A scoping review of neuroendocrinological biomarkers with potential for use in equine behavioral medicine practice

Equine emotion and welfare are of increasing concern to the general public, equestrians and researchers. There is growing demand for more objective indicators of specific emotional states, as well as the well-being of horses in a variety of contexts. Such markers may also assist in both the diagnosis and selection of biological therapies within equine behavioral medicine. This scoping review summarizes and appraises the published findings relating to relevant biological measurements from the last two decades. Particular attention is given to methodological, subject-related and context-dependent factors that might affect reference values. The most common biomarkers found were ACTH, cortisol and serotonin. Other biomarkers found, but with fewer reports, were oxytocin, melatonin, dopamine and tryptophan. We conclude that research on these topics has used diverse methodological approaches which makes their current clinical value hard to determine. This could explain the common recommendation for the need for further research. To create useful, valid and applicable indicators for the study of emotional states in clinical environments, there is a need for both agreed definition of relevant subjective states and greater consensus on the standardized protocols to be used for sample collection, analysis and units of measurement reported. Our findings provide a solid basis for the further development of the assaying of these biomarkers for more routine use in both clinical practice and equine welfare research.