Are Equine Assisted Interactions (EAI) a Dark Horse Bet? A Systematic Review on Measuring the Psychological and Physiological Effects of EAI on PTSD, Anxiety and/or Mood Disorders

Abstract

Background:

Conventional forms of trauma-focused psychological interventions seem to be less effective in military veterans with post-traumatic stress disorder (PTSD). Equine Assisted Interventions (EAI) have become increasingly popular as an additional treatment, resulting in increasing research on the effects of EAI. However, no systematic review on the effects of EAI has focused on PTSD, anxiety and/or mood disorders and whether the effects are reflected in psychological and physiological measures. This systematic review investigated the treatment effects, study design, population, intervention, and psychological and physiological measurements in EAI research.

Methods:

The search strategy yielded 1028 original articles from 3 electronic databases: PubMed, Scopus, and CAB Abstracts. These articles were screened by 2 individual blinded researchers with Rayyan according to Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines and rendered 31 eligible studies.

Results:

Although all quantitative studies reported 1 or more positive treatment effects on symptoms of PTSD, anxiety and/or mood disorders, well-designed studies were sparse. This review demonstrated a lack of methodological information and few randomized controlled studies, in addition to a large heterogeneity in population characteristics, interventions, terminology, and questionnaires.

Conclusion:

Future studies should address these methodological considerations in order to advance the understanding of EAI and optimize their efficacy in treating PTSD, anxiety and/or mood disorders.

Main Points

• The majority of the included papers on the effects of Equine Assisted Interventions (EAI) focus on post-traumatic stress disorder (PTSD) and use psychometric questionnaires such as a version of the post-traumatic stress disorder Checklist (PCL) questionnaire or the Clinician-Administrated PTSD Scale for DSM V (CAPS-5) to measure effects, whereas only a few include physiological parameters.

• Although most papers found positive effects on symptoms of PTSD, anxiety, and/or mood disorders, methodological issues impede establishing the effectiveness of EAI.

• Future studies should include randomized, controlled quantitative studies with a waitlist or positive control group that use physiological parameters in addition to psychometric parameters during pre-, mid-, post-treatment, and 3-month follow-up.

• Attention should be paid to reporting complete information on participant selection and co-morbidity, dropout rates, and information on the psychotherapeutic elements of the EAI program.

Introduction

Post-traumatic stress disorder (PTSD) can be effectively treated with conventional forms of trauma-focused psychological interventions.¹ However, this seems less effective in military veterans and war-affected populations.¹ Equine-Assisted Interactions (EAI) have become increasingly popular as an additional treatment option for patients with mental and emotional health issues^2,3 and seem effective in military veteran populations.^4-6 The umbrella terms EAI or Equine-Assisted Activities and Therapies (EAAT) can be divided into different forms of treatment with a wide range of definitions, see e.g., Marchand et al⁷ for a nomenclature based upon the work of Hallberg.⁸ A distinction can be made between Equine Assisted Psychotherapy (EAP) and Therapeutic Horse Riding (THR), and there are 2 main associations EAI study programs adhere to: the Equine-Assisted Growth and Learning Association (EAGALA, https://www.eagala.org/) and the Professional Association of Therapeutic Horsemanship International (PATH Intl., https://pathintl.org/) that certifies instructors. Despite the development of specific terminology and the practical development of the field, research on EAI is still in its early stages.

Review articles on the effects of EAI are inconclusive regarding treatment effects and either focus on a broad range of mental and physical disorders or a very specific disorder, such as PTSD. Systematic reviews on the effectiveness of EAI that include studies on both mental and physical disorders, such as autism, ADHD, schizophrenia, eating disorders, multiple sclerosis, breast cancer, and stroke in children, adults, and elderly people, report equivocal evidence.^7,9-11 Other more recent reviews on the effects of EAI on a specific mental disorder indicate that EAI seems to improve mental health and reduce PTSD symptoms.^3,7 However, as the body of literature that reports these positive effects on mental health is sparse and reflects early scientific development, including methodological flaws, additional research is needed to draw more robust conclusions.^3,7,12 Better insight into possible methodological pitfalls may provide advice for future studies on this topic. This calls for a thorough assessment of the study designs and methodologies used, including investigations of the study population, the intervention itself, and the measures used to determine treatment effects. To this day, no systematic review has been performed on the effectiveness of EAI on psychological disorders, including PTSD, anxiety, and/or mood disorders. This may reduce heterogeneity in the studies included in this review, as it focuses on a specific range of psychological disorders, and it may include a larger number of methodologically sound scientific studies compared to reviews that focus on PTSD only.

The physiological working mechanisms behind the effects of EAI in patients with PTSD, anxiety and/or mood disorders require further investigation as well. In EAI studies focusing on patients with neuromuscular disorders, physiological parameters have indicated improved physical functioning.^13,14 A review on the psycho-physiological effects of Animal Assisted Interventions (AAI) proposed oxytocin as an underlying mechanism for beneficial effects of EAI on both mental and physical health.¹⁵ The neuropeptide oxytocin is released during social interactions that are perceived as positive and can reduce stress and improve mental and psychological welfare.^16,17 Studies on dogs have indicated that interactions such as petting and stroking can lead to oxytocin production in both the animal and handler and a subsequent decrease of the stress hormone cortisol.^18,19 Similar effects could occur during EAI. A review on the physiological effects of EAI showed a favorable effect on heart rate variability by activating the parasympathetic nervous system;²⁰ however, this study included research focusing on a range of conditions and disorders. A review focusing on physiological, in addition to psychological measures of EAI, specifically addressing people with PTSD, anxiety and/or mood disorders seems lacking. Nevertheless, physiological measures may be an important measure in determining the effectiveness of EAI in people with PTSD, anxiety and/or mood disorders. Therefore, this systematic review investigates the study design, study population, intervention characteristics, measurements, and treatment effects in EAI research for PTSD, anxiety, and/or mood disorders. It focuses on both treatment outcomes and psychological and physiological measures.

Materials and Methods

Literature Search

This section provides a detailed account of the search strategy and search results according to Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines (https://www.prisma-statement.org/). In September 2022, the electronic databases PubMed (https://pubmed.ncbi.nlm.nih.gov/), Scopus (https://www.scopus.com), and CAB Abstracts (https://www.cabidigitallibrary.org/product/ca) were searched for relevant research articles. Search terms included: “therapeutic riding” OR “therapeutic horse*” OR “hippotherapy*” “therapeutic riding” OR “therapeutic horse*” OR “hippotherap*” OR “horseback riding therap*” OR “equine assisted*” OR “equine facilitated*” OR “equine therapy” OR “horse therapy”; and the [MeSH Term]: “equine assisted psychotherapy”. In Figure 1 a flow diagram of the search and articles retrieved and eliminated is presented, resulting in 1371 original research articles. The literature review tool Rayyan (https://www.rayyan.ai/) was used for screening the articles by 2 independent blind reviewers. A total of 1028 original research articles published after 1990 were included in the review process, of which 9 were not retrievable. These articles had English or German as the primary language. Reviews, meta-analyses, conference abstracts, books, or dissertations were not included, resulting in 352 excluded articles. All the included articles focused on the effects of EAI on patients with PTSD, anxiety, and/or mood disorders. Furthermore, 181 articles were excluded because they did not report on the therapeutic effects of horses. Articles that studied the effects of EAI on physical disorders (n = 164), autism (n = 47), addiction (n = 12), ADHD (n = 11), dementia (n = 7), Down Syndrome (n = 6), eating disorders (n = 4), Parkinson’s disease (n = 2), schizophrenia (n = 2), psychotic disorder (n = 2), and a group of other subjects that are not the right target population (n = 198) were excluded. This selection resulted in 31 eligible studies included in the current systematic review.

Figure 1.

