Technical note: Comparison of salivary and serum cortisol concentrations after adrenocorticotropic hormone challenge in ewes

An ACTH challenge was conducted to determine if salivary cortisol concentration reflects serum cortisol concentration in ewes. Twelve yearling ewes (64.0 ± 1.2 kg) were administered ACTH (100 IU, intravenously) or saline. Serum and salivary samples were collected at 30-min intervals for 2 h before ACTH administration, at 15-min intervals for 2 h after treatment, and at 30-min intervals for an additional 3 h, and cortisol concentration was determined by RIA. Although ewes responded to ACTH and saline, cortisol concentration was greater (P < 0.001) in ACTH-treated ewes from 15 to 120 min and tended to be greater (P = 0.054) at 150 min after challenge in serum. In saliva, cortisol concentration was greater (P < 0.001) in ACTH-treated ewes from 30 to 120 min and tended to be greater (P = 0.092) at 15 min after challenge. No difference was observed between ACTH-treated ewes and controls for time to peak serum cortisol concentration (P = 0.126) and time to peak salivary cortisol concentration (P = 0.109), or between saliva and serum for time to peak cortisol concentration (P = 0.220) and return to baseline cortisol concentration (P = 0.341). The serum (P = 0.009) and salivary (P = 0.050) cortisol areas under the curve between 0 and 150 min were greater for ACTH-treated ewes than controls, and serum (P = 0.002) and salivary (P < 0.001) cortisol return to baseline concentration was longer for ACTH-treated ewes. The correlation coefficient between serum and salivary cortisol concentrations was 0.88 (P < 0.001). These data indicate that salivary cortisol concentration is closely related to serum cortisol concentration and that the former may represent a suitable noninvasive alternative to blood collection for measurement of cortisol in sheep.