Salivary cortisol as an indicator of stress

Already in 2006, Marie Peeters, under the supervision of professors Marc Vandenheede (Faculty of Veterinary Medicine – Veterinary Ethology and Animal well-being Division) and Pascal Poncin (Faculty of sciences, biological behavior unit) of the University of Liège, devoted herself to a first ever study “Behavioral and physiological appreciation of stress levels in hospitalized domestic horses”, which served as a springboard to her doctoral research. For six months, the doctoral student studied 13 horses hospitalized in an equine clinic in the context of benign surgical intervention. The temperament of the animal as well as its stress levels generated by the handling and environment of the clinic were assessed during the period of hospitalization. The quality of induction and waking, stages marking a general anesthetic and considered to be “particularly sensitive moments”, were also the subject of monitoring and qualitative assessment. Given the impossibility of interviewing the animal about how it felt, the researcher based her studies on measurable clues known to be related to stress. In this case, physiological variables like the dosage of hormones or measurement of the pulse rate and its variability, but also behavioral variables, measured during rest and during reactivity tests when confronted by humans or a new object. “Taking account of the fact that stress levels can be altered by factors other than the stressful situation-sickness or an effort, for example-it is not advisable to base ones study only on the analysis of a physiological function. In a stressful situation, the reaction of the horse is both behavioral-modification of normal behavior-and physiological, caused by the nervous system: increase in the pulse and breathing rate, decrease in digestion, etc.” explains Marie Peeters.

Fortunately for researchers, one of the responses to stress is – neuroendocrine, caused by the hypothalamic pituitary adrenal axis (HPA): on sight of the stressful element this axis is stimulated. This provokes an increase in the secretion of various hormones such as cortisol, largely recognized as been directly related to the stress level in both humans and horses. In the blood, most of the cortisol attaches itself to proteins and a small portion remains free. In a stressful situation this attached cortisone increased slightly, until saturation, while the free cortisol increases more strongly.

This unattached cortisol exits the blood vessels and makes its way into tissue and saliva. “Despite the fact that, methodically, the saliva sample is painless and more easily preserved than the blood sample, the concentration in salivary cortisol is a better representation of the importance of the stress level than that measured in the blood. For the same stress, the concentration in cortisol increases by a factor of 10 to a factor of 1 in the blood”, explains Marie Peeters.  “The objectives of this end-of-study work were to assess the stress level in a hospitalized horse –by means of behavioral and physiological measurements-, assessing its temperament and to relate these measurements to the quality of induction and waking”.