Behavioural and physiological responses of therapy ...

Applied Animal Behaviour Science 205 (2018) 61–67

Contents lists available at ScienceDirect

Applied Animal Behaviour Science journal homepage: www.elsevier.com/locate/applanim

Behavioural and physiological responses of therapy horses to mentally traumatized humans

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Katrina Merkiesa,b, , Marnie J. McKechniea, Emily Zakrajseka a b

Department of Animal Biosciences, University of Guelph, Guelph, ON, N1G 2W1, Canada Campbell Centre for the Study of Animal Welfare, University of Guelph, Guelph, ON, N1G 2W1, Canada

A R T I C LE I N FO

A B S T R A C T

Keywords: Equine-assisted therapy PTSD Horse Behaviour Cortisol Heart rate

The benefits to humans of equine-assisted therapy (EAT) have been well-researched, however few studies have analyzed the effects on the horse. Understanding how differing mental states of humans affect the behaviour and response of the horse can assist in providing optimal outcomes for both horse and human. Four humans clinically diagnosed and under care of a psychotherapist for Post-Traumatic Stress Disorder (PTSD) were matched physically to four neurotypical control humans and individually subjected to each of 17 therapy horses loose in a round pen. A professional acting coach instructed the control humans in replicating the physical movements of their paired PTSD individual. Both horses and humans were equipped with a heart rate (HR) monitor recording HR every 5secs. Saliva samples were collected from each horse 30 min before and 30 min after each trial to analyze cortisol concentrations. Each trial consisted of 5 min of baseline observation of the horse alone in the round pen after which the human entered the round pen for 2 min, followed by an additional 5 min of the horse alone. Behavioural observations indicative of stress in the horse (gait, head height, ear orientation, body orientation, distance from the human, latency of approach to the human, vocalizations, and chewing) were retrospectively collected from video recordings of each trial and analyzed using a repeated measures GLIMMIX with Tukey’s multiple comparisons for differences between treatments and time periods. Horses moved slower (p < 0.0001), carried their head lower (p < 0.0001), vocalized less (p < 0.0001), and chewed less (p < 0.0001) when any human was present with them in the round pen. Horse HR increased in the presence of the PTSD humans, even after the PTSD human left the pen (p < 0.0001). Since two of the PTSD/control human pairs were experienced with horses and two were not, a post-hoc analysis showed that horses approached quicker (p < 0.016) and stood closer (p < 0.0082) to humans who were experienced with horses. Horse HR was lower when with inexperienced humans (p < 0.0001) whereas inexperienced human HR was higher (p < 0.0001). Horse salivary cortisol did not differ between exposure to PTSD and control humans (p > 0.32). Overall, behavioural and physiological responses of horses to humans are more pronounced based on human experience with horses than whether the human is diagnosed with a mental disorder. This may be a reflection of a directness of movement associated with humans who are experienced with horses that makes the horse more attentive. It appears that horses respond more to physical cues from the human rather than emotional cues. This knowledge is important in tailoring therapy programs and justifying horse responses when interacting with a patient in a therapy setting.

1. Introduction Over the past few decades, there has been a growing interest in equine-assisted therapy (EAT) as the benefits of interacting with horses for the treatment of individuals with mental illnesses become apparent (Bachi, 2012; Frewin and Gardiner, 2005). The basic premise of such programs is that the immediate and direct feedback from the horse allows the participant to gain awareness of his/her own behaviour or mental state. This presumes that horses perceive and respond to ⁎

emotional changes in the human, and requires an understanding of horse behavior on the part of program facilitators to foster a positive learning environment and guide mutually constructive interactions between horse and human (Hausberger et al., 2008). However there is little research that analyzes these interactions from the viewpoint of the horse. More attention needs to be accorded to the benefits the animals receive in these programs (Hatch, 2007) as there is the potential for a therapy animal’s welfare to be negatively affected (Wensley, 2008). Equine-assisted therapy programs begin by introducing the human

Corresponding author at: Department of Animal Biosciences, University of Guelph, Guelph, ON, N1G 2W1, Canada. E-mail address: [email protected] (K. Merkies).

https://doi.org/10.1016/j.applanim.2018.05.019 Received 18 October 2017; Received in revised form 20 March 2018; Accepted 10 May 2018 Available online 14 May 2018 0168-1591/ © 2018 Elsevier B.V. All rights reserved.


