Longer study length, standardized sampling techniques, and broader geographic scope leads to higher likelihood of detecting stable abundance patterns in long term black-legged tick studies

Introduction

Understanding how study design and monitoring strategies shape inference within, and synthesis across, studies is critical across biological sciences. This is especially important as climatic conditions and ecosystems change in stark and unpredictable ways, requiring scientists to tease apart drivers of population and community trends from the past to understand possible futures (Bahlai et al., 2021). Many biological and field studies are short term and limited in scope, leading to concerns about how to interpret the many short-term studies in comparison with the relatively fewer long term ecological studies (Wauchope et al., 2019; Bahlai et al., 2021). Shorter term studies increase the likelihood of finding misleading trends, potentially leading to misinformed management approaches (White & Bahlai, 2021). Natural variability in a stable population might lead a 2–3 year study to appear to capture a strong upward trend in abundance for a given taxon, simply as a result of natural population cycling (Fournier, White & Heard, 2019).

Monitoring studies are critical for informing public health about potential vectors of concern. Surveys indicate that the control and management of vector borne diseases is of immediate concern for public health in the United States (Hill, Whitford & MacDonald, 2009). Public health officials must know the geographical range of infectious vectors and their level of abundance to understand potential disease risk (Townson et al., 2005). However, standardizing results from multiple studies on vectors can be problematic. Novel sampling approaches and methodologies are required to effectively monitor variable vector populations (Sougoufara, Ottih & Tripet, 2020). Reliable inference about population trends from single studies, or synthesizing across them, must account for variation in monitoring methods.

An important vector of concern for public health in the US is ticks. They are common vectors that can transmit multiple diseases to humans and are responsible for vectoring the greatest number of human diseases in the US (Eisen et al., 2017). Increased outdoor activity, especially during the ongoing COVID-19 pandemic, may be exposing more people to potential tick-borne illnesses (United States Department of Health and Human Services, 2020). This pattern matches broader tick trends. Globally, both the known range of tick species and the reported cases of tick-vectored illnesses are increasing (Paddock et al., 2016).

Ticks and tick-vectored diseases are commonly monitored using dragging, flagging, and carbon dioxide baited traps (Falco & Fish, 1992). Dragging is a method where a person pulls a horizontal pole behind them across the ground and ticks latch onto the cloth attached to the pole (Salomon, Hamer & Swei, 2020). Another common tick survey method is passive surveillance, where individuals send in ticks they have found on themselves to public health departments, but do not deliberately search for ticks (Ripoche et al., 2018). While these are common, there is a wide variety of methods reported across studies. For example, one method of surveying ticks has been collecting them from hunter-killed deer in the midwestern US (Egizi et al., 2018; Lee et al., 2013). Additionally, some studies measure density instead of abundance (Kahl et al., 2002), use distance-based sampling (Gray et al., 1992), or are modeled projections of where ticks are likely to be (Baldwin et al., 2022), based on varying assumptions about climate and landscape factors (Estrada-Peña et al., 2013). The lack of a shared and standardized sampling method may be preventing effective understanding of changes in abundance and distribution of ticks synthesized across surveys (Dennis et al., 1998; Eisen, Eisen & Beard, 2016). Recent research has identified the lack of standardized data collection, and subsequent data sharing, as impeding researchers’ understanding of tick-transmitted pathogens (Estrada-Peña et al., 2021).

This is critical, as Lyme disease is a common and increasing illness impacting US residents. Recent research estimates that more than 400,000 people annually have been diagnosed in the US between 2010 and 2018 (Kugeler et al., 2021). The primary vectors of Borrelia burgdorferi, bacteria that cause Lyme disease, are black-legged ticks (Des Vignes & Fish, 1997). Black-legged ticks are common throughout the US - and both widespread and abundant. Established black-legged tick populations are most concentrated in upper north-central, northeastern, and west-coast states (Eisen, Eisen & Beard, 2016). Black-legged ticks transmit seven human pathogens of concern, including B. burgdorferi, and both the tick and pathogen transmission have increased over the past two decades in the US (Eisen & Eisen, 2018). In addition, black-legged ticks are spreading rapidly, found in more than twice the number of counties in the United States compared to twenty years ago (Eisen, Eisen & Beard, 2016). Systematic surveillance of ticks and tick-borne pathogens is important for understanding Lyme disease risk (Eisen & Paddock, 2021). Variation in black-legged tick monitoring, and gaps in data, are currently considered major barriers to predicting Lyme disease risk (Kugeler & Eisen, 2020). Understanding tick prevalence and potential Lyme disease risk can help shift the burden of tick vectored disease management from individual prevention to states or other public health agencies (Eisen, 2020).