Flow diagram of the identification, screening, and selection of eligible studies for this systematic review, according to Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines. ADHD, attention deficit hyperactivity disorder; EAI, equine assisted interactions.

Literature Processing

This review focused on the type of study, the study population (sample), the intervention, the psychological and/or physiological measures, and the statistical significance of treatment effects. An overview of these aspects will be presented in Table 1. For the type of study, specific attention was given to whether the study was quantitative (considered if studies had >1 participant and used statistics), qualitative (case studies and/or studies that merely focused on subjective experiences), or included aspects of both. In addition, information was provided on the presence of a control group (CG) and, if so, whether group assignment was randomized. For the study population, attention was given to the sample size, dropout rate (focusing on attrition during the EAI program), age and gender of the participants, symptoms, and diagnosis of the participants (including selection criteria of symptoms/diagnosis), their comorbidities, and ongoing other treatments during the EAI program. For the intervention, the focus was on the number, frequency, and duration of sessions, the association the program adhered to (EAGALA or PATH), the type of EAI according to the paper (due to varying nomenclature), key elements, tasks with the horse (grooming, groundwork and/or riding), and whether the program used individual elements or elements of group therapy. For the psychological and physiological measures, it was assessed which questionnaires and/or physical measures were used in the studies. The assessment of statistical significance was restricted to effects retrieved with statistical methods (e.g., studies providing P-values or confidence intervals), and only expected for quantitative studies and not for case studies. These quantitative studies have a higher position in the pyramid of evidence and are considered of higher scientific quality and empirical evidence.²¹ Qualitative study outcomes, outcomes of single case studies, and clinically relevant outcomes that were not analyzed statistically were not reported as significant effects. Further attention was paid to group differences, if applicable. When statistics were used, but no treatment effects or group differences were found, this was also indicated. The above aspects could only be included in Table 1 if they were mentioned in the article.

Table 1.

General Descriptions of the 31 Included Studies Focusing on the Effects of EAI on Participants with PTSD, Anxiety and/or Mood Disorders. The table provides detailed information on the type of study, study population, intervention type, psychological measures, physiological measures, and the significant treatment effects. Apparent missing information could not be retrieved from the studies.