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around horses be reviewed with staff in all horse organizations. A survey of businesses offering equine-assisted therapy in Florida revealed that all of them reported challenges with their horse training program, often having to resort to outside professionals (Rankins et al., 2017). Although all organizations that certify equine-assisted therapy practitioners consider the interpretation of horse behaviour critical to program success, there exist no standards for teaching or understanding equine behaviour in these certification programs (Kieson and Abramson 2016). The purpose of this study was to quantify behavioural and physiological responses of therapy horses exposed to humans who have been clinically diagnosed with a psychological or emotional illness, namely Post-Traumatic Stress Disorder (PTSD), compared to those who have not been diagnosed with psychological ailments. Based on previous research in this lab showing less stress behaviours in horses exposed to humans who were nervous of horses (Merkies et al., 2014), it was hypothesized that horses would display fewer signs of behavioural and physiological stress when exposed to humans with PTSD compared to neurotypical individuals. Understanding the therapy horse’s role is fundamental for furthering research, providing suitable training for facilitators, and ensuring appropriate safety measures for all participants while gaining additional insight into the human-animal bond.

patient to the horse he/she will interact with. The appropriate pairing of a horse with a participant is essential in developing suitable learning and discovery opportunities for the individual patient. Some EAT practitioners believe that a successful bond between the human and horse is built upon reciprocated trust and dependability that can help establish a successful therapy program (Frewin and Gardiner, 2005). Many practitioners consider the horse itself as the catalyst for therapeutic changes (Kendall et al., 2014). A widely held belief is that the horse can intuit what emotional support the human patient requires and the human is paired with a horse that can fulfill these needs. Anthropocentric labeling of human-horse interactions, such as “the horse knows what the human wants” or that horses are “willing to please” assumes that horses will act benevolently to achieve mutual goals (McGreevy et al., 2009). This assumption can lead to miscommunication and compromise welfare for both human and equine participants. Literature abounds with the effects of stress placed on humans working in careers of social work, psychology and psychiatry (reviewed in Lloyd et al., 2002). It is reasonable to assume that animals placed in similar environments could also experience stress, and that a number of variables influence a horse’s response to his human partner. It is known, for example, that both the temperament of the horse and the attitude of the human (Hausberger et al., 2008) play a role in the horse-human relationship. Heart rate increased in horses that were petted by humans who were thinking negative thoughts (Hama et al., 1996) and horses being led through a maze by a handler with a negative attitude were less cooperative (Chamove et al., 2002). Humans that were anticipating the occurrence of a frightening incidence while leading or riding a horse caused an increase in both their own and the horse’s heart rate (Keeling et al., 2009), while humans that were nervous around horses caused a decreased heart rate in the horses themselves (Merkies et al., 2014). What is unclear is whether horses are responding to physical or emotional cues from the rider/handler. Programs utilizing EAT invariably encounter humans with physical and emotional emanations, as the focus of the therapy exercises is to allow the participants to recognize and face their mental limitations or fears. Only a handful of studies evaluating the effect of humans on horses involved in EAT have been published. None of these studies (Johnson et al., 2017; Kaiser et al., 2006; McKinney et al., 2015) found differences in mean behaviour scores in horses ridden by recreational riders or physically- or psychologically-handicapped riders. Fazio et al. (2013) found lower concentrations of circulating stress hormones (β-endorphin, ACTH and cortisol) in horses engaged in riding sessions with disabled riders than with recreational riders whereas Johnson et al. (2017) showed higher levels of cortisol in horses when ridden by veterans suffering from Post-Traumatic Stress Disorder (PTSD) compared to experienced riders. Post-Traumatic Stress Disorder is described as a complex pathological reaction to trauma wherein traumatic experiences may compromise feelings of safety in the affected person’s own body or environment (Tsur et al., 2018). Trauma occurring during childhood can lead to disrupted neurological development which can have long-term impacts on cognitive function (Mueller and McCullough, 2017). These impacts, coupled with fear, anxiety and lack of trust can impede treatment of PTSD. Equine-assisted therapy is an attractive treatment modality as interactions with horses can provide a feeling a safety and modulate arousal and fear responses in humans. The simple act of grooming a horse can overcome touch avoidance and increase body awareness of the individual suffering from PTSD (Mueller and McCullough, 2017). The importance of fully understanding the variables that affect the horse-human relationship is essential considering that the most significant factor contributing to the risk of human injury when working around horses is the relationship between the horse and the human (Hawson et al., 2010; Keeling et al., 2009). One study reported over 80% of equestrians experienced an injury due to horse riding (Mayberry et al., 2007), leading Gombeski et al. (2017) to recommend that safety

