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Animal-Assisted Intervention for Autism Spectrum Disorder: A Systematic Literature Review

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Abstract

The inclusion of animals in therapeutic activities, known as animal-assisted intervention (AAI), has been suggested as a treatment practice for autism spectrum disorder (ASD). This paper presents a systematic review of the empirical research on AAI for ASD. Fourteen studies published in peer-reviewed journals qualified for inclusion. The presentation of AAI was highly variable across the studies. Reported outcomes included improvements for multiple areas of functioning known to be impaired in ASD, namely increased social interaction and communication as well as decreased problem behaviors, autistic severity, and stress. Yet despite unanimously positive outcomes, most studies were limited by many methodological weaknesses. This review demonstrates that there is preliminary “proof of concept” of AAI for ASD and highlights the need for further, more rigorous research.

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References

  • Adams, T. (2011). A resource for studying human-animal interaction. In P. McCardle, S. McCune, J. A. Griffin, & V. Maholmes (Eds.), How animals affect us: Examining the influence of human–animal interaction on child development and human health (pp. 203–214). Washington, DC: American Psychological Association.

    Google Scholar 

  • Aman, M. G., Singh, N. N., & Turbott, S. H. (1987). Reliability of the aberrant behavior checklist: A behavior rating scale for the assessment of treatment effects. American Journal of Mental Deficiency, 89(5), 485–491.

    Google Scholar 

  • Barak, Y., Savorai, O., Mavashev, S., & Beni, A. (2001). Animal-assisted therapy for elderly schizophrenic patients: A one-year controlled trial. American Journal of Geriatric Psychiatry, 9(4), 439–442.

    PubMed  Google Scholar 

  • Baron, G. M., Lipsitt, L. P., & Goodwin, M. S. (2006). Stress and coping in autism: Scientific foundations for research and practice. In G. M. Baron, J. Groden, G. Groden, & L. P. Lipsitt (Eds.), Stress and coping in autism (pp. 53–92). New York: Oxford University Press.

    Google Scholar 

  • Bass, M. M., Duchowny, C. A., & Llabre, M. M. (2009). The effect of therapeutic horseback riding on social functioning in children with autism. Journal of Autism and Developmental Disorders, 39(9), 1261–1267.

    Article  PubMed  Google Scholar 

  • Basu, S., Kafkes, A., Giest, R., & Kielhofner, G. (2002). Pediatric volitional Questionnaire. Chicago, IL: Model of Human Occupation Clearinghouse.

    Google Scholar 

  • Beck, A. M. (2010). Animals and child health and development. In P. McCardle, S. McCune, J. A. Griffin, L. Esposito, & L. Freund (Eds.), The role of pets in children’s lives: Human–animal interactions in child development, health, and therapeutic interventions (pp. 43–52). Baltimore, MD: Brookes Publishing.

    Google Scholar 

  • Ben-Itzchak, E., & Zachor, D. A. (2007). The effects of intellectual functioning and autism severity on outcome of early behavioral intervention for children with autism. Research in Developmental Disabilities, 28(3), 287–303.

    Article  PubMed  Google Scholar 

  • Bergstrom, R., Tarbox, J., & Gutshall, K. A. (2011). Behavioral intervention for domestic pet mistreatment in a young child with autism. Research in Autism Spectrum Disorders, 5(1), 218–221.

    Article  Google Scholar 

  • Brakes, P., & Williamson, C. (2007). Dolphin-assisted therapy: Can you put your faith in DAT? Wiltshire, UK: Whale and Dolphin Conservation Society.

  • Breitenbach, E., Stumpf, E., Fersen, L. V., & Ebert, H. (2009). Dolphin-assisted therapy: Changes in interaction and communication between children with severe disabilities and their caregivers. Anthrozoös, 22(3), 277–289.

    Article  Google Scholar 

  • Burrows, K. E., Adams, C. L., & Spiers, J. (2008). Sentinels of safety: Service dogs ensure safety and enhance freedom and well-being for families with autistic children. Qualitative Health Research, 18(12), 1642–1649.