To understand how variable methodology in black-legged tick studies may influence which population patterns researchers find, we conducted a data synthesis experiment. To do this, we used a novel nonrandom sampling approach (Bahlai et al., 2021) to analyze publicly available black-legged tick datasets from long term monitoring studies to understand the impacts of study parameters on inferences about black-legged tick abundance and trajectory. Specifically, we examined study length, sampling technique, life stage, geographic scope, and sampling metrics. We anticipated that studies using dragging would have less variable findings than other sampling techniques, due to existing research showing that dragging is more consistent for some life stages of black-legged tick collection (Falco & Fish, 1992). We also anticipated that the tick life stage monitored could influence expected trends. Nymphs are most likely to spread Borrelia burgdorferi (Rulison et al., 2013), and may have different abundance patterns than adults or larvae. Results from this analysis can help researchers contextualize existing monitoring data and the potential opportunities and challenges when extrapolating from the past to the future. This data experiment may also help researchers better understand how to design black-legged tick monitoring studies to avoid misleading inferences, potentially leading to improved surveillance of a public health risk.

Materials & Methods

Results

Study length

Using the ‘broken window’ algorithm, we found that across all datasets, years to stability time ranged from 4 to 23 years (Fig. 1). The proportion of datasets that reached stability by 4 years is very low; by 10 years, the majority of datasets had reached stability (Fig. 1). We found that study length was positively correlated with years to stability (Pearson product moment correlation test, r² = 0.9, p-value <0.001) (Fig. 2). From our analysis, we also found that the proportion of windows significantly wrong before datasets reached stability ranged from 0 to 100% (Fig. 3). We also found that the association between proportion significantly wrong before stability time and overall proportion wrong were not strongly correlated (t = −1.2, df = 571.46, p-value = 0.23, Fig. 3).

Sampling technique

We found that datasets with dragging techniques reached stability time faster than datasets collected using opportunistic, passive sampling methods (t = −8.53, df = 236.23, p-value <0.001) (Fig. 4A). The median number of years to reach stability was 7 for dragging techniques and 12 for opportunistic techniques. However, we did not find a significant difference when we compared the proportion significantly wrong before stability by sampling technique (t = 0.08, df = 155.58, p-value = 0.93, Fig. 4B). Datasets with dragging sampling techniques in our data experiment also exhibited a smaller range of years to stability, compared to datasets with opportunistic passive sampling methods.

Life stage

Datasets measuring black-legged tick larvae reached stability slower than datasets measuring adult or nymph abundance data (adults and larvae, t = −5.96, df = 10.11, p-value <0.001; nymphs and larvae, t = −5.56, df = 10.33, p-value <0.001, Fig. 5A). We found no difference in the time to stability between datasets measuring adults or nymph blacklegged ticks (t = −0.63, df = 128.99, p-value = 0.53). The median time for datasets measuring larvae to reach stability was 11.5 years, compared to 7 years for adults and nymphs.

The mean proportion wrong before stability for adults was 0.35, compared to nymphs (0.16) and larvae (0.28). Datasets measuring nymph tick life stages had a significantly lower proportion of windows wrong before stability time compared to adults (t = 2.99, df = 112.55, p-value = −0.003, Fig. 5B). We found no difference in the proportion wrong before stability time between larvae and nymphs (t = −0.78, df = 7.85, p-value = 0.46) and larvae and adults (t = 0.44, df = 8.77, p-value = 0.67).

Geographic scope

Blacklegged tick datasets from studies reporting data from counties reached stability in fewer years than datasets reported at smaller spatial scales (town, state forest, or grid) (county and grid, t = −17.21, df = 33.87, p-value <0.001; county and state forest, t = −5.55, df = 5.22, p-value = 0.002; county and town, t = −17.03, df = 243.09, p-value <0.001; Fig. 6A). The median number years to stability for datasets of county scope was 7 years, compared to 11.5 years for grid and 12 years for town and state forest. We found no difference in years to stability comparing across datasets reported from towns, state forests, or grids (town and state forest, t = 0.47, df = 6.12, p-value = 0.65; town and grid, t = 0.22, df = 86.18, p-value = 0.82; state forest and grid, t = −0.38, df = 6.03, p-value = 0.72; Fig. 6A).

