Effects of season, age, sex, and housing on salivary cortisol concentrations in horses
Introduction
Analysis of salivary cortisol concentrations is increasingly used to assess the adrenocortical response of horses to potentially stressful situations, and several cortisol assays have been validated for equine saliva [1], [2], [3], [4]. From horses accustomed to routine handling by humans, saliva can be collected easily, repeatedly, and without restraint of the animal. Salivary cortisol mirrors the unbound, that is, biologically active fraction of total plasma cortisol while plasma cortisol is largely bound to carrier proteins [5].
In horses, as in other species including humans [6] and rhesus monkeys [7], cortisol release into blood follows a diurnal rhythm with the highest concentrations in the morning and a nadir in the late afternoon and evening [8], [9], [10]. This rhythm can be disrupted by even minor perturbations resulting in a damping of the daily oscillations and elevated cortisol concentrations, especially around the time of the daily nadir [10]. The diurnal rhythm in cortisol release is well reflected by concentrations in saliva [11], [12], [13], [14]. However, salivary cortisol concentrations have previously been determined in horses during specific experiments only, not allowing the comparison of undisturbed cortisol release patterns among different horse groups or times of the year. Cortisol release into blood or saliva may be physiologically influenced also by reproductive state and the sex of the individuals (humans [15] and dog [16]). Although most phases of pregnancy were without effect on plasma cortisol in horses [17], salivary cortisol increased in mares shortly before foaling [18]. Elevated plasma cortisol concentrations have also been demonstrated in pregnant women [19].
Marked increases in salivary cortisol concentrations as occurring in mares at foaling [20], in horses transported by road [21], [22], [23], or in foals at weaning [11] clearly indicate increased adrenocortical activity. However, experimentally induced increases in salivary cortisol concentrations are often relatively small and hardly exceed the range of physiological diurnal changes. This is especially true for studies aimed at assessing stress in horses submitted to equestrian training [3], [24], [25], [26]. Whether such increases, although statistically significant, indicate that horses perceive a particular challenge or procedure as an acute stressor often can be doubted, thus, for stress studies, proper controls are required. The accuracy of studies evaluating adrenocortical activity in horses could be improved by a better understanding of the factors affecting salivary cortisol concentrations.
In this study, we have analyzed salivary cortisol concentration in horses of the same breed and genetic background and kept on the same premises over a 6-mo period from December to May. Horses have a seasonal sexual activity (reviewed in [27]), and the study period included the nonbreeding season and the breeding season. Horses were divided by age, sex, sexual activity, and reproductive state. We hypothesized that basal cortisol concentrations not only follow a diurnal but also a circannual rhythm. We further tested if salivary cortisol is affected by age of the horses, their reproductive state, and by the housing system with increased cortisol release in single housed vs group-stabled horses.
Section snippets
Animals
A total of 94 warmblood sport horses of the Brandenburg State Stud at Neustadt (Dosse), Germany, were included into the study. They were kept either in group stables on straw or in individual loose boxes on straw or wood chippings. Horses were fed oats, concentrates, and mineral supplements 3 times daily and hay twice daily. Water was freely available at all times. From December to April, all group-housed horses had daily access to a paddock for 4 to 5 h. Horses housed in individual boxes had
Results
When data from all 94 horses irrespective of group and housing system were combined, cortisol concentrations in saliva followed a diurnal pattern with the highest concentrations in the morning and a decrease throughout the day (P < 0.001). Comparison between months showed significant differences over time (overall effect P < 0.001) and post hoc tests revealed higher cortisol concentration in December vs all other months (P < 0.01) except March (Fig. 1).
Salivary cortisol concentrations in 1-,
Discussion
In this study, effects of season, time of day, age, sex and reproductive stage, and group housing vs individual stabling on cortisol concentrations in saliva of horses were analyzed. As could be expected from nontreated control group horses in previous studies [11], [12], [13], [14], salivary cortisol concentrations showed a clear diurnal rhythm with the highest values in the morning and a nadir in the late afternoon and evening. A diurnal rhythm in salivary cortisol concentrations existed also
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