Authors (year)	Research Design	Population	Intervention	Psychometric Parameters	Physiological Parameters	Significant Effects
Acri et al36 (2021)	Qualitative development, fidelity and reliability study	Youth Gender unspecified Childhood anxiety	10 weekly sessions lasting 1 hour PATH, EAAT, cognitive behavioural therapy, psychoeducation, relaxation techniques Grooming, groundwork, riding group elements	N/A	N/A	N/A
Alfonso et al22 (2015)	Quantitative Controlled Randomized	n = 11 after 1 dropout (TG n = 6 non-treatment CG n = 5) Age: 18-29 years, mean = 21.36, SD = 3.30 Females Self-reported having >4 symptoms of social anxiety	6 weekly sessions lasting 2-2.5 hours EAAT with cognitive-behavioural strategies Grooming, groundwork, riding Individual and group elements	Collected pre- and post-treatment, and 6-week follow-up: LSAS Acceptability questionnaire	N/A	Treatment effect on social anxiety immediately post-treatment and at follow-up compared to pre-treatment in TG, not in CG
Arnon et al27 (2020)	Quantitative No control	n = 8; n = 6 at 3-month follow-up Six male, 2 female Age: 30-61, mean 45.0, SD = 10.2 Military veterans with PTSD symptoms (PCL-5 ≥ 30; CAPS-IV≥50)	8 weekly sessions lasting 1.5 hours EAGALA, EAAT, experiential learning, relaxation exercise, focusing on physical sensations Grooming, groundwork Group elements	Collected pre-, mid-, post-treatment and at 3-month follow-up: BDI-II, CAPS-5, HAM-D, PCL-5, Q-LES-Q-SF Collected post-treatment: CSQ	N/A	Treatment effect on PTSD and depressive symptoms and quality of life from pre- to post-treatment (all questionnaires) Treatment effects for CAPS-5 and HAM-D evident from mid-treatment with additional effects at post-treatment Treatment effect at follow-up compared to pre-treatment for CAPS-5 and HAM-D
Burton et al6 (2019)	Quantitative Controlled Not randomized	n = 20; TG and CG (n = 10), after missing data of 1 CG Per group 8 male, 2 female Age: mean = 47, SD = 14 Military veterans with PTSD diagnosis (PCL-M ≥ 29) All participants had ongoing, active PTSD treatment during the study	6 weekly sessions lasting 1 hour CG: only standard previously established PTSD treatment TG: additional EAGALA, EAP, horses as metaphor Groundwork Group elements	Collected pre- and post-treatment: CD-RISC, PCL-M	Collected pre- and post-treatment: salivary cortisol	Treatment effect on PTSD symptoms (in TC and CG) and resilience (only in TG), but not on salivary cortisol No significant difference between TG and CG
Chakales et al28 (2020)	Quantitative No control	n = 28 5 male, 23 female Age unspecified, medical students No diagnosed mental disorders, meeting criteria for depression (BDI-II): mild (14-19) n = 3, moderate (20-28) n = 2, severe (29-63) n = 1	7 sessions lasting 3-4 hours, over a span of 2 months PATH, EAAT, interpersonal (communication) skills, leadership qualities, teamwork and self-care techniques, stress reduction and reflection, insight into: self-regulation of emotion, mindfulness, and behaviour Groundwork Group elements	Collected pre- and post-treatment: BDI-II, ESF, PSS	N/A	Treatment effect on stress and depression and subscales for loneliness, relationships with the opposite sex, sleeping difficulty, exercise, difficulty reading textbooks
Craig47 (2020)	Qualitative No control	n = 11 Female Age: 13-17 Diagnosed with mental health conditions, including anxiety, depression, PTSD, eating disorders, self-harm, due to adverse childhood experiences	3 hours mid-week and 8 hours Saturday, no. unspecified EAP	Observations and informal conversations for 4 months (>60hours) Interviews (30-45 minutes) with participants and staff	N/A	N/A
Craig et al48 (2020)	Qualitative No control	n = 11 Females Age: 13-17 years Diagnosed with mental health conditions, including anxiety, depression, PTSD, eating disorders, self-harm, due to adverse childhood experiences	3 hours mid-week and 8 hours Saturday, no. unspecified EAP	Observations and informal conversations for 4 months (>60 hours) Interviews (30-45 minutes) with participants and staff	N/A	N/A
Earles et al29 (2015)	Quantitative No control	n = 16 4 male, 12 female age: 33-62, mean = 51.25, SD = 9.99 PTSD symptoms (PCL-S≥31) and ≥1 criterion A traumatic event (LEC) Allowed to continue their current treatment or medication regimen	6 weekly sessions lasting 2 hours EAT Groundwork Individual and group elements	Collected pre- and post-treatment: AUDIT, FFMQ, GSE, LEC, LOT-R, PCL-S, PHQ (PHQ-9, PHQ-15, GAD), PCI, SSS, SWLS, TEQ	N/A	Treatment effects on PTSD, anxiety and depression, less severe emotional responses to trauma, increased mindfulness strategies, and decreased alcohol use
Ferruolo39 (2016)	Qualitative No control	n = 8 in total (n per group unknown) Males Military veterans being treated for combat-related depressive and anxiety disorders and reintegration issues	1 or 2 day program (2 day program had a week in between) lasting 7 hours each day Equine-facilitated mental health therapy Cognitive–behavioural therapy (CBT), motivational interviewing (MI), mindfulness-based stress reduction (MBSR), and mindfulness-based cognitive therapy (MBCT) frameworks, psychoeducation, guided experiential equine activities, horse used as metaphor and mirror, group processing Group elements	Collected post-treatment: Self-report evaluation with open-ended questions about the experience of the program	N/A	Treatment effect N/A No significant difference between the programs
Fisher et al30 (2021)	Quantitative No control Open trial	n = 63, dropout = 5 during program, 10 more at follow-up 40 male, 23 female Age: 27-50, mean = 50.01, SD = 12.38 Military veterans with PTSD diagnosis (PCL-5 ≥ 30, ascertained with SCID-5-RV and confirmed with CAPS-5) Co-morbid diagnosis (in total n = 48 at least one): major, persistent and other depressive disorder, generalized and social anxiety disorder, obsessive-compulsive disorder, alcohol use disorder, substance use disorder, adjustment disorder, attention-deficit/hyperactivity disorder Current medication permitted if stable for min. 3 months (n = 47 in total): psychotherapy (supportive, not treatment), pharmacotherapy or both	8 weekly sessions lasting 1.5 hours EAT, experiential, not trauma focused, awareness of self and others, communication, emotional response, critical thinking/problem solving, self-confidence/self-efficacy, psychoeducation Groundwork Group elements	Collected pre-, mid- and post-treatment and at 3-month follow-up: BDI-II, CAPS-5, HDRS, PCL-5	N/A	Treatment effect evident at midpoint (all questionnaires), with further reduction of PTSD and depression at post-treatment (all but HDRS) and maintained at follow-up
Johnson et al23 (2018)	Quantitative Controlled Randomized	n = 29 participants (TG n = 15, CG n = 14), dropout of 1 during WL-CG and 9 during THR Age eligible participants (38): 29-73, mean = 54.35, SD = 12.85 Eligible participants (38): 32 male, 6 female Military veterans with PTSD diagnosis or PTSD and traumatic brain injury	6 weekly sessions lasting 1 hour CG: wait list CG TG: PATH, THR, self-efficacy, emotion regulation, social and emotional loneliness Grooming, riding	TG+CG: Collected pre-, mid- and post-treatment CG: additionally 6 and 3 weeks pre-treatment CSES, DERS, PCL-M, SELSA	N/A	In TG treatment effect on only PTSD symptoms (PCL-M) evident at mid-treatment, with further reduction of PTSD at post-treatment
Johnson et al46 (2021)	Qualitative No control	n = 20 veterans, after dropout of 9 (from pop. of Johnson et al23 2018) 18 male (90%) Age: 29-68, mean = 52.90, SD = 12.71 Military veterans with PTSD diagnosis and/or traumatic brain injury	6 weekly sessions lasting 1 hour PATH, THR with human-horse interaction Grooming, riding	Collected post-treatment: 8-item riding questionnaire about the benefits, drawbacks, perceptions of experiences and recommendations	N/A	N/A
