Abstract
Valid measures of stress are needed in horses to determine over-all stress levels and to identify stress triggers, used to ensure management is kept within the animals’ ability to cope and welfare is not compromised. Levels of circulating cortisol reflect HPA-axis activity, and excretion into saliva and faeces allow non-invasive sampling. We validated an enzyme-linked immunoassay (ELISA) for horse salivary and faecal cortisol and validated these as indicators of acute and over-all stress levels in riding horses. Saliva was swabbed every 30-minutes over three days in N=15 horses with eight in light exercise. Faeces were collected from N=9 working horses on stabled workdays and at rest at grass for three consecutive weeks. Immunological validity of the ELISA was demonstrated by high specificity, accuracy, precision and sensitivity. Biological validity of salivary cortisol was demonstrated by diurnal decline and elevation post-exercise both mirroring known patterns in plasma cortisol; and by a trend towards elevation following 10 minute exposure to a known stressor. Faecal cortisol was biologically validated by decline between working and rest days. Large individual differences in assay values were found and not all individuals followed the group means. Salivary cortisol was labile, and although it has a close temporal relationship to circulating cortisol, measures may be confounded by environmental disturbance, pulsatile release patterns and diurnal rhythm. Careful sampling protocols are therefore needed. Faecal cortisol as an index of circulating cortisol has a 24-hour time lag to excretion, and collection protocols must evenly sample total faecal mass due to uneven hormone distribution and be frozen immediately post-excretion to avoid degradation. With careful sampling, salivary cortisol may be used to measure acute stress responses to identify stress triggers, and faecal cortisol may be used to compare over-all stress levels over longer-term conditions.
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