Response: The Geographic Distribution of Ixodes scapularis (Acari: Ixodidae) Revisited: The Importance of Assumptions About Error Balance

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.

Evaluation of the association between climate warming and the spread and proliferation of Ixodes scapularis in northern states in the Eastern United States

Ixodes scapularis (the blacklegged tick) is widely distributed in forested areas across the eastern United States. The public health impact of I. scapularis is greatest in the north, where nymphal stage ticks commonly bite humans and serve as primary vectors for multiple human pathogens. There were dramatic increases in the tick's distribution and abundance over the last half-century in the northern part of the eastern US, and climate warming is commonly mentioned as a primary driver for these changes. In this review, we summarize the evidence for the observed spread and proliferation of I. scapularis being driven by climate warming. Although laboratory and small-scale field studies have provided insights into how temperature and humidity impact survival and reproduction of I. scapularis, using these associations to predict broad-scale distribution and abundance patterns is more challenging. Numerous efforts have been undertaken to model the distribution and abundance of I. scapularis at state, regional, and global scales based on climate and landscape variables, but outcomes have been ambiguous. Across the models, the functional relationships between seasonal or annual measures of heat, cold, precipitation, or humidity and tick presence or abundance were inconsistent. The contribution of climate relative to landscape variables was poorly defined. Over the last half-century, climate warming occurred in parallel with spread and population increase of the white-tailed deer, the most important reproductive host for I. scapularis adults, in the northern part of the eastern US. There is strong evidence for white-tailed deer playing a key role to facilitate spread and proliferation of I. scapularis in the US over the last century. However, due to a lack of spatially and temporally congruent data, climate, landscape, and host variables are rarely included in the same models, thus limiting the ability to evaluate their relative contributions or interactions in defining the geographic range and abundance patterns of ticks. We conclude that the role of climate change as a key driver for geographic expansion and population increase of I. scapularis in the northern part of the eastern US over the last half-century remains uncertain.

Changes in the geographic distribution of the blacklegged tick, Ixodes scapularis, in the United States

Ixodes scapularis (the blacklegged tick) was considered a species of no medical concern until the mid-1970s. By that time, the tick's geographic distribution was thought to be mainly in the southeastern United States (US), with additional localized populations along the Eastern Seaboard north to southern Massachusetts and in the Upper Midwest. Since 1975, I. scapularis has been implicated as a vector of seven human pathogens and is now widely distributed across the eastern US up to the border with Canada. Geographic expansion of tick-borne diseases associated with I. scapularis (e.g., Lyme disease, anaplasmosis, and babesiosis) is attributed to an expanding range of the tick. However, due to changes in tick surveillance efforts over time, it is difficult to differentiate between range expansion and increased recognition of already established tick populations. We provide a history of the documented occurrence of I. scapularis in the US from its description in 1821 to present, emphasizing studies that provide evidence of expansion of the geographic distribution of the tick. Deforestation and decimation of the white-tailed deer (Odocoileus virginianus), the primary reproductive host for I. scapularis adults, during the 1800s presumably led to the tick disappearing from large areas of the eastern US where it previously had been established. Subsequent reforestation and deer population recovery, together with recent climate warming, contributed to I. scapularis proliferating in and spreading from refugia where it had persisted into the early 1900s. From documented tick collection records, it appears I. scapularis was present in numerous locations in the southern part of the eastern US in the early 1900s, whereas in the north it likely was limited to a small number of refugia sites during that time period. There is clear evidence for established populations of I. scapularis in coastal New York and Massachusetts by 1950, and in northwestern Wisconsin by the late 1960s. While recognizing that surveillance for I. scapularis increased dramatically from the 1980s onward, we describe multiple instances of clearly documented expansion of the tick's geographic distribution in the Northeast, Upper Midwest, and Ohio Valley regions from the 1980s to present. Spread and local population increase of I. scapularis, together with documentation of Borrelia burgdorferi sensu stricto in host-seeking ticks, was universally followed by increases in Lyme disease cases in these areas. Southward expansion of northern populations of I. scapularis, for which the host questing behavior of the nymphal stage leads to substantially higher risk of human bites compared with southern populations, into Virginia and North Carolina also was followed by rising numbers of Lyme disease cases. Ongoing surveillance of ticks and tick-borne pathogens is essential to provide the data needed for studies that seek to evaluate the relative roles of land cover, tick hosts, and climate in explaining and predicting geographic expansion of ticks and tick-borne diseases.

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.