2. Method 2.1. Participants 2.1.1. Humans The procedures in this study were approved by the Research Ethics Board (REB) at the University of Guelph for use of human research subjects (REB#13MY036). Four mentally-traumatized females (47.5 ± 17.3 years, 60.3 ± 2.2 kg, 164.3 ± 8.2 cm) clinically diagnosed with PTSD and under current care of a psychotherapist contacted the principal researcher directly after a local media article regarding the research project was released, volunteering to be part of the study. Four neurotypical “control” females (44.5 ± 18.5 years, 60.5 ± 2.6 kg, 163 ± 8.7 cm) with no clinical diagnoses for a psychological illness were recruited by direct solicitation or word of mouth. Treatment humans (PTSD) were physically matched with control humans (CON) for height, weight, body type, hair colour and clothing worn during trials. None of the participants had interacted with the research horses prior to the study. All participants ranked themselves on a 1–10 scale regarding their equine experience (1 = no experience, 10 = abundant experience) before trials began. All volunteers received a monetary honorarium for their participation. 2.1.2. Horses The procedures in this study were approved by the University of Guelph Animal Care Committee under the auspices of the Canadian Council of Animal Care for the use of animals in teaching and research (AUP#3344). Seventeen animals were able to be recruited for this study leading to a calculated statistical power level of 0.65 with a large anticipated effect size (Cohen’s d = 0.5) and an α-level of 0.05 (Soper, 2010). All horses were trained therapy animals provided by Sunrise Therapeutic Riding and Learning Centre in Puslinch, ON. Five mares and 12 geldings (16.4 ± 3.6 years) of varying breeds (Arabian, Haflinger cross, Welsh, Holsteiner, Pony of the Americas, Appaloosa cross, Quarter Horse, Morgan/Thoroughbred/Saddlebred cross, Rocky Mountain Horse, Swedish Warmblood, Paint, Shire cross) and heights (149.4 ± 9.1 cm) were used. Horses had access to grass pasture 24/7 and received daily vitamin/mineral supplements or senior sweetfeed, beet pulp, and low protein timothy-alfalfa hay specific to their needs. Each horse was used eight times (once per human). 62


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monitor (Polar RS800, Lachine, QC) at least 30 min before each trial. The HR transmitter attached around the human’s chest with a band containing the electrodes moistened with water to detect the signal. The human wore the receiver around her wrist for the duration of her trials. The horse HR transmitter was placed around the horse’s girth using an elastic surcingle designed for horses with built in electrodes moistened with water. The receiver was attached to the surcingle between the horse’s front legs. Recording of horse and human HR began 10 min prior to the start of each trial. HR data from both horses and humans were collected in 5 s intervals throughout the duration of each trial.

