Pupillometry to show stress release during equine sports massage therapy

Abstract

Abstract Anecdotal reports state that wellness treatment for horses, such as massage therapy, relaxes the treated animal. Massage therapists and horse owners typically report an ”improvement” without verifying or quantifying the treatment results. This paper shows that the effect of wellness treatment and stress release can be measured. One of the horse’s pupils was photographed at the experiment's beginning and end. Horses were randomly assigned to two experimental groups, animals receiving a massage (N=18) and horses standing for the time of a massage in the stable lane (N=10). We opened the images of the pupil with Fiji (ImageJ) and used the elliptical selection tool to measure the pupils’ and iris’ areas. The ratio between the pupils’ aperture and the iris’ area was a normalized measure for pupil size. At the end of the experiment, we compared the normalized size of the pupils with a two-tailed paired t-test within groups and a two-tailed t-test between groups. For the experimental group, it was before and after the treatment, and for the control group, before and after the horse was placed in the stable lane. Comparisons between the experimental and control groups were made at the procedure's beginning and end. The treatment decreased the normalized pupil area in the experimental group, on average, by a factor of 0.78 ± 0.15. The change was statistically significant. For the horses in the control group, the pupil size increased, on average, by a factor of 1.14 ± 018. The differences are not statistically different. The initial pupil size of the horses in the experimental group was 1.88-times larger than that in the control group. After the treatment, the difference was reduced to a factor of 1.25. In conclusion, massage therapy decreases the pupil area in horses. At the beginning of the experiment, the horses in the experimental group had larger pupil sizes than the horses in the control group, indicating that the horses in the experimental group were more stressed before the treatment compared to the control animals. The observed changes in pupil size in the experimental group likely resulted from enhanced parasympathetic and diminished sympathetic activity resulting from the treatment. Observed changes in pupil size agree with the anecdotal horse owner reports and the therapist’s treatment notes.