    Article  PubMed  Google Scholar 

  • Carter, E. W., Cushing, L. S., & Kennedy, C. H. (2008). Peer support strategies for improving all students’ social lives and learning. Baltimore, MD: Paul H. Brooks.

    Google Scholar 

  • Celani, G. (2002). Human beings, animals and inanimate objects: What do people with autism like? Autism, 6, 93–102.

    Article  PubMed  Google Scholar 

  • Chambless, D. L., Baker, M. J., Baucom, D. H., Beutler, L. E., Calhoun, K. S., & Crits-Christoph, P. (1998). Update on empirically validated therapies, II. The Clinical Psychologist, 51(1), 3–16.

    Google Scholar 

  • Charlop-Christy, M. H. (2008). How to do incidental teaching. Austin, TX: Pro-Ed.

    Google Scholar 

  • Christon, L. M., Mackintosh, V. H., & Myers, B. J. (2010). Use of complementary and alternative medicine (CAM) treatments by parents of children with autism spectrum disorders. Research in Autism Spectrum Disorders, 4(2), 249–259. doi:10.1016/j.rasd.2009.09.013.

    Article  Google Scholar 

  • Constantino, J. N. (2002). The social responsiveness scale. Los Angeles: Western Psychological Services.

    Google Scholar 

  • Davis, B. W., Nattrass, K., O’Brien, S., Patronek, G., & MacCollin, M. (2004). Assistance dog placement in the pediatric population: Benefits, risks, and recommendations for future application. Anthrozoos, 17(2), 130–145.

    Article  Google Scholar 

  • Dilts, R., Trompisch, N., & Bergquist, T. M. (2011). Dolphin-assisted therapy for children with special needs: A pilot study. Journal of Creativity in Mental Health, 6(1), 56–68.

    Article  Google Scholar 

  • Dunn, W. (1999). The sensory profile: Examiner’s manual. San Antonio, TX: Psychological Corporation.

    Google Scholar 

  • Edwards, N. E., & Beck, A. M. (2002). Animal-assisted therapy and nutrition in Alzheimer’s disease. Western Journal of Nursing Research, 24(6), 697–712.

    Article  PubMed  Google Scholar 

  • Endenburg, N., & van Lith, H. A. (2011). The influence of animals on the development of children. The Veterinary Journal, 190(2), 208–214.

    Article  PubMed  Google Scholar 

  • Endicott, J., Nee, J., Harrison, W., & Blumenthal, R. (1993). Quality of life enjoyment and satisfaction questionnaire: A new measure. Psychopharmacology Bulletin, 29(2), 321–326.

    PubMed  Google Scholar 

  • Esposito, L., McCardle, P., Maholmes, V., McCune, S., & Griffin, J. A. (2011a). Introduction. In P. McCardle, S. McCune, J. A. Griffin, L. Esposito, & L. S. Freund (Eds.), Animals in our lives: Human–animal interaction in family, community, and therapeutic settings (pp. 1–5). Baltimore, MD: Brookes Publishing Co.

    Google Scholar 

  • Esposito, L., McCune, S., Griffin, J. A., & Maholmes, V. (2011b). Directions in human–animal interaction research: Child development, health, and therapeutic interventions. Child Development Perspectives. doi:10.1111/j.1750-8606.2011.00175.x.

  • Friedmann, E., & Son, H. (2009). The human-companion animal bond: How humans benefit. Veterinary Clinics of North America: Small Animal Practice, 39(2), 293–326.

    Article  PubMed  Google Scholar 

  • Gabriels, R. L., Agnew, J. A., Holt, K. D., Shoffner, A., Zhaoxing, P., & Ruzzano, S. (2012). Pilot study measuring the effects of therapeutic horseback riding on school-age children and adolescents with autism spectrum disorders. Research in Autism Spectrum Disorders, 6(2), 578–588. doi:10.1016/j.rasd.2011.09.007.

    Article  Google Scholar 

  • Gardner, N. (2008). A friend like Henry. London: Hodder & Stoughton.

    Google Scholar 

  • Gast, D. L. (2009). Single-subject research methodology in behavioral sciences. New York: Routledge.