Lanning et al34 (2017)	Multi-method (quantitative and qualitative) No control	n = 51 33 male, 18 female Age: 22-57, mean = 36.8, SD = 11.26 Over 75% (n = 39) completed all 8 weeks of the program and 55% (n = 28) completed the 2-month follow-up surveys Military veterans and active duty service members with PTSD diagnosis or PTSD symptoms (PCL-M or PCL-5) 67% currently in therapy, 27% completed therapy	8 weekly sessions lasting 1.5 hours PATH, THR, balance and flexibility, memory, coping mechanisms, sensory integration, core muscle tone, trust, respect, and power conflicts Grooming, groundwork and riding Group elements	Collected pre-, mid- and post-treatment and at 2-month follow-up: PCL-M, PCL-5, SF-36v2, WHODAS 2.0 7 structured questions regarding experience of the program at week 8	N/A	Treatment effects on PTSD and mental health evident at mid- and post-treatment and follow-up compared to pre-treatment Treatment effects on vitality, social functioning and role limitations due to emotional problems at post-treatment compared to pre-treatment Decrease in PTSD symptoms was related to an improvement in mental health components and body function
Lanning et al25 (2018)	Quantitative Controlled Not randomized	n = 89 (TG n = 51, CG n = 38) At the end of the study 39 and 29 participants left in each group resp. (at follow-up 28 TG) 70% (n = 62 male) and 30% (n = 27 female) Age: 22-57, mean = 38.8, SD = 11.08 Military veterans with PTSD diagnosis Overall 69% completed and/or were currently in therapy for PTSD (75% TG, 61% CG)	8 weekly sessions lasting 1.5 hours CG: treatment-as-usual (i.e. cognitive behavioural therapy, exposure therapy) TG: PATH, THR, natural horsemanship philosophy, methods of communication, mutually respectful relationship Grooming, groundwork and riding Group elements	Collected pre-, mid- and post-treatment (all) and 2-months post-intervention (only TG): MDI, PCL-M, PCL-5, SF-36v2, WHODAS 2.0	N/A	Significant treatment effects n/a In TG compared to CG significantly higher scores for role limitations due to emotional problems and lower scores for functional disability pre-treatment; higher scores for limitations due to physical health problems mid-treatment; higher scores for mental health (summary score) and vitality post-treatment
Malinowski et al4 (2018)	Quantitative No control	n = 7 6 male, 1 female age: 31-68, mean = 58, SD = 13.14 Military veterans with PTSD diagnosis	5 daily sessions lasting 1 hour PATH, EAAT, mindfulness and active feelings exercises and equine biofeedback Grooming, groundwork	Collected pre- and post-treatment: BSI, PCL-5	Collected before, during and after each session: Heart rate Respiration rate Systolic and diastolic blood pressure	Treatment effect on overall PTSD (and particularly hyperarousal symptoms) and overall psychological distress The HR was significantly reduced on day 2 during the actual EAAT session
McCullough et al35 (2015)	Quantitative No control	n = 11, after dropout of 1 Age: 10-18 6 male, 5 female PTSD symptomatology (CRIES-13 ≥ 12 or ≥4 Level 3 or “ sometimes” answers)	8 weekly sessions lasting 1.5-2 hours EFP, experiential, cognitive-behavioural intervention	Collected pre-, mid- and post-treatment: CRIES-13, HABS	N/A	Treatment effect on PTSD symptoms Human-animal bond shows a significant increase Possible relationship between PTSD symptomatology and the human-animal bond (depending on the statistics used)
Meyer & Sartori (2019)	Qualitative No control	n = 5 Gender unspecified Age: 59-63 Military veterans with PTSD diagnosis Co-morbid diagnosis: depression, alcohol dependence in remission (n unspecified)	EFP, sensory awareness, communication Groundwork Group elements	Semi-structured interviews about human-horse attachment and the experience of the interactions	N/A	N/A
Monroe et al31 (2019)	Quantitative No control	n = 48, 42 completed all sessions (dropout = 6), 38 completed (most) pre-, post-measures 27 male, 13 female, 8 unknown Age: 24-64, mean = 43 Military veterans with PTSD: screened diagnosis: PCL-M > 50 n = 29, no diagnosis PCL-M < 50 n = 9, self-reported symptoms without screener n = 10	8 weekly sessions lasting 3 hours EAAT consisting of Cognitive Processing Techniques Grooming, groundwork, riding Group elements	Collected pre- and post-treatment: BAI, BDI-II, PCL-M, RAAS, QOLI	N/A	Overall treatment effects on PTSD, depression, quality of life and attachment anxiety Only significant PTSD symptom reduction in participants who met the PTSD diagnosis cut-off score (PCL-M>50) compared to who did not
Mueller & McCullough (2017)	Quantitative Controlled Quasi-randomized	n = 36 in TG, n = 32 in CG (after dropout of n = 14, TG: n = 25 and CG: n = 29) After dropout 45 male, 9 female Age: 10-18 Baseline score of 12 or higher on the CRIES-13 and a need for mental health treatment related to trauma-recovery	10 sessions, divided over 12 weeks, lasting 2 hours CG: trauma-focused cognitive behavioural therapy TG: PATH, EFP Grooming, groundwork, riding Group elements	Collected pre-, mid- and post-treatment: CRIES-13 HABS (only treatment group)	N/A	Treatment effects on PTSD (but not more compared to the CG that received traditional treatment) Human-animal bond scores remained stable across time and correlated negatively with change in PTSD symptoms
Naste et al41 (2018)	Qualitative No control	3 case studies with n = 1 Females Age: 10, 11 and 12 PTSD diagnosis Co-morbid diagnosis: Reactive Attachment Disorder and intellectual impairment (n = 1), learning disabilities (n = 1)	2 years (n = 1), others unspecified EFP for Complex Trauma, embedded within the trauma-informed framework called Attachment, Regulation and Competency (ARC), core components: safety, attachment, regulation and in addition routines & rituals Overall, but not in each case clearly mentioned: grooming, groundwork, riding	Collected at unspecified timepoints: Narrative information provided by clinicians ADI, A-DES, BRIEF-P, CAM, CBCL, CDC-3, CDI-2, PTSD-RI, SAM	N/A	N/A
Nevins et al42 (2013)	qualitative no control	n = 1 male age: 52 military veteran who met criteria for PTSD and major depression (CAPS = 103) ongoing treatment: Citalopram	3 daily sessions lasting 4 hours Saratoga WarHorse Connection method (nonverbal language of the horse), educational classes groundwork individual elements	Collected: pre- and post-treatment and at follow-up after 2, 4, 6 and 12 weeks: BDI-II, MSSS, PCL-C, RSES, QOLI	N/A	N/A
Romaniuk et al5 (2018)	Quantitative No control Different TG’s not randomized	n = 47 (n = 25 individual veterans [IP] and n = 22 couples program [CP]) 100% completed all measures (except PCL-5 of 5 participants, due to error), at follow up 66.67% returned PLC-5 and 59.57% other measures IP: 19 male, 6 female, age: 26-72, mean = 50.28, SD = 14.59 CP: 10 male, 1 female, age: 29-68, mean = 41.12, SD = 10.03 Partners: 1 male, 10 female, 31-57, mean = 40.82, SD = 9.42 Military veterans and their partners who identify as wounded, injured or ill including a self-report of a physical and/or psychological condition	5 consecutive days, reside on site EAT Relational Gestalt Therapy, mindfulness, grounding techniques, natural horsemanship, experiential, social engagement skills Additionally for couples: couples date and counselling Grooming, groundwork Group elements	Collected pre- and post-treatment and at 3-month follow-up: DASS-21, OHQ, PCL-5, Q-LES-Q-SF	N/A	Treatment effects at post-treatment on depression, stress, PTSD, happiness, quality of life for both IP and CP, for IP also anxiety Treatment effects at follow-up compared to pre-treatment on depression, anxiety, stress and PTSD for CP but not for IP At follow-up the CP had fewer symptoms of depression, stress and PTSD than the IP, and a lower percentage met criteria for provisional PTSD diagnosis
