Seasonal Dynamics of Ocular Infrared Thermography (OST) in Heifers under Tropical Conditions: Links with Cortisol and Physiological Stress

Abstract

Heat stress presents a significant challenge for livestock production in tropical climates, creating a need for reliable non-invasive indicators to monitor animal welfare. This study assessed the seasonal variability of ocular surface infrared thermography (IRT), which has potential to assess an animal’s stress response, as well as its association with serum cortisol and physiological parameters. A total of 36 heifers [Sahiwal (SW) = 18; Karan Fries (KF) = 18] were randomly assigned each season, with six animals observed twice per season. OST was documented at three ocular locations—the inner canthus (EYETi), central ocular surface (EYETc), and outer canthus (EYETo) were obtained via IRT, and rectal temperature and respiratory rate were recorded, and serum cortisol was obtained simultaneously using ELISA. Among all the ocular regions, the EYETi region demonstrated the most consistent and responsive thermal variation across seasons, with higher mean temperatures of 33.53 ± 0.30°C in winter to 35.96 ± 0.14°C in spring, and reaching a peak at 40.02 ± 0.17°C during summer in KF. These values corresponded with seasonal increases in RT (from 100.68 °F to 102.72°F) and serum cortisol levels, which were highest during summer, particularly in Karan Fries, indicating heightened heat sensitivity in them. Ocular surface temperature correlated positively with established stress indicators, including RT (r = 0.89, p < 0.001), respiration rate RR (r = 0.92, p < 0.001), and serum cortisol level (r = 0.88, p < 0.001) suggesting that infrared thermography of the inner ocular region can serve as a non-invasive, reliable indicator of physiological stress in growing animals.