2.2. Procedures 2.2.1. Testing area set-up The testing area consisted of a round pen (13 m in diameter) set up inside an indoor arena familiar to the horses. Two video cameras (Panasonic HC-X900 M, Guelph, ON) were positioned on either side of the round pen to continuously record all trials. Inside the round pen pylons marked a radius 4 m from the center to aid in estimating the distance between the horse and human. Researchers were located outside the pen approximately 10 m away during each trial. 2.2.2. Trial procedures One neutral experienced horse handler who was blind to the treatments brought in horses from their paddock approximately 1 h before initiation of each trial. Horses had access to hay and water while in the stalls. All horses and humans were equipped with a heart rate (HR) monitor at least 30 min before each trial, and horses had their initial salivary sample collected as described below. For each trial, horses were led individually into the round pen by the neutral handler. The trial started after the handler released the horse and exited from the pen. After being released, the horse was free to move about the round pen as it chose while being observed for 5 min to collect baseline data (BASELINE period; Fig. 1). After 5 min of baseline data recording, the handler entered the round pen and held the horse exactly opposite and facing the gate while the test human was led into the center of the round pen by a research assistant. Both horse handler and research assistant then left the pen and the test period began. Humans were instructed to move about the pen as they chose, however they were not to initiate physical contact with the horse; if the horse made physical contact, the human could ignore the horse or move away as she chose, but could not touch the horse. Each period with the human lasted for 2 min (TEST period; Fig. 1). After the 2 min period with the human, the handler entered the pen and held the horse while the human was escorted out of the round pen by a research assistant. Another 5 min of post-test data was collected on the horse (POST-TEST period; Fig. 1). After the POST-TEST period the horse was led out of the round pen and back to the stall. A subsequent salivary sample was obtained from each horse 30 min post-test. PTSD humans were tested first in each instance to allow a professional acting coach to instruct the neurotypical control humans how to replicate physical body movements of their matched PTSD individual based on the video recordings. This reproduction of movements did not necessarily result in mirror scenes, as the horse was not expected to perform exactly the same movements. However, the neurotypical human was coached to move around the pen in the same fashion as her paired treatment human and react to initiated horse contact in the same manner. Each human was tested with all 17 horses over a consecutive 2day period. Control humans were tested one week following their PTSD human.

2.3.1.2. Horse salivary cortisol. Saliva samples were collected from the horse 30 min prior to each trial and 30 min post-trial. Saliva was collected using a long cotton swab (Salivette; Sarstedt Inc. Montreal, QC) held in the horse’s mouth for 1 min until saturated. Swabs were placed in the Salivette tubes and kept cool until transported to the lab where they were centrifuged (1000G × 2 min) and stored at −15 °C until assayed. Samples were thawed to room temperature, diluted 1:1 with Trizma buffer (0.02 M Trizma, 0.300 M NaCl, 0.1% BSA; pH 7.5) and then assayed in duplicate (50ul/well) using a modified enzyme immunoassay (EIA) employing a polyclonal cortisol antibody made in rabbits (R4866) and cortisol conjugated to horse-radish peroxidase (CJ Munro, UC Davis) (Graham et al., 2001). In brief, microtitre plates (Nunc; Fisher Scientific, Ottawa, ON) were coated with affinity purified goat anti-rabbit gamma globulin (25 μg/plate; Sigma Chemicals, St. Louis, MI) dissolved in coating buffer (0.015 M Na2CO3, 0.035 M NaHCO3; pH 9.6) and incubated overnight at room temperature. Wells were emptied and refilled with Trizma assay buffer and stored at room temperature for at least 30 min prior to use to block nonspecific binding. Coated plates were washed (0.04% Tween 20) and samples and standards were dispensed. Horse-radish peroxidase-labeled cortisol was dispensed followed by anti-cortisol antibody (R4866); CJ Munro. Plates were incubated overnight at 4 °C. Following incubation plates were washed and incubated with substrate solution (0.5 ml of 0.016 M tetramethylbenzidine in dimethylsulphoxide and 100 μl of 0.175 M H2O2 diluted in 24 ml of 0.01 M C2H3O2Na; pH 5.0). After incubation (45 min, room temperature) the enzyme reaction was stopped with 3 M H2SO4. The optical density was measured at 450 nm (reference 595 nm). The standard curve of corticosterone ranged from 1.9–250 pg/well. The intra and interassay coefficients of variation were < 15%. All samples were diluted in assay buffer and assayed in duplicate. 2.3.2. Behavioural data collection Behaviours were determined retrospectively using data captured by both video cameras. Two independent observers recorded all instances of vocalization and chewing, while gait, head height, ear and body position in relation to the human, and distance from the human recorded in 5 s sampling intervals. Kappa values of inter-rater agreement were acceptable for all measures (0.81–0.98). Interval behaviours were scored on an arbitrary scale as follows: gait (lying/rolling = 0, halt = 1, walk = 2, trot = 3, canter = 4), head height (lying/rolling = 0, below the withers = 1, at the withers = 2, above the withers = 3), ear

