Prolonged latency of pupillary light reflex in confined sows: Possible stress-related symptom?

This study aimed to compare pupillary light reflex (PLR) of stall-housed sows with group-housed sows because PLR is a sensitive indicator reflecting psychological state in humans and animals. Six hundred thirty-seven pregnant sows confined in gestation stalls were examined and PLR times (PLRTs) were measured. The results of the chi-square test showed that no significant difference was found in the pupils with PLRT ≥10 seconds between the left and the right pupils (χ2 = 0.03, P > 0.05), and the right-sided pupils with PLRT ≥10 seconds was significantly correlated to left-sided ones (χ2 = 181.85, P < 0.01). Among the 637 individuals, only 38 individuals were found whose PLRT showed >10 seconds on both pupils, accounting for 5.5% of the total. With the treatment of 0.125% of pilocarpine, neither 64 pupils with PLRT ≥10 seconds on the left nor 66 pupils with PRT ≥10 seconds on the right showed sensitivity to pilocarpine. The results also showed that the confined sows had longer PLRT than that of the grouped sows on both the left (13.97 ± 2.96 vs. 8.15 ± 0.97, P < 0.001) and the right (14.01 ± 2.87 vs. 8.19 ± 0.82, P < 0.001) pupils. However, the difference in ratio of the initial pupil size to maximally constricted size was not significant between the 2 groups (P > 0.05). In conclusion, the prolonged latency of PLR or increased PLRT found in the confined sows appeared to be a stressful response to chronic stress. PLRT has the potential to be an indicator used to measure stressed animals.