Introduction There are persistent demands for well-designed randomized controlled trials (RCTs), including follow-up measurements, in studies on animal-assisted treatment (AAT). In addition, a possible dose-response relationship is under discussion. The aim of the present study was to investigate the efficacy of a single-session AAT with sheep, including a booster exercise, over a follow-up period of four weeks. Methods In an RCT, a single-session AAT with sheep in a group setting, including an imaginative booster exercise conducted in the week following the AAT session, was compared to treatment as usual (TAU). Sixty psychiatric inpatients with severe mental illness were assessed for positive and negative emotions, mindfulness, and self-efficacy expectancy at baseline (PRE), immediately after the intervention (POST), and at one-week and four-week follow-ups. Results The results indicate significant differences between the two groups at POST and still in the one-week follow-up (FU1) in three of four outcomes. Within the intervention group, within-group analyses demonstrated significant improvements from PRE to POST and from PRE to FU1 across all outcomes, with large effect sizes. At the four-week follow-up, all significant effects had diminished. Conclusions An imaginative booster exercise conducted within one week after an AAT session was effective in maintaining large effect sizes for up to one week. However, the results did not persist at the four-week follow-up. Longer follow-up periods, variations in the number of sessions, and the inclusion of active control groups are therefore necessary for further AAT studies. Trial registration https://drks.de/search/de/trial/DRKS00031347 , identifier DRKS 00031347
Schmid et al. (Thu,) studied this question.