likelihood of such bites generally increases with increasing nymphal densities. National tick surveillance was initiated in 2018 to track changes in the distribution and abundance of medically important ticks at the county spatial scale throughout the United States. Tick surveillance records, including historical data collected prior to the initiation of the national program, are collated in the ArboNET Tick Module database. Through exploration of ArboNET Tick Module data, we found that efforts to quantify the density of host-seeking I. scapularis nymphs (DON) were unevenly distributed among geographic regions with the greatest proportion of counties sampled in the Northeast and Upper Midwest. Submissions covering tick collections from 2004 through 2022 revealed extensive variation in DON estimates at collection site, county, state, and regional spatial scales. Throughout the entire study period, county DON estimates ranged from 0.0 to 488.5 nymphs/1,000 m2 . Although substantial variation was recorded within regions, DON estimates were greatest in the Northeast, Upper Midwest, and northern states within the Southeast regions (Virginia and North Carolina); densities were intermediate in the Ohio Valley and very low in the South and Northern Rockies and Plains regions. The proportion of counties classified as moderate or high DON was lower in the Northeast, Ohio Valley, and Southeast regions during the 2004 through 2017 time period (prior to initiation of the national tick surveillance program) compared to 2018 through 2022; DON estimates remained similarly low between these time periods in the South and the Northern Rockies and Plains regions. Despite the limitations described herein, the ArboNET Tick Module provides useful data for tracking changes in acarological risk across multiple geographic scales and long periods of time.

Willingness and ability of existing mosquito control and public health agencies in New Jersey to assume responsibilities for management of ticks and tick-borne disease

We conducted surveys of New Jersey mosquito control and public health agencies to determine their willingness and ability to expand or create and maintain publicly funded tick and tick-borne disease (T/TBD) management programs. Nearly all (86%) of 21 county mosquito control agencies (MCAs) completed the survey, while only 25% of the 102 health departments (HDs) responded, probably reflecting traditional agency responsibilities. Although few of either group had formal programs, many were engaged in T/TBD-related activities. Many MCAs rated their ability to assume T/TBD responsibilities as high or moderate, while most HDs rated their capabilities as low. With the exceptions of lack of sustainable funding and possible legal constraints, the groups differed regarding perceived barriers to program creation and maintenance. Both groups envisioned comprehensive programs emphasizing public education, but program priorities differed between the groups. MCAs were willing to include most program activities, while HDs felt that some activities should be the responsibility of other agencies. MCAs were generally more familiar than HDs with tick control methods and while both groups would include control in a comprehensive program, both would limit control to public lands. Estimated program costs varied widely, probably reflecting responding agency size and complexity of envisioned programs. These results in a state with a system of existing agencies staffed by highly competent professionals suggest that more than simply additional funding (e.g., established guidelines for tick control and surveillance) is needed to create a network of practice necessary to address the growing incidence of TBD.

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

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

Detection of Borrelia burgdorferi sensu lato species in host-seeking Ixodes species ticks in the United States

