17 Use of hair cortisol analysis to evaluate the influence of housing conditions on long-term stress in horses

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Horses are kept in a variety of housing systems which often reflect the needs of both horses and their owners as well as available resources. Housing management practices not only impact horse health and performance but may also influence a horse's physiological and psychological stress. Traditional sampling methods using blood, saliva, urine, and feces are often utilized to evaluate stress in animals by measuring acute cortisol levels. Hair cortisol analysis allows for the quantification of long-term cortisol which may be a better indicator of prolonged periods of elevated stress. The objectives of this study were to evaluate the influence of transitioning horses from outdoor group housing to individual box stalls on hair cortisol concentrations (HCC) and determine the relationship between physiological and behavioral stress responses. Hair samples were collected approximately every 30 d from the same location on 11 Quarter Horses (5 mares, 6 geldings; 3.5 ± 1.3 yr) over a period of 180 d (90d group housing/90d box stalls). Hair cortisol production was measured via enzyme-linked immunosorbent assay (ELISA) and horse heart rate (HR) and behavior were recorded throughout a series of novel object and sensory sensitivity tests. Data were analyzed using PROC MIXED and Pearson correlations in SAS v. 9.4. HCC ranged from 0.6 pg/mg to 3.0 pg/mg throughout the study with an average concentration of 1.3 ± 0.4 pg/mg. Overall, there was no difference in HCC due to the type of housing system. However, HCC increased during the 30-d period immediately following the transition from group housing to individual housing and then decreased during the next 30 d period ( P < 0.05). These results suggest that the horses in this study experienced an increase in physiological stress due to the change in housing system but were able to adapt to their new environment. Interestingly, HCC were influenced by the color of the horse's mane and tail ( P < 0.01). Analysis of the relationship between physiological and behavioral responses during the novel object and sensory sensitivity tests revealed mixed results. Horse HR and reactivity were positively correlated during both sound sensitivity tests ( P < 0.05). However, behavioral observations and reactivity during the novel object, smell, and taste sensitivity tests were influenced by the stimulus ( P < 0.05). As behavioral responses may not always reflect physiological responses and only provide short-term measures of stress, hair cortisol has the potential to serve as a biomarker of long-term stress in horses and provides a tool to monitor the effects of management practices such as housing systems.