Rosing et al44 (2022)	Qualitative No control	n = 12, after dropout of 1 (from pop. of Shelef et al45 2019) Gender unspecified Military and police veterans with PTSD diagnosis (DSM-5) Age: 27-64, mean = 41.25, SD = 11.19 Other medication: all participants (antidepressants n = 6, antipsychotic medication n = 8, benzodiazepines n = 9, mood stabilizers n = 1, medical cannabis n = 9)	Weekly sessions for 6 months lasting 3 hours EAGALA, EAT, experience based therapy, attachment theory, narrative approach Grooming, groundwork, riding Group elements	1-2.5 hours long semi-structured in-depth interviews about the participant’s experiences in the original EAT program of about 3 years before	N/A	N/A
Schroeder & Stroud37 (2015)	Qualitative No control Practice and feasibility study	n = 4 Females Age: mid 20 - early 60 Symptoms of posttraumatic stress and a history of interpersonal violence victimization	9 weekly sessions lasting 2 hours PATH, Equine Facilitated Group Psychotherapy (EFGP) Experiential, safety/trust building, mind/body awareness, reconnection and intimacy, creating and maintaining healthy boundaries, emotional regulation and distress tolerance, empowerment, intra- and interpersonal awareness Grooming, groundwork Group elements	N/A	N/A	N/A
Shelef et al45 (2019)	Quantitative No control	n = 23, of which 13 completed Completers: 11 male, 2 female; non-completers 9 male, 1 female Age completers: mean = 38.8, SD = 10; non completers: mean = 34.7, SD = 10.6 Established DSM-5 PTSD diagnosis (referred by Israel Ministry of Defense) Other medication: antidepressant (6 completers, 6 non-completers), antipsychotic (8 completers, 2 non-completers), benzodiazepine (9 completers, 4 non-completers), mood stabilizer (1 completer, 0 non-completers), medical cannabis (9 completers, 5 non-completers)	Weekly sessions for 6 months lasting 3 hours EAT, trust, communication Grooming, groundwork, riding Group elements	Collected pre- and post-treatment: SPRINT Collected pre-treatment, after 1 month and post-treatment: SDS	N/A	Treatment effect on PTSD symptoms, re-establishing functioning (ability to work and perform daily tasks), and efficiency (reduction of inefficiency in work/study/house work) Treatment effect on disability, evident from 1 month
Steele et al32 (2018)	Quantitative No control	n = 85 60 male, 25 female Age: 22-72, mean = 42.94, SD = 11.63 Military veterans with PTSD	7 consecutive days, reside on site EAGALA, therapeutic treatment designed to address the effects of combat and incorporating EMDR, yoga, narrative writing and equine-assisted psychotherapy (EAP), enhance skills for problem solving, communication, relational attachment, confidence and trust, self-control, setting healthy and safe boundaries, and calming the nervous system Groundwork Group elements	Collected pre- and post-treatment: DES, DTS, MIES, M-PTSD, PHQ-9, RAAS	N/A	Treatment effect on distress, PTSD, depression, moral injury, dissociation and adult attachment
Sylvia et al49 (2020)	Qualitative No control	n = 65 military veterans and n = 49 family members (partner, parent, friend, other) 3 veterans and 5 family members did not complete questionnaires Veterans: 54 male, 11 female Veterans: age 25-57, mean = 37.95, SD = 8.02 Veterans: PTSD (n = 53) and/or traumatic brain injury (n = 9), also Alcohol abuse (n = 1) and Depression (n = 2)	3 sessions within 2 subsequent days lasting 2 hours EAAT, mindfulness, caretaking, non-horse based activities Grooming, groundwork, riding Group elements	Collected post-treatment: Satisfaction survey with 11 open-ended questions	N/A	N/A
Wharton et al38 (2019)	Quantitative No control Also feasibility	n = 27, dropout of n = 2 (7%) after 1 session 78% male Age: mean = 51 Military veterans with PTSD diagnosis (PCL-M) Co-morbid diagnosis: n = 4 dissociative identity disorder (2 men, 2 women), all participants but 1 had additional mood-related disorders	12 weekly sessions Equine Facilitated Cognitive Processing Therapy (EF-CPT), identifying emotions, retelling of the traumatic event, identifying thinking patterns, and addressing trust, intimacy, power, and control Grooming, groundwork Individual elements	Collected pre- and post-treatment: HABS, PCL-M, TRGI, WAI-SF	N/A	Treatment effect on PTSD symptoms, trauma related guilt HABS and WAI-SF indicated a good relationship at baseline and significant improvement throughout the therapy
Willmund et al24 (2021)	Quantitative Controlled Not randomized	n = 36 opposite-sex couples, divided in n = 20 therapy group and a n = 16 couples no-treatment CG 95% response rate at end program Every couple included 1 partner that was a military veteran with PTSD diagnosis TG: 19 male, 1 female, age: mean = 41.20, SD = 10.90 CG: 15 male, 1 female, age: mean = 39.50, SD = 9.24 No ongoing other treatment	3 consecutive days, reside on site EAGALA, EAI, experiential, trauma therapeutic (DeGPT), psychodynamic psychotherapeutic guidelines, focus on increased insight, resource activation, and overall stabilization Groundwork Group elements	Collected pre- and 6 weeks post-treatment: FPD (PL), LEC-5, PCL-5 (only to PTSD patients), PFB, PHQ (PHQ-9, PHQ-15 and PHQ-stress)	N/A	Treatment effect in TG for the veterans with PTSD on negative alterations in cognition and mood, total symptom severity, problems in the relationship and depressive symptoms Treatment effect in TG for the partners without PTSD on problems in the relationship, somatic symptoms and current problems, communication and relationship quality
Yorke et al43 (2008)	Qualitative No intervention	n = 6 2 male, 4 female Age: 18-51 All participants experienced debilitating physical and/or psychological trauma	No program Caretaking, groundwork, riding, self-considered as being therapeutic during recovering from trauma Had been riding since childhood, resumed horseriding at least 2 to 3 months before participating in the study Riding duration: 30 minutes - several hours Riding frequency: 3-7 times per week (except 1 participant)	2 semi-structured, audio taped interviews (each ~2 hours long), exploring experienced trauma and the human-horse relationship which was videotaped (30-60 minutes) and discussed	N/A	n/a
Zhu et al33 (2021)	Quantitative No control	n = 19, after dropout of 1 9 male, 10 female Age: mean = 46.64, SD = 12.34 Military veterans with PTSD (PCL-5, ascertained with SCID-5-RV) Co-morbid diagnoses: depressive disorder (n = 10 major, n = 4 persistent, n = 1 other) and alcohol use disorder (n = 3) Ongoing other treatment allowed (n = 11 medication, n = 11 psychotherapy, n = 14 any treatment)	8 weekly sessions lasting 1.5 hours EAT, psychoeducation, team feedback and direction Group elements	Conducted pre- and post-treatment and at 3-month follow-up: BDI-II, CAPS-5, HAM-D, PCL-5	Conducted pre- and post-treatment: Longitudinal multimodal MRI assessments / neuro-imaging including structural magnetic resonance imaging (sMRI), resting state-fMRI (rs-fMRI) and diffusion tensor imaging (DTI)	Treatment effect on PTSD and depression (BDI-II, CAPS-5, PCL-5, but not HAM-D) at post-treatment and follow-up compared to pre-treatment Post-treatment effects include an increase in caudate functional connectivity (FC), and a reduction in the gray matter density of the thalamus and the caudate Other effects include positive associations between improved caudate FC with clinical improvements post-treatment and at follow-up, and between a higher baseline caudate FC and PTSD symptom reduction (both effects depending on the questionnaire)