Lyme disease is the most commonly reported vector-borne disease in the United States and is transmitted by Ixodes scapularis in the eastern US and I. pacificus in the west. The causative agents, Borrelia burgdorferi sensu stricto (Bbss) and B. mayonii belong to the B. burgdorferi sensu lato (Bbsl) species complex. An additional eight species of Bbsl have been identified in Ixodes species ticks in the US, but their geographic distribution, vector associations, human encounter rates and pathogenicity in humans are poorly defined. To better understand the geographic distribution and vector associations of Bbsl spirochetes in frequent and infrequent human-biting Ixodes species ticks in the US, we previously screened 29,517 host-seeking I. scapularis or I. pacificus ticks and 692 ticks belonging to eight other Ixodes species for Borrelia spirochetes using a previously described tick testing algorithm that utilizes a combination of real-time PCR and Sanger sequencing for Borrelia species identification. The assay was designed to detect known human pathogens spread by Ixodes species ticks, but it was not optimized to detect Bbsl co-infections. To determine if such co-infections were overlooked particularly in ticks infected with Bbss, we retested and analyzed a subsample of 845 Borrelia infected ticks using a next generation sequencing multiplex PCR amplicon sequencing (MPAS) assay that can identify Borrelia species and Bbsl co-infections. The assay also includes targets that can molecularly confirm identifications of Ixodes species ticks to better inform pathogen-vector associations. We show that Bbss is the most prevalent species in I. scapularis and I. pacificus; other Bbsl species were rarely detected in I. scapularis and the only Bbsl co-infections identified in I. scapularis were with Bbss and B. mayonii. We detected B. andersonii in I. dentatus in the Mid-Atlantic and Upper Midwest regions, B. kurtenbachii in I. scapularis in the Upper Midwest, B. bissettiae in I. pacificus and I. spinipalpis in the Northwest, and B. carolinensis in I. affinis in the Mid-Atlantic and Southeast, and B. lanei in I. spinipalpis in the Northwest. Twelve of 62 (19.4%) Borrelia-infected I. affinis from the Mid-Atlantic region were co-infected with Bbss and B. carolinensis. Our data support the notion that Bbsl species are maintained in largely independent enzootic cycles, with occasional spill-over resulting in multiple Bbsl species detected in Ixodes species ticks.

Ticks (Acari: Ixodida) on synanthropic small and medium-sized mammals in areas of the northeastern United States infested with the Asian longhorned tick, Haemaphysalis longicornis

The northeastern United States (US) is a hotspot for tick-borne diseases. Adding to an already complex vector landscape, in 2017 large populations of the invasive Haemaphysalis longicornis, the Asian longhorned tick, were detected in New Jersey (NJ) and later found to be widespread from Connecticut to Georgia. In its native range in northeastern Asia, H. longicornis is considered an important vector of deadly pathogens to humans, companion animals, and livestock. To identify the primary hosts of H. longicornis, we surveyed synanthropic small and medium-sized mammals in three different sites in suburban New Brunswick, NJ. Specifically, we collected approximately 9,000 tick specimens belonging to nine species from 11 different species of mammals sampled between May and September 2021. We found that H. longicornis feeds more frequently on rodents than previously thought, and that this invasive tick is likely exposed to important enzootic and zoonotic pathogens. Overall, we obtained detailed information about the seasonal dynamics and feeding patterns of six tick species common in the northeastern US, Haemaphysalis longicornis, Amblyomma americanum, Dermacentor variabilis, Ixodes scapularis, Ixodes texanus and Ixodes cookei. We found that unlike I. scapularis that feeds on mammals of all sizes, H. longicornis feeds on hosts following the general pattern of A. americanum, favoring larger species such as skunks, groundhogs, and raccoons. However, our survey revealed that unlike A. americanum, H. longicornis reaches high densities on Virginia opossum. Overall, the newly invasive H. longicornis was the most numerous tick species, both on multiple host species and in the environment, raising significant questions regarding its role in the epidemiology of tick-borne pathogens, especially those affecting livestock, companion animals and wildlife. In conclusion, our findings provide valuable insights into the tick species composition on mammalian hosts in NJ and the ongoing national expansion of H. longicornis.

Comparison of the initial and residual speed of Ixodes scapularis kill on dogs treated with a single dose of Bravecto® Chew (25 mg/kg fluralaner) or Simparica TRIO® (1.2 mg/kg sarolaner, 24 µg/kg moxidectin, 5 mg/kg pyrantel)

BACKGROUND

Compliant ectoparasiticide product use is a comprehensive way to control ticks and reduce the risk of tick-borne pathogen transmission to dogs. Because the systemically acting isoxazoline ectoparasiticides require tick attachment for drug delivery, fast speed of kill is essential to minimize tick-borne pathogen transmission risk.