    Google Scholar 

  • Gee, N. R. (2011). The role of pets in the classroom. In P. McCardle, S. McCune, J. A. Griffin, L. Esposito, & L. Freund (Eds.), Animals in our lives: Human–animal interaction in family, community, and therapeutic settings. Baltimore, MD: Brookes Publishing.

    Google Scholar 

  • Grandin, T. (2008). Foreword. In M. Pavlides (Ed.), Animal-assisted interventions for individuals with autism (pp. 7–10). London: Jessica Kingsley Publishers.

    Google Scholar 

  • Grandin, T. (2010). The roles that animals can play with individuals with autism. In P. McCardle, S. McCune, J. A. Griffin, L. Esposito, & L. Freund (Eds.), The role of pets in children’s lives: Human–animal interactions in child development, health, and therapeutic interventions. Baltimore, MD: Brookes Publishing.

    Google Scholar 

  • Grandin, T. (2011). The role that animals can play with individuals with autism. In P. McCardle, S. McCune, J. A. Griffin, L. Esposito, & L. S. Freund (Eds.), Animals in our lives: Human–animal interaction in family, community, and therapeutic settings (pp. 183–195). Baltimore, MD: Brookes Publishing Co.

    Google Scholar 

  • Grandin, T., Fine, A. H., & Bowers, C. M. (2010). The use of therapy animals with individuals with autism. In A. H. Fine (Ed.), Handbook on animal-assisted therapy: Theoretical foundations and guidelines for practice (3rd ed., pp. 247–264). San Diego, CA: Academic Press.

    Chapter  Google Scholar 

  • Grandin, T., & Johnson, C. (2005). Animals in translation: Using the mysteries of autism to decode animal behavior. New York, NY: Scribner.

    Google Scholar 

  • Griffin, J. A., McCune, S., Maholmes, V., & Hurley, K. (2011). Human–animal interaction research: An introduction to issues and topics. In P. McCardle, S. McCune, J. A. Griffin, & V. Maholmes (Eds.), How animals affect us: Examining the influence of human–animal interaction on child development and human health (pp. 3–9). Washington, DC: American Psychological Association.

    Google Scholar 

  • Heimlich, K. (2001). Animal-assisted therapy and the severely disabled child: A quantitative study. Journal of Rehabilitation, 67(4), 48–54.

    Google Scholar 

  • Herzog, H. (2010). Some we love, some we hate, some we eat: Why it’s so hard to think straight about animals. New York: Harper.

    Google Scholar 

  • Herzog, H. (2011). The impact of pets on human health and psychological well-being: Fact, fiction, or hypothesis? Current Directions in Psychological Science, 20(4), 236–239. doi:10.1177/0963721411415220.

    Article  Google Scholar 

  • Isaacson, R. (2009). The horse boy. New York, NY: Little, Brown and Company.

    Google Scholar 

  • Johnson, S. C. (2003). Detecting agents. Philosophical Transactions of the Royal Society of London. Series B, Biological sciences, 358, 549–559.

    Article  PubMed  Google Scholar 

  • Kasari, C. (2002). Assessing change in early intervention programs for children with autism. Journal of Autism and Developmental Disorders, 32(5), 447–461.

    Article  PubMed  Google Scholar 

  • Katcher, A. H., Friedmann, E., Beck, A. M., & Lynch, J. J. (1983). Looking, talking, and blood pressure: The physiological consequences of interaction with the living environment. In A. H. Katcher & A. M. Beck (Eds.), New perspectives on our lives with companion animals (pp. 351–359). Philadelphia: University of Pennsylvania Press.

    Google Scholar 

  • Katcher, A. H., & Wilkins, G. G. (1998). Animal-assisted therapy in the treatment of disruptive behavior disorder in childhood. In A. Lundberg (Ed.), The environment and mental health (pp. 193–204). Mahwah, NJ: Lawrence Erlbaum Associates, Inc.

    Google Scholar 

  • Keino, H., Funahashi, A., Keino, H., Miwa, C., Hosokawa, M., & Hayashi, Y. (2009). Psycho-educational horseback riding to facilitate communication ability of children with pervasive developmental disorders. Journal of Equine Science, 20(4), 79–88.