Datasets from studies reporting data from state forests had a significantly lower proportion of wrong windows, as measured by the proportion wrong before stability time, compared to datasets reported from counties and towns (state forest and county, t = 5.24, df = 74.29, p-value <0.001; state forest and town, t = 7.42, df = 46.44, p-value = p-value <0.001; Fig. 6B). Studies reported from grids did not have a higher proportion of windows wrong before stability time (t = −1.91, df = 25.10, p-value = 0.068). The mean proportion significantly wrong for data at the scale of state forest is 0.077, compared to 0.35 of county datasets, 0.31 of town datasets, and 0.23 of grid datasets. The proportion of wrong windows did not significantly vary between county, town, and grid studies (county and town, t = 0.72, df = 115.03, p-value = 0.48; county and grid, t = 1.33, df = 43.07, p-value = 0.19; town and grid, t = 1.00, df = 28.52, p-value = 0.32; Fig. 6B).

Sampling metric

We found no difference in the number of years until a dataset reached stability time comparing datasets measuring the abundance of black-legged ticks and datasets that tested for B. burgdorferi infection (t = −1.29, df = 283.9, p-value = 0.20; Fig. 7A). The datasets also did not significantly vary in the proportion of wrong windows, as measured by the proportion wrong before stability time (t = −1.18, df = 232.98, p-value = 0.24; Fig. 7B).

Discussion

Overall, we found several aspects of how blacklegged ticks are monitored and data reported may have an impact on the likelihood of a study reflecting longer-term population trends. This is supported by our findings about datasets reaching stability time, as well the proportion of significantly wrong windows, varying by study methods. Specifically, datasets using dragging, counting nymph or adult life stages, and reported by county, reached stability faster than other datasets. These findings support our hypothesis that the dragging method would be associated with more consistent patterns than other sampling methods. We also predicted that datasets collected on different blacklegged tick life stages may vary in stability time; however, we did not anticipate that larvae would reach stability time slower than other life stages. Our results also indicate the importance of long term monitoring, as we found that the longer datasets were correlated with more years to reach stability and many datasets with higher proportions of misleading windows before stability time. As none of the datasets assessed reached stability time before 4 years, our analyses indicate that short term studies may be insufficient to explain long term dynamics in blacklegged ticks.

Our findings are consistent with previous research supporting the importance of long term ecological monitoring for understanding populations in field biology. Several other studies provide support for using long term studies to understand patterns (White, 2019; Cusser et al., 2020; Cusser et al., 2021). For example, White found that at least 72% of the vertebrate populations they studied required at least 10 years of study to detect a significant trend in abundance, while Cusser et al. (2020) determined that it would take at least 16–19 years to detect a significant effect from no-till management on crop yield and soil moisture, emphasizing the importance of long term studies. Similarly, we predicted and found that the dragging sampling technique would produce more consistent abundance trends than datasets associated with opportunistic surveying. This was expected given research indicating that dragging can be a consistent way to monitor black-legged ticks (Falco & Fish, 1992). However, other research has found that data produced using different tick sampling methods is similar (Rulison et al., 2013). Yet even more recent research attempting to validate dragging with other tick species in Sweden concluded that dragging was not recommended for tick adults or larvae, due to low repeatability and agreement (Kjellander et al., 2021). This may be due to the lack of standardization within dragging cloths or other within-method variability between surveys (Newman et al., 2019).