METHODS

Dogs of satisfactory tick-carrying capacity were randomly allocated to treatment groups and administered, per label instructions, Bravecto® Chews (minimum 25 mg/kg fluralaner), Simparica TRIO® (minimum 1.2 mg/kg sarolaner, 24 µg/kg moxidectin, 5 mg/kg pyrantel), or no treatment. Dogs were infested with approximately 50 unfed adult (35 female, 15 male) Ixodes scapularis on Day -2, 21 and 28. Live tick counts were performed at 4, 8, 12 and 24 h post-treatment (Day 0) and post-infestation on Day 21 and 28. Tick control efficacy was determined by comparing live tick means for each product-treated group to the untreated control group and each other at all time points using a linear mixed model. The percent of dogs free of live ticks was analyzed using the Fisher's exact test for treatment group comparison.

RESULTS

The untreated control group maintained adequate tick infestations throughout the study. Using geometric means, an existing I. scapularis infestation was controlled by 99.7% and 93.0% 12 h post-treatment and by 100% and 99.5% 24 h post-treatment, for Bravecto® and Simparica TRIO®-treated dogs, respectively. Ixodes scapularis infestations were controlled more quickly for Bravecto®- compared to Simparica TRIO®-treated dogs on Day 21 at 8 h (efficacy 74.0% vs. 0.0%, p = 0.003) and 12 h (efficacy 99.2% vs. 39.4%, p < 0.001) post-infestation and Day 28 at 8 h (efficacy 92.2% vs. 0.0%, p < 0.001) and 12 h (efficacy 99.6% vs. 27.7%, p < 0.001) post-infestation. On Day 28 post-treatment, the efficacy of Bravecto® and Simparica TRIO® to control a new I. scapularis infestation was 100% and 96.6%, respectively, by 24 h post-infestation. Of product-treated dogs, 100% of Bravecto®-treated dogs were free of live ticks by 24 h post-treatment or post-infestation. No treatment-related adverse reactions occurred during the study.

CONCLUSIONS

Ixodes scapularis infestations are controlled more quickly 21 and 28 days post-treatment for dogs administered a single dose of Bravecto® compared to dogs administered a single dose of Simparica TRIO®.

Tick-Borne Co-Infections: Challenges in Molecular and Serologic Diagnoses

Co-infections are a poorly understood aspect of tick-borne diseases. In the United States alone, nineteen different tick-borne pathogens have been identified. The majority of these agents are transmitted by only two tick species, Ixodes scapularis and Amblyomma americanum. Surveillance studies have demonstrated the presence of multiple pathogens in individual ticks suggesting a risk of polymicrobial transmission to humans. However, relatively few studies have explored this relationship and its impact on human disease. One of the key factors for this deficiency are the intrinsic limitations associated with molecular and serologic assays employed for the diagnosis of tick-borne diseases. Limitations in the sensitivity, specificity and most importantly, the capacity for inclusion of multiple agents within a single assay represent the primary challenges for the accurate detection of polymicrobial tick-borne infections. This review will focus on outlining these limitations and discuss potential solutions for the enhanced diagnosis of tick-borne co-infections.

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

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

Characteristics of Hard Tick Relapsing Fever Caused by Borrelia miyamotoi, United States, 2013-2019

Borrelia miyamotoi, transmitted by Ixodes spp. ticks, was recognized as an agent of hard tick relapsing fever in the United States in 2013. Nine state health departments in the Northeast and Midwest have conducted public health surveillance for this emerging condition by using a shared, working surveillance case definition. During 2013-2019, a total of 300 cases were identified through surveillance; 166 (55%) were classified as confirmed and 134 (45%) as possible. Median age of case-patients was 52 years (range 1-86 years); 52% were male. Most cases (70%) occurred during June-September, with a peak in August. Fever and headache were common symptoms; 28% of case-patients reported recurring fevers, 55% had arthralgia, and 16% had a rash. Thirteen percent of patients were hospitalized, and no deaths were reported. Ongoing surveillance will improve understanding of the incidence and clinical severity of this emerging disease.

