Static electricity passively attracts ticks onto hosts

Abstract

Most terrestrial animals naturally accumulate electrostatic charges, meaning that they will generate electric forces that interact with other charges in their environment, including those on or within other organisms. However, how this naturally occurring static electricity influences the ecology and life history of organisms remains largely unknown.¹ Mammals, birds, and reptiles are known to carry appreciable net electrostatic charges, equivalent to surface potentials on the order of hundreds to tens of thousands of volts.^{1,2,3,4,5,6,7} Therefore, we hypothesize that their parasites, such as ticks, are passively attracted onto their surfaces by electrostatic forces acting across air gaps. This biophysical mechanism is proposed by us to assist these ectoparasites in making contact with their hosts, increasing their effective "reach" because they are otherwise incapable of jumping. Herein, experimental and theoretical evidence show that the tick Ixodes ricinus (Figure 1A) can close the gap to their hosts using ecologically relevant electric fields. We also find that this electrostatic interaction is not significantly influenced by the polarity of the electric field, revealing that the mechanism of attraction relies upon induction of an electrical polarization within the tick, as opposed to a static charge on its surface. These findings open a new dimension to our understanding of how ticks, and possibly many other terrestrial organisms, find and attach to their hosts or vectors. Furthermore, this discovery may inspire novel solutions for mitigating the notable and often devastating economic, social, and public health impacts of ticks on humans and livestock.^{8,9,10,11,12,13,14,15}.

Keywords: Acari; Ixodes ricinus; charge; ectoparasites; electric ecology; electric fields; electrostatics; parasitism; polarization; triboelectricity.

Figure 1. Ixodes ricinus and its ability to be attracted by static electric fields

(A) Macrophotograph of an I. ricinus nymph. (B) Frames from a 240-fps video recording of a tick passively attracted by the static charge of a rabbit foot across an air gap of several millimeters. Purple arrows indicate tick location. See Video S2. (C) 240-fps video frames of a tick passively attracted by the static charge of an acrylic sheet triboelectrically charged with rabbit fur across an air gap of several centimeters. Pink arrows indicate tick location. See Video S3. See also Video S1 for electrostatic attraction of a freely walking tick.

Figure 2. Mapping electrostatic interactions between tick hosts and vegetation

Three-dimensional finite element models of electric fields around a host and between host and vegetation. Vegetation is electrically grounded, hosts have a uniform surface potential of +750 V. See Table S1 for model input parameters. (A) Electric field strength surrounding a charged cow. (B) Electric field strength between a charged host and grounded tuft of grass. (C) Electric field strength between a charged host and grounded blade of grass, separated by 2.5 mm.

Figure 3. Number and timing of live ticks attracted electrostatically across a 3 mm air gap against gravity

(A) Model of experimental apparatus and electric fields (strength as color gradient). Field strength data are generated computationally using the finite element method. See Table S1 for model parameters. (B) Number of live ticks fully or partially lifted in treatment and control conditions. In the control group, no voltage (0 V) was applied to the electrode and in the treatment group, +750 V was applied. (C) The time taken for each tick to fully lift off from the ground plate following electric field onset (median time = 0.79 s; IQR = 2.05 s). Midline of boxplot is the median, edges of box are the first and third quartiles, and whiskers are the minimum and maximum, defined by the first and third quartiles ± 1.5 × interquartile range.

Figure 4. Threshold and mechanism of electrostatic tick attraction

(A) The relationship between distance to the electrode (mm) and the voltage (V) required to fully lift freshly dead ticks across air gaps against gravity. Light gray points indicate one data point at value, dark gray indicates two data points, and black indicates three data points. Red points show the mean voltage for each distance measured. Straight line shows the linear regression model with forced zero intercept (R² = 0.97, F_1,71 = 2371.1, P < 0.0001). (B) Voltage magnitudes (V) required to lift dead ticks across a 2.5 mm air gap against gravity for both positive and negative voltages. n = 12 for each group. Midline of boxplots is the median, edges of boxes are the first and third quartiles, and whiskers are the minimum and maximum, defined by the first and third quartiles ± 1.5 × interquartile range.