Overnight dexamethasone suppression and cortisol index tests in clinically healthy horses and with crib-biting

The dexamethasone suppression test (DST) has been used in humans for the diagnosis of psychiatric problems such as depression, anxiety and Cushing's syndrome, and in horses for the ante mortem diagnosis of dysfunction of the intermediate part of the pituitary.

This study aimed to examine the functionality of the hypothalamic-pituitary-adrenal axis through the DST and the cortisol index (CI), to evaluate the rhythmicity of the circadian cycle of cortisol in healthy horses with crib-biting or windsucking.

A total of 20 Colombian Creole horses of both sexes, under similar management conditions and a complete stabling system, formed two study groups: crib-biting horses (G1) (n = 10) and non-crib-biting horses (G2) (n = 10). Blood samples were taken in the morning and afternoon to determine the CI, and immediately after collection in the afternoon, the DST protocol was established. The serum cortisol concentration was determined through sandwich ELISA at all established times and after dexamethasone treatment for each group.

Serum cortisol concentrations were equal in both groups (G1: 9.52 ± 7.01 µg/dl; G2: 8.4 ± 5.30 µg/dl), with no difference in CI, and DST was positive in all animals regardless of their clinical condition.

The DST and the CI did not distinguish horses with and without crib-biting. It is necessary to implement techniques with greater precision in longitudinal studies and establish specific reference values and cutoff points for the interpretation of cortisol suppression for cases of crib-biting, accompanied by other hormones such as adrenocorticotropic hormone and corticotropin-releasing hormone, owing to their multifactorial nature and high cortisol variability.