Socio-ecological determinants of rickettsial seroprevalence in a rural community of Yucatán, Mexico

Rickettsial diseases have seen a re-emergence in the Americas in the last few decades, with concerning morbidity, mortality and economic implications that result from loss of productivity, income, curbs in liberal trade agreements, and reduction in agricultural practices. The aim of this study is to determine the socioecological determinants and seroprevalence for Rickettsia typhi and Rickettsia rickettsii among residents of Teabo, a rural community of Yucatán, Mexico. Sociodemographic data and serum samples were obtained from 180 consenting participants. Antibody titers for R. typhi and R. rickettsii were determined by indirect immunofluorescence assay (IFA). Participants also submitted tick samples collected from their residential area. We conducted logistic regression models to evaluate the association between exposure variables and seroprevalence. Rhipicephalus sanguineus s.l. (37%; n = 65), and Amblyomma cajennense Fabricius (17%; n = 29) were the predominant tick species in peri-domestic areas. Out of the 180 participants, there was significantly higher seroprevalence of R. typhi (n = 77; 46%) compared to R. rickettsii [n = 27, 15%, (p < 0.05)]. Pearson's chi-square test of independence revealed significant differences in R. rickettsii seroprevalence by gender (X² [n = 175, df = 4, (p < 0.001)] = 180.26), level of education, (X² [n = 180, df = 4, (p < 0.001)] = 44.0), and by tick species found in residential area, (X² [n = 180, df = 4, (p = 0.050)] = 9.48). After adjusting for other variables in a logistic regression model, for each unit increase in the number of dogs present in the residential area, there was a 27% increase in the odds of human seroprevalence for R. typhi IgG (AOR = 1.27, 95% CI: 1.01-1.63). Compared to study participants living in residential areas with a 'low' height of vegetation, those living in residential areas with a 'medium' height of vegetation had 2.5 times greater odds of human seroprevalence for R. typhi IgG (AOR = 2.51, 95% CI: 1.19-5.40). Potentially modifiable existing factors in the peri-domestic area may constitute a high-risk source of seroprevalence for rickettsial antibodies among residents of the rural community of Teabo, Yucatán, Mexico.

Tick and Tickborne Pathogen Surveillance as a Public Health Tool in the United States

In recent decades, tickborne disease (TBD) cases and established populations of medically important ticks have been reported over expanding geographic areas, and an increasing number of tickborne bacteria, viruses, and protozoans have been recognized as human pathogens, collectively contributing to an increasing burden of TBDs in the United States. The prevention and diagnosis of TBDs depend greatly on an accurate understanding by the public and healthcare providers of when and where persons are at risk for exposure to human-biting ticks and to the pathogens these ticks transmit. However, national maps showing the distributions of medically important ticks and the presence or prevalence of tickborne pathogens are often incomplete, outdated, or lacking entirely. Similar deficiencies exist regarding geographic variability in host-seeking tick abundance. Efforts to accurately depict acarological risk are hampered by lack of systematic and routine surveillance for medically important ticks and their associated human pathogens. In this review, we: 1) outline the public health importance of tick surveillance; 2) identify gaps in knowledge regarding the distributions and abundance of medically important ticks in the United States and the presence and prevalence of their associated pathogens; 3) describe key objectives for tick surveillance and review methods appropriate for addressing those goals; and 4) assess current capacity and barriers to implementation and sustainability of tick surveillance programs.

Modeling future climate suitability for the western blacklegged tick, Ixodes pacificus, in California with an emphasis on land access and ownership

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 21^st 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.

Validating Species Distribution Models With Standardized Surveys for Ixodid Ticks in Mainland Florida

Tick-borne pathogens are of growing concern. The U.S. Centers for Disease Control and Prevention (CDC) developed guidelines standardizing surveys of tick vectors to better monitor the changes in their occurrences. Unbiased surveillance data, from standardized surveys, are presumed critical to generate valid species distribution models (SDMs). We tested previously generated SDMs from standardized protocols for three medically important ticks [Amblyomma americanum (Linnaeus, Ixodida, Ixodidae), Ixodes scapularis (Say, Ixodida, Ixodidae), and Dermacentor variabilis (Say, Ixodida, Ixodidae)]. These previous models ruled out a quarter to half of the state as having these species, with consensus occurrence in about a quarter of the state. New surveys performed throughout 2019 on 250 transects at 43 sites indicated the rule-out functions were 100% accurate for I. scapularis and D. variabilis and 91.9% for A. americanum. As SDM concordance increased, the proportion of transects yielding ticks increased. Independent surveys of SDMs provide external validation-an aspect missing from many SDM studies.

Predicting the current and future distribution of the western black-legged tick, Ixodes pacificus, across the Western US using citizen science collections

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

Modeling Climate Suitability of the Western Blacklegged Tick in California

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.

The Blacklegged Tick, Ixodes scapularis: An Increasing Public Health Concern

In the United States, the blacklegged tick, Ixodes scapularis, is a vector of seven human pathogens, including those causing Lyme disease, anaplasmosis, babesiosis, Borrelia miyamotoi disease, Powassan virus disease, and ehrlichiosis associated with Ehrlichia muris eauclarensis. In addition to an accelerated rate of discovery of I. scapularis-borne pathogens over the past two decades, the geographic range of the tick, and incidence and range of I. scapularis-borne disease cases, have increased. Despite knowledge of when and where humans are most at risk of exposure to infected ticks, control of I. scapularis-borne diseases remains a challenge. Human vaccines are not available, and we lack solid evidence for other prevention and control methods to reduce human disease. The way forward is discussed.