2.3. Data collection 2.3.1. Physiological data collection 2.3.1.1. Heart rate. All humans and horses were equipped with a HR

Fig. 1. Timeline of events for each trial with horse (n = 17) and human (n = 8) (136 individual trials). 63


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3. Results 3.1. Physiological data 3.1.1. Horse heart rate Horse HR remained relatively constant throughout the BASELINE period (49 ± 15bpm; F(1,9458) = 1.83; P > 0.17). When the horse was exposed to the human, average horse HR did not differ between treatments (48 ± 16bpm in PTSD vs. 46 ± 14bpm in CON; F(1,3725) = 1.56; P > 0.21) even with gait as a covariate. Over the TEST period, horse HR tended to increase by 1.6bpm when exposed to the PTSD individuals (F(1,1934) = 3.51; P < 0.06) whereas when exposed to the CON individuals, horse HR decreased by 1.7bpm (F(1,1812) = 7.07; P < 0.008). During the POST-TEST period, horse HR continued to increase by 1.6bpm after exposure to the PTSD individuals (average 52 ± 23bpm; F(1,4290) = 66.91; P < 0.0001) whereas horse HR remained constant after exposure to the CON individuals (47 ± 14bpm; F(1,4068) = 0.02; P > 0.88). The post-hoc analysis of human experience with horses showed a lower average horse HR during the TEST period when the horse was paired with individuals (n = 4) who rated themselves as less experienced around horses (45 ± 11.5bpm) compared to those individuals (n = 4) with more horse experience (49 ± 18bpm; F(1,3725) = 2.09; P < 0.037; Fig. 2).

Fig. 2. Mean horse HR (bpm) when therapy horses (n = 17) were individually exposed in a round pen for two minutes to a human who was inexperienced with horses (n = 4) compared to a human who was experienced with horses (n = 4). a,b differ, P < 0.037.

position (towards human = 0, away from human = 1), body position (facing toward human = 0, facing away from human = 1), distance from human (within half a horse length of human) = 0, < 6.5 m from human = 1, 6.5–13 m from human = 2. Latency (time measurement in seconds from time of release by handler until horse physically touched human) and character (whether the human approached the horse or the horse approached the human) of approach to the human, and other behaviours such as interacting with the pylons were noted.

3.1.2. Human heart rate When exposed to the horse in the round pen for two minutes, PTSD individuals had a lower average HR (83 ± 20bpm) than CON individuals (85 ± 11bpm; F(1,3754) = 83.83; P < 0.0001). Human HR did not change over time regardless of treatment (F(1,3771) = 0.10; P > 0.75). Human HR did not correlate to horse HR (r (3732) = −0.102, P < 0.0001). Post hoc analysis showed HR of inexperienced humans (91 ± 17bpm) was higher than HR of experienced humans (77 ± 12bpm; F(1,3754) = 1198.95; P < 0.0001)

