Eye surface infrared thermography usefulness as a noninvasive method of measuring stress response in sheep during shearing: Correlations with serum cortisol and rectal temperature values

During shearing, animals' welfare is adversely affected and acute stress occurs. Once animal perceives a threat, it develops behavioral, autonomic, endocrine or immune responses to maintain homeostasis. This study aimed to investigate the usefulness of eye temperature assessment by infrared thermography (IRT) to evaluate acute stress response in sheep undergoing a shearing procedure. From each animal, blood sampling, rectal and eye temperature assessment were performed before shearing (T-PRE), 5 (T-POST5) and 60 (T-POST60) minutes after the end of shearing procedure. On blood samples the serum cortisol concentrations were evaluated. Rectal temperature (T-RECTAL) was measured using a digital thermometer. Thermographic acquisitions of eye temperature were performed from the eye total area (T-EYE) and from three regions of interest (lateral canthus, TROI-1; central cornea, TROI-2; medial canthus, TROI-3). One-way analysis of variance showed a significant increase of serum cortisol concentration, T-RECTAL, T-EYE and TROI-3 (p < 0.001). Serum cortisol was positively correlated with TTRECTAL and TROI-3 at T-POST5 and T-POST60. T-RECTAL resulted positively correlated with TROI-3 at TPRE, T-POST5 and T-POST60. Agreement between TRECTAL and each eye temperature considered (TEYE, TROI-1, TROI-2, TROI-3) has been shown by Bland-Altman plots at each time point of monitoring period. The findings obtained in the current survey suggest that the medial canthus is the most suitable region for eye temperature measurement to asses stress response in animals. Moreover, this study highlighted the usefulness of IRT as an immediate and noninvasive physiological measure to assess stress response in sheep.