Seasonal activity patterns of host-seeking Ixodes scapularis (Acari: Ixodidae) in Minnesota, 2015-2017

As the primary vector of Lyme disease spirochetes and several other medically significant pathogens, Ixodes scapularis presents a threat to public health in the United States. The incidence of Lyme disease is growing rapidly in upper midwestern states, particularly Michigan, Minnesota, and Wisconsin. The probability of a tick bite, acarological risk, is affected by the phenology of host-seeking I. scapularis. Phenology has been well-studied in northeastern states, but not in the Upper Midwest. We conducted biweekly drag sampling across 4 woodland sites in Minnesota between April and November from 2015 to 2017. The majority of ticks collected were I. scapularis (82%). Adults were active throughout our entire 8-month collection season, with sporadic activity during the summer, larger peaks in activity observed in April, and less consistent and lower peaks observed in October. Nymphs were most active from May through August, with continuing low-level activity in October, and peak activity most commonly observed in June. The observed nymphal peak corresponded with the typical peak in reported human Lyme disease and anaplasmosis cases. These findings are consistent with previous studies from the Upper Midwest and highlight a risk of human exposure to I. scapularis at least from April through November. This information may aid in communicating the seasonality of acarological risk for those living in Minnesota and other upper midwestern states as well as being relevant to the assessment of the ecoepidemiology of Lyme disease and the modeling of transmission dynamics.

A serological assay to detect and differentiate rodent exposure to soft tick and hard tick relapsing fever infections in the United States

Human cases of relapsing fever (RF) in North America are caused primarily by Borrelia hermsii and Borrelia turicatae, which are spread by argasid (soft) ticks, and by Borrelia miyamotoi, which is transmitted by ixodid (hard) ticks. In some regions of the United States, the ranges of the hard and soft tick RF species are known to overlap; in many areas, recorded ranges of RF spirochetes overlap with Lyme disease (LD) group Borrelia spirochetes. Identification of RF clusters or cases detected in unusual geographic localities might prompt public health agencies to investigate environmental exposures, enabling prevention of additional cases through locally targeted mitigation. However, exposure risks and mitigation strategies differ among hard and soft tick RF, prompting a need for additional diagnostic strategies that differentiate hard tick from soft tick RF. We evaluated the ability of new and previously described recombinant antigens in serological assays to differentiate among prior exposures in mice to LD, soft or hard tick RF spirochetes. We extracted whole-cell protein lysates from RF Borrelia cultures and synthesized six recombinant RF antigens (Borrelia immunogenic protein A (BipA) derived from four species of RF Borrelia, glycerophosphodiester phosphodiesterase (GlpQ), and Borrelia miyamotoi membrane antigen A (BmaA)) to detect reactivity in laboratory derived (Peromyscus sp. and Mus sp.) mouse serum infected with RF and LD Borrelia species. Among 44 Borrelia exposed mouse samples tested, all five mice exposed to LD spirochetes were correctly differentiated from the 39 mice exposed to RF Borrelia using the recombinant targets. Of the 39 mice exposed to RF spirochetes, 28 were accurately categorized to species of exposure (71%). Segregation among soft tick RF species (Borrelia hermsii, Borrelia parkeri and Borrelia turicatae) was inadequate (58%) owing to observed cross-reactivity among recombinant BipA protein targets. However, among the 28 samples accurately separated to species, all were accurately assigned to soft tick or hard tick RF type. Although not adequately specific to accurately categorize exposure to soft tick RF species, the recombinant BipA protein targets from soft and hard tick RF species show utility in accurately discriminating mouse exposures to LD or RF Borrelia, and accurately segregate hard tick from soft tick RF Borrelia exposure.

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

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

Detection of Ehrlichia muris eauclairensis in Blacklegged Ticks (Ixodes scapularis) and White-Footed Mice (Peromyscus leucopus) in Massachusetts

In 2011, Ehrlichia muris eauclairensis (EME) was described as a human pathogen spread by the blacklegged tick, Ixodes scapularis. Until very recently, its reported distribution was limited to the upper midwestern United States, mainly in Minnesota and Wisconsin. In this study, we report the detection of EME DNA in 4 of 16,146 human biting I. scapularis ticks submitted from Massachusetts to a passive tick surveillance program. Active tick surveillance yielded evidence of EME local transmission in the northeastern United States through detection of EME DNA in 2 of 461 host-seeking I. scapularis nymphs, and in 2 white-footed mice (Peromyscus leucopus) of 491 rodent samples collected in the National Ecological Observatory Network (NEON) Harvard Forest site in Massachusetts.

