Thermal Asymmetry of the Eyes and Ears in Holstein Dairy Cows: Association with Physiological Stress During the Postpartum Period

Guilherme Violin, Simon Abraham Warju, Nanako Mochizuki and Takahiro Aoki

Background and Objective: Animal welfare has increased interest in developing simple, non-invasive methods for detecting stress and disease in farm animals. Infrared thermography (IRT) has emerged as a promising tool for monitoring physiological and pathological conditions. Although emotional stress has been associated with thermal asymmetry between the left and right sides of the face, the effect of physiological stress, particularly calving, on ocular and auricular thermal asymmetry remains unclear. This study investigated whether calving induces thermal asymmetry in the eyes and ears of Holstein dairy cows and evaluated the influence of subclinical hypocalcemia on these thermal responses. Materials and Methods: A total of 29 Holstein dairy cows were monitored using infrared thermography at four sampling points: Approximately one week before the expected calving date and once daily during the first three days postpartum. Cows with clinical disease or recent treatment were excluded. Based on serum Ionized Calcium (iCa²⁺) concentrations, cows were classified as healthy (n = 13; iCa²⁺ >1.00 mmol/L at all sampling points) or having subclinical hypocalcemia (n = 16; iCa²⁺ <1.00 mmol/L at least once). Paired t-tests were used to compare left and right eye and ear temperatures, with statistical significance set at p<0.05. Results: No significant thermal asymmetry was detected before calving. In healthy cows, significant left-right differences in eye temperature were observed on the first and second days postpartum, while ear temperature showed significant asymmetry on the second postpartum day (p<0.05). Thermal symmetry returned by the third postpartum day. In contrast, cows with subclinical hypocalcemia exhibited no significant thermal asymmetry at any sampling point. Conclusion: Physiological stress associated with calving induces a transient thermal asymmetry in the eyes and ears of healthy Holstein dairy cows, which can be detected using infrared thermography. However, this response is not evident in cows with subclinical hypocalcemia, suggesting that metabolic disorders may alter thermographic patterns. These findings support the application of infrared thermography as a non-invasive tool for monitoring calving-related stress while highlighting the importance of considering calcium status when interpreting postpartum thermal assessments.

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