Tick-borne illnesses are a persistent and increasing public health concern, accounting for the vast majority of reported vector-borne infections in the United States (U.S.). Lyme disease, caused primarily by Borrelia burgdorferi sensu stricto and spread by the blacklegged tick, Ixodes scapularis, is the most commonly reported tick-borne disease (Eisen et al., 2017; Rosenberg et al., 2018). Ixodes scapularis serves as a vector of several additional human pathogens including Borrelia mayonii (a less common Lyme disease agent in the Upper Midwest), Anaplasma phagocytophilum (anaplasmosis), Babesia microti (babesiosis), Borrelia miyamotoi (hard tick relapsing fever), Ehrlichia muris eauclairensis (ehrlichiosis), and Powassan virus disease (Powassan virus lineage II or deer tick virus that causes a neuroinvasive viral disease) (Eisen and Eisen, 2018). Over the past two decades, the geographic range of I. scapularis and associated human pathogens has expanded considerably, contributing to an increase in reported cases of tick-borne diseases, particularly in the Northeast, Ohio Valley, and Upper Midwest regions (Brinkerhoff et al., 2014; Kugeler et al., 2015; Lantos et al., 2017; Fleshman et al., 2022; Eisen and Eisen, 2023).

Effective prevention and diagnosis of tick-borne diseases hinge upon awareness by the public and healthcare professionals regarding when and where individuals are at risk for exposure to human-biting ticks and the pathogens they carry. Traditional public health surveillance that tracks the occurrence of notifiable tick-borne disease cases provides critical information to assess trends in where and when cases are reported but is currently limited in its ability to differentiate local exposures from travel-associated exposures to infected ticks. This is because human cases are reported to the patient home of residence, which may or may not be the site of exposure to infected ticks. Tick surveillance aims to supplement tick-borne disease surveillance by providing county estimates of the presence and abundance of medically important tick species, and presence and prevalence of associated human pathogens in human biting ticks (Eisen and Paddock, 2021). Previously, we updated maps showing the county level distributions of I. scapularis (Eisen et al., 2016; CDC, 2023), the pathogens they transmit (Fleshman et al., 2021, 2022) and the prevalence of pathogens in host-seeking nymphs and adults (Lehane et al., 2021; Foster et al., 2023).

Mapping the presence of medically important ticks and their associated human pathogens serves as a crucial initial step in defining the risk of acquiring tick-borne diseases. Additionally, gaining insight into regional variation in the density of host-seeking ticks provides useful information on the likelihood of human encounters with ticks across regions of the U.S. where ticks are established (Campbell et al., 1998; Falco and Fish, 1998; Pepin et al., 2012). Previous work demonstrated that I. scapularis nymphs are the most epidemiologically significant life stage in the transmission of B. burgdorferi. The timing of most human infections coincides with when nymphal ticks are host-seeking; nymphs are smaller than adults and more difficult to detect, potentially leading to a longer duration of feeding that is more likely to result in infection (Eisen, 2018; Mead, 2022). The host-seeking behavior of I. scapularis nymphs differs across the tick’s range in the eastern U.S., resulting in a more limited geographic range over which humans are likely to encounter questing nymphal ticks (Diuk-Wasser et al., 2010; Arsnoe et al., 2019; CDC, 2023). An earlier systematic effort to map the distribution and abundance of host-seeking I. scapularis nymphs was undertaken nearly two decades ago (Diuk-Wasser et al., 2006, 2010). Recent dramatic changes in the tick’s reported geographic range necessitated an update (Eisen et al., 2016; Eisen and Eisen, 2023).

Here, we assessed geographic and temporal trends in the collection of I. scapularis nymphal density data submitted to the U.S. Centers for Disease Control and Prevention (CDC) ArboNET Tick Module. The module was created in 2018 to house data collected as part of national tick surveillance efforts that were initiated that year (Eisen and Paddock, 2021; CDC, 2018), but historical records were also included in the database. Based on ArboNET Tick Module submissions, we report county, state, and regional variation in the density of host-seeking I. scapularis nymphs in the eastern U.S. and compare regional density estimates during two surveillance time periods: 2004 through 2017 (prior to initiation of the national tick surveillance program) and 2018 through 2022 (records collected as part of national tick surveillance efforts).