    Article  Google Scholar 

  • Kern, J. K., Fletcher, C. L., Garver, C. R., Mehta, J. A., Grannemann, B. D., & Knox, K. R. (2011). Prospective trial of equine-assisted activities in autism spectrum disorder. Alternative Therapies in Health and Medicine, 17(3), 14–20.

    PubMed  Google Scholar 

  • Kortschal, K., & Ortbauer, B. (2003). Behavioral effects of the presence of a dog in a classroom. Anthrozoös, 16(2), 147–159.

    Article  Google Scholar 

  • Kršková, L., Talarovičová, A., & Olexová, L. (2010). Guinea pigs-The “small great” therapist for autistic children, or: Do guinea pigs have positive effects on autistic child social behavior? Society and Animals, 18, 139–151.

    Article  Google Scholar 

  • Kruger, K. A., & Serpell, J. A. (2010). Animal-assisted interventions in mental health: Definitions and theoretical foundations. In A. H. Fine (Ed.), Handbook on animal-assisted therapy: Theoretical foundations and guidelines for practice (3rd ed., pp. 33–48). San Diego: Academic Press.

    Chapter  Google Scholar 

  • Law, S., & Scott, S. (1995). Tips for practitioners: Pet care: A vehicle for learning. Focus on Autistic Behavior, 10(2), 17.

    Google Scholar 

  • Liberati, A., Altman, D. G., Tetzlaff, J., Mulrow, C., Gotzsche, P. C., & Ioannidis, J. P. A. (2009). The PRISMA statement for reporting systematic reviews and meta-analyses of studies that evaluate healthcare interventions: Explanation and elaboration. BMJ, 339, b2700. doi:10.1136/bmj.b2700.

    Article  PubMed  Google Scholar 

  • Lovaas, O. I. (2002). Teaching individuals with developmental delays: Basic intervention techniques. Austin, TX: Pro-Ed.

    Google Scholar 

  • Marino, L. (2012). Construct validity of animal-assisted therapy and activities: How important is the animal in AAT? Anthrozoös, 25(3), 139–151. doi:10.2752/175303712x13353430377219.

    Article  Google Scholar 

  • Marino, L., & Lilienfeld, S. O. (2007). Dolphin-assisted therapy: More flawed data and more flawed conclusions. Anthrozoös, 20(3), 239–249.

    Article  Google Scholar 

  • Martin, F., & Farnum, J. (2002). Animal-assisted therapy for children with pervasive developmental disorders. Western Journal of Nursing Research, 24(6), 657–670.

    Article  PubMed  Google Scholar 

  • Maurice, C., Green, G., & Luce, S. C. (Eds.). (1996). Behavioral intervention for young children with autism: A manual for parents and professionals. Austin, TX: Pro-Ed.

    Google Scholar 

  • McConnell, S. R. (2002). Interventions to facilitate social interaction for young children with autism: Review of available research and recommendations for educational intervention and future research. Journal of Autism and Developmental Disorders, 32(5), 351–372. doi:10.1023/a:1020537805154.

    Article  PubMed  Google Scholar 

  • McNicholas, J., & Collis, G. M. (2000). Dogs as catalysts for social interaction: Robustness of the effect. British Journal of Psychology, 91(1), 61–70.

    Article  PubMed  Google Scholar 

  • Melson, G. F. (2003). Child development and the human-companion animal bond. American Behavioral Scientist, 47(1), 31–39.

    Article  Google Scholar 

  • Melson, G. F. (2011). Principles for human–animal interaction research. In P. McCardle, S. McCune, J. A. Griffin, L. Esposito, & L. S. Freund (Eds.), Animals in our lives: Human–animal interaction in family, community, and therapeutic settings (pp. 13–33). Baltimore, MD: Brookes Publishing Co.

    Google Scholar 

  • Memishevikj, H., & Hodzhikj, S. (2010). The effects of equine-assisted therapy in improving the psychosocial functioning of children with autism. Journal of Special Education and Rehabilitation, 11(3–4), 57–67.

    Google Scholar 

  • Moher, D., Liberati, A., Tetzlaff, J., & Altman, D. G. (2009). Preferred reporting items for systematic reviews and meta-analyses: The PRISMA statement. BMJ, 339, b2535. doi:10.1136/bmj.b2535.