2.4. Statistical analysis Descriptive statistics for all behaviours were performed using SPSS Statistics (Version 24, IBM Corp, Markham, ON) while all other analyses were conducted with SAS (Version 9.4, Toronto, ON). Both horse and human HRs were log transformed since they did not achieve a normal distribution (Kolmogorov-Smirnov test, P < 0.01). A mixed procedure with horse and human as random factors was used to analyze cortisol concentrations, vocalizations, chewing, and character of approach. A Generalized Linear Mixed Model (GLIMMIX) with repeated measures was used to determine the effect of period (BASELINE, TEST, POST-TEST) and treatment (PTSD or CON human) on HR and behavioural responses according to the following equation where Υ indicates the behaviour being analyzed:

3.2. Salivary cortisol Horse salivary cortisol concentration did not differ when the horse was exposed to PTSD or CON individuals (average 2.31 ± 5.65 ng/ml; F(1,237) = 1.27, P > 0.26). Horse salivary cortisol concentration also did not differ between pre-trial and post-trial sampling (mean 2.30 ± 5.62 ng/ml; F(1,237) = 0.01, P > 0.90). 3.3. Behavioural data

Υijklmnop = μ + treatmenti + periodj + treatment*periodij + gaitk + humanl + horsem + timen + humanHRo + horseHRp + eijklmnop

When the horse was partnered with the human in the round pen, no differences were evidenced between CON and PTSD individuals for gait (F(1,3781) = 0.03, P > 0.87), head height (F(1,3757) = 0.28, P > 0.59), ear orientation (F(1,3713) = 0.16, P > 0.68), body orientation (F(1,3765) = 0.41, P > 0.52), distance from the human (F(1,3765) = 0.01, P > 0.92), latency to approach (horse to human (F(1,49) = 0.71, P > 0.40) or human to horse (F(1,6) = 0.67, P > 0.44)), horse vocalizations (F(1,108) = 0.01, P > 0.91), or chewing (F(1,107) = 0.02, P > 0.88). The post-hoc analysis on human experience with horses showed that horses approached quicker (F(1,49) = 6.20; P < 0.016), moved faster (F(1,3781) = 5.80; P < 0.0161), and stood closer (F(1,3786) = 7.00; P < 0.0082) to humans who were experienced with horses. Head height and gait were influenced by period. Horses carried their head lowest during the TEST period and highest in the POST-TEST period (F(1,21719) = 140.53; P < 0.0001; Fig. 3). Horses slowed their gait during the TEST period compared to BASELINE (F(2,21864) = 52.76; P < 0.0001), and this effect carried over to the POST-TEST period, which did not differ from the TEST period (F(2,21904) = −0.17; P > 0.98; Fig. 4). Horses chewed less during the TEST period

Tukey’s multiple comparisons of least squared means characterized differences in behaviours, HR and cortisol between treatments and periods with a probability value of P < 0.05 denoting statistical significance. When analyzing HR, gait was used as a covariate to account for increases in HR concomitant with increases in gait. A linear regression analyzed changes in HR over time and a Pearson’s correlation analyzed the relationship between horse and human HR. Upon analyzing the data, it became apparent that two of the human pairs were very experienced with horses and two of the human pairs were inexperienced. For this reason, a post-hoc effect of human experience with horses was carried out using the equation below to determine if this factor affected any behavioural or physiological outcomes. Υijklmnopq = μ + treatmenti + periodj + treatment*periodij + gaitk + humanl + horsem + timen + humanHRo + horseHRp + human experienceq + eijklmnopq 64