For abbreviations of the questionnaires see Table 2.

CG, control group; EAAT, Equine Assisted Activities and Therapies; EAGALA, Equine-Assisted Growth and Learning Association; EAP, Equine Assisted Psychotherapy; EAT, Equine Assisted Therapy; EFP, Equine Facilitated Psychotherapy; PATH, Professional Association of Therapeutic Horsemanship International; PTSD, post-traumatic stress disorder; TG, treatment group; THR, Therapeutic Horseback Riding.

A more detailed overview of all questionnaires used is given in Table 2, focusing on the name and abbreviation of the questionnaire, the number of items of the questionnaire, whether the questionnaire is self-reported or used by a clinician, the aim of the questionnaire, and which studies included in this review used a particular questionnaire. This was done to provide insight into which questionnaires are used in the field, which may aid standardization of future studies.

Table 2.

A List of Questionnaires Used to Investigate the Effects of EAI on Participants with PTSD, Anxiety and/or Mood Disorders. The table includes the name and abbreviation of the questionnaires, the number of items, whether it is self-reported or not, the aim of the questionnaire, and lists which studies in this review used the questionnaires. When the number of items is missing, it could not be retrieved from the study. The table differentiates between questionnaires used for adults and children.

Name		Items	Reported	Aim	References
Adults
AUDIT	Alcohol Use Disorders Identification Test	10	Self	Alcohol consumption	Earles et al (2015)
BAI	Burns Anxiety Inventory	33	Self	Anxiety	Monroe et al (2019)
BDI-II	Beck Depression Inventory II	21	Self	Depression	Arnon et al (2020), Chakales et al (2020), Fisher et al (2021), Monroe et al (2019), Nevins et al (2013), Zhu et al (2021)
BSI	Brief Symptom Inventory		Self	Psychological distress	Malinowski et al (2018)
CAPS	Clinician-Administrated PTSD Scale (IV: DSM-4, V: DSM 5)		Clinician	PTSD	Arnon et al (2020), Fisher et al (2021), Zhu et al (2021)
CD-RISC	Connor-Davidson Resilience Scale	25	Self	Resilience	Burton et al (2019)
CSES	Coping Self Efficacy Scale	26	Self	Coping	Johnson et al (2018)
CSQ	Client Satisfaction Questionnaire		Self	Satisfaction	Arnon et al (2020)
DASS-21	Depression Anxiety Stress Scale-21		Self	Depression, anxiety and stress	Romaniuk et al (2018)
DERS	Difficulties in Emotion Regulation Scale	36	Self	Emotional arousal	Johnson et al (2018)
DES	Dissociative Experiences Scale	28	Self	Dissociation	Steele et al (2018)
DTS	Davidson Trauma Scale	17	Self	PTSD	Steele et al (2018)
ESF	Evaluation of Stress Factors (original 29 items: this study selected 12 items)	12	Self	Stress	Chakales et al (2020)
FFMQ	Five Facet Mindfulness Questionnaire	39	Self	Mindfullness	Earles et al (2015)
FPD (PL)	Fragebogen zur Partnerschaftsdiagnostik (Problemliste)	23	Self	Partnership quality	Willmund et al (2021)
GSE	General Perceived Self-Efficacy Scale	10	Self		Earles et al (2015)
HABS	Human-Animal Bond Scale	15	Caregiver	Human animal bond	Wharton et al (2019)
HAM-D / HDRS	Hamilton Depression Rating Scale	17	Clinician	Depression	Arnon et al (2020), Fisher et al (2021), Zhu et al (2021)
LEC	Life Events Checklist for DSM-5 (LEC-5: version for DSM V)	17	Self	Potentially traumatic events in a respondent’s lifetime	Earles et al (2015), Willmund et al (2021)
LOT-R	Revised Life Orientation Test (LOT-R) is a .	10	Self	How optimistic or pessimistic people feel about the future	Earles et al (2015)
LSAS	Liebowitz Social Anxiety Scale	24	Self	Social anxiety	Alfonso et al (2015)
MDI	Major Depression Inventory	10	Self	Depression	Lanning et al (2018)
MIES	Moral Injury Events Scale	11	Self	Moral injury	Steele et al (2018)
M-PTSD	Mississippi scale for Combat-related PTSD	35	Self	Combat-related PTSD in veteran populations	Steele et al (2018)
MSSS	Modified Social Support Survey	18	Self	Support	Nevins et al (2013)
OHQ	Oxford Happiness Questionnaire	29	Self	Happiness	Romaniuk et al (2018)
PCI	Proactive Coping Inventory (original 55 items, this article selected 14-item pro-active coping subscale)	14	Self	Proactive approaches to coping	Earles et al (2015)
PCL	Posttraumatic Stress Disorder (PTSD) Checklist (PCL C: civilian, M:military, S: specific, 5: DSM V)	17	Self	PTSD	Arnon et al (2020), Burton et al (2019), Earles et al (2015), Fisher et al (2021), Johnson et al (2018), Lanning et al (2017 & 2018), Malinowski et al (2018), Monroe et al (2019), Nevins et al (2013), Romaniuk et al (2018), Wharton et al (2019), Willmund et al (2021), Zhu et al (2021)
PFB	Partnership Questionnaire	30	Self	Partnership quality	Willmund et al (2021)
PHQ	Patient Health Questionnaire (PHQ-9, PHQ-15, GAD-7: Generalized Anxiety Disorder, Stress)	9, 15, 7, 10	Self	Depression, somatic symptoms, anxiety, stress	Earles et al (2015), Steele et al (2018), Willmund et al (2021)
PSS	Perceived Stress Scale		Self	Stress	Chakales et al (2020)
RAAS	Revised Adult Attachment Scale	18	Self	Attachment	Monroe et al (2019), Steele et al (2018)
RSES	Response to Stressful Experiences	22	Self	Resiliency	Nevins et al (2013)
SCID-5-RV	Structured Clinical Interview DSM V Research Version		Clinician	Psychiatric disorders	Fisher et al (2021), Zhu et al (2020)
SDS	Sheehan Disability Scale	3	Self	Functional disability	Shelef et al (2019)
SELSA	Social and Emotional Loneliness Scale for Adults-short version	15	Self	Loneliness	Johnson et al (2018)
SF-36v2	Quality of Life Assessment	36	Self	Quality of life	Lanning et al (2017 & 2018)
SPRINT	Short Post Traumatic Stress Disorder	8	Self	PTSD	Shelef et al (2019)
SSS	Social Support Scale	27	Self	Social support	Earles et al (2015)
SWLS	Satisfaction With Life Scale	5	Self	Life satisfaction	Earles et al (2015)
TEQ	Trauma Emotion Questionnaire	18			Earles et al (2015)
TRGI	Trauma-Related Guilt Inventory	32	Self	Trauma related guilt	Wharton et al (2019)
Q-LES-Q-SF	Quality of Life Enjoyment and Satisfaction Questionnare-Short Form	16	Self	Quality of life, enjoyment, and satisfaction	Arnon et al (2020), Romaniuk et al (2018)
QOLI	Quality of Life Inventory	32	Self	Quality of life	Monroe et al (2019), Nevins et al (2013)
WAI-SF	Working Alliance Inventory - Short Form	12	Self	Quality of alliance	Wharton et al (2019)
WHODAS 2.0	World Health Organization Disability Assessment Schedule 2.0	36		Functioning	Lanning et al (2017 & 2018)
Children
A-DES	Adolescent Dissociative Experience Scale	30	Self	Dissociation, age 12-18 years	Naste et al (2018)
ADI	Abbreviated Dysregulation Index	30	Self	Dysregulation	Naste et al (2018)
BRIEF-P	Behaviour Rating Inventory of Executive Functioning - Parent	86	Caregiver	Executive functioning	Naste et al (2018)
CAM	Children’s Alexithymia Measure	14	Parent	Recognizing and expressing feelings, age 5-17 years	Naste et al (2018)
CBCL	Child Behavioural Checklist	120	Caregiver	Functioning and behavior problems, age 6-18 years	Naste et al (2018)
CDC-3	Child Dissociation Checklist	20	Parent	Dissociation	Naste et al (2018)
CDI-2	Children’s Depression Inventory	28	Self	Depression, age 7-17 years	Naste et al (2018)
CRIES-13	Children’s revised inventory of Events Scale	13	Self	PTSD	McCullough et al (2015), Mueller & McCullough (2017)
HABS	Human-Animal Bond Scale	15	Caregiver	Human animal bond	McCullough et al (2015), Mueller & McCullough (2017)
PTSD-RI	UCLA-PTSD Stress Reaction Index	31	Self	PTSD	Naste et al (2018)
SAM	Somatic Awareness Measure	24	Self	Bodily and somatic awareness	Naste et al (2018)

PTSD, post-traumatic stress disorder.

Results

Treatment Effects

Controlled studies investigating the effects of EAI were sparse, but overall indicated positive treatment outcomes. Most of the controlled studies found treatment effects restricted to the treatment group (TG) that received EAI.^6,22-24 One study directly compared the TG and the CG and found significantly larger post-treatment effects on quality of life (mental components and vitality) in veterans with PTSD in the TG compared to the CG.²⁵ The CG participants in these studies did not receive additional therapy or they continued already ongoing therapy that was also allowed in the TG, indicating beneficial effects of EAI. Mueller et al²⁶ did not find differences between a TG and a CG that received conventional therapy. Both groups showed PTSD symptom reduction, suggesting that EAI did not provide additional effects.

Most studies investigated the effects of EAI based on questionnaires and found less severe symptoms of post-traumatic stress, less severe symptoms of depression or impairment on daily life,^5,24,27-33 less severe symptoms of anxiety,^5,22,29,31 improved quality of life^{5,25,27,31,34} and less stress^4,5,28 (see Table 1). Significant treatment effects were less evident in the few studies that used physiological measures. Burton et al⁶ did not find significant differences in salivary cortisol in participants with PTSD pre- and post-treatment, or between the TG and CG. Malinowski et al⁴ only found a significant reduction of heart rate on day 2 during the EAI session, which was explained by the calmer type of activity conducted on that day. Zhu et al³³ found structural and functional changes in the basal ganglia/limbic network (reward circuitry) post-treatment, and this was positively associated with clinical improvements. These findings suggested that enhanced reward responsiveness may underlie clinical improvements.³³

Most studies investigated treatment effects of EAI over time and found 1 or multiple psychological effects comparing post- to pre-treatment (see Table 1). Some studies also investigated significant treatment effects at mid-treatment, and most of these found effects evident from mid-treatment^23,27,30,34 and/or additional treatment effects from mid- to post-treatment.^23,27,30 Two studies that found treatment effects post-treatment did not find effects at mid-treatment.^26,35 The studies that investigated follow-up effects showed that there was a follow-up effect compared to pre-treatment or that post-treatment effects were maintained at follow-up.^{5,22,27,30,33,34} See Table 1 for details. To objectively assess the effectiveness of EAI, this review not only discusses the treatment effects found by included studies but also puts the results into perspective by investigating the methodology.

Research Designs

The majority of the studies were from the last decade and had a quantitative approach (see Table 1). Eleven studies were considered qualitative (see Table 1) and 1 multi-method study included both qualitative and quantitative elements.³⁴ Some qualitative studies also investigated the development and application of EAI programs^36,37 or their feasibility.³⁸ The quantitative studies that included a CG either used a no-treatment CG,^22,24 a wait-list CG,²³ or a treatment as usual CG.^6,25,26 Half of the controlled studies randomly assigned the participants to the different groups.^22,23,26

Study Population

The sample size of the included studies differed greatly, but 8 studies had a relatively small sample size (n <10).^{4,27,37,39-43} Sometimes studies mentioned the sample size without including dropout rate, and sometimes the dropout rate was not clearly described at all (see Table 1). This hinders a clear summary of the sample sizes and dropout numbers in this review. In half of the studies that did mention dropout rates, the dropout rate was relatively small (<10%),^{22,30,33,35,38,44} ranging up to 10 of 23 participants (indicating 43%) in the study of Shelef et al⁴⁵ Furthermore, some studies in this review seem to have reused samples from other studies, based on comparable study descriptions and authorship.^{23,25,34,44-48} Studies on military veterans are overrepresented in this review.^{4-6,23,25,27,30-34,38-40,44,46,49} and mostly focused on PTSD. Most studies investigated the effects of EAI on adult participants, but some on children (age <18).^{26,35,36,41,47,48} Overall, the gender of the participants was skewed to males, especially in veteran studies (see Table 1).