Our review of ArboNET Tick Module submissions covering tick collections from 2004 through 2022 revealed extensive variation in estimates of the density of host-seeking I. scapularis nymphs at collection site, county, state, and regional spatial scales. The high degree of variation reported at larger spatial scales was expected and is consistent with previous studies that reported substantial differences in nymphal densities among sampling sites even across very small distances, within a single day of sampling, or across seasons (Pardanani and Mather, 2004; Jordan et al., 2022; Schulze and Jordan, 2022). Variation in DON is attributable to the tick’s response to weather conditions, host availability, seasonality of host-seeking, site characteristics, and sampling effort (Schulze et al., 1997; Schulze and Jordan, 2003; Allan et al., 2003; Rand et al., 2003; Ostfeld et al., 2006; Dobson, 2013, 2014; Kilpatrick et al., 2014; Burtis et al., 2016, 2019, 2023; Borgmann-Winter and Allen, 2020; Ginsberg et al., 2020). Although variation in reported DON was considerable among counties, we show that average DON estimates are greatest in the Northeast, Upper Midwest, and northernmost states in the Southeast (Virginia and North Carolina) regions; densities were intermediate in the Ohio Valley and very low in the South and Northern Rockies and Plains regions. Despite the overall higher densities observed in northern states of the eastern U.S., DON estimates ranged from zero nymphs to as many as 488.5 nymphs per 1000 m2 among collection sites. The public health community and the public should be aware that the likelihood of encountering I. scapularis nymphs is spatially and temporally dynamic, but risk of encounters can be significant in these areas.

Though I. scapularis has been recorded as present in numerous counties in the South and southern states within the Southeast regions (Eisen et al., 2016; Figs. 2 and 3), in counties where drag sampling was conducted and data were submitted to ArboNET, no nymphs or very few were collected. As a result, risk of human encounters with I. scapularis nymphs in these areas is expected to be very low. In previously published maps, several counties lacked records of the tick’s presence in the Northern Rockies and Plains region (Eisen et al., 2016; CDC, 2023). However, maps of tick presence do not typically account for sampling effort, making it difficult to determine if the tick is truly absent or if surveillance was not conducted. Recent drag sampling efforts reported here suggest that encountering nymphal ticks is rare, even in the eastern counties of the Northern Rockies and Plains region where the tick has been reported previously. Despite sampling at least 750 m2 at 94 sites from 29 counties in Nebraska, North Dakota and South Dakota, no nymphs were collected from 2004 through 2022. However, it is important to highlight that data inclusion criteria for this study required drag or flag sampling of at least 750 m2 per collection site within fixed time periods and therefore, observations were excluded (Table 1). In some instances, host-seeking nymphs were collected by drag sampling within smaller sampling grids or using timed collections. Such observations are depicted in the tick presence maps presented here, but densities were not estimated. Nonetheless, these efforts demonstrate that questing nymphs are present and sometimes abundant within limited forested locations in the eastern reaches of the Northern Rockies and Plain region (Russart et al., 2014; Nielsen et al., 2020; Black et al., 2021). For these reasons, it was not possible within the scope of the limited dataset presented here to compare absence and presence of I. scapularis over time.

