Technical note: Effects of an epinephrine infusion on eye temperature and heart rate variability in bull calves

Changes in autonomic nervous system (ANS) activity are one of the first phases of a stress response, but they are rarely used to assess the welfare of farm animals. Eye temperature measured using infrared thermography (IRT) is proposed as an indicator of ANS activity because it may reflect changes in blood flow in the capillary beds of the conjunctiva. The aim was to determine whether epinephrine infusion would initiate eye temperature changes in calves. Sixteen 4-mo-old Friesian calves (124 ± 5 kg) were assigned randomly to receive a jugular infusion of either epinephrine (4 μg/kg per min for 5 min) or saline. Eye temperature (°C), heart rate (HR), and HR variability (HRV) were recorded from 15 min before infusion until 10 min after it was completed. Blood samples collected via jugular catheter were assayed for epinephrine, norepinephrine, and cortisol concentrations, and packed cell volume (PCV) was measured. No changes in any variable were observed with the saline infusion. Plasma epinephrine concentrations increased 90-fold with epinephrine infusion, which was associated with a decrease in eye temperature of 1.4 ± 0.05°C. During epinephrine infusion, plasma norepinephrine concentrations decreased by half and HR decreased by 9.3 ± 3.3 beats/min. The HRV measure, the root mean square of successive differences, increased by 49.7 ± 9.2 ms, indicating a compensatory increase in parasympathetic activity. After epinephrine infusion, plasma cortisol concentrations increased by 10.4 ± 1.7 ng/mL and PCV was higher (38 vs. 31 ± 0.1%, epinephrine vs. saline, respectively). These results support the hypothesis that changes in eye temperature are mediated by the sympathetic component of the ANS. Infrared thermography is a noninvasive method to assess ANS activity for evaluating welfare of cattle.