We anticipated, and found, that datasets studying different life stages of ticks would vary in how many years it took to reach stability time. Specifically, time to stability was longer for datasets monitoring larvae, the youngest life stage, compared to adults and nymphs. Additionally, datasets monitoring adult black-legged ticks had higher proportions of significantly wrong windows. This finding is critical for researchers trying to glean insights into broad scale black-legged tick trends across disparate study methods. This suggests that datasets that focus on adults will be more likely to lead to misleading results if the study has not reached stability, which supports longer-term studies. This finding is critical for understanding how to apply black-legged tick monitoring data to potential Lyme disease risk, as different life stages may pose different levels of exposure risk to humans. Nymph, not adult, abundance has been shown to be the primary factor in determining risk of exposure (Falco et al., 1999). Due to this potential variation in transmission by life stage, the recent US Centers for Disease Control and Prevention (CDC) report on black-legged tick surveillance provides distinct guidance on collecting and interpreting data from each life stage (Centers for Disease Control and Prevention , 2021). Our results support this distinction. However, life stage is distinct from other categorizations of tick life history thought to impact abundance and disease transmission, such as ‘physiological age,’ a measure that groups ticks based on environmental stress events (Pool et al., 2017). There may be important variation among ticks of the same life stage, based on their physiological age, that surveyors are not assessing in the field, as this may require the addition of molecular biology approaches (Estrada-Peña et al., 2021). And even though we found no difference in our results when comparing the number of years to reach stability and proportion wrong before stability in datasets that measured black-legged tick abundance and the percent with Borrelia burgdorferi, this may be due to additional variation within studies measuring pathogen prevalence within ticks. Some studies test pools of ticks for infection to reduce cost and labor, and variation in the number of ticks processed can impact the perceived estimation of prevalence (Estrada-Peña et al., 2021).

Our results indicate that sampling a larger geographic area leads to more consistent patterns in black-legged tick population dynamics than studies conducted on smaller areas. This is supported by previous work finding high levels of fine-scale spatial distribution of black-legged ticks (Dumas et al., 2022; Mathews-Martin et al., 2020). Fortunately, county-level information is a common geographic scale for documenting the presence and abundance of blacklegged ticks and Lyme disease (e.g., Fleshman et al., 2021). Given that ecological monitoring requires tradeoffs in effort, cost, and insight (Bennett et al., 2018), this finding may indicate that studies at larger spatial scales may be able to give indication of population trajectories faster than studies at smaller scales.

Our findings also support the collection of long term tick datasets. Even with the improved insights from longer monitoring studies, managers and policymakers will still need to make decisions without complete information. Determining what is ’enough’ data for evidence-based decision making is a challenge (Canessa et al., 2015). As the lack of shared, standardized sampling methods may hamper the ability to monitor changes in abundance and distribution of ticks (Dennis et al., 1998; Eisen, Eisen & Beard, 2016), understanding patterns and differences between datasets may help. Our results can help inform if and how black-legged tick monitoring data can be reasonably interpreted based on study parameters to make informed, rigorous claims to support management decisions that can help reduce Lyme disease exposure and risk. Moreover, we further demonstrate the value of resampling existing data to give insights into analyzing existing data and designing future monitoring programs (White & Bahlai, 2021).

Understanding population trajectories is also critical for using ecological data for ecological forecasting. Ecological forecasting is considered the next critical frontier in broad scale environmental science (Dietze et al., 2018). Ecological forecasting initiatives are increasing, with an NSF-funded Research Coordination Network (NSF, 2019), conferences (EFI , 2019; Ecological Forecasting Initiative, EFI & National Ecological Observation Network, , 2020a), and ongoing ecological forecast challenges (Ecological Forecasting Initiative, EFI, Research Coordination Network, RCN & National Ecological Observation Network, NEON , 2020). With study organisms such as black-legged ticks, forecasting could be used to proactively protect human health. As standardized data has been identified as critical for understanding tick-transmitted pathogens (Estrada-Peña et al., 2021), future work should examine how these study parameters, identified as critical through retrospective data resampling, shape forecasted patterns and simulations. This is especially important for forecasting disease risk, as data gaps and variation in monitoring data are already considered major challenges (Kugeler & Eisen, 2020). Our results highlight the need to better understand and explicitly incorporate that variability into the interpretation of black-legged tick data.

Conclusions

We used 289 datasets from seven long term black-legged tick studies to conduct a moving windows data resampling experiment, revealing that sampling design does shape inferences in black-legged tick population trajectories and the number of years to reveal stable patterns. Specifically, our results show the importance of study length, sampling technique, life stage, and geographic scope in understanding black-legged tick populations, in the absence of standardized surveillance methods. Current public health efforts based on existing black-legged tick datasets must take monitoring study parameters into account, to better understand if and how to use monitoring data to inform decisioning. We also advocate that potential future forecasting initiatives consider these parameters when projecting future black-legged tick population trends. Understanding how study design and monitoring shapes synthesis is critical across many biological disciplines.

Supplemental Information