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humans showed an increase in HR, even while the PTSD human heart rates were lower than control humans. There was no evidence of synchronization between horse and human heart rates in any instance. When the PTSD human left the pen, horses continued to have a higher heart rate compared to the control humans. While it may not be possible to determine the exact mechanisms surrounding horses’ responses to humans in varying psychological states, it was clear that the presence of any human was preferred by the horse to being alone. Horses alone in the round pen displayed a variety of stress behaviours including increased vocalizations and faster gaits, however when any human female joined them regardless of her mental state, stress levels decreased as evidenced by horses lowering their head, moving slower, and chewing and vocalizing less. A raised head position is typically classified as a sign of heightened awareness of surroundings (Visser et al., 2009) while a lowered head position can indicate submission especially when followed by lip licking and chewing (Krueger, 2007). Chewing and lip licking were recorded in this study, yet there was no relationship to head height, as also documented by Warren-Smith et al. (2007). When the human that joined the horse in the round pen happened to be experienced around horses, the horse approached her more quickly, stood closer to her, and displayed an increase in heart rate. If the human in the round pen was not experienced around horses, the horse remained aloof with a decreased heart rate. This may indicate a directness about the mannerisms of a human who is experienced around horses that makes the horse more attentive, perhaps in anticipation of being ridden or worked as experience would predict. Humans who were purposeful in their body positioning in relation to the horse in a round pen also produced definitive responses in the horse (Merkies et al., 2013). Although appeased by the human company, there appeared to be no expectation or evidence of a relationship by the horse with the inexperienced human present. Similarly Baragli et al. (2009) demonstrated that horses perceive and respond to humans based on their past interactions. While human self-ranking of experience with horses is subjective, the results presented here corroborate previous results from this lab showing humans who ranked themselves as more fearful of horses were associated with lower horse HR (Merkies et al., 2014). While the effects of mental illness and experience with horses were analyzed in the current study, it cannot be discounted that there may be other tacit effects, e.g. human attitude toward horses, that influence the behavioural responses of horses as previously shown by Hama et al. (1996) and Chamove et al. (2002). These are intriguing aspects to research further to expand our understanding of how horses view humans. No differences in horse salivary cortisol were noted throughout this study. Similarly Fazio et al. (2013) did not show any significant changes in average basal β-endorphins or adrenocorticotropic hormone in horses while working with a disabled group of riders versus a recreational group of riders. Numerous studies have shown physiological and behavioural effects of humans on horse behaviours, however issues with appropriate equipment and sensitivity to measure physiological responses of horses to various riding and training methods have led König von Borstel et al. (2017) to suggest that behavioural measures are more reliable indicators of stress in horses. Cortisol measures must be interpreted carefully as acute stress may cause an increase in cortisol measurements while chronic stress may be associated with decreased cortisol related to coping mechanisms (Pawluski et al., 2017). Therapy horses are accustomed to being utilized in sessions with unfamiliar and unpredictable humans thus may not show changes in cortisol concentrations associated with acute scenarios. Other limitations to this study include the low number of human subjects and the fact that the PTSD humans were tested first with each horse. Although a week elapsed between the horse being exposed to the PTSD human and the control human, there may have been interactions between the PTSD human and the horse that carried over to the horse‘s interactions with the paired control human.

Fig. 3. Horse (n = 17) head height score (mean ± SD) assigned to an arbitrary scale (1 = below withers, 2 = level with withers, 3 = above withers) observed for five minutes during BASELINE, two minutes during exposure to a human (TEST), and for five minutes after the human left the round pen (POST-TEST). (a,b,c, differ P < 0.0001).

Fig. 4. Horse gait score (mean ± SD) assigned to an arbitrary scale (0 = rolling, 1 = halt, 2 = walk, 3 = trot, 4 = canter) observed for five minutes during BASELINE, two minutes during exposure to a human (TEST), and for five minutes after the human left the round pen (POST-TEST). (a,b differ P < 0.0001).