The selection criteria of the research populations varied and were not always clear. Assessment of eligibility for participation varied, as some studies used self-reported symptoms or questionnaires (e.g. ^5,22,31,34), while others used a clinician to assess symptoms or to confirm diagnosis (e.g. ^27,30,33). A diversity of questionnaires was used to assess symptoms and diagnoses (Tables 1 and 2). Across studies, cut-off scores for diagnosis were not always the same even when they used the same questionnaire^6,31 and some studies did not report which cut-off scores were used (e.g. ^34,38). Multiple studies indicated co-morbidity, such as military veterans that were physically or psychologically injured,⁵ patients with PTSD that also had brain injury,^23,49 or a broader range of symptoms including anxiety, depression, eating disorders, and self-harming behaviour.^47,48 In addition, some studies allowed participants that had other (ongoing) treatment (such as psychotherapy, pharmacotherapy, and supportive therapy) during the EAI programs.^{6,25,29,30,33,34,39,42,44,45} The level of detail about ongoing treatment varied, as well as the number of participants on a certain treatment (see Table 1).

Intervention

The timespan of the program, the duration, and frequency of the sessions varied greatly, but generally, programs entailed 6-8 sessions of 1-3 hours on average (see Table 1). Although most programs were given on a weekly basis, some entailed multiple days where the participants resided on site.^5,24,32 A clear description of the program and/or a classification of the interventions was not always provided. About half of the studies adhered to either EAGALA models or used PATH certified instructors.^{4,23,25,26,28,34,36,37,46} Articles also differed in the terminology used to describe their program. Many used similar umbrella terms such as Equine Assisted Therapy (EAT) or EAAT (see Table 1). Some used the term EAP,^6,32,47,48 others Equine Facilitated Psychotherapy (EFP),^26,35,40,41 some Equine-Facilitated Mental Health Therapy,³⁹ and a few papers used THR.^23,25,34,46 Articles described different psychotherapeutic elements within their programs, such as experiential therapy,^{5,24,27,30,35,37,39} cognitive behavioral (therapy, processing or strategies),^{22,25,26,31,35-39} and relaxation techniques, mindfulness and/or yoga.^{4,5,27,28,32,36,39,49}

In most studies, EAI programs included groundwork and grooming (see Table 1), and a smaller number included riding.^{22,26,31,36,41,43-45,49} A few studies did not clearly state what activities were included in their program.^{33,35,39,47,48}

Most of the included papers indicated that their program included group elements (see Table 1) and a few (also) included individual elements.^22,29,38,42 Eight papers did not (clearly) mention whether there were any individual or group elements.^{4,23,35,41,43,46-48}

Psychological Measures

All studies included psychological measures such as the use of questionnaires or open interviews, except the studies that focused on the development of EAI programs.^36,37 Most studies that investigated PTSD in adults used a version of the PCL questionnaire, such as the PCL-5,^{4,5,24,25,27,30,33,34} the PCL-M,^{6,23,25,31,34,38} the PCL-S²⁹ or the PCL-C.⁴² To a lesser extent, PTSD symptoms were measured by the CAPS-5,^27,30,33 the M-PTSD,³² the DTS,³² and the SPRINT.⁴⁵ Other questionnaires related to trauma were the Moral Injury Events Scale for moral injury,³² LEC for trauma history^24,29 and Trauma-Related Guilt Inventory for trauma-related guilt.³⁸ Most papers that focused on evaluating depression in adults, used the BDI-II^{27,28,30,31,33,42} and to a lesser extent the HAM-D/HDRS,^27,30,33 PHQ-9,^24,29,32 DAS-21⁵ and MDI.²⁵ Dissociation in adults was measured with DES.³² The human-animal bond was assessed with the HABS.^26,35,38 Anxiety was assessed by the GAD part of the PHQ,²⁹ the BAI,³¹ DAS-21⁵ and LSAS.²² Several questionnaires that focused on quality of life (including life satisfaction and happiness) were the Q-LES-Q-SF,^5,27 QOLI,^31,42 SWLS,²⁹ OHQ⁵ and SF-36V2.^25,34 Resiliency was measured by the CD-RISC⁶ and RSES.⁴² Functioning was assessed with SDS⁴⁵ and WHODAS.^25,34 Other questionnaires, that were used for children or were only used once by the studies in this review, as they addressed specific aspects of (mental) health (e.g., relationship quality, expressing feelings, behavioral problems), can be found in Table 2.

Physiological Measures

Very few papers included physiological measures to investigate the effects on EAI on humans with anxiety and mood disorders.^4,6,33 Burton et al⁶ measured salivary cortisol in participants with PTSD at the start and end of the program. Malinowski et al⁴ measured heart rate, respiration rate, and systolic and diastolic blood pressure before, during, and after each session in veterans with PTSD. Zhu et al³³ is the only scientific study that used neuroimaging techniques to investigate the effects of EAI on humans with PTSD. Measurements occurred pre- and post-treatment and included resting-state fMRI (rs-fMRI), structural magnetic resonance imaging (sMRI), and diffusion tensor imaging to measure neural changes following EAI. For more detailed information see Table 1.

Discussion

The aim of this systematic review was to determine the effects of EAI on patients with PTSD, anxiety and/or mood disorders. Furthermore, this review provided insight into the research designs, study populations, the interventions, and the psychological and physiological measures used. Although all quantitative studies within this review that used psychological measures found 1 or more positive treatment effects on symptoms of PTSD, anxiety and/or mood disorders, well-designed studies were sparse. Treatment effects were less evident for the few studies that used physiological measures. Due to methodological issues, interpretation of study results could be a dark horse bet and should be done carefully. These issues will be discussed below, and possible solutions and future directions will be provided.

Research Design

This review shows that poor research design is still an important issue within the field of EAI. Eighteen quantitative studies, twelve qualitative studies, and 1 multimethod study were included. Quantitative studies with a randomized CG are of higher quality evidence for clinical decision-making,²¹ but in this review, many of these studies showed limitations such as a small sample size, lack of CGs, no randomization, and/or no long-term follow-up. Other reviews that measured the effects of EAI on a wide range of mental disorders^9-11 and PTSD specifically^3,7,12 also emphasized the early development of the field with resulting methodological flaws and the need for randomized controlled trials. Randomizing a research design addressing military veterans with PTSD may be difficult since participants may indicate that they do not want to be part of a process where they or their comrades would not receive treatment.²⁵ Veterans specifically are more likely to participate when everyone will receive active treatment.⁵⁰ Practical solutions could be comparing a randomized wait-list CG to a TG and then giving the CG treatment afterward as part of the trial,²³ or a positive CG in which participants receive conventional treatment without a horse present. Additionally, this may provide insight into the working mechanisms of EAI, as it allows to investigate the effects of the presence of a horse separately from other program components.

Study Population

The included papers show ambiguity in dropout rates, selection criteria, co-morbidity inclusion, and ongoing other treatments. Studies were often based on small sample sizes, and high dropout rates may decrease the power of the study even more. It is often not clearly described what the dropout rate was, and there was a large variability in the dropout rates that were mentioned. Reporting dropout rate and the definition of it is important because results may be biased towards participants that completed the program and completed all measurements. This issue was also addressed by Varker et al,⁵¹ who found that only 7% of studies properly defined dropout and that only half of the included studies mentioned reasons for dropout. Literature indicates a higher dropout rate in military populations with PTSD compared to other populations⁵¹ and, for this population, a higher dropout for trauma-focused therapy compared to non-trauma-focused therapy.^51,52 The meta-analysis of Imel et al⁵³ showed that for non-trauma-focused treatment, dropout ranged from 0% to 48%, which is comparable to the range in dropout rate in this review. This indicates the potential of EAI programs having relatively low dropout rates compared to conventional therapy.