    Article  PubMed  Google Scholar 

  • Nathanson, D. E. (1998). Long-term effectiveness of dolphin-assisted therapy for children with severe disabilities. Anthrozoös, 11(1), 22–32.

    Article  Google Scholar 

  • Nathanson, D. E. (2007). Reinforcement effectiveness of animatronic and real dolphins. Anthrozoös, 20(2), 181–194.

    Article  Google Scholar 

  • Nathanson, D. E., de Castro, D., Friend, H., & McMahon, M. (1997). Effectiveness of short-term dolphin-assisted therapy for children with severe disabilities. Anthrozoös, 10(2), 90–100.

    Article  Google Scholar 

  • National Standards Report. (2009). The national standards project—addressing the need for evidence-based practice guidelines for autism specturm disorders. Randolph: National Autism Center.

    Google Scholar 

  • Nimer, J., & Lundahl, B. (2007). Animal-assisted therapy: A meta-analysis. Anthrozoös, 20(3), 225–238.

    Article  Google Scholar 

  • Odom, S. L., Collet-Klingenberg, L., Rogers, S. J., & Hatton, D. D. (2010). Evidence-based practices in interventions for children and youth with autism spectrum disorders. Preventing School Failure, 54(4), 275–282.

    Article  Google Scholar 

  • O’Haire, M. E. (2010). Companion animals and human health: Benefits, challenges, and the road ahead. Journal of Veterinary Behavior: Clinical Applications and Research, 5(5), 226–234.

    Article  Google Scholar 

  • Pavlides, M. (2008). Animal-assisted interventions for individuals with autism. London: Jessica Kingsley Publishers.

    Google Scholar 

  • Prince-Hughes, D. (2005). Songs of the gorilla nation. New York: Three Rivers Press.

    Google Scholar 

  • Prothmann, A., Ettrich, C., & Prothmann, S. (2009). Preference for, and responsiveness to, people, dogs and objects in children with autism. Anthrozoös, 22(2), 161–171.

    Article  Google Scholar 

  • Redefer, L. A., & Goodman, J. F. (1989). Brief report: Pet-facilitated therapy with autistic children. Journal of Autism and Developmental Disorders, 19(3), 461–467.

    Article  PubMed  Google Scholar 

  • Reichow, B., Volkmar, F. R., & Cicchetti, D. V. (2008). Development of the evaluative method for evaluating and determining evidence-based practices in autism. Journal of Autism and Developmental Disorders, 38(7), 1311–1319. doi:10.1007/s10803-007-0517-7.

    Article  PubMed  Google Scholar 

  • Reid, D. H., & Parsons, M. B. (2002). Facilitating play dates for children with autism and typically-developing peers in natural settings: A training manual. Morganton, NC: Habilitative Management Consultants.

    Google Scholar 

  • Rimland, B., & Edelson, S. M. (1999). Autism treatment evaluation checklist. San Diego, CA: Autism Research Institute.

    Google Scholar 

  • Sams, M. J., Fortney, E. V., & Willenbring, S. (2006). Occupational therapy incorporating animals for children with autism: A pilot investigation. American Journal of Occupational Therapy, 60(3), 268–274.

    Article  PubMed  Google Scholar 

  • Schopler, E., Reichler, R. J., & Renner, B. R. (1994). The childhood autism rating scale. Los Angeles, CA: California Western Psychological Services.

    Google Scholar 

  • Serpell, J. A. (2006). Animal-assisted interventions in historical perspective. In A. H. Fine (Ed.), Handbook on animal-assisted therapy: Theoretical foundations and guidelines for practice (2nd ed., pp. 3–20). San Diego, CA: Academic Press.

    Chapter  Google Scholar 

  • Serpell, J. A., Coppinger, R., & Fine, A. H. (2006). Welfare considerations in therapy and assistance animals. In A. H. Fine (Ed.), Handbook on animal-assisted therapy: Theoretical foundations and guidelines for practice (2nd ed., pp. 453–474). San Diego, CA: Academic Press.

