Effects of white-tailed deer habitat use preferences on southern cattle fever tick eradication: simulating impact on "pasture vacation" strategies

Abstract

Background: Rhipicephalus (Boophilus) annulatus and Rhipicephalus (Boophilus) microplus (southern cattle fever tick; SCFT), collectively known as cattle-fever ticks (CFTs), are vectors of protozoal parasites (Babesia bigemina and Babesia bovis) that cause bovine babesiosis (also known as cattle fever). One traditional strategy for CFT eradication involves the implementation of a "pasture vacation," which involves removing cattle (Bos taurus) from an infested pasture for an extended period of time. However, vacated pastures are often inhabited by wildlife hosts, such as white-tailed deer (WTD; Odocoileus virginianus), which can serve as alternate hosts for questing CFTs. We hypothesized that the distribution of host-seeking larvae among habitat types post-pasture vacation would reflect habitat use patterns of WTD, and in turn, affect the subsequent rate of pasture infestation by CFT.

Methods: We adapted a spatially explicit, individual-based model to simulate interactions among SCFT, cattle, and WTD as a tool to investigate the potential effects of WTD habitat use preferences on the efficacy of a pasture vacation. We parameterized the model to represent conditions typical of rangelands in south Texas, USA, simulated a 1-year pasture vacation under different assumptions regarding WTD habitat use preferences, and summarized effects on efficacy through (1) time post-vacation to reach 100% of pre-vacation densities of host-seeking larvae, and (2) the ecological conditions that resulted in the lowest host-seeking larval densities following pasture vacation.

Results: Larval densities at the landscape scale varied seasonally in a similar manner over the entire simulation period, regardless of WTD habitat use preferences. Following the removal of cattle, larval densities declined sharply to < 100 larvae/ha. Following the return of cattle, larval densities increased to > 60% of pre-vacation densities ≈ 21 weeks post-vacation, and reached pre-vacation levels in less than a year. Trends in larval densities in different habitat types paralleled those at the landscape scale over the entire simulation period, but differed quantitatively from one another during the pasture vacation. Relative larval densities (highest to lowest) shifted from (1) wood/shrub, (2) grass, (3) mixed-brush during the pre-vacation period to (1) mixed-brush, (2) wood/shrub, (3) grass or (1) wood/shrub, (2) mixed-brush, (3) grass during the post-vacation period, depending on WTD habitat use preferences.

Conclusions: By monitoring WTD-driven shifts in distributions of SCFT host-seeking larvae among habitat types during simulated pasture vacation experiments, we were able to identify potential SCFT refugia from which recrudescence of infestations could originate. Such information could inform timely applications of acaricides to specific refugia habitats immediately prior to the termination of pasture vacations.

Keywords: Cattle Fever Tick Eradication Program; Host–parasite interaction; Individual-based model; Integrated tick management research; Rhipicephalus sp.; Spatially explicit model.

Fig. 1

a Schematic representation of the hierarchical relationships among the 900-ha simulated landscape, host activity ranges, and weekly host movements. b Conceptualization of the tick-host-landscape-climate system indicating important model components and processes

Fig. 2

Overview of the sequence of events and processes involved in the execution of the model simulating the spatial and temporal dynamics of southern cattle fever ticks (SCFTs) (Adapted and modified from Wang et al. [10])

Fig. 3

a Spatial distribution of habitat types across the simulated landscape, consisting of 30% wood/shrub (good SCFT) habitats (green with W0), 30% mixed-brush (fair SCFT) habitats (red with B0), and 40% grass (poor SCFT) habitats (blue with M0). Pink dots with the number 1 represent cattle, pink dots with the number 2 represent white-tailed deer (WTD). b Weather profile during the 3-year simulation period based on historical temperature, saturation deficit, and precipitation data for Corpus Christi, Texas, USA. Vacation “Pasture vacation”

Fig. 4

a–d Temporal response of SCFT host-seeking larval densities under different WTD habitat use preferences during the vacation and post-vacation years (pre-vacation year not shown). Weekly mean numbers of host-seeking larvae per ha are shown at the landscape scale (a) and by habitat type: wood/shrub (b), mixed-brush (c), and grass (d). Results are from simulations in which WTD habitat use preferences for wood/shrub (good SCFT) habitats, mixed-brush (fair SCFT) habitats, and grass (poor SCFT) habitats were 0, 0.5, 0.5 (gray solid line), 0.2, 0.4, 0.4 (dashed line), and 1, 0, 0 (black solid line), respectively. Dashed vertical lines representing weeks 10, 21, and 46 of the post-vacation year are included for reference

Fig. 5

SCFT host-seeking larval densities at the landscape scale and in each habitat type at their lowest point, 10 weeks post-pasture vacation, under the indicated assumptions regarding WTD habitat use preferences. Preference values represent the proportion of time spent in wood/shrub (good SCFT) habitats, with the remaining portion of time divided equally between mixed-brush (fair SCFT) habitats and grass (poor SCFT) habitats

Fig. 6

Temporal shifts in spatial distributions of SCFT host-seeking larval densities [host-seeking larvae (HSL)/ha] across a simulated landscape assuming different WTD habitat use preferences. The simulated landscape consisted of 30% wood/shrub (good SCFT) habitats (green), 30% mixed-brush (fair SCFT) habitats (red), and 40% grass (poor SCFT) habitats (blue). Results are from simulations in which WTD habitat use preferences for wood/shrub, mixed-brush, and grass were a 0, 0.5, 0.5, b 0.2, 0.4, 0.4, and c 1, 0, 0, respectively. Darker shading indicates higher densities of HSL. Note differences in shading scales