A recent review described changes in the tick’s distribution with significant recolonization of counties in the Upper Midwest, Ohio Valley, Northeast, and northern states in the Southeast region over the past half century (Eisen and Eisen, 2023). Numerous small-scale studies have noted trends of increasing DON immediately after colonization, then stabilization in long-term density estimates (Hamer et al., 2010; Serra et al., 2013; Brinkerhoff et al., 2014; Tran et al., 2021; Christie et al., 2022). As a result of these tick colonization and establishment processes, it was not surprising to see differences in the proportions of counties classified as moderate or high between time periods. The proportion of counties classified as moderate or high DON was greater in the Northeast, Ohio Valley, and Southeast regions from 2018 through 2022 compared with 2004 through 2017; DON estimates remained similarly low between these time periods in the South and the Northern Rockies and Plains regions. The timeframes examined in this study are somewhat arbitrary relative to range expansion of the tick. Instead, they were derived based on the timing of establishment of the national tick surveillance program and the ArboNET Tick Module database. In addition, the sampling strategy was not optimized to detect changes in DON at small spatial scales. For the most part, the same sites were not sampled repeatedly, and county estimates were often derived from different collection sites over separate years. These factors, along with describing trends at regional scales, may have obscured real changes in nymphal densities over time that were reported in previously published studies. For example, the number of counties classified in the moderate or high DON category was similar between time periods in the Upper Midwest. However, previous studies showed significant expansion of the tick’s range and local increases in DON in newly colonized areas mostly prior to 2018 in Iowa (Oliver et al., 2017), Minnesota (Diuk-Wasser et al., 2010; Johnson et al., 2018), Wisconsin (Lee et al., 2013), Indiana (Pinger et al., 1996), Michigan (Hamer et al., 2010; Lantos et al., 2017), and Ohio (Wang et al., 2014). Owing to the lack of consistent sampling effort over time and space, we are limited in our ability to use ArboNET Tick Module data to track changes in DON at fine spatial and temporal scales. However, we were able to detect significant differences in DON categories at coarse (regional) spatial scales and between broad temporal periods, but differences in sampling effort (represented in part by the GR scores) should be taken into consideration when viewing these records.

Although the density of host-seeking I. scapularis is generally considered a critical component in estimating acarological risk for exposure to Lyme disease spirochetes and other I. scapularis-borne pathogens (Mather et al., 1996; Falco and Fish, 1998; Campbell et al., 1998; Diuk-Wasser et al., 2010, 2012; Pepin et al., 2012), it is a notoriously difficult value to measure precisely. Previous studies have highlighted significant variability in estimates at the community level (Dobson 2013; Pardanani and Mather, 2004; Hahn et al., 2018; Schulze and Jordan, 2022). The precision of DON estimates are greatly improved by repeated drag sampling of the same sites on multiple occasion throughout the year (Diuk-Wasser et al., 2006; Dobson, 2013). Understanding spatial trends in DON is improved by careful selection of sampling sites (Diuk-Wasser et al., 2006, 2010; Johnson et al., 2018). Although such stringent criteria were met in several research studies included in historical ArboNET data (e.g.; Diuk-Wasser et al., 2010; Johnson et al., 2018), adherence to such costly and labor-intensive implementation schemes are rarely achieved in public health surveillance (Mader et al., 2021). We found that efforts to quantify DON were unevenly distributed among geographic regions with the greatest proportion of counties sampled being in the Northeast and Upper Midwest. The majority of county DON estimates from 2004 through 2022 were based on sampling a single site within the county only a single year across the observation period. The median number of visits per site per year to estimate DON declined from 5 visits per year from 2004 through 2017 to only 2 visits per year from 2018 through 2022. However, despite declines in the intensity of sampling per collection location, the proportion of counties sampled within regions vastly increased from after 2018 in the Northeast, Southeast, Northern Rockies and Plains, and Ohio Valley. We did not observe large changes in effort between time periods in the remaining regions.

Given the limitations of sampling conducted as part of national tick surveillance efforts, we cannot estimate DON with precision at fine spatial and temporal scales. Comparisons between time periods can be difficult based on inconsistent sampling and high variation in DON estimates. Despite these limitations, ArboNET Tick Module data provide useful information for tracking changes in acarological risk categories across broad geographic scales and long periods of time. Such efforts aid in monitoring expansion in the distribution of host-seeking nymphs, particularly into the Southeast where human encounters with I. scapularis nymphs and Lyme disease cases have each steadily increased over the past two decades (Brinkerhoff et al., 2014; Lantos et al., 2015, 2021).