(0.24 ± 0.92 chews/min; F(2,367) = 12.62; P < 0.0001) compared to BASELINE (0.61 ± 1.89 chews/min) or POST-TEST period (0.59 ± 1.38 chews/min) which did not differ from each other (F(2,367) = 0.27; P > 0.99) with no effect of treatment (F(3,367) = 0.12; P > 0.27). There was no effect of treatment (F(1,108) = 0.01; P > 0.90) on vocalizations, but horses vocalized less during the TEST period (mean 0.11 ± 0.61 VOX/min; F(2,368) = 29.14; P < 0.0001) compared to, BASELINE (mean 0.84 ± 2.72 VOX/min) or POST-TEST period (mean 0.94 ± 3.15 VOX/min) which did not differ from each other (F(2,368) = 0.82; P > 0.79). The length of time a horse had been used for therapy did not affect any behaviours (P > 0.10). 4. Discussion This study exposed horses individually to pairs of humans who physically resembled each other and moved in a similar way, however one of those humans suffered from PTSD while the other did not. Physiological and behavioural measures showed some differences in responses of the horses to the two treatment groups, however more salient results came from analyzing horse responses to any human presence and to the presence of humans experienced with horses. Horses did not differentiate between PTSD and control individuals for any of the measured behaviours, however horses exposed to PTSD 65


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Conflicts of interest

The onus that some EAT program facilitators place on therapy horses to distinguish and empathize with individuals of differing mental states may not be ethical. Anthropocentric labeling of horse-human interactions assumes that horses will sympathize with riders or handlers to achieve mutual goals (McGreevy et al., 2009). In this study the only difference horses displayed between treatment groups was HR, suggesting that for the most part the horse exhibits typical equine behaviours regardless of the mental status of the human. If two humans physically appear and move similarly, they may be perceived as equivalent to the horse from a physical aspect, thus, any differences the horse displays in how it interacts with various humans is likely not due to a transmitted emotional or mental human need. Although humans who are mentally traumatized may have therapeutic needs, it might be misunderstood that the horse can “sense” the human mental state of mind (McGreevy et al., 2009). This misunderstanding may lead to unrealistic expectations placed on the horse to empathize with the afflicted human. The horse should not carry the idealistic responsibility of acting as the primary caregiver to the human in times of need (DeAraugo et al., 2014). Instead, this study showed that horses responded to the mentally traumatized humans similarly to those humans with a healthier mental state. Corrigan (2016) notes that the stigma surrounding mental illness often results in prejudice and discrimination of people who suffer from mental illness. One way to ease the stigma is to account for similarities between individuals who have mental illness compared to those who do not. This study could be a positive implication for therapy programs, as horses do not appear to differentiate between a human’s mental state, which could provide a sense of normalcy to the human patient. If horses do not respond differently to PTSD versus neurotypical humans, then, as Nimer and Lundahl (2015) suggested, future studies may utilize control humans to examine research questions surrounding horse-human interaction in a therapy setting. In equine-assisted therapy, the horse provides benefits to humans experiencing mental or psychological trauma, but it is equally important to understand the horse’s needs to ensure the best possible welfare for the horse and safety of the participants while improving therapy outcomes. Although human injuries are most often caused by fear responses in horses (Hawson et al., 2010), if these fear responses are not instigated by human inexperience with horses or the mental state of the human, then trainers and handlers can apply training models appropriately based on understanding of horse behaviour to prevent injuries. Although EAT is growing in importance to society, there is still a wide gap between practice and knowledge. Nimer and Lundahl’s (2007) meta-analysis concluded that the small amount of research that currently exists often has methodological errors, such as a lack of control groups, ignoring outliers in the data set, small sample sizes, overgeneralized findings, author/research bias, and an inconsistency between observations and actual discoveries. Limitations to this study include a small sample size and the described interactions between horses and humans may not reflect a typical therapy session; therefore similar experiments with larger sample sizes could prove useful in the future.

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5. Conclusions This foundational study provided a quantitative basis for behavioural responses of therapy horses and found no major differences in typical equine stress behaviours indicated by gait, head height, ear position, and distance from the human between PTSD and neurotypical individuals. Horses were less stressed when any human was present with them, and more attentive toward humans who were more experienced with horses. Horses appear to respond more to physical cues from the human rather than implied emotional needs. This knowledge is important in tailoring therapy programs and justifying horse responses when interacting with a patient in a therapy setting. 66


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