Borrelia miyamotoi Infection in Immunocompromised Man, California, USA, 2021

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.

A citizen science approach to investigate the distribution, abundance, and pathogen infection of vector ticks through active surveillance

Tick-borne disease poses a growing public health burden in the United States and understanding the patterns of presence and density of infected vector ticks is key to developing and implementing effective public health management strategies. Citizen science has emerged as a highly effective means to generate data sets on the geographical distribution of tick species. But to date, nearly all citizen science studies of ticks are 'passive surveillance' programs in which researchers accept reports of ticks, together with either physical specimens or digital images, found opportunistically on people, pets, and livestock from community members for species identification and in some cases also tick-borne pathogen detection. These studies are limited because data are not collected systematically, making comparisons among locations and over time challenging, and introducing considerable reporting bias. In this study, we engaged citizen scientists in 'active surveillance' of host-seeking ticks, training volunteers to actively collect ticks on their woodland properties in an emergent region of tick-borne disease in the state of Maine, USA. We developed volunteer recruitment strategies, materials to train volunteers in data collection methods, field data collection protocols based on techniques used by professional scientists, and a variety of incentives to promote volunteer retention and satisfaction with their experiences, and we communicated research findings to participants. A total of 125 volunteers in 2020 and 181 volunteers in 2021 collected 7,246 ticks in southern and coastal Maine, including the American dog tick (Dermacentor variabilis, 4,023 specimens), the blacklegged tick (Ixodes scapularis, 3,092 specimens), and the rabbit tick (Haemaphysalis leporispalustris, 102 specimens). We demonstrated the feasibility of citizen scientists collecting ticks using active surveillance methods and found that volunteers were motivated to participate largely by their interest in the scientific problem and a desire to learn about ticks on their properties.

Human Borrelia miyamotoi Infection in North America

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

White-Tailed Deer Serum Kills the Lyme Disease Spirochete, Borrelia burgdorferi

Borrelia burgdorferi is a human pathogen vectored by Ixodes ticks and maintained in nature by a suite of competent vertebrate reservoirs. White-tailed deer (WTD) are considered to be noncompetent reservoirs for B. burgdorferi. Sera from other deer species have been found to be borreliacidal, and similar mechanisms could explain the lack of reservoir competence of WTD. Therefore, we determined whether WTD serum can kill B. burgdorferi. Using an in vitro serum sensitivity assay and subculturing of spirochetes, we demonstrated that WTD serum effectively kills B. burgdorferi. The borreliacidal activity of WTD serum likely contributes to the inability of WTD to efficiently harbor and transmit B. burgdorferi.

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

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

Using next generation sequencing for molecular detection and differentiation of Anaplasma phagocytophilum variants from host seeking Ixodes scapularis ticks in the United States

Anaplasmosis is increasingly common in the United States, with cases being reported over an expanding geographic area. To monitor for changes in risk of human infection, the U.S. Centers for Disease Control and Prevention monitors the distribution and abundance of host-seeking vector ticks (Ixodes scapularis and Ixodes pacificus) and their infection with Anaplasma phagocytophilum. While several variants of A. phagocytophilum circulate in I. scapularis, only the human-active variant (Ap-ha) appears to be pathogenic in humans. Failure to differentiate between human and non-human variants may artificially inflate estimates of the risk of human infection. Efforts to differentiate the Ap-ha variant from the deer variant (Ap-V1) in ticks typically rely on traditional PCR assays coupled with sequencing of PCR products. However, laboratories are increasingly turning to Next Generation Sequencing (NGS) to increase testing efficiency, retain high sensitivity, and increase specificity compared with traditional PCR assays. We describe a new NGS assay with novel targets that accurately segregate the Ap-ha variant from other non-human variants and further identify unique clades within the human and non-human variants. Recognizing that not all investigators have access to NGS technology, we also developed a PCR assay based on one of the novel targets so that variants can be visualized using agarose gel electrophoresis without the need for subsequent sequencing. Such an assay may be used to improve estimates of human risk of developing anaplasmosis in North America.

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

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