Deer management generally reduces densities of nymphal Ixodes scapularis, but not prevalence of infection with Borrelia burgdorferi sensu stricto

Abstract

Human Lyme disease-primarily caused by the bacterium Borrelia burgdorferi sensu stricto (s.s.) in North America-is the most common vector-borne disease in the United States. Research on risk mitigation strategies during the last three decades has emphasized methods to reduce densities of the primary vector in eastern North America, the blacklegged tick (Ixodes scapularis). Controlling white-tailed deer populations has been considered a potential method for reducing tick densities, as white-tailed deer are important hosts for blacklegged tick reproduction. However, the feasibility and efficacy of white-tailed deer management to impact acarological risk of encountering infected ticks (namely, density of host-seeking infected nymphs; DIN) is unclear. We investigated the effect of white-tailed deer density and management on the density of host-seeking nymphs and B. burgdorferi s.s. infection prevalence using surveillance data from eight national parks and park regions in the eastern United States from 2014-2022. We found that deer density was significantly positively correlated with the density of nymphs (nymph density increased by 49% with a 1 standard deviation increase in deer density) but was not strongly correlated with the prevalence of B. burgdorferi s.s. infection in nymphal ticks. Further, while white-tailed deer reduction efforts were followed by a decrease in the density of I. scapularis nymphs in parks, deer removal had variable effects on B. burgdorferi s.s. infection prevalence, with some parks experiencing slight declines and others slight increases in prevalence. Our findings suggest that managing white-tailed deer densities alone may not be effective in reducing DIN in all situations but may be a useful tool when implemented in integrated management regimes.

Keywords: Borrelia burgdorferi sensu stricto; Ixodes scapularis; Lyme disease; blacklegged tick; density of infected nymphs; white-tailed deer density.

Fig. 1.

Eastern United States national park locations where annual tick surveillance occurred between 2014–2022. (A) Eight national parks were surveyed routinely across four states (VA, MD, PA, and NY) and one territory (District of Columbia). (B) Seven of the parks are located inland in the mid- and south- Atlantic regions, including Gettysburg National Military Park (GETT), Catoctin Mountain Park (CATO), Monocacy National Battlefield (MONO), Chesapeake and Ohio Canal National Historic Park (CHOH), Rock Creek National Park (ROCR), Manassas National Battlefield (MANA), and Prince William Forest Park (PRWI). (C, inset) Fire Island National Seashore (FIIS) is a barrier island off New York state. National Park Service boundaries are shown in green and transect locations are shown by the black points. (Basemap source: ESRI, 2022)

Fig. 2.

Densities of Ixodes scapularis nymphs (per 750 m²), Borrelia burgdorferi sensu stricto infection prevalence, and white-tailed deer densities (per km²) at eight United States national parks from 2012–2022. (A) Tick density data represent the average of the maximum densities detected at each transect within a given park or park region (transect number surveyed ranged from 1 to 9) per year and were available from 2014–2022. Mean tick densities are plotted as points (see Supplemental Material X for mean log[tick density] and S.D.). (B) Average B. burgdorferi s.s. infection prevalence among I. scapularis nymphs per park from 2014–2022. (C) White-tailed deer density estimates (±S.E.) were provided by the National Capital Region Wildlife Resources Program for each national park from 2012–2020. Horizontal gray lines are placed at deer density values of 5 and 20 per km². See Table 1 for full park names.

Fig. 3.

The effect of white-tailed deer reduction efforts on Ixodes scapularis nymph density and Borrelia burgdorferi sensu stricto infection prevalence from 2014–2022. (A) The average density of nymphs (per 750 m²; ±S.D.) from field observations pre- and post- deer management. Individual observations are shown by the semi-transparent points. (B) The average white-tailed deer density (±S.D.) pre- and post- management efforts for each park or park region. (C) Percent change in nymph density between each post-management year and the average nymph density observed pre-management. Each data point represents percent changes for a single transect within each park relative to the average nymph density for that respective transect pre-management. The x-axis represents the 2-year lagged number of years following the first year of management (e.g., if management started in 2013, x-axis value of 1 indicates change in nymph densities observed in 2015). (D) Predicted density of infected nymphs (per 750 m²; with 95% credible interval) pre- and post- deer management efforts.

Fig. 4.

Model results and predictions assessing the effect of deer density on Ixodes scapularis nymph densities. (A) Coefficient plot for model 01, investigating the impact of 2-year lagged deer density on nymph densities. The plot shows the mean point estimate, the 50% probability mass interval (thick, gray line), and the 95% probability mass interval (thin, grey line). (B) Nymph density predictions across a range of deer densities (5–85 per km²) using model 01 results, extrapolated to the maximum deer density observed for each park. Mean predicted values are presented by the solid lines and raw data are shown by the points (both predicted means and raw data colored by park). See Table 1 for full park names.

Fig. 5.

The density of Borrelia burgdorferi sensu stricto-infected Ixodes scapularis nymphs (DIN) at varying deer densities for eight United States national parks. DIN was estimated for each park at deer densities of 5 (light blue) and 20 (dark blue) deer per km², and at the average deer density at each park from 2012–2020 (gold). The mean predicted densities are shown by points and the credible intervals are shown by the vertical semi-transparetn bars. The average deer density for each park was: CATO 10.50, CHOH 28.17, FIIS 37.55 (region SH), GETT 9.40, MANA 26.56, MONO 51.58, PRWI 13.40, and ROCR 15.13. See Table 1 for full park names.