Comparison between blood serum and salivary cortisol concentrations in horses using an adrenocorticotropic hormone challenge

P>Reasons for performing study: In horses, serum cortisol concentration is considered to provide an indirect measurement of stress. However, it includes both free and bound fractions. The sampling method is also invasive and often stressful. This is not the case for salivary cortisol, which is collected using a more welfare-friendly method and represents a part of the free cortisol fraction, which is the biologically active form. Objectives: To compare salivary and serum cortisol assays in horses, in a wide range of concentrations, using an adrenocorticotropic hormone (ACTH) stimulation test, in order to validate salivary cortisol for stress assessment in horse. Methods: In 5 horses, blood samples were drawn using an i.v. catheter. Saliva samples were taken using swabs. Cortisol was assayed by radioimmunoassay. All data were treated with a regression method, which pools and analyses data from multiple subjects for linear analysis. Results: Mean +/- s.d. cortisol concentrations measured at rest were 188.81 +/- 51.46 nmol/l in serum and 1.19 +/- 0.54 nmol/l in saliva. They started increasing immediately after ACTH injection and peaks were reached after 96 +/- 16.7 min in serum (356.98 +/- 55.29 nmol/l) and after 124 +/- 8.9 min in saliva (21.79 +/- 7.74 nmol/l, P < 0.05). Discharge percentages were also different (225% in serum and 2150% in saliva, P < 0.05). Correlation between serum and salivary cortisol concentrations showed an adjusted r2 = 0.80 (P < 0.001). The strong link between serum and salivary cortisol concentrations was also estimated by a regression analysis. Conclusions: The reliability of both RIAs and regression found between serum and salivary cortisol concentrations permits the validation of saliva-sampling as a noninvasive technique for cortisol level assessment in horses.