Toward a diagnostic framework for the equine depressive phenotype: A narrative review

Human clinical depression is a heterogeneous affective disorder that results from a myriad of genetic and environmental influences. Although stress-induced rodent models of depression have existed for some time, it has not yet been fully established whether depression exists in out-bred populations of non-rodent animal species such as the horse. Research strongly suggests that, where species-appropriate tests of depression are applied, the co-occurrence of specific criteria has the ability to identify animals in depressive states. This is critically important because depressive-like states are known to be a sign of poor health and/or welfare, and for some captive and domestic animal species, may also affect performance and productivity. This review article focuses on the domestic horse and uses an integrated approach to assess species-appropriate tests that have the potential capability to identify depressive individuals for this species. In line with human and other animal studies, the review concludes that depression in the horse can potentially be identified through the co-occurrence of specific biomarkers. These are primarily measures of inactivity, measures of non-reactivity, anhedonia, changes in sleep parameters and reduction in cognitive attention. This information provides a platform for further research to a) establish baseline parameters and criteria thresholds for the depression-related biomarkers b) validate the multi-biomarker co-occurrence approach to identify the equine depression phenotype and c) establish protocols for the practical and logistical application of measuring depression-related biomarkers in the horse.