This systematic review showed large differences in inclusion criteria, and the words “symptoms” and “diagnosis” are sometimes unclear and interchangeably used. This also suggests a difference in symptom severity in participants. Differences in symptom severity may also occur within a certain disorder; for example PTSD may differ from chronic and/or therapy-resistant PTSD. In addition, it is not always clear by whom and on what grounds the diagnosis was made, but the type and validity of questionnaires varied, as well as the cut-off points that were used. This further emphasizes the recommendation of Boss et al³ to standardize the set of psychometric instruments used and stresses the importance of a proper description of admission criteria for future repeatability. To aid this, the review provided a table with questionnaires mostly used in research on EAI. In this review, 6 out of 17 studies that found significant treatment effects allowed other ongoing treatments outside EAI. This makes it questionable to attribute the positive treatment effects only to EAI in these studies. Detailed information on comorbidity within the study population is important, as this may affect treatment outcomes. In conclusion, it is recommended for future studies to have a sufficiently large sample size to reach statistical power, provide information on dropout rates and their definition, use clear selection criteria based on diagnoses with the recommended questionnaires in this review, provide information on comorbidity, and exclude patients that have ongoing other treatments.

Intervention

Willmund et al. (2021) showed that not only veterans benefited from EAI interventions, but also their partners showed improved somatic symptoms and improved relationship quality. In addition, in children who experienced maltreatment social support from family members mediates the symptom severity of PTSD. This may indicate that social support is important and that involving the partners of the patients may be more effective in reaching long-term effects, because the system around the participant changes as well.

The current review showed a large variety in frequency and duration of EAI, the used nomenclature, the type of (psycho)therapeutic interventions, the type of practical exercises with the horse, and whether the program contained individual or group elements. Standardization and reporting on these aspects are essential to make future studies on EAI repeatable, comparable, and of higher quality, leading to more effective implementations of EAI programs. This review showed that most programs entailed 6-8 sessions of 1-3 hours. This is in line with the study of Marchand et al,⁷ that proposed a standardized model EAI intervention for PTSD based on 6 sessions of 2 hours. However, a dose-response curve for EAI is currently unknown. In addition, this review showed that a clear description of the program is often not provided, and it is unclear what type of psychotherapeutic interventions and/or practical exercises were included. Marchand et al⁷ pleaded for the use of a standardized nomenclature based on the work of Hallberg.⁸ In contrast, it is recommended to use standardized terms to classify EAI within conventional therapeutic nomenclature. This would enable EAI to connect with conventional therapy, instead of dismissing it as an alternative therapeutic intervention. For example, several studies within this review could be classified as non-trauma-focused therapy, as they sometimes include relaxation techniques or mindfulness. Other studies could be classified as trauma-focused therapy as they include cognitive-behavioral therapeutic interventions. Therefore, EAI might be classified as an additional non-trauma-focused therapy that may include (trauma-focused) psychotherapeutic elements.

So far, trauma-focused therapies such as cognitive-behavioral therapy or exposure therapy are the first choice and seem effective in PTSD patients.^1,54 However, certain populations, such as military veterans, experience reduced improvement after these conventional types of therapy.¹ Additionally, trauma-focused therapies may have higher dropout rates and poorer treatment compliance due to the aversive nature of the procedures, which drives the need for alternative treatment options.⁵⁴ Non-trauma-focused therapies such as mindfulness can be clinically useful in patients with PTSD,^55-57 and could be incorporated into empirically supported treatments.⁵⁸ This is in line with the study by Burton et al⁶ that combined EAI with standard treatment and found additional effects of EAI on resilience. Although more research is needed, this suggests that EAI can be incorporated into standard (trauma-focused) therapies and may render additional effects.

This review showed insufficient reporting considering the activities and/or elements included in the EAI program. This hindered drawing conclusions on these aspects based on this review. Marchand et al⁷ recommended an intervention model without riding due to the associated risk and complexity with riding, and the lack of evidence that riding has additional positive effects. This review does not provide evidence that riding had additional effects or not. It could be a focus for future studies to investigate the efficacy of specific EAI elements, as this could provide more insight into the working mechanisms of EAI. In addition, investigating differences between individual and group-based EAI programs would be a starting point to indicate whether social support from other participants may contribute to the effects.

Measures

This review provided insight into questionnaires that are mostly used for certain mental disorders. Standardizing the use of these questionnaires as psychological measures should promote comparability and repeatability in the future. It is recommended to collect measures at least at pre-, mid-, and post-treatment, at 3-month follow-up, and preferably also after a year to investigate long-term effects. Establishing a dose-response curve is needed to optimize treatment efficacy while minimizing participant investment and program costs.

The current review also showed that it is not customary to use physiological measures when determining the effects of EAI in human patients. Only 3 studies in this review combined psychological measures with physiological measures. Although psychometric self-reports are seen as the gold standard in assessing PTSD symptoms, these results can be compromised by memory, emotional, or cultural bias.⁵⁹ Using psychophysiological measures can be complementary, as heart rate, heart rate variability, skin conductance, electromyography (EMG), and blood pressure are potential autonomic indices of emotion regulation and treatment related changes.^59-61 More research on the physiological effects of EAI may be especially important as the stress-reducing effect of oxytocin is a hypothesized working mechanism of AAI.¹⁵ For example, petting and stroking dogs led to oxytocin production and a subsequent decrease of the stress hormone cortisol in their human owners.^18,19 This finding is consistent with other findings in the literature that illustrated a decrease in subjective arousal and heart rate when petting horses.⁶² Including psychophysiological measures will contribute to the comparability of studies, expose mechanisms of action^7,12 and further substantiate the effects found by psychological measures.

Limitations

The main limitations of this review are that the number of rigorous studies included is small and essential information was lacking. By keeping the search strategy broad, most papers on the subject should have been retrieved, but papers published after September 2022 might have been missed. Another limitation of this review is that it is likely that published studies tend to bias towards reporting positive results, and negative effects tend not to be published, which may also affect the results of this review. Other sources of bias may include the overrepresentation of male military veterans. Participants were self-selected, leading to a bias towards people who are willing to work on their mental health and feel attracted to the program. This review included both qualitative and quantitative studies, but the reported treatment effects focused mainly on the quantitative studies, which used statistical methods. Treatment effects from the qualitative studies were considered to contain lower empirical evidence. Nevertheless, these qualitative studies may provide valuable information on the effects of EAI.

Although EAI seems to be promising in reducing symptoms in participants with PTSD, anxiety and/or mood disorders, there are many methodological issues that impede establishing the effectiveness of EAI. Within this review, there is a lack of well-designed quantitative studies that use randomized and controlled setups, and there is a large heterogeneity in how the effects of EAI are measured and reported. A waitlist or positive CG could be a practical solution for future studies. In addition, participant selection should be improved by standardizing questionnaires and using solid criteria, but also by excluding ongoing conventional treatments of the participants. Future studies should also improve their reporting on methodologies, such as dropout rates, comorbidity, and the included (psycho)therapeutic elements and activities in the program. In order to prevent EAI from being dismissed as an alternative intervention, it is recommended to use terms within conventional therapeutic nomenclature (e.g., trauma-focused and non-trauma-focused) to classify EAI. Optimizing the field of EAI requires establishing a dose-response curve and investigating which elements are required for optimal treatment outcomes, both regarding program activities and individual vs. group-based programs. It is therefore recommended that future studies include measurements at pre-, mid-, and post-treatment, at 3-month follow-up, and preferably also after a year to investigate long-term effects. By using physiological measures in addition to psychological measures, bias could be reduced, and psychometric results could be placed into perspective.

Funding Statement

This study was funded by Vrienden Diergeneeskunde, Dr. C.J. Vaillantfonds, K.F. Hein Fonds, and Karel Doormanfonds. These funding bodies did not have any influence on the design, data collection, analysis, or interpretation of the results within this study.

Data Availability Statement:

The data that support the findings of this study are available on request from the corresponding author.