    Google Scholar 

  • Silva, K., Correia, R., Lima, M., Magalhães, A., & de Sousa, L. (2011). Can dogs prime autistic children for therapy? Evidence from a single case study. The Journal of Alternative and Complementary Medicine, 17(7), 1–5. doi:10.1089/acm.2010.0436.

    Article  Google Scholar 

  • Smith, T., Scahill, L., Dawson, G., Guthrie, D., Lord, C., & Odom, S. (2007). Designing research studies on psychosocial interventions in autism. Journal of Autism and Developmental Disorders, 37(2), 354–366.

    Article  PubMed  Google Scholar 

  • Solomon, O. (2010). What a dog can do: Children with autism and therapy dogs in social interaction. Ethos, 38(1), 143–166.

    Article  Google Scholar 

  • Sparrow, S. S., Cicchetti, D. V., & Balla, D. A. (2005). Vineland adaptive behavior scales (2nd ed.). Livonia, MN: Pearson Assessments.

    Google Scholar 

  • Taylor, R. R., Kielhofner, G., Smith, C., Butler, S., Cahill, S. M., & Ciukaj, M. D. (2009). Volitional change in children with autism: A single-case design study of the impact of hippotherapy on motivation. Occupational Therapy in Mental Health, 25, 192–200.

    Article  Google Scholar 

  • Tissen, I., Hergovich, A., & Spiel, C. (2007). School-based social training with and without dogs: Evaluation of their effectiveness. Anthrozoös, 20(4), 365–373.

    Article  Google Scholar 

  • Umbarger, G. T., I. I. I. (2007). State of the evidence regarding complimentary and alternative medical treatments for autism spectrum disorders. Education & Training in Developmental Disabilities, 42(4), 437–447.

    Google Scholar 

  • Viau, R., Arsenault-Lapierre, G., Fecteau, S., Champagne, N., Walker, C.-D., & Lupien, S. (2010). Effect of service dogs on salivary cortisol secretion in autistic children. Psychoneuroendocrinology, 35(8), 1187–1193. doi:10.1016/j.psyneuen.2010.02.004.

    Article  PubMed  Google Scholar 

  • Wells, D. L. (2009). The effects of animals on human health and well-being. Journal of Social Issues, 65(3), 523–543.

    Article  Google Scholar 

  • White, S. W., Oswald, D., Ollendick, T., & Scahill, L. (2009). Anxiety in children and adolescents with autism spectrum disorders. Clinical Psychology Review, 29(3), 216–229.

    Article  PubMed  Google Scholar 

  • Wilson, C. C. (1991). The pet as an anxiolytic intervention. Journal of Nervous and Mental Disease, 179(8), 482–489.

    Article  PubMed  Google Scholar 

  • Winchester, P., Kendall, K., Peters, H., Sears, N., & Winkley, T. (2002). The effect of therapeutic horseback riding on gross motor function and gait speed in children who are developmentally delayed. Physical and Occupational Therapy in Pediatrics, 22(3–4), 37–50.

    PubMed  Google Scholar 

  • Wood, M. W. (2006). Techniques for searching the animal-assisted therapy literature. In A. H. Fine (Ed.), Handbook on animal-assisted therapy: Theoretical foundations and guidelines for practice (2nd ed., pp. 413–424). San Diego, CA: Academic Press.

    Google Scholar 

  • Wood, L., Giles-Corti, B., & Bulsara, M. (2005). The pet connection: Pets as a conduit for social capital? Social Science and Medicine, 61(6), 1159–1173.

    Article  PubMed  Google Scholar 

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Acknowledgments

Marguerite E. O’Haire gratefully acknowledges Virginia Slaughter, Samantha J. McKenzie, Mitchell T. Zischke (The University of Queensland), and Chelsea C. O’Haire (George Washington University) for their thoughtful and constructive comments regarding drafts of this manuscript. This research was funded by The University of Queensland Research Scholarship and Graduate School International Travel Award.

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    Marguerite E. O’Haire

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O’Haire, M.E. Animal-Assisted Intervention for Autism Spectrum Disorder: A Systematic Literature Review. J Autism Dev Disord 43, 1606–1622 (2013). https://doi.org/10.1